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Let me pose a general fud-4-thot question.Helpful Member!(6) 

wktaylor (Aeronautics) (OP)
13 Apr 11 20:16
OK Guys...

Let me pose a general fud-4-thot [I loved "the Farside"] question.

Note. This thread has the intent of raising the hairs on Your neck... and stimulating discussion. Hopefully it will also raise general awareness as to how amazingly vulnerable airframe structures are to a variety of damaging factors. Relatively new engineers... or those not involved with mishap investigations and damage repairs [depot liaison, field support, etc]... will gain some insights from those of us "gray-hairs" who have "been-there, seen-that, done-that...

For all airframe structures only... NOT including engines, landing gear, instruments/electronics, mechanical/electrical/environmental systems, etc [maybe we'll try this same question for each of these later]...

Identify obvious, and NOT-so-obvious mechanisms, for structural damage.

Sub points to carefully consider:

There ARE substantial differences regarding the aircraft Type, IE: para-sails; Ultra-lights; GA; Commuters/corporate; Medium and heavy transports; Cargo; Military [USAF, USN, USCG, USA, IE: Trainers, Fighters, Bombers, Helos, etc]; Light helos; Heavy helos; LTAs, etc...

There are substantial differences regarding the aircraft construction Type, IE: all fabric; Tube-fabric; Wood; Sheet metal; Machined-metal; Composites-metal mixes; all composite;  etc...

There are differences regarding Mission type, IE: training, commuting, airlines, cargo, tactical, etc...

I'll lead the parade...

Examples of damage mechanisms.

Some environmental factors. Ice/slush, "Arizona dusty road", rain, hail, airborne volcanic dust, bird/critter-nests...

Operational factors: overstress [g, airspeed], hard landings, taxi collisions, severe turbulence, bird-strikes, lightning strikes...

Assembly and Maintenance factors: force-fitting parts, poorly drilled holes, loose/incorrect fastener installations, sealant adhesion failures...

Exposure to corrosive or abusive fluids/materials: urine, salt water, detergent wash-water, jet-fuel additives, hydrazine, deicing compounds...

GOT the concept??? Your comments appreciated.
 

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

MintJulep (Mechanical)
13 Apr 11 20:47
Collision from service vehicle.  Closing door or hatch on obstruction.  Counterfeit parts.  Over tightening fasteners.  Poorly written or ambiguous maintenance instructions. Misinterpretation of inspection results.
KENAT (Mechanical)
13 Apr 11 20:48
Deliberate sabotage be it by malcontent employees or terrorists.

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MikeHalloran (Mechanical)
13 Apr 11 21:22
Collateral damage from salvage ops.
{
Super Cub banner plane went down off local beach some years ago.
Cylinder head stud failed, cylinder fell off, engine lost power.
It looked okay until they pulled it through the surf to get it on a truck.
Probably should have torn off the fabric while it was still in deep water.
}
 

Mike Halloran
Pembroke Pines, FL, USA

SparWeb (Aerospace)
13 Apr 11 21:36
Turboprop airplanes landed regularly on unpaved runways: propellers throw rocks at the fuselage and "golf-ball" the skin around the nose and break pilot's window.

Airplanes in arctic/antarctic land on ice runways, park overnight, then the skis freeze into the ice.  Ground crews try to free the aircraft by pushing sideways on the tail (there's a FAR for this load condition!).  Sometimes works sometimes breaks the oleo scissor.

Speaking of the oleo scissors:  usually broken by ground crew putting the tow bar on the nose wheel, then driving the tractor away without un-pinning the scissor.  Meanwhile the control locks inside some airplanes (eg Piper Navajo) prevent the nosewheel from turning.  Something breaks.  Sometimes many things break.

Boss was replacing fabric on his Piper Pacer.  Discovered some previously fabric covered sheet metal part with a bullet hole in it.  Hole too big to be a screwdriver drop so...

I could go on and on and on.

Steven Fahey, CET

thruthefence (Aerospace)
13 Apr 11 22:19
De-Bogging an aircraft in an unapproved fashion. Tow straps on anything that looks handy, & so forth. Damage is ofter hidden, for example, a powerful enough tug will snatch most any airplane out of the mud, by pulling on the nosewheel, there's some stuff up in there that ain't designed to be yanked on.

exceeding turn limits while towing.
KENAT (Mechanical)
13 Apr 11 22:42
On military aircraft problems on release of items in flight.  

Not just the classic of the store dropping down, getting caught in some kind of slip steam or similar and then coming back up to the A/C but also issues like the snatch disconnect on MIL38999C connectors on smart stores failing and the store pulling a bunch of cabling out of the A/C and damaging associated structure or similar.

Also accidental detonation of stores on A/C.

Heck, just loading the stores in the first place is fought with potential issues of driving weapons loaders into aircraft on the ground, connecting lanyards etc incorrectly and so on.

Did anybody say FOD yet?

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berkshire (Aeronautics)
13 Apr 11 23:40
Wil did not put sailplanes on the list, but they are not exempt.

Example: several years ago an FAA Licensed repair station in Virginia undertook the annual inspection of a sailplane made by Glasflugel in Germany. They took the rudder off to inspect the hinge mechanism as part of the annual. On this aircraft the rudder is operated by a pushrod and swash plate mechanism, not rudder cables. It is possible to re-install this upside down, and the station did. The aircraft was signed off and returned to service. The owner discovered that the system was upside down on take off (The control sense is reversed.) he released and almost got the aircraft stopped before instinct took over and he pushed the rudder pedal the wrong way. The resulting course deviation put the glider through a barbed wire fence tearing holes in both wings.
 At no point did the repair station before release or the owner do a control check to see that the controls were working in the correct sense I.E. right foot right rudder left foot left rudder.

 This same manufacturer sent me an aileron belcrank for another aircraft that was for the wrong wing, upon doing a control check we noticed that the ailerons were going up and down in unison like flaps, not one up one down like they should.

 For those in light aircraft in the USA who think this cannot happen to them, Bellanaca used the same knuckle drive on their ailerons.
B.E.
moon161 (Mechanical)
14 Apr 11 1:22
I've flown around 20% of the miles i've driven, statistically, I'm due for a plane accident. I'll still sleep if I don't get the window seat.  
lisa247 (Aerospace)
14 Apr 11 3:34
How does that make you due a plane accident moon161?
rb1957 (Aerospace)
14 Apr 11 7:55
bogus material
faked certs
effed up analysis
invalid testing
over-confidence (analysts or pilots mainly)
political influence in the design/cert process
IRstuff (Aerospace)
14 Apr 11 10:02
Getting one's tail assembly whacked by an A380 wingtip
Whacking a CRJ700 tail assembly with one's wingtip
Vibration
Pressurization cycles
Wing loading and flexure
Not following engine R&R procedures, ala AA DC-10
Not following maintenance procedures in general

TTFN

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MiketheEngineer (Structural)
14 Apr 11 11:16
Without my knowledge - nose wheel fell off on take off - metal fatigue.

Didn't need much braking upon landing.. and quite surprised when composite propeller shattered into a thousand slivers.
berkshire (Aeronautics)
14 Apr 11 15:28
IR stuff.
I guess the A380 rates as a taxi collision, as refered to in Wil's original post. We have also had the 737 unzipping problem.
   I wonder what will show up next.
B.E.
KENAT (Mechanical)
14 Apr 11 15:50
Over pressurization during fueling.

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verymadmac (Mechanical)
14 Apr 11 18:05
Landing slightly short and having fence posts leaving marks on the underside of the wing.

Forgetting to set park break on a sloping strip and having the aircraft roll off down hill.

Using rope as a long line on a helicopter, managing to break it & having it recoil up through the rotor-head.

Use of SRM repairs outside of limits.

Overloaded operations
i.e ag spraying in R22
with Instructor + student + spray gear + min fuel = over gross and still nothing in the spray tanks.

 
SparWeb (Aerospace)
14 Apr 11 18:33
Have all you guys seen the photo of the Cessna Caravan with a caved-in wing leading edge, and 50 feet back, a dead giraffe on the runway?

If anyone wants to see I can dig up the photo.
 

Steven Fahey, CET

der8110 (Aeronautics)
14 Apr 11 20:58
Let's not bring the internet to a grinding halt... but here's a few unusual ones, with no FAR (yet!):
- wire rope indentation on leading edge (radio station guy wire)
- seat belt half hanging out door, beating up door and fus.
- antenna not screwed down, beat up fairing until departing aircraft
- overzealous pressure refueling
- 3" long fully threaded screw, more than long enough for interior trim (poked through)
- door stay chains, caught between door and frame
- passenger stepping on stairs before fully lowered, tore out chunk of door frame
- antenna removed by sawing off screws (hacksaw between fus. and base)
- antenna pried off with what must have been a can opener

 
Andries (Aeronautics)
15 Apr 11 0:40
You have convinced me - I'm not flying any more!

Andries
verymadmac (Mechanical)
15 Apr 11 2:42
Parachute rig fouling tail plane
Parachutists hanging onto / climbing onto tail plane / fin
Helpful Member!  debodine (Electrical)
15 Apr 11 10:19
I too loved Far Side.  My favorite was the first deer in the forest has a target rondelle on his side, and the second deer says, "Bummer of a birthmark, Hal."

 
KirbyWan (Aerospace)
15 Apr 11 12:26
One of the damage mechanisms I face alot is where two pieces of metal are pressed up against each other but not bolted.  The interfaces between parts with such things as finger seals where relative motion impresses the shape of one part into the other, usually to the point that both need to be replaced.  When repaired on a normal schedule these do not present a problem, but I have seen some components where you know the usual preventive maintenance that would have found the damage was ignored and ended up being such severe damage that the cost of repair was far in excess of what it would have cost if they had performed consistent maintenance.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

Kwan (Aerospace)
15 Apr 11 12:34
I'll throw in a couple.

lighning strike.  I get to see these once in awhile.  I've seen chunks missig from the trailing edge.

Another issue is vibration.  Acoustic cracking, excessive engine vibration.

Heat treat issues.  These can lead to lower corrosion resistance, allowables, alpha case, etc.

The root cause can be a result of one (or more) of thousands (can I say millions?) of abmormalties.

One last thought about the glider.  The pilot should be verifying control system direction before flying (everytime!).  This would have shown the mis-rig of the rudder.  It also shows that full travel occurs (imagine FOB jamming a control cable).

Those are my thoughts Wil.  What kinda food do I get?

 
MintJulep (Mechanical)
15 Apr 11 13:01

Quote:

One last thought about the glider.  The pilot should be verifying control system direction before flying (everytime!).  This would have shown the mis-rig of the rudder.  It also shows that full travel occurs (imagine FOB jamming a control cable).

I don't have to imagine it.  It's happened to me.

And yes, I did a full control check before take-off.

Mechanic left a flashlight under the seat pan.  Control function was normal during pre-flight checks.  However in bumping down the runway for take-off it dislodged from its benign location and wedged itself between the fuselage and aileron bell crank.

The result was that I could not move the stick to effect any left roll.  Fortunately right roll and all pitch control remained available to me.  
berkshire (Aeronautics)
15 Apr 11 13:24
Kwan,
""The pilot should be verifying control system direction before flying (every time!).""

 This really applies to every aircraft not just gliders, "controls free, clear and correct is or should be part of every pilots check list."

 However this thing about looking for misrigging after maintenance is not often checked, control cables, can, get crossed.  I personally have been guilty of it. I assembled the trim tab mechanism backwards in a Cessna 172 and did not catch it. The pilot told me about it after test flying the plane, boy was my face red. Pilots take it on faith, that when an aircraft comes out of a maintenance facility, that everything is correct and working as it should be.
B.E.
 
Helpful Member!(2)  wktaylor (Aeronautics) (OP)
15 Apr 11 14:23
It Is positively amazing the crazy thinks that happen to acft.

How about gear retraction on the ground? I had (2) C-141s where the Mains stayed in place, but the Nose gears dutifully folded-up crushing fuselage structure and doors. I developed quick/dirty ABDR repairs to fly them back to home-station.

Pilot raising the gear handle on TO roll with the "intent" of having a crisp/quick gear retraction just after lift-off... but then settling back onto the RW as the gear fold-up [show-offs]. Also pilots forgetting the LDG on landing... sliding on the belly. T-37s were famous for crazy stuff that student pilots did during take-offs and landings!

Anyone worked tire over-pressure explosions or rolling disintegration [throwing tire shrapnel into flaps and lower wing skins/spars]? The Concorde disaster is a classic example of catastrophic tire damage penetrating the integral fuel tanks and starting a raging fire just at lift-off. I think there were also wheel fragments involved with this incident.

Anyone dealt with obvious or suspected sabotage? Amazing what a full paint can can do when thrown onto a thin skin.

Titanium engine-bay panels, attached to titanium fuselage structure [flanges] on the F-15 are notorious for loosening slightly in-service...then wear/chafing/galling under sustained engine vibration and air-load buffeting.

Speaking of F-15s... the honeycomb wing tips regularly folded-up [or down]: In-fligh buffeting at High AOA was so severe, the were experiencing close to 100-G peek amplitudes.

Honeycomb structure invaded by moisture... corroding aluminum core & skin... disbonding then literally "blowing apart" after a seam opens up. Or... exploding [steam formation] when hit by a lighting stroke. Oh yeah... found out that leaking fuel, cleaning solvents, wash-water, etc... will find it's way into honeycomb.

CAUTION!!! NEVER water-jet blast adhesive bond-lines or fillet/fay sealant... intrusive water will open the joint to further moisture intrusion and peel the primer!

What about steel or CRES nuts/nutplates mounted to aluminum structure without being installed wet with primer or sealant? the corrosion [especially with high-nickel-ferrous alloys against aluminum can be mind-boggling [especially in a seacoast atmosphere].

Anyone seen the effects of using sanding disks over a broad skin area, to clean-off corrosion and/or rust from the heads of flush steel fasteners? Grinding across the skin and fastener heads spreads/embeds steel particles in-the-skin. REALLY bad corrosion pitting in 7075-T6 or 7178-T6 sheet material... especially at/around each of the steel particles and along the grinding marks... with a fine red-rust peppering every where!

Found a production-run of titanium fittings that had been "hand-cleaned-up" using either a sanding disk or CRES wire-wheel. There were fine score-marks perfectly aligned with the tight fillet radii ... and a deep-blue discoloration within the score-marks. Fatigue-cracking failed the main-flange... and was identified in other flanges by FPI NDI.    

How about abusive anodizing of end-grains on forged-aluminum, sheet or extruded parts? An engine mechanic whined at me that he "broke" the [0.25-thick] flange off of a forged [7075-T73] engine mount [C-130 truss] with a rubber mallet. Huhh? With just 30X magnification I was able to ID a checkered-crazed surface along the cracked machined fillet, where end-grains where exposed. Looking all over the E-Mount it wasn't rocket science to realize a Type III [hard] sulfuric acid anodize had been applied ILO the required [thin] Type IIB.  

Speaking of -130s... The [MS20012, I think] mount bolts/washers/nuts from one engine were sent to the NDI shop for inspection... but the NDI guys "couldn't inspect the hardware". I asked Why? He said come take a look!!! Every bolt had threads that were bent/cracked, the shanks were bent and scored circumstantially... along with the underside of the heads. The nuts and washers were no better-off. Checking with the -130 operators, I found-out they were concerned because the engine was vibrating badly... so they were changing the engine/prop. The hardware NDI was mandatory because of the engine change. I hand-carried the parts to the -130 squadron maintenance chief... who was stunned when I showed him the junk hardware. He agreed with me that the engine was probably vibrating because [at least] one or more of the engine mount bolts/nuts had become OBVIOUSLY loose.

Anyone ever seen a fabrication Jig slip and miss-align a major component such as a door frame or cabin skin? A production liaison engineer's nightmare! Funniest [after all was said and done] was the "completed aircraft ready for customer delivery". The customer felt like the aircraft was leaning left. Mechanics looked it over carefully and did an alignment inspection. Everything seemed OK... but even they had a "funny feeling" about it. That when I was called in. Checking the WINDOW water line locations, left and right sides, it was obvious that the Lt side windows were ~0.5 inches below design... and the Rt side windows were ~0.5 above design. Found out that the jig that sets the window cutouts was subject to "slipping around slightly". Inexperienced mechanics had NOT checked it for perfect alignment before fixing the window position. The entire upper cabin skin had to be replaced.

Keep-up the in-put guys.. this is going to be interesting... see You again in a couple of days!

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

MintJulep (Mechanical)
15 Apr 11 14:35
It doesn't only happen with airframes.

Your typical stick-and-skin subway car is built up from floor, wall and roof sub-assemblies.

Found one somewhat "twisted" once.

Turns out that the roof section was rather badly out of "flat" prior to final assembly.

But hey, with enough big enough clamps they were able to get all the gaps closed and weld things together.

When it was rolled out along the bumpy rail it went "boink".
SparWeb (Aerospace)
15 Apr 11 14:54
Yes Wil,
For every action there is an equal and opposite screw-up.

I have a structural analysis report on my desk where I've gone through and marked in red all the places there the author erred:
bad arithmetic,
missed a fitting factor,
used the wrong assumption,
used the wrong gross weight of the rotorcraft,
changed an assumption between the line it's declared and the line where it's used,
ignored advisory material on the subject,
declares potential failure modes that are not checked anywhere,
invents formulas without quoting the source (eventually I found in Bruhn),
uses strength data from a unqualified source.

All that in ONE report.  Signed and approved by an engineer with supposedly higher experience and status than me.

But it does have a pretty colour FEA model in the appendix so it must be okay.

Customer brings this to me a few months ago.  He paid the guy a lot of money for that work.  I have to be honest with him.  If I'm going to touch it, I'm going to load test his product.  And throw all the engineering work the other guy did in the garbage.

Because I like making people happy, you know.

Steven Fahey, CET

evelrod (Automotive)
15 Apr 11 16:47
Haven't seen too many screwups, personally.  Belonging to the Planes of Fame for a few years I heard all to many scary stories.  Only saw one...a Beech Stagerwing.  Gear would not crank down and pilot thought it would be 'safer' to put down in the adjacent "grass".  I put that in quotations for those of you who have not seen the grass next to the runways at Chino.  Bad idea. Prop dug in and pulled the cowl down flipping the ac on it's back.  About a year and a hundred grand and it was back in the air again.  No big deal ! ??????

I was at Douglas Long Beach Bldg 55 when the contractors built the wing jig for the C-17 backwards and engineering did not catch it until it was almost finished.  That was fun.  It was even more fun when the rigging contractor, (rigging co. name omitted), made a big deal, televised for Douglas of course, to pick it up and could not budge it.  

Rod
thruthefence (Aerospace)
15 Apr 11 19:15
"antenna removed by sawing off screws (hacksaw between fus. and base)"

This ties my favorite:

Lower wing inspection panels on a transport category aircraft required removal,but the heads of the threaded fasteners stripped by poor technique of contract "technicians", who were told by the overseer to simply "drill 'em out!" (meaning the screw heads);  This was interpreted to "chain drill", in a circular fashion, around the head of each fastener, allowing the panel to be pried off (with difficulty) with a large screw driver. Three panels were removed in this fashion before the "supervisory personnel" discovered the "work-around".

I'll leave it to your imagination what the underlying structure looked like after the panel removal.

I must add that the "panel removers" were furnished by a contract labor company, and were said to be 'experienced' mechanically minded workers. It turned out they were rounded up a few days before off of a street corner in Ft Worth.

 
berkshire (Aeronautics)
15 Apr 11 23:17
Thru the fence,
I had one of those," be careful how you give instructions moments", some years ago.
The shop was slow and I was finding make work projects for the technicians .
  One of these involved breaking down a wooden wing to salvage the components.
The wing had a large number of good ribs and enough spar cap material to make a good splice on another wing if needed.
 I showed the tech how to remove the ribs by prying them away from the face of the spar with a chisel.
 The work was tedious and slow going, but that was ok, he had nothing else to do.
  I got a phone call and had to leave the building.
  When I got back the place reeked of gasoline fumes and there was a neat pile of ribs on the floor next to a large pile of sawdust.
  After I had left,  the tech had a brainwave, he went out to his truck and retrieved a chainsaw he kept there. He took the saw and sliced down either side of the rib through the spar cap, pried the remaining block off with his fingers, voila job done.
He just did not understand that I wanted to save the spar cap as well
B.E.
FastMouse (Aeronautics)
18 Apr 11 7:46
Some of my favorite moments over the years...

Mechanics doing engine run-up of large jet.  Used chocks, but not the correct ones.  Aircraft jumped chocks, rolled into a hangar, wing LE impacted a massive steel I-beam.  Wing damage surprisingly light.  Steel I-beam:  mangled!  On that day we learned that there is nothing wrong with mid-70s aircraft design philosophies.

Someone with a problem threw handfuls of carbide chips into titanium melt at a material supplier.  A long way downstream, lots of expensive parts were found to contain carbide inclusions.

Someone with a problem threw acid into an empty wing box.  Maybe mercury was also involved, can't recall.  Significant damage.

Airline based in hot and very humid location.  Plus significant industrial pollution.  Plus bubbling sulfur springs upwind of main hub.  Plus aircraft flying very, very short missions and "cruising" at 10.000 ft, so were never dry.  Ideal environment for significant corrosion.
 
Fresh fish container burst in hold of widebody twin.  Airline swilled out the entire compartment with salt water.  Noted corrosion some years later.

Mechanic working in fuselage dropped tool between floor beams.  Made loud clatter when it hit the hangar floor.  Surprising, since the belly skin was between the mechanic and the floor.  Corrosion had reduced skin integrity to zero.

To achieve weight target, "they" needed to reduce the anticipated loads and so justify a lighter weight structure.  Did so by setting unrealistically low values of typical TOW, etc, in the fatigue missions used for analysis.  Witnessed early in-service degradation due to the assumed typical TOW, etc, being too low.

To accelerate the full-scale fatigue test, smaller "non-damaging" load cycles were neglected from test.  In-service fatigue lives turned out to be approx 1/3rd to 2/3rds of lives measured on test.

Probe mounted on long, springy arm was subjected to a big, inertial thud every time the system was operated, exactly as anticipated and catered for.  Subsequent dynamic response of long, springy arm not anticipated and not catered for.  Additional stress cycles caused premature fatigue failure.

Mechanic observed surface pitting corrosion and blended it away.  Had actually blended away a region of shot-peened material.

It is easy to laught as some of this stuff, but as a tip-of-the-hat to the industry, I am happy to say that on every occasion, the subsequent response of those involved to overcome the issues was rational and appropriate.  Thank heavens for people who still care about professionalism!
Helpful Member!(3)  blakmax (Aeronautics)
18 Apr 11 8:57
Don't know if this is an urban myth, but here goes.

Some Australian Army pilots were having a similar discussion to this thread, and boasted that they had experienced a kangaroo strike on a Nomad while landing at a remote airfield in outback Australia. "Beat that."
A Canadian P3 crew immediately claimed bragging rights by claiming a salmon strike which damaged the windshield. Evidently they flew under a large eagle which had just caught the fish and they scared it so it released the fish which hit the aircraft.

Canadians 1, Aussies 0.

Regards

Blakmax
KENAT (Mechanical)
18 Apr 11 17:37
Impact with the ocean (or possibly some flotsum etc.) when going low level in a Buccaneer.  By low level I mean wave top height.

The nail used to hang a copy of the Koran from the fuselage bulkhead in an Arab VIP chopper.

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berkshire (Aeronautics)
18 Apr 11 20:53
I got a good one today,
 a wing skin with a persistant crack on a fiberglass aircraft.
It had been repaired several times before, per the SRM, according to the logbook, by several different facilities.
 On stripping back the gelcoat and paint to the fiberglass, what should show up, but a perfect 50/1 scarf per the SRM with no glass in it. Somebody had cleaned and scarfed the repaired area then puttied and gelcoated over it.
 Repairs are now under way including the required fiberglass.

I guess the other repairmen Did not answer the following questions:
 Did I perform the job task without pressures, stress and distractions?
 Did I reinspect my work or have someone inspect my work before return to service?
 Did I make the proper record entries for the work performed?
B.E.
wktaylor (Aeronautics) (OP)
19 Apr 11 18:56
I worked on an overseas USAF installation... the C-130 guys always surprised me...

C130 ramp skin and sub structure was over-heated with strips of black rubber fused to the surface. A special ops team had trouble getting their ATV back on board during an "urgent" situation. Ramp was replaced [sent back for depot "overhaul"].

Note. Same aircraft was also getting a fair number of lightning-strike holes patched [30-caliber size].

Urine soaked substructure around the urinals on the aft cargo-door longerons. positively the worst stink and corrosion, I have ever seen/smelled on critical primary structure. I suggested that try lining the area under/around the urinal with puppy-pee-pads. Never did.

Old C-130 landed late PM after a really long mission. The crew chief was trying to do a post flight inspection in the dark. Kept hearing snap/crackle/pop along one area of upper wing-skin as he walked across it. Finally found a few loose/pulled-through fasteners. Mechanics opened the wing and pulled-out the ballistic foam: found SEVERAL broken/bent truss ribs and gussets and many more broken loose fasteners. I was called. NOT GOOD. "Lets look at the other wing!" Better condition outside... but a LOT worse inside. Called engrs stateside. This acft had old wings that were subject to damaging ground resonance [flapping effect] during fast taxi on rough surfaces with a certain waviness contour. The resonance could over-stress the wings under certain conditions. Funny thing.... the acft had encountered exactly those conditions just prior to takeoff at the foreign Air Base where it had operated earlier. Double outer-wing change was mandatory before next flight.

Wing commander was demonstrating aggressive off-field landings. Drove the acft at above max sink-rate into loose gravel. Dragged the aft belly for ~150-yds [very nose high], before lifting-off. found. The aft MLG pair and belly structure trashed. NOT counted as a mishap... just operational damage [major depot repair required].

Other fun stuff...

FA-18 lost a wing-tip missile launcher rail. Forward bolts holding it to the spar came loose [probably under-torqued]. The bolts simply loosened [spinning in place], until one failed... then so did the others.    

Due to the Extreme humidity, fiberglass radomes [fighter and transport] absorbed moisture like crazy if the coatings were allowed to deteriorate. When this happened radar signals notably degraded... and static electricity arcing marks would start appearing all-over the interior, which lead to disbonding and soft spots.

I worked on a small tactical jet. We started getting urgent calls from one unit that the pylon-attach [3/8"] nutplates were breaking. Some broke just after torquing... others a few days later. Lab evaluation confirmed that the nutplates had been overheated during the post cadmium plating embrittlement relief bake [lab estimated exposure up to 525F]. Cadmium infusion, and possible "blue-brittle" embrittlement were to blame. Found-out the NP manufacturer had already been disbarred from government work and the entire production lot was "suspect"... and the company had been out of business for a year.     

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

wktaylor (Aeronautics) (OP)
25 Apr 11 16:03
Sorry... been jammed-up recently.

Anyone every dealt with biological contamination/damage in integral fuel tanks? Nasty stuff.

During depot maintenance of CH-53 we found the entire Bilge area coated with mold, slime, fungus, urine, fuel, hydralic oil and sea water. Muck, corrosion and stink every where.

Anoyone found loose/missing nuts on spinning bolts in class 1 holes?

What about loose/migrating bushings or plain bearings?

Elongated (worn-out) or cracked-out holes in piano hinge nodes?

Shear [small-] head hi-loks or small-head bolts or blind-fasteners [rivets or bolts] that have been over-stressed [tension] and are dished or circumfrentially cracked and pulling through?

Unsealed airframe electrical grounding points [fasteners, studs, etc] that are corroded, loose and/or arced.

Crazed transparent plastics, such as windshields, canopies, windows, light lenses, etc.

Cracked, torn, spalled fiberglass fairings and cowlings.

Fuel and pressurization leaks due to loose [non-adhereing], torn or overheated fay, fillet and cap-sealant.

Titanium structure or parts [ducts, etc], that have been heat or fire damaged from straw-yellow to blue-purple to chocholate brown?

Or epoxy primer that has sustained High heat? and has turned from clean yellow/yellow green [etc] to tan-green to tan to tan-brown to tan-brown-scorched black?

Magnesium castings riddled with pitting corrosion. PS: don't even attempt to weld on these old cast-mag parts without 48-hour bake at 180--200F to drive out moisture. Pockets of trapped moisture in the casting matrix may "explode" [steam-burst] when hit with temperatures over 220F.

Parts overloaded in bending that exhibited anticlastic behavior? F-15 speed brake [carbon-fiber-epoxy skins with titanium backbone/hinge beams] that bent/deformed >>forward [upward when laid on back of jet]<< as it was over-loaded around the (2) length-wise hinge beams [speed-brake failed to load-relieve during a massive over-G and over-speed emergency pull-out so it "saw" the full load].

Small bolts/screws over-torqued and sheared or distorted during installation.

Hi-G aircraft flexing so severely that butt-gaps between skin edges compress together crushinhg the environemntal butt/fillet sealant.

Exposed edges on panels, skins, etc of extremely high performance acft that erode [paint and metal] due to wind/sand/dust/rain blast/impingement at high speeds?

Oppps... Gotta go... more later.
 

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

KENAT (Mechanical)
25 Apr 11 16:09
"Crazed transparent plastics" we used to make money at my last place repairing the laser seeker dome on Paveways, so yeah I've seen it.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

blakmax (Aeronautics)
25 Apr 11 21:01
Main spar bolts on a Mirage III which tightened into nuts in a captive nut strip. Bolts has snapped off in the nut strip, so the bolts were ground flat in installed with Locktite.

Taperlock fasteners on a steel carry through box that were fitted into reamed tapered holes. Some when removed has about 0.020 in. of bearing blue on one side. Some reaming job. huh?

An A4 destined for a museum was transported down a highway, but was too wide to fit under a bridge. Someone "trimmed" the wing tips off with a gas axe. He didn't realise that the wings tips could be folded up.

Two seater jet trainer experienced problems with the canopy locking mechanism, which resulted in the canopy departing the aircraft in flight. Forward pilot saw the canopy go and assumed that the aft pilot had ejected, so he ejected. Aft pilot followed soon after. Perfect aircraft apart from the missing canopy soon hit the deck.  
thruthefence (Aerospace)
25 Apr 11 22:19
"Main spar bolts on a Mirage III which tightened into nuts in a captive nut strip. Bolts has snapped off in the nut strip, so the bolts were ground flat in installed with Locktite."

Surely you jest?
blakmax (Aeronautics)
26 Apr 11 4:45
Thruthefence, I saw it on two aircraft, one with two bolts "repaired" that way. Mind you it was in the early eighties, and thankfully things have changed for the better.
blakmax (Aeronautics)
26 Apr 11 7:00
Oh I should say that we found out about it when we removed the bolts as part of a bonded composite repair program to address fatigue cracks in the adjacent skins. We did not do the bolt modification ourselves, we had to fix it.

Regards

Blakmax
Kwan (Aerospace)
26 Apr 11 12:30
Customer bought airplane and performed inspection.  Found a nacelle fairing fastener installed into lower spar cap.  Someone actually tapped a hole in the spar cap for the fairing screw.

Long ago I was liaisoning a mod.  We were building 16g racks.  The shop asked for a shelf trim note on unit 6 or something.  After much measuring and scratching my head I backed up and looked over the whole rack.  The whole rack was bowed because the shear panel on the back was installed incorrectly.  The mechanic installed the rivets one at a time and the panel pulled.  A big clamp was used to line up the panel with the holes.  Needless to say when the panel was remove it sounded like a gun going off.



 
blakmax (Aeronautics)
27 Apr 11 7:52
Another one. Super whiz kid engineer was tasked to address cracking in a 7049-T6 structure (thank goodness not many are in sevice these days). "There is an SRM repair for that structure, implement that!" Pity the crack was stress corrosion, which grew perpendiculer to the fatigue crack envisaged by the SRM repair. The addtional fasteners located ahead of the crack not only increased the Kt ahead of the crack, they exposed fresh grain boundaries to induce even more SCC.  
wktaylor (Aeronautics) (OP)
27 Apr 11 13:58
O god... "Taper-Loks" the ingenious fastening idea that is a devil to execute properly! I have never been more frustrated in my entire life than removing and replacing TLs. Situation ONLY gets worse if over-sizes are demanded by circumstances. Spent many hours observing blue-pin checks of TLs for reinstallation and new installation. F-15 lower skins were loaded with them.

Heavy rain wind and flooding have also taken toll of light and heavy aircraft. Recent fly-in at Sun-N-Fun had a tornado blow-thru damaging dozens of acft [overstressed in-place... plus failed tie-downs]. http://www.youtube.com/watch?v=DNt1BKqF3a0

Repaired many aircraft damage by ground-taxi collisions. The AirBus A380 and CRJ700(?) collision at JFK is a classic. http://www.youtube.com/watch?v=uogmKMUrbT4

The saddest taxi accident was an HH-3 Jolly Green Giant [Viet Nam Era] that was taxiing "close-enough" to a "taxi-line" adjacent to the ramp edge. Low visibility and a  "sense of urgency" due to deteriorating late afternoon conditions with fast-moving gray overcast skies [incoming typhoon]. The crew failed to see a gray concrete light pole off to the right side.  When the rotor-tips collided with pole, the reaction actually torqued the helo toward the pole starting a cascade of massive damage and flying parts "all around the scene". One mechanic just happened to walking to work was killed by a piece of blade-tip. The investigation uncovered a series of seemingly insignificant taxi accidents all over the base. Analysis by safety office and civil engineering uncovered the fact that taxi lines [which should include extra margins for clearance with parked aircraft and obstacles] hadn't been re-evaluated since the late 1970s. The ramp where the mishap occurred was "striped" for F-4s... not larger-span acft such as helos and C-130s.

Anyone ever investigated flutter or severe vibration problems?

I was engineering investigator on an O-2A that flew into a thunderstorm. It entered a violent down draft and came screaming [over airspeed-limit] out of the bottom of the draft. Aileron flutter occurred during attempted pull-out, resulting in the aileron tearing in half [fore-aft at ~ aileron mid-span], then departed the wing from centrifugal force as the wing failed [upward] after massive up-down-up flapping cycle. Found the aileron had 0.023 thick paint build-up [should not have exceeded 0.004 thick without re-balance]. NOTE: the aileron skin was only 0.020 or 0.025 thick.

Worked an F-15 that had severe unknown vibration at high subsonic/transonic. Pilots were leary of it and maintenance crews were baffled. When the called me [last resort], I couldn't find anything else to investigate. HOWEVER, I had just read the latest F-16 LMTAS bulletin that described similar severe vibrations on an F-16 caused by a loose 20-mm Vulcan gun installation. I mentioned it to the shop chief who blew-it-off... at that time. A few weeks later I saw the same shop chief and asked "what happened to  'old-shaky'": He said he had second-thoughts about my suggestion... especially when he remembered who was assigned that task... and decided to re-inspect it personally. Sure-as hell he found the gun, main fittings loose. Mechanics went through the entire gun system "tightening/replacing" as needed. Vibrations completely disappeared.

On a similar "warning note"... I worked on a large jet that has thin spot-welded skin panels, lots of sheet-metal longitudinal stiffeners and relatively few ring-frames and bulkheads. The spot-welded skin-panels [skins + SW doublers, tripplers, etc] were being replaced with monolithic thick sheet skins, same alloy-temper as the originals. 'A bit heavy... but so-what"??? Well stress and vibrations/flutter guys ran the numbers. Excessively stiff panels at certain locations drove adjacent or opposite [thin] skin-sections into potential resonance frequencies [panel and possibly the entire fuselage section. As a result of this revelation, there was an immediate push to make sure new skin panels were tailored similar to the old skin panels [thick skins were chem-milled AR]... and/or thin sheets were stacked/field-fastened.

Blakmax... Ahhhh... the infamous SCC crack that starts on the surface, runs for a short distance... then turns 90-degrees and runs laminar [sideways] down the sheet center... what a pain. The old 7XXX-T6 sheet and plate alloys [usually machined across grains] are notorious for that.  

 On a strange note. I had a mechanic call about a crack in a machined-frame on a HH-60. This frame supports the roof and runs past the side window sill to the floor structure. It is critical for crash-worthiness. Hmmmm: not good.  The mechanics wanted a temporary repair to fly for "just a couple of weeks more". I said "OK use eddy current to find the crack-tip and I'll work the repair". A couple of days later they called and told me they'd traced the crack down the frame to the floor. Maintenance supervisor grounded the helo and sent it back a few weeks early for depot repair/overhaul.   

How many of You have see terrible workmanship that causing multiple drilled and/or elongated holes at critical locations?? Replacing riveted nutplates thru thick members, blind is a classic YGBSM story (all because a nutplate rusted or spun-in place; but was absolutely essential to hold a flimsy fairing in-place)!!!

Anyone have their own horror stories regarding Elliptical Blind Nuts [NAS1734 or NAS1735 per NAS1736?] used at critical locations on F-16 wings? Thank-god for FTI [etc] Rivetless Nutplates with replaceable nut elements.

Gotta go back-to-work!

 

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

kontiki99 (Electrical)
27 Apr 11 16:44
fire damage that ruins heat treat

leakage of battery acid, mercury,

 
thruthefence (Aerospace)
28 Apr 11 9:00
The maintenance level "bodges" alluded to here should be required reading for any maintenance training curriculum, be it Military or Part 147.
FastMouse (Aeronautics)
28 Apr 11 10:37
I sure do wish that people would stop removing installed rivet heads with chisels.
MikeHalloran (Mechanical)
28 Apr 11 10:55
You mean there's another way?  winky smile

 

Mike Halloran
Pembroke Pines, FL, USA

wktaylor (Aeronautics) (OP)
28 Apr 11 11:32
When I was 17, I was hired for a summer to take a Beech-Baron wing apart. The shop was rebuilding the outboard end and leading edges that were crash-damaged. It had both solid and blind rivets. The shop owner taught me correct way(s) to remove both with absolute minimum damage and "speed". Text Book USAF T.O. process... so I found-out later.

NOT only were the correct ways precise, but they could also be very fast and easy when You got the hang-of-it and got a rhythm going. The principle problems were: (a) stripping the accumulating rivet heads from the drill-bits; (b) spinning blind rivets; and (c) hand or back fatigue due to working odd positions. Even changing drill bit sizes was quick/easy [spin chucking].

Tears me apart seeing the amateurish rivet removal methods used by so-called sheet metal technicians.

On a similar note. I work with an overhaul shop that is now building brand-new rudders and elevators for the acft I'm working. Our Assys are made from extremely thin sheet metal [0.012--0.040 thicknesses, 2024-T3/-T42, 7075-T6 & T62, etc] hand-riveted together in jigs/fixtures. In today's world of thick machined parts, aluminum honeycomb or composites, the "art" of lightweight sheet metal assembly had to be practically re-learned. I found old [WWII] sheet-metal Assy texts [theory and practice; topics ranging from handling, fixturing, drilling/reaming, dimpling, countersinking, hand riveting, etc] and sent them digital copies. The shop assembly techs were struggling and were racking-up discrepancy after discrepancy, such as: rivet gun or bucking bar marks/skids/gouges, wrinkling, oil-canning misalignment, etc. PAINFUL.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

der8110 (Aeronautics)
28 Apr 11 18:20
WWII texts and old T.O.'s are indeed fabulous.  Another medium from the same era is animated training films on sheet metal techniques done by Walt Disney:

http://www.imdb.com/title/tt0036497/

Well done, and entertaining too

 
wktaylor (Aeronautics) (OP)
28 Apr 11 18:40
Anyone seen titanium heated [in air] like these samples?

I have: F-15 that experienced a catastrophic engine failure and had a sustained engine bay fire [~15-minutes]. The engine bay was primarily Titanium 6AL-4V. Many parts were oxidized purple to chocolate brown. Aft fuselage was removed and replaced.

NOTE: these samples were made by a HS student in the sheet metal shop on base by special permission. Her dad was in the safety office and he/I wanted samples to hold in our hands. She won a science award for the project.

 

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

thruthefence (Aerospace)
28 Apr 11 22:41
"animated training films on sheet metal techniques done by Walt Disney"

any idea if these are available online anywhere?


OT, regarding the "old texts", I have accumulated a pretty good collection of old training & technical books from the late 'teens through the early '50's. They make fascinating reading.

An example: From a '30's Aircraft engine school text; they describe how to disassemble, clean, inspect, and test "spark plugs". not "remove", but disassemble to it's various component parts! These were the days when engine overhaul periods were measured in the hundreds of hours (if not less) rather then thousands.

One paragraph describes how to build a siphon type sprayer to "clean engine components" with gasoline!!  Apparently an "approved method'! And these were the days when everyone had a lucky Strike hanging out of his mouth!

A bunch of manly men, in those days!  
MNLiaison (Aerospace)
28 Apr 11 23:26
Curled blind rivet heads due to use of single action rivet gun for double action rivets.

Carbon fiber repairs installed over painted surface.

Aircraft slipping off jacks and receiving punctures in the wings / fuselage due to faulty jacking equipment and inattentive operators.

Double drilled holes at 40 locations due to improper removal technique by mechanic. Same mechanic allowed to install repaired panel onto aircraft resulting in short edge margin.



 
blakmax (Aeronautics)
29 Apr 11 5:34
Let's try an advanced technology one! There have been heaps of PhD studies on embedding optic fibres in a composite structure to detect impact damage. Broken fibres, no light comes out the other end. Great idea. Saves all of that ultrasonics etc.

Now look at the practical side. Once broken, these fibres can not be re-joined in any manner which provides a compatible surface profile. Further, when scarfing the structure to implement a repair, you cut even more optic fibres. Now every station outside the damage and repair is no longer monitored by the technology. The operator must revert to ultrasonics etc, and carry the cost and equipment for two inspection methods. Eventually the aircraft is weighed down by many pounds of useless fibres.

Similar comments for an alternative idea of dye-filled glass micro-spheres for impact detection in composites. Impact releases the dye and the damage is easily seen. However, when the area is sanded back, heaps of paint released over the site where you want to bond. Not good for adhesion!

Sorry to get all boffinish here, but these whiz kid ideas really bug me. Hey WKTaylor, this reminds me of the Garry Larson cartoon where a group of men in white coats are standing beside a very badly built rocket shaped object, with the caption "Let's face it. We are not rocket scientists!". Love it.

Regards

Blakmax
Kwan (Aerospace)
29 Apr 11 6:10
Blakmax,

I can relate to the advanced technology solving the worlds problems!  The fiber optics are wonderful!  But I bet they don't detect delams.  Delams happen more than broken fibers, at least in my world.

I've worked on WWII aircraft.  Had to work out external/intenal loads.  Back in the day they used a handful of loadcases.  The engineers understood the corners of the flight envelope.

Now what do we do?  We run ten thousand loadcases.  It's not an elegant solution anymore.  And I think any weight savings isn't worth it.

Anyone ever see peel ply left in the layup?  Happens more than I would have guessed.   
blakmax (Aeronautics)
29 Apr 11 9:04
Kwan

I have seen peel ply left in the lay-up, and the release ply on the adhesive. However, there is an even more sinister peel ply problem. For those who rely totally on peel ply removal for surface preparation, be warned. For the peel ply to be able to be removed, the peel strength of the bond to the peel ply must be lower than the peel strength of the laminate, otherwise the first ply is stripped off.

To achieve that there are two metods for treating the peel ply fibres in the process of making the peel ply material. The first involves coating the fibres with a release material (e.g. silicone). This material transfers to the bonding surface, despite the claims of the manufacturers that the material does not transfer. This contaminates the bond surface, resulting in poor bond performance in service.

The alternative is to heat scour the fibres which produces a glazed surface which does not bond to the resin. Unfortunately, the cast surface left by the fibres also does not bond well to the adhesive.

The crux of the problem is the belief that removing a peel ply leaves a clean surface for bonding. This mantra ignores the fact that surfaces must not only be clean, they must be chemically active to enable chemical reactions between the adhesive and the surface. Silicone (or teflon) treated fibres contaminate. Heat scoured fibres leave a chemically inactive surface. Conculsion? Peel plies are not an adequate surface prearation for composite bonds.

See the paper at the link below.

Regards

Blakmax
thruthefence (Aerospace)
29 Apr 11 10:21
I'd like to comment on "Paint Shop" bodges.

My fav, and I've seen it for 40 years, and continue to see it. (must be the high turnover in paint prep jobs):
Specific Aircraft, Small Beechcraft, Bonanzas, Barons, ect. After stripping the paint, Aluminum etch & alodyne is applied to the bare metal. The problem being the ailerons & elevators are magnesium sheet. No matter how many water rinses you do, it keeps cooking the magnesium, and after a year or so, you have these little "termite tunnels" creeping under the paint film.

The stripping crew carefully masks the periphery of the cabin windows with metal duct tape to protect the acrylic from stripper. They then tape over the windows themselves with crepe tape & paper. In order to do a "neat" job, they trace around the window edges with an exacto knife, or box cutter. Makes for a very workman-like masking job. Until you pull the paper off & see the kerfs the knives have made in the transparency. We're talking pressurized aircraft.

Paintshop owner mortgages his house to buy one of those new (at the time) plastic media blasting stripper rigs. Instead of waiting for the tech rep, he does some "OJT" on a Cessna 414A. Works great on the fuselage, "Shucks, who needs training, let's get this puppy stripped! Time is money!!"

Until you get aft of the pressure bulkhead, where the sheetmetal is say .020", (and the flight controls were the same). Stretched the metal so bad between structural elements it looked like a Dope & Fab airplane!

Operator brought it into my shop so we could "fix it". Every affected panel would "oil can" when pushed. Insurance Ended up totaling the aircraft.   
Kwan (Aerospace)
29 Apr 11 12:15
Thank Blakmax, that's good info, a star for you.

It's amazing how easy it is to make a mistake in this industry.  Problems can occur from any direction.

Wil mentioned fastener removal.  However, it's friday and I don't want to talk about that subject.  I like to go into the weekend happy!  But here is a link to EDM fastener removal.  Anyone have any experience in this?



 
wktaylor (Aeronautics) (OP)
29 Apr 11 12:22
I worked with a shop repairing boron-fiber-epoxy and carbon-fiber-epoxy control surfaces. The usual technique for repair was to grind/scarf the damage areas, clean with IPA and hot-bond preformed patches or lay-up in-place. We learned a hard lesson early-on.

Immediately after drying the parts of the IPA, the tech added the repair plies as advertised. He then set the equipment for cure... 375--400F + Vacuum pressure. Just as the repair was reaching ~350F, we ALL heard a muffled but unique "krumph"; and found the vacuum bag around the part heating-up had been blown apart. In the twisted core I found concentrated carbon-black residues, suggesting a detonation. Consulting with experts lead to an obscure report recommending vacuum bagging and low-heat [180-200F] extended drying of composite parts after they had been extensively cleaned with solvents. Light solvents, especially IPA, tend to wick into the core areas by capillary action thru the porous fiber matrix. At the perfect stochastic ratio voila... an explosion during repair heat-up, typically close to 350F.

OH yeah, the shop guys [and I] learned a bit late about the possible hazards of breathing composite material sanding dust. Hopefully the long term effects from boron & carbon fiber dust] are more benign than asbestos fiber dust.

Anyone worked over-pressurization incidents? I've worked (2) wings and (1) fuselage that were over-pressurized during testing. What a mess. The wings were repairable with great effort... the fuselage was scrapped.

I know of (2) acft that were blown apart by oxygen tank ruptures, caused by servicing low pressure gaseous O2 systems [800-PSI max] with high pressure [3000-PSI] O2 by mechanics who ingeniously figured-out how to install high pressure servicing-fitting onto the acft's low-pressure O2 servicing fitting.

Also had one bird heavily damaged by a freshly serviced LOX tank. The sequence of events went something like this: the empty LOX bottle had been stored in a open-air shed; was then serviced with LOX and placed onto the transport cart [exposed] which encountered brief/light rain. The tank was then loaded into the jet as part of pre-flight servicing, without checking the tank vent. As the LOX warmed slightly, the tank couldn't vent-off building gaseous O2 pressure, which quickly over-pressurized and ruptured the tank. Investigation revealed that the over-pressure vent was probably plugged by hard ice, due to condensation and/or rain intrusion into the vent port!?!?#$@$$^^$. It was a good thing there weren't any arcing/sparking sources near-by which could have made this mishap MUCH worse... and the same thing could have happened to the other acft serviced at the same time [but didn't, thankfully].

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

rb1957 (Aerospace)
29 Apr 11 12:22
other than covering every occurrence of murphy's law, is this thread going anywhere ... it's more like a pub discussion

my 2c ...
debodine (Electrical)
29 Apr 11 14:08
rb1957 the next round is yours, we look forward to your generosity in setting up the pints for us!
wktaylor (Aeronautics) (OP)
29 Apr 11 14:29
rb1957...

Amazing bad things happen, subtle and not-so-subtle... daily... when working as an aero engineer on REAL aircraft.

Lots of "office-bound" engrs are blissfully unaware how obscure and/or brutal these events and be. This thread was started with the intent of highlighting real-world examples of the "bad things that can happen"... as fud-4-thot.

Let me know if this thread should be put-out-of-it's-misery. Otherwise... get Your experiences in writing!!!

PS: I prefer Foster's in an Oil Can.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

debodine (Electrical)
29 Apr 11 15:04
wktaylor has a very good point.  It always amazes me that most engineers in our company have rarely ever walked an aircraft, much less watched mechanics try to comply with a drawing or turned a wrench themselves beefore becoming an engineer.

I have found my designs ALWAYS work better when I get an experienced lead to walk through my pre-release design with me to see if it can be accomplished with available tools and skills and access points.  I am often shocked at how difficult my design would be to accomplish per the drawing, and how easy it usually is to make my design better with a few small changes.  And I had 15 years working on aircraft before I became an engineer.  I certainly have not seen it all yet!

So a forum like this helps me see things I have not yet personally experienced.
berkshire (Aeronautics)
29 Apr 11 17:53
The post today by Black Max on peel plies confirms thoughts that I have had for years,
about the adequacy of just ripping off a peelply and bonding straight to the spot.

 I have only ever used the un coated polyester peel plies. Since I mostly work with room or low heat cure resins
The feeling in the Company where I used to work, building fiberglass sailplanes, was that the primary use of the peel ply was to remove the amine blush scum left by the room temperature curing resins. The technique used, was to peel off the ply then hand sand to get a bonding surface. Prior to the use of peel ply the bonding surface had been scraped with a hook scraper to remove the amine scum, then hand sanded to expose the fibers for bonding.
 After we started using peel ply, after stripping the surface would be sanded to remove the visible weave, then to be bonded to surfaces were blown off, with dry filtered air, but not wiped down because it was believed that solvents like acetone carried more contaminants into the bond line than they removed.
  Now the report Max produced, is saying that even that may not be adequate to get a good bond line. That hand sanding to remove the visible weave, may still leave un bondable sites in the joint.
B.E.
  
 

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

blakmax (Aeronautics)
29 Apr 11 19:53
Berkshire

We found that the best way to treat composite surfaces was to use a heat scoured peel ply, and after removcing that we grit blasted using 50 micron aluminium oxide in a dry nitrogen gas. Only needs a light blast to remove the surface molecules so you get a chemically active surface. No need to blast the impression of the ply off. Dust was blown off the surface with nitrogen.

Never solvent degrease after the grit blast. Any small pockets of contamination are dissolved by the solvent and spread over the entire surface. Besides, it is impossible to get the solvent resdiue off the surface.

Regards

Blakmax
berkshire (Aeronautics)
29 Apr 11 21:16
Only needs a light blast to remove the surface molecules so you get a chemically active surface. No need to blast the impression of the ply off.

Duly noted Max.

B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

MikeHalloran (Mechanical)
29 Apr 11 21:27
Uh, Max, could you write that up as a FAQ and put it in the "Composite engineering" section?  That kind of hard-won knowledge never finds its way into books.



 

Mike Halloran
Pembroke Pines, FL, USA

blakmax (Aeronautics)
30 Apr 11 7:50
Mike Halloran

This information is in the FAA Tech Center Library in DOT/FAA/AR – TN06/57, Apr 2007 BEST PRACTICE IN ADHESIVE BONDED STRUCTURES AND REPAIRS. I also added a lot more information on adhesive bonding technology, especially in-situ repairs.

Regards

Blakmax
PHovnanian (Electrical)
30 Apr 11 11:47
Structural damage due to arcing electrical faults as a result of the omission of differential bus/feeder protection.
SparWeb (Aerospace)
1 May 11 0:47
Betting back to "critter" stories...

The boss was looking at a Mooney parked in the grass at the end of a hangar one day.  For Sale sign in the windshield.  I was there for some other reason, but he's looking for a new "project".  He was starting to get excited about the condition of the engine, and instruments that the owner had added.

I was paying more attention to the outside.  When the walk-around got me to the tail I noticed a brown streak coming from behind an inspection panel in the tail.  I got a screwdriver from somewhere.  Just putting the screwdriver against the screw was enough pressure to "squeeze" another brown drip out from behind the panel.  Three screws later the panel was off, and the bay inside was completely packed with straw and grass.  The bird had made its home here for a while because the bay was saturated with poop and water too, hence the brownich goo.

Some kind of bad karma came to haunt me, when about a year later we found ourselves re-skinning a Beech Baron wing, where some squirrels had taken up resisdence.
 

Steven Fahey, CET

berkshire (Aeronautics)
1 May 11 1:20
Sparweb,
I had one of those, I had a pair of wings off a Schweizer 1-26 that had been stored in a barn, I had one of my guys removing feathers and straw from them when the Feds walked in to inspect my station.
 They came back 3 times in the following week to check on my progess with those wings.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

wktaylor (Aeronautics) (OP)
1 May 11 18:58
blackmax: great Post! Thanks for the link to a fine document.

I was tasked to re-qualify a very expensive adhesive for our shop. It was just over the out-of date limit.

Working with a repair technician and pre-primed coupons, we set-up and cooked several Test lots.

The goal was to attain 3500-psi lap-shear, minimum. The tests revealed 2200, 3100, 2950, 3350, etc... The scatter made me concerned about configuration issues during setup: so I carefully specified the set-up and witness the technician doing work. That's I began to feel less comfortable about the tech: he rushed certain steps, misaligned a few and was a little careless in cleaning and handling processes. There was relatively little change in the scatter... but there was some overall improvement in the lap-shear values. I re-evaluated coupon set-up with advice from some other bonding engineers... and went back to the shop [yes this adhesive was really expensive and hard to get in-theater... we were working the issue hard to re-Qual it]. The original tech had already started another job and couldn't be pulled-off to help... so another tech, who I was unfamiliar with, took over. He worked in a totally different style: methodical, clean precise lay-up measured work, etc. The test coupons failed between 3250--3400-psi lap-shear. never did qualify the adhesive... but I qualified the new technician for all future testing and recommended the original technician keep hands-off.

NOTE. Later I found the original tech had a reputation for cutting corners. When I discovered he had applied a corrosion protective finish after assembly, I asked him in front of his lead to disassemble the bolted together pieces. He and I got in-to-it. When all was said and done the lead forced the issue and the assembly was taken apart: there was NO alodine or primer in the fay surfaces of 7075-T6 parts bolted together that would be used on a NAVY HELO [hoist system fitting] over the ocean. He was a proud guy... but was absolutely un-trustworthy and had consistently low workmanship standards. He was finally forced to leave by the shop chief.

Another quirky guy was our welder. Didn't take me long to realize that if it was weldable and there was an established good practice, he knew every detail of the technology and "had the touch" [exceptional workmanship]. Literally the finest aerospace welder I've ever met: fully trustworthy and filled with knowledge and "the hands-on gift". However, as an employee, he was a eccentric hand-full. Played by this own rules one-to-many-times and was "fired". Another Company doing aircraft overhauls on contract across the base hired him immediately [OK, I put in a "good-word or two for him"]. We eventually lost a LOT of work he used to do for us [he/I continued our friendship till I left].  

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

wktaylor (Aeronautics) (OP)
2 May 11 11:22
Just released to the web... a frightening display of flight control system failure...

Tu-154 "wallowing" [roll-pitch-yaw] around the sky due to major controllability issues [first flight after long-term storage?]

I'm sure this flight took everything the crew had just to maintain mental control of themselves... much less the acft... which they eventually get back on the ground.

Unfortunately this may have been what happened to [as I recall] a few F-15s, F-16s, DC-10 [Sioux City], -135 [Germany] and a DC-9 [California] with serious controllability issues... just before they eventually lost total control and crashed. The Fighter guys had the "luxury" of ejection seats... everyone else was along for the ride strapped in their seats.

Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

VIDEO: Tu-154 struggles against in-flight oscillation
By David Kaminski-Morrow
01/05/11

Extraordinary video images have emerged of a Tupolev Tu-154 apparently suffering serious in-flight oscillations shortly after departing from an air base in Moscow.

Text accompanying the images indicates that the aircraft - whose registration is given as RA-85563 - was flying from the Moscow Chkalovsky base on 29 April.

Details of the incident remain sketchy and unconfirmed but a series of video clips shows the aircraft departing, before it appears to encounter problems in lateral and longitudinal control.

It eventually returns to the airfield to land. Unconfirmed reports suggest the aircraft was conducting a maiden flight after long-term storage.

[Two videos on the following webpage...]

http://www.flightglobal.com/articles/2011/05/01/356137/video-tu-154-struggles-against-in-flight-oscillation.html

NOTE: the ground personnel standing around (second video, when the aircraft is on final approach from the right) appear to be watching [raptured] in disbelief... waiting for the fireball on the other side of the tree-line.... which [thankfully] never came... aircraft is finally seen rolling-out in the distance.
 

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

KENAT (Mechanical)
2 May 11 14:12
rb1957, I actually think this thread is potentially pretty informative.  Seems that a number of folks have a real depth of knowledge to share.

Sure some of them are 'Murphy's' but some of them show patterns etc. that could be avoided.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

cloa (Petroleum)
12 May 11 22:30
Who wants to distill a summary and post as FAQ otherwise it will be surely hidden by new threads?

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