jacking cases need ultimate factor? 1

[airmail]

Hi guys,
just a confirmation... for large aeroplanes (CS25), when the plane is in Jack position; does it need to withstand just limit loads, isn´t it? So, once I have put the adequate reaction factors stated in CS25.519 in my analysis, no need for applying the typical UF=1.5 on top of them...

 

[Sparweb]

Although you could probably take that argument to the point where, logically, you would think you don't need the safety factor of 1.5, reading 25.303 doesn't actually give you that latitude. The safety factor is not used for accelerations or external forces - that's what limit load is for. The SF is to ensure that the structure does not fail when exposed to loads slightly greater than limit load. The SF makes a clear distinction between supporting load without failure and supporting load without deformation. Even when jacking the aircraft, neither should happen, but if exposed to a load greater than the limit, but less than 1.5 * limit, the structure must not fail either.

STF

 

[airmail]

thanks SparWeb,interesting post for the limit load understanding...
I guess that, for some failure scenarios, limit load capability is enough for certificate the structure because load level is known (or accepted) not to overcome the associated to the failure case. So, isn´t this rational valid for jacking (where loads are known, controlled and without any "unexpected" exceedance)? Do you know any practical UF people use for Jack? is UF=1.5 widely used even though?

thanks again!

 

[Sparweb]

Jacking points aren't just used for holding the plane up inside a hangar.
On many aircraft they are also used for holding the aircraft down when stored outside.
Now the loads aren't so easy to predict.

The "unexpected" exceedance should always be expected. Do you think no aircraft has ever been dropped off of its jacks? You should spend more time in hangars - it won't take long to hear some stories.

Go look at the jacking points of an aircraft that's more than 20 years old. They probably look a little beat up.

If memory serves... There is a life-extension package for a certain type of aircraft requiring certain members to be installed only when the aircraft is on jacks with thousands of pounds of lead stacked on the wing, in order to pre-set the load on the spar while a reinforcement member is installed. (If anyone can jog my memory on exactly which aircraft that is, I'd appreciate it.) Anyway the point is that who would have foreseen the need for the jacks to support that load?



STF

 

[tbuelna]

The jacking point loads are clearly described as limit loads, and as SparWeb noted:
" CS 25.303 Factor of safety- Unless otherwise specified, a factor of safety of 1·5 must be applied to the prescribed limit load which are considered external loads on the structure. When loading condition is prescribed in terms of ultimate loads, a factor of safety need not be applied unless otherwise specified.

 

[SAITAETGrad]

If a plane's jack pads can't handle 3x 1g static load with a 0.5g side load before it falls...who can we ask to stand under it? The jacks themselves must have what- a factor of safety of 8? Is there maybe some other OHS requirements that could require higher factors than 25.519? Interesting discussion...I've never worked on a jack pad repair (or similar mod) before.

 

[tbuelna]

SAITAETGrad-

Aircraft structures must be very light, so they are designed and analyzed to very slim margins. If you consider just how heavy a commercial aircraft structure is, and how high the forces/moments applied to the jacking points can be, you would appreciate the design and analysis factors used. The 1.5 FoS required for analysis is used to account for any inaccuracies in the analysis procedures. If an excessive load was applied to a jacking point, it could damage the fuselage structure and the damage might go undetected. A good example is that of the DC-10 engine pylon failure of AA flight 191, that was allegedly caused by excessive loading on the engine pylon structure during maintenance.

 

[airmail]

So maybe the question is where does the 3 times its own weight requirement come from? I also see quite much to add 1.5 on top of that, despite whatever incident could have occurred in the past.

 

[Sparweb]

So maybe the question is where does the 3 times its own weight requirement come from?

Once the aircraft is on jacks, it becomes part of a "workplace". I may be using North American terminology here. Think of your local civil building and worker safety codes. When a load is elevated, the people working in it and under it become vulnerable to injury or death if it falls. Assurance that the support points have adequate strength in that situation must be shown to people responsible for worker safety, not flight safety. They live by their own set of rules. I don't know much about them. What's the difference between jacking up an aircraft for maintenance, or a truck, or a bulldozer? Not much to the guy whose leg is pinned under it, if it comes off the jacks.


STF

 

[rb1957]

my 2c ... use ultimate. not applying the ultimate factor means you can accept yielding and deformation (as in the crash cases).

i don't know where 3g comes from ... i'd've used (without thinking about it) 1g ??

Quando Omni Flunkus Moritati

 

[WKTaylor]

On rare occasions, uncoordinated jacking procedures, or sudden loss of hydraulics to one or more jacks, have caused structural damage or jack-jumping: this is when extra strength margins all locations are all-mighty important.

Lufthansa had this happen to a 747-200... scary. Thank God the the remaining jacks/jack-points didn't catastropically fail.

http://avtales.wordpress.com/2004/02/16/lufthansa-747-jacking-incident/

http://www.airliners.net/aviation-forums/general_aviation/read.main/1415398/

Regards, Wil Taylor

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[rb1957]

no factor can cover the potential "stupidity" of users ...

Quando Omni Flunkus Moritati