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Indonesian 737 Max 8
8

Indonesian 737 Max 8

Indonesian 737 Max 8

(OP)
Indonesian 737 Max 8 - Splashed

Most reports I've seen mention the aircraft had an 'undisclosed issue' before take-off. Does anyone know what that was?

Surely the crew was in contact and reporting something during the relatively long disaster.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Indonesian 737 Max 8

Seems that there had been some technical issues reported for the prior flight, people reporting unusual engine noise, etc. Can't find the link to the article right now.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: Indonesian 737 Max 8

Couple of days ago, the BBC was reporting a discrepancy between captain's and first officer's altitude and ASI displays.

A

RE: Indonesian 737 Max 8

One recorder has been recovered, they didn't say which one.

Another thing not reported yet that I have seen is whether the same crew was on the doomed flight as on the one before. There were some issues on that flight, and were said to have been fixed. If the pilots carried over to this flight, they can't help, but if they were relieved, they could probably shed some light.

RE: Indonesian 737 Max 8

Here's a link to a forum where lots of aviation experts are discussing this crash. I follow the forum regularly and you'll get breaking there first.

https://www.airliners.net/forum/viewtopic.php?f=3&...

Brad Waybright

It's all okay as long as it's okay.

RE: Indonesian 737 Max 8

Reports are that the pilot of this aircraft's previous flight issued a "Pan Pan" alert, requesting to return to the airport, then cancelled it when the plane started flying satisfactorily. It was also reported that passengers on that previous flight saw a pilot walking down the aisle with a big manual. Before this crash, Lion Air had a seven star safety rating, now reduced to six stars.

RE: Indonesian 737 Max 8

(OP)
A prior flight passenger said the plane dropped suddenly "panicking the passengers". Seems like the next load of passengers got the complete ride.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Indonesian 737 Max 8

I think it is highly unusual, maybe in violation of aviation rules in most countries, for a Pan Pan alert to be overridden with the flight continued as if nothing had happened.

RE: Indonesian 737 Max 8

Quote (hokie)

I think it is highly unusual, maybe in violation of aviation rules in most countries, for a Pan Pan alert to be overridden with the flight continued as if nothing had happened.

Possibly against the airlines internal operational specifications but not against any aviation rules I'm aware of.

Pilots actually frequently cancel emergencies. For example; sudden icing beyond the capabilities of the airplane to fly through. Declare emergency and use pilot authority to descend without getting clearance from ATC. Get below the icing and then you can cancel your emergency and continue on like nothing happened. You will of course have to explain later why you called an emergency and could get in trouble if they find you either caused an emergency through reckless action or canceled an emergency when the emergency still existed.

Pan-pan is even more likely to be canceled; it essentially indicates you have a priority situation that requires special attention. Something like a broken radio or a standby instrument failure or a jammed landing gear. It's not an emergency yet but you may need something special (direct route to destination, going into a holding pattern, emergency equipment standing by, etc.), if you're able to fix the problem in the air you can let ATC know that you fixed it and proceed on.

Clearly they made a mistake to continue on. It's just a matter of whether that mistake was reasonable and/or not their fault.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

There have been a number of instances of instrument failure due to clogged pitot tubes. Insects? But surely properly trained pilots would be able to overcome that type problem.

RE: Indonesian 737 Max 8

6
hi all just registered to answer some questions

I used to be Finite Element Mech Eng in a previous life. Now I fly aircraft for a living.

I am not rated on the 737 only Dash 8 Q400 and previous Jetstream 31/32/41. The Q400 is a modern digital efis system aircraft and the Jetstreams old analogue tech.

First of all the emergency declaring. There is nothing stopping you trying to cancel if you sort out a problem. But its a two way thing once ATC knows you have an issue they can adjust the emergency level as they see fit and the pilot has no control over it. I won't go into the ins and outs of various situations but I have in the past declared and then cancelled and continued to destination and even operated the next flight immediately. 2 mins filling a report out and not heard a thing about it again. Also somethings such as Hydraulics failures are major issues on certain aircraft types and on others such as the Jetstream its more of an annoyance. This is to do with how the controls are powered or not in the case of the Jetstream. BUt if ATC hears anything to do with hydralic failure they will presume the worst thinking you going to go off the end of the runway due to needing to do a flapless high speed approach and have no brakes or steering on the ground. Jetstream we would pump the gear and flaps down, land and taxi using the prop reverse and front wheel castoring. Not a huge issue.

Instrument failures are a bit of a bitch. Analogue instrument aircraft in some ways are easier to trouble shoot and work out what's wrong and deal with.

Digital aircraft with efis the instrumentation comes from the airdata computers which take the environmental data and anther box does magnetic heading and navigation and displays it on the screens. There are at least two and they cross reference each other and when they disagree they tell the pilots. There is also a back up system which has its own pitot and static system and is run on a backup electrical supply with sperate battery support for if the main electrics go down.

What the pilots sees if something goes wrong is xxxxxx mismatch on the primary flight screen it won't trouble shoot. You then have to go into a quick reference handbook to reconfigure the displays to remove the faulty data input after comparing the 3 information sources and hopefully going with the two that agree. Its a two person check and double check. One person is meant to fly the machine the other one trouble shoots.

Now the departure is high work load and everything has to work towards getting away from the ground. The further away from the ground you are the more time and space you have to sort things out. Something happens below minimum safe altitude and you have a problem.

Now not all pilots think the same way, as an engineer flying a plane is just an energy equation which you can change the variables using the controls. So what to do is very logical to me with the flight profile. Your just playing with 4 variables thrust drag lift and weight.
The if you stick the nose at an up angle to the horizon with a set amount of thrust and the wings level it will climb within a range of speeds depending on weight. You don't actually need to know your airspeed to know you are safe. This to me is glaringly obvious and how I have treated instrument failures in the past from single engine piston right the way through to the 30 ton q400 (although I haven't had a problem with the Q's instruments yet with a mismatch). It works every time. To be honest its my normal method of flying pitch power equals performance. The speeds vary with weight but what your actually doing is setting a most efficient angle of attack which never changes.

Now other pilots brains don't seem to work the same way or as fast working out what's wrong. They "chase" the needles so they apply control inputs to make the numbers right. Once the numbers are right they then realise that the other data is not as expected then after a period of recognition then transfer to other methods of defining the flight profile. Now this goes back to the very first lessons flying how they fly. Personally when I was a flying instructor I didn't give my students any instruments and they had to learn to look out the window for attitude and set the power by ear. Then when they had that sorted they got the instruments. But because the habit was set initially they were not fixated on airspeed or other instruments depending what they were doing. Just attitude and power. Flying in cloud with the attitude indicator failed is another issue but we can do it.

This dealing with instrumentation failures is a hot topic in the sim sessions and has been since the Air France AF 477 accident.

Please note in no way am I inferring anything to do with this incident, At least once a year depending on the training syllabus sometimes more there will be an item in the sim to do with instrument failures. The low level just as the gear comes up are shall we say interesting. Personally I learn more about dealing with the human side of things during them with the reaction of the other pilot than I do from the actual handling of it. You can have two people with completely different ideas what's actually happening and what to do to fix it. And what you don't want is two people fighting over control inputs that's always fatal.

Most incidents are a combination of human and equipment, we will have to wait until the CVR and FDR are analysed to see what actually happened and no doubt the year after next there will be something in the sim training syllabus so we can all learn from it.





RE: Indonesian 737 Max 8

Thanks for sharing that with us, Alistair_Heaton. Very good perspective on the challenges and responses of properly trained pilots.

RE: Indonesian 737 Max 8

Avionics trouble shooting is always a problem as a lot of the time they test out SATIS on the ground and only show issues when the aircraft is in flight. vibration thermal expansion pressure changes are all factors.

Thankfully these days most machines have huge number of variables data recorders which get downloaded every 30 hours of flying anyway. They record stupid numbers of variables from the primary flight instruments through to the amount of times the toilet pump is used.

This data is used to monitor the pilots performance ie they run it all through a computer after its downloaded and checks to see if we have been sticking to SOP's or handling the aircraft in a none approved manner.
The other side is all the tech data for the engines and other systems. At least these days a tech problem the technicians can see the raw data instead of having to use pilot reports of problems. We also have a bookmark ability in the cockpit so if the engines fart or something weird happens we can press it and it dumps a marker in the datalog so it flags where in the 30 hours of data the issue occurred.

There are all sorts of cultural issues going on as well and shall we say philosophy's. I am lucky that I work for a company that the CEO was a pilot in his younger days and has dictated a clean techlog policy.

tech faults with aircraft get stuck into 5 classes.

AOG plane grounded until its fixed

A requires fixed in a small number of flights or time period.

B 3 days

C 10 days

D 120 days.

Its all control by a thing called minimum equipment list the more safety related the issue the shorter the period there is to fix it. If the problem is not mentioned in the MEL then your grounded.

Clean techlog means that the problems are fixed ASAP and things are not aloud to build up or left until the end of the limit. It means the engineering stores have to carry a huge number of spares which some company's would only order on demand knowing that they could get them inside the MEL limits. It costs money. But... I have only had to ground an aircraft down route twice in 2 years/1000 flights working for them. We are at the top of the league tables for on time departures and its a rare event with have to cancel a flight due aircraft tech. For me its obvious that his policy works. If its more cost effective than ordering on demand I really don't have a clue.

Training we spend 25% more time in the sim that legal min and also have 20% more ground school every year.

Some pilots are really not so lucky. Every entry in the techlog is an argument and training is considered a box ticking exercise. I really don't have a clue what this company's operating philosophy is like.

RE: Indonesian 737 Max 8

Any chance you could share what airline that is?

RE: Indonesian 737 Max 8

It may not be one selling tickets. Several years ago I’d heard from a pilot on one of the major carriers that the FedEx pilots got far more simulator time than any of the people haulers.

RE: Indonesian 737 Max 8

New posts on airliners.net forum cite a recent press conference stating that the previous 3 flights experienced unreliable airspeed data.

Brad Waybright

It's all okay as long as it's okay.

RE: Indonesian 737 Max 8

Quote (hokie)

But surely properly trained pilots would be able to overcome that type problem.

Short answer, yes. I'm not trained in large aircraft but am an instrument rated pilot so can share generalities. Pilots are definitely trained to fly without accurate airspeed or altimeter indications. I've actually had an in-flight altimeter failure due to water getting into the static air port during a foggy day. Quick fix as the plane was equipped with an alternate static air system, so it was a non-emergency. Generally these can be trained for safely and resolved without issue. You may have even been on a flight that had a failed instrument in-flight and not even known.

That said, instrument failures can be some of the most dangerous problems in aviation because they're sudden, possibly intermittent, and difficult to diagnose in a chaotic situation. There are redundancies of course, but the redundancies are not always complete replacements for the failed instrument. I'm not qualified to talk about the full procedure in a large, modern airline, but generally the procedure is to identify the instrument giving false readings, disable or ignore it (hard to do!), and utilize reliable alternative sources of info of which there should always be at least one. The plane will keep flying just fine but it becomes much harder to ensure the plane can be flown safely by reference to instruments alone. Due to this danger the systems are very robust and capable, but it still isn't perfect. A lot of flight training is spent focusing on preparing for sudden instrument failure.

Air France 447 was the extreme example of sudden instrument failure essentially startling the crew into an improper response, combined with some shortcomings in the design of the Airbus: https://en.wikipedia.org/wiki/Air_France_Flight_44...

Not sure if that happened here as well.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

Alistair_Heaton and TehMightyEngineer have both touched on an important point when it comes to the pilot/machine interface for diagnosing problems in the stress of an emergency. Some instrumentation, at least with regard to the signals/data, are closed loop systems. For example, GPS. The GPS satellites contain data streams that works with the on-board receivers and the mix of various GPS satellites being used, typically 4-5 satellites for the heavy iron on which I have most of my experience. The GPS receivers on-board and the satellites in the constellation being used can talk back and forth to detect, isolate, ignore and/or call up redundant satellites without pilot intervention. Thus the data presented to the pilot is reliable unless a flag and/or warning notifies the pilot not to use the data.

Air data systems are (at least as far as the data provided to the pilot) not closed loop systems. What I mean is there is no satellite constellation, ground station, airborne station or any other system in place outside and independent of the aircraft against which the air data system can compare what it THINKS is correct, and thus no feedback outside the aircraft itself to say to the air data system, "Whoa, the data you are feeding to the pilot sucks, throw a flag!"

Thus the air data systems can really only compare against each other. On some aircraft there are actually four air data systems: Pilot, CoPilot, Standby and Alternate. This gives a lot of redundancy unless the situation that is affecting one system affects all systems in a similar manner.

To illustrate: In a GPS constellation the probability (I don't have hard data in front of me, using relative terms) is very unlikely that more than one satellite will develop the same malfunction at the same time as they are thousands of miles apart and fully independent from each other. However, even on an aircraft with four air data systems, if all the pitot tubes and/or all of the static ports are affected in a similar manner by the same phenomenon (say for example rapid onset of icing at altitude), the various systems still agree with each other even though all of the systems are now lying to the pilots.

Of course physical separation on the exterior of the aircraft and other means (software processing) are used to try and provide as much independence as possible along with opportunities to find deviations that can provide warnings to the crew, but it is not as easy to provide independent comparisons between on-board systems as it is with signals and data originating outside the aircraft to use for cross check.

RE: Indonesian 737 Max 8

Debodine is 100% correct. In my in-flight incident with my altimeter, water in the static port causes the altimeter to get stuck at an altitude. Because it's rare to be stuck perfectly stable, I saw my altimeter fluctuate from a steady change during a climb to suddenly stopping and then jumping a few hundred feet. Clearly a failed instrument and obviously unreliable. Verified my standby altimeter was showing the same (they share the static port), and selected alternate static air supply. Problem went away.

However, if it stuck perfectly at one altitude and I wasn't paying attention, I might have kept climbing or even started descending without knowing it. I might see an airspeed that was then too high or too low for what I expected and start messing with the engine or pitch to try to "fix" it. This confusion could cost precious time while the airplane is slowly getting out of control. I would like to think I'm smart enough to avoid this but better pilots than myself have messed this one up.

Until they develop a closed loop system for the primary instruments (attitude, heading, airspeed, altimeter) the only thing I can do it keep training as much and as often as possible. Thankfully GPS has made things way easier and newer tech like synthetic vision is greatly simplifying instrument flying.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

GPS is still an afterthought in aircraft, I think.

GPS provides both groundspeed and an independent altitude. Ever since Air France, it struck me as odd that we're not able to built a better air speed sensor and/or figure out some way to use the GPS altitude and speed data to cross-check the air data.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Indonesian 737 Max 8

(OP)
Ian; Thanks for that description of sliding towards uncontrolled flight. Pretty subtle. Nasty! I know high performance airliners have a surprisingly small speed window at altitude for not stalling from the 447 disaster. I can see how in fairly short time you could exit that window, especially in a moonless night over an ocean.

For pitot speed instrument failures isn't GPS more than capable to get you air speed close enough to work. Oh, it's the fact that the air mass itself could be moving significantly isn't it?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Indonesian 737 Max 8

Quote (smoked)

Oh, it's the fact that the air mass itself could be moving significantly isn't it?

That. And density.

RE: Indonesian 737 Max 8

(OP)
Ah, that's right! A rather key aspect of lift and drag.

Thanks Mint.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Indonesian 737 Max 8

Density tends to cancel itself out when the aircraft is in the air. And being in a moving mass of air is more a navigational issue than a flight performance issue until you get a sudden change in that movement which we term wind shear. Then the momentum of the aircraft comes into play and you can get suddenly relative drops or increases in airspeed. But steady state the aircraft doesn't know its in a moving mass of air.

Ground speed off the GPS is used on the flyby wire aircraft namely airbus but I have only ever flown one of them twice in the sim for an hour and haven't done the 5 weeks ground school conversion onto a airbus type so don't want to comment on how that works I just know on approach its involved somehow but quiet what it does I have no clue.

GPS quality is governed by a thing called RAIM the system automatically works out a quality factor and tells you when it drops below a certain value for a particular phase of flight or airspace. In the case of doing GPS approaches (RNP) we have indicators of what class of signal we are receiving and if its below whats mandated the box will refuse to give us the guidance to carry out the approach.

Now the thing that really kills lift is when the angle of attack of the aircraft exceeds the critical angle and the aircraft goes into a stall. There is a sudden drop in the coefficient of lift as the air flow breaks away from its lamina flow over the aerofoil and turbulent flow starts.

Now as such on civilian aircraft the only indication of angle of attack (AoA) is the attitude of the aircraft. The data is available because the stall protection system has usually 2 or more AoA vanes to get data from. And when it senses your near the limits it triggers various responses in the cockpit which are tactile(thing called a stick shaker) noise, and visual. If the AoA increases further it can then trigger a thing called stick push which applied nose down control input thus reducing the AoA.

Now what your actually doing in the cockpit when you fly a speed is set a AoA for what you want. For takeoff and landing its defined from the critical angle of attack and after departure you are setting the most efficient AoA from the lift drag curve. Its the same angle of attack its just the speed varies with weight. It is possible to have zero airspeed and not be at the critical angle of attack. But we don't do that sort of nonsense in transport aircraft.

No why do pilots fly airspeed not AoA…… historical and the whole training setup is geared towards nailing an airspeed.

I don't want to name my airline. The media dept does search the net for the airline name and its part of our contract that us pilots don't make statements on the company's behalf. Although nothing I have said is controversial they don't tell me how to fly a plane and I don't step on there toes.

Now I have to go throw an aircraft at the ground 4 times.

I will leave you with a training video which covers this subject.

https://youtu.be/WfNBmZy1Yuc?list=PLAjYL1_ZduxTtSk...

This series of training lectures are extremely good. Note the date they were made. These issues have been current for years and years. They are long every year we have 5 days of similar ground school looking at accident causes so we can hopefully trap the errors before they occur. IN the sim which occurs every 6 months for 2 day we recreate situations to see how to deal with them.

https://www.youtube.com/playlist?list=PLAjYL1_Zdux...

please note my comments above are extremely simplistic from both a pilots point of view and engineering. To behonest I really don't have time to provide a more comprehensive reply.







RE: Indonesian 737 Max 8

Contrast Air France 447 (crashed in the Atlantic) and US Airways Flight 1549 (landed in the Hudson).

An obvious and highly significant difference is that Capt Sullenberger concentrated on flying the airplane (while copilot Skiles was busy troubleshooting). According to the information I've seen, that division of responsibilities, ensuring that somebody was 'flying the plane', perhaps didn't occur on other incidents that ended badly.

In such emergencies, perhaps the displays on the left side of the cockpit should flash "Fly the plane!" in large letters.

Let the right seat do the system failure troubleshooting.

(Acknowledge in advance this is a vast oversimplification; it's intended to make the rather obvious point about CRM, user interfaces, distractions of too many error messages and Human Factors.)

RE: Indonesian 737 Max 8

That type of tunnel vision has been the root cause of more than a few accidents. Another, similar, cause is when the cockpit crew is afraid of the captain and doesn't articulate that there's a problem.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Indonesian 737 Max 8

GPS is an open loop system as well- but the inputs into any system processing GPS information are less corruptible than something like a pitot tube that can suffer from water or ice.

I can't imagine that GPS would ever be able to fully substitute for an on-aircraft, physical way of measuring airspeed. Just put an aircraft in the jet stream, and your ground speed and airspeed could be different by more than 100 kn; the position, velocity, etc of the jet stream aren't known precisely enough that you'd be able to use GPS to cancel out its effect all the time.

RE: Indonesian 737 Max 8

I have to agree with jgKRI about GPS usage for airspeed not being viable. GPS can be used to precisely determine position, and from precise position to precise position, the velocity between the positions (ground speed), and I have even read of airborne experiments where four GPS receivers on an aircraft (nose, tail, left wingtip, right wingtip) did a pretty good job of determining roll, pitch and yaw. But to determine the airspeed within a moving mass of air that can't currently be accurately characterized with available technology, that remains a goal yet to be established and reached.

RE: Indonesian 737 Max 8

Why do you need airspeed?

There is age old system that pilots are taught.

Aviate navigate then communicate.

Btw that's my day just finished 280 pax moved. 10 cups of tea. 7 tons of fuels burnt. And the aircraft in the same state as when I signed for it this morning.



RE: Indonesian 737 Max 8

(OP)

Quote:

cockpit crew is afraid of the captain
Tenerife captain of the Rhine.

I wonder if a simple heated probe being a mass-flow sensor coupled with a altimeter couldn't be used as a speed system. The barometer/altimeter to give air density the massflow with the density to give speed.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Indonesian 737 Max 8

smile you got where I am coming from. There are other ways of instrumenting aircraft to get what you need. But we are still using basically ww1 tech.

RE: Indonesian 737 Max 8

But you also missed the time difference between my posts. I started in the dark and last landing was in the dark.


The dealing with issues at the beginning of the shift is shall we say different to the last sector of the day. The human limitations is also a major influence.

RE: Indonesian 737 Max 8

" four GPS receivers on an aircraft (nose, tail, left wingtip, right wingtip) did a pretty good job of determining roll, pitch and yaw."

Using differential GPS (don't ask me) we no longer need conventional instruments (accelerometers, gyros, correvits) to do much of my vehicle dynamics work. But we can't measure windspeed using them!

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Indonesian 737 Max 8

that's called spurious stick shaker.

Normally when the stick shaker goes off it means your approaching the stall. So you do the stall recovery memory actions

Which are

1. reduce the pitch attitude.
2. roll wings level
3. power to max
4. once airspeed increase pitch for climb.
5. clean up
6. normal flight at safe altitude.

I might add this is the European recovery procedure. American pilots do it differently normally even though NASA and the FAA have told them to do it the same as us.

Spurious stick shaker we can kill with a single button push. But we are back to recognising that it is indeed spurious.

During departure your going to be at high power levels anyway and if you reduce the attitude to below the horizon the airspeed will within a couple of second exceed Vmo (max operating speed) and another alarm will start going off. At this point you have conflicting alarms one for high speed and one for low.

I have had this issue 3 times in my 8000 hours. Twice on the analog machine which has tabs on the wing which lift when the static point starts going under the foil. And once on a AoA vane stall system. Again for me the biggest problem was trying to convince my colleague we were no where near stalling.

Pitot tube blockage is another one which is interesting it makes the airspeed indicate that its rapidly increasing ie it turns the airspeed indicator into an altimeter. Increasing airspeed in the climb would normally lead to a pitch up to reduce it. If you pitch up to much you hit critical angle of attach and stall. Had that a couple of times on the analogue machine.

RE: Indonesian 737 Max 8

Alistair, from what I'm reading it sounds like the AOA discrepancy kicked on an automated nose-down trim, kind of like a stick pusher but more gradual and able to be overrode by the pilot either with yoke forces or via manually actuating the trim system. This kind of stall protection system is beyond my experience, though I do have training regarding runaway trim. Can you shed some light on what this article and the service bulletin from Boeing is discussing?

Quote (Alistair)

American pilots do it differently normally

We do? Your memory action procedure is the same as mine; though again I don't fly anything bigger than 4 seats so not sure if the airlines are doing anything different.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

Yes there is a couple of areas which differ one is this stall recovery and the other upset recovery.

There is a fixation about not loosing altitude. So the power is applied while lowering the nose as a combined action. Then the wings levelled and then the pitch for climb. The way we are taught is almost a pause between each item. The problem with applying at the same time especially in swept wing jets is the pitch power couple. By putting the power in your getting a huge up couple which increases your AoA and can make maters worse. Then the fixation of not loosing altitude means that the pitch for climb occurs to abruptly which can and does frequently result in a secondary deeper stall. Most will try and power out which works on the stickshaker because its a incipient stall recovery and the plane isn't actually stalled yet. The only way to unstall a plane is to reduce the AoA. No difference between a C150 and a 747. if you have been taught the above method then its slowly working its way through.

The upset recovery is another issue, a lot of US pilots have been taught to lift the wing using rudder. This is fine if its a singular event ie you will get away with it even though there is more drag and the plane is completely out of balance. But if you have hit vortex with rapidly changing air flow it can result in a cyclic application of flight controls sometimes full deflection at way over rough airspeed. The classic example of this is AA587, after 5-6 cyclic applications of full rudder the tail failed.

Last BFR I did the FAA instructor really was not happy with me doing it the European way and that was 10 years ago. And some of our new pilots who did their initial training in the US are still coming into the sim with the American way of dealing with these situations so its still being taught.

RE: Indonesian 737 Max 8

As I said previously I am not 737 rated but again we have a trim runaway procedure on all aircraft fitted with electric trim. Its one button/switch to kill the electrical trim system. The Boeings have a manual trim wheel as well and its scary loud and fast when it moves. The q400 if the trim system is active more than 5 seconds then an audible alarm sounds. And if ours fail we just have to hold it untrimmed which if its been allowed to go fully forward is a bit of sweat and swearing flaring.

RE: Indonesian 737 Max 8

Quote (Alistair)

There is a fixation about not loosing altitude.

Ah, yep, I see the difference you're talking about and indeed I was trained the "American" way. When I did my instrument training there was some emphasis on unloading the wings first prior to rolling and recovering. We're still grilled about altitude loss, though. If we pause too much we fail the procedure due to altitude loss. I think part of it is primary training occurs in piston singles which are much more forgiving to recovery technique.

Quote (Alistair)

The upset recovery is another issue, a lot of US pilots have been taught to lift the wing using rudder.

I believe this has been changed; when I did my instrument training recently the upset recovery was taught to me as: unload the wings, level via ailerons and coordinated rudder, recover from the vertical upset, and set power to return to previous altitude. Still, I imagine many older instructors are teaching the older methods.

Quote (Alistair)

trim runaway procedure on all aircraft fitted with electric trim

In the G1000 Cessna I fly the procedure is identical. Holding A/P disc kills electric trim, if trim is in motion for more than a few seconds we get an audible warning, and if it gets out of hand we can manually override the A/P clutches with a complimentary arm workout. Funny how the technically advanced GA aircraft are becoming more and more like a single-pilot jet in terms of procedures and avionics.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

to be honest mate I am legal to fly this. And your G1000 is fancier than the Q400



And this

RE: Indonesian 737 Max 8

opps chose the wrong picture of the Jetstream cockpit its has a FMS and auto pilot the ones I fly have no autopilot and this is the gps.

RE: Indonesian 737 Max 8

Heh, the G1000 makes single pilot IFR so simple. I occasionally go up and do some IFR work in an old C152 just to make sure I can still fly without a computer doing 90% of the work for me. Not going to be one of those pilots who can only follow the magenta line.

Anyway, back to the topic; what I'm reading some component was replaced by the airline prior to the fatal flight. I'm sure the accident chain will include all parties but this is seeming more and more like a Boeing design issue. Too soon to make that judgement you think?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

For those playing at home; here's the cockpit of the G1000 system Alstair and I are talking about:



Modern single engine trainer aircraft these days often have more advanced avionics than many of the older airline fleets. Nothing wrong with the older tech of course and the cost to update doesn't make sense; still amusing though.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

Speed Trim SystemThe Speed Trim System (STS) is a speed stability augmentation system designed to improve flight characteristics during operations with a low gross weight, aft center of gravity and high thrust when the autopilot is not engaged. The purpose of the STS is to return the airplane to a trimmed speed by commanding the stabilizer in a direction opposite the speed change. The STS monitors inputs of stabilizer position, thrust lever position, airspeed and vertical speed and then trims the stabilizer using the autopilot stabilizer trim. As the airplane speed increases or decreases from the trimmed speed, the stabilizer is commanded in the direction to return the airplane to the trimmed speed. This increases control column forces to force the airplane to return to the trimmed speed. As the airplane returns to the trimmed speed, the STS commanded stabilizer movement is removed.

STS operates most frequently during takeoffs, climb and go-arounds. Conditions for speed trim operation are listed below:•STS Mach gain is fully enabled between 100 KIAS and Mach 0.60 with a fadeout to zero by Mach 0.68
•10 seconds after takeoff
•5 seconds following release of trim switches
•Autopilot not engaged
•Sensing of trim requirement

RE: Indonesian 737 Max 8

Found that on the system in question although quiet how that links into the AoA system is obviously a need to know system knowledge.

And way way to soon to make anything other than interest comments as we have been doing on the general systems how they work and human performance issues.

One of the reason why I registered was partly to do with my background as an engineer but also because with hindsight I realised that my engineering training was really quiet poor at highlight the human interaction and reaction aspects of the projects I was involved with. And from my lurking particularly this forum section I saw what a great bunch of thinking and opened minded people were participating.

You just have to look at that ergonomic heap of shite which is the Jetstream (35 year old design) and compare it with the Q400 which is 15 years old to see how things have progressed. But have they progressed too far? Is there too much data being pushed towards the user. This has implications in several applications I have "gone back" to an old haunt in Nuclear to teach Crew resource management and Threat Error Management (TEM), also I have taught medics as well and got them to use checklists.

As engineers its something to think about.

I am opened minded if its a design issue or human factors/training. You have to remember that the 737 would not pass current certification standards if it was a green fields design. Its been stretched digitised and tinkered with all on the back of a 1960's design and the certification standards of the day. Similar to the DXB EK521k which is now going through the courts claiming design failure. As a punter involved yes have your day in court.... but honestly as a pilot you don't just press a button and expect the machine to do as you expect. You verify its the correct button, then you press it and then you verify again that its had the effect you want. You press a button wanting toga power and you don't see the needles rising and you don't feel a kick in the pants as the power winds up... you bloody well shove the power levers forward and pitch the nose to the required attitude.

RE: Indonesian 737 Max 8

(OP)
The PhyOrg article states differing "AoA of 20°".

That seems huge to me. What are the typical values of AoA seen in airliner operations?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Indonesian 737 Max 8

yes 0 to about 15 degrees anything more than that and your into critical angle of attack which means the stall system is triggered.

I maybe guilty of presuming that people know what the critical angle of attack is and its approximate value in my posts... I do apologise.

As soon as I saw 20 deg split I presumed the stick shaker system was triggered.

0 is extremely rare and would be extremely uncomfortable to the pax. The wing incidence which is shall we say welded is about 1-2 degs and we would normally cruise with 1-2 deg nose pitch up giving about 3 deg in the cruise but this does depend on what cost index we are using and what the "sweet" AoA is for coefficient of lift and Coefficient of Drag.

RE: Indonesian 737 Max 8

If pitot tubes keep causing problems (system failures, ice build-up, water, mud dauber wasps, masking tape leftover from painting....the list goes on and on), then it may be time to consider back-up systems, based on an alternate technology, with a well-designed method of combining the information (Kalman filter concept).

Quora says, "Hot wire anemometers arranged in a circular pattern can determine both wind speed and direction. Another method is using ultrasonic based sensors." These used to measure wind.

Lasers are also mentioned, in terms of tracking microscopic airborne particles.

And with GPS as another input for our purposes.

Strings attached to the windscreen and sidescreens like gliders?

RE: Indonesian 737 Max 8

0 is extremely rare and would be extremely uncomfortable to the pax really, really fun!

But, yes, most (all?) planes cruise with a few degrees of AoA. Bernoulli and Newton generally both are at work to make a wing fly.

Quote (Alistair)

cost index we are using and what the "sweet" AoA is for coefficient of lift and Coefficient of Drag.

Oh, do expand on this. Is AoA how you target efficiency for cruise flight? (dragging us off topic, sorry)

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

Quote (VE1BLL)

then it may be time to consider back-up systems

You're not wrong but how would any of these systems not also be susceptible to the same failures as a pitot-static system?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

20 degree differential between two AoA data streams is huge.

There's no exact numbers- optimal angle of attack for an airfoil in any particular steady-state flight condition depends on aircraft weight/CG position, mach number, density altitude, and position on the wing. Position on the wing is interesting to keep in mind, because most aircraft wings are swept combinations of several, sometimes many, individual foil shapes and there is typically twist in the chord angle between the root and the tip of the wing; so the 'true' AOA is different at any given point on the wing. The reason for the airfoil twist in the wing is actually to control where stall conditions first arise on the wing to make the stall more progressive and easier to control/recover.

Anyway. Most conventional airfoil shapes will have a stall angle between 15 and 20 degrees, and actual operating aoa will be much lower, say 2-4 degrees.

So yeah, having two aoa streams, one telling you that the aoa is 2 degrees and the other saying it's 22 degrees is potentially really, really bad.

RE: Indonesian 737 Max 8

to be honest I reckon aircraft fly because of newton but are controlled because of Bernoulli.

If you have a flat piece of wood at an angle to an airflow it will produce lift and give a force vector.

We don't get AoA in the cockpit but most performance stuff is related to AoA and where you are in the drag curve Vy will give you the best performance because it gives you the least drag. But there are other financial cost and because the increase in drag is not linear then they adjust the cost index to take into account the fuel price.

Because we can't set a AoA we get an app or pages of performance data which links weight to power requirements which is a round about way leads to a AoA being set. There is another fudge factor built in which takes into account head and tail winds so we fly faster in a head wind and slower in a tail wind which factors in the maint costs per hour of operation. Also the air temp changes the settings and the fuel burn. IN the jets with auto throttles its all done by the Flight management system. Turboprop drivers have to do it manually we look at the temp, head/tail wind, weight and altitude and put them into an app on a Ipad and then it spits out a torque setting. We set it and order a cup of tea. last flight of the day we may choose to ignore the app especially if out mates are flying and its a race to get home first.

RE: Indonesian 737 Max 8

Quote (TehME)

...susceptible to the same failures as a pitot-static system?

The goal would be to reduce the odds, so that Airspeed Indicator issues might be persuaded to move way down the list.

Commonality can allow common failure modes, in spite of redundancy. e.g. AF447.

(By way of counter-example: If I recall correctly, the Space Shuttle famously had intentionally diverse flight control computers. Triple redundant, but all three different. Different hardware and different software.)

As has been noted, Human Factors is a major contributing factor with these incidents. So how backups are all integrated and presented seems like the difficult aspect. Multiple diverse sensors is the relatively easier design aspect.

The complex interactions between bad or confusing data and the pilots, leading to an otherwise functional aircraft to crash, seems to be a recurring theme. Engineering or designing out these sorts of issues seems like an issue worth addressing.

I'm not an SME on this topic. These are just opinions based on what I've seen or read.

RE: Indonesian 737 Max 8

The space shuttle even with its redundancy still ended up with instrument failures and some hairy landings.

All the pilots on the stick were test pilots who have super human data processing ability's. They still screwed it up occasionally.

And as the poster above says the changing of the design and the presentation of data is constantly evolving.

In the next 12 months I will be getting trained up on this aircraft.



Still fly's the same as the Jetstream. No paper no checklists, only memory item is to put your o2 mask on if there is smoke.

RE: Indonesian 737 Max 8

Looks like an A350. What's the giant red button do?

Has Airbus finally made it so you have some feedback on what each side stick is doing? (i.e. Airfrance crash)

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries https://www.facebook.com/AmericanConcrete/

RE: Indonesian 737 Max 8

That's the eject button, obviously

RE: Indonesian 737 Max 8

It is an airbus but a Canadian one not a French one winky smile

I haven't got a clue what the big red button does. I suspect its a flight test aircraft. They are set up a bit weird because they have huge water tanks in the back and they can pump between them to do the CofG testing and variable weight for the envelope testing.

Always gets the chemtrail lot excited when they see funny buttons and water tanks in the back. closely followed by the flat earth lot asking if the water is flat or at an angle.

I don't remember seeing it on our fleet the couple of times I have jumpseated.

RE: Indonesian 737 Max 8

Quote (jgKRI)

That's the eject button, obviously
I think it's an emergency stop.

Brad Waybright

It's all okay as long as it's okay.

RE: Indonesian 737 Max 8

Strikes me that the big red buttons are associated with the special red panel in the center console.
Shifting water back/forward, with a valve Open/Close/Fail annunciator on the red panel... perhaps a "dump" on the panic button? Just guessing.
Back in the day, Canadair (predecessor to Bombardier) had a test flight crash, due to CG too far aft.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

RE: Indonesian 737 Max 8

The word out from Boeing to RTFM was real subtle.
Why they were still flying a plane with wonky sensors, well it is Lion.
In that part of the world there are not a lot of good options.
I flew Garuda once, it was an old 737 that they couldn't pressurize because of gaps in the skin panels.
It didn't matter we were island hopping, about 15 min in the air and never above 3,000ft.
The livestock on the plane was very interesting.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Indonesian 737 Max 8

Aft CofG is a killer, forward C of G is just a work out of your arms and a rather firm arrival.

On the Q400 None public transport we get a additional lump of performance graph to play with in the forward part. Rear is a hard limit and don't screw around with it. Thankfully its extremely hard to get into even with the pax cabin empty and the rear hold full. You would have to fill the rear hold and only seats behind the wing to get near it.

RE: Indonesian 737 Max 8

Isn't that big red knob the tow rope release?

RE: Indonesian 737 Max 8

I was on a 45 minute flight on a King Air that had the CoG so far back that we were sitting on the tail prop with the nose in the air.
The pilot and co-pilot boarded and climbed uphill to their seats. As they were strapping in the nose slowly came down and the nose wheel bounced once on the ground. I guess three point contact meant good to go.
For 45 minutes we were at such an extreme AoA that I kept thinking;
"If the engines fail, we are going to slide backwards down into the ocean."
As you may have deduced, we made it.
Some of the King Airs had a radar set mounted low down on the center of the dash.
One plane had a full size colour picture of a radar set glued in the place where the radar set would normally be mounted.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Indonesian 737 Max 8


Emergency AD Issued On B737 Max






The FAA has issued an Emergency Airworthiness Directive (PDF) that directs the owners of all Boeing 737 Max aircraft to amend their operating manuals, to avoid a control problem like the one that apparently caused the fatal crash of a Max 8 last week. “Possible erroneous angle-of-attack inputs on Boeing 737 Max aircraft … can potentially make the horizontal stabilizers repeatedly pitch the nose of the airplane downward, making the aircraft difficult to control," the FAA says in an emergency AD dated Nov. 7. The airplanes are not grounded, and the owners have three days to comply with the AD, which requires a revision to the airplane flight manual.
“This emergency AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer,” the AD reads. “This condition, if not addressed, could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain.” The AD requires revising certificate limitations and operating procedures to provide the flight crew with runaway horizontal stabilizer trim procedures to follow under certain conditions.
B.E.

You are judged not by what you know, but by what you can do.

RE: Indonesian 737 Max 8

How exactly the LionAir 737's AP could have gotten confused is not yet clear to me, even with the text of the AD to refer to. It seems a declaration that the AP system's functional hazard analysis has been found to be invalid, or relied on a backup source that doesn't do what it's supposed to. The AD says nothing about the configuration of the AP system, as if one faulty sensor should be expected to provoke a wild ride. It absolutely should not. The text of the AD makes mockery of a reliability requirement that has been standard practice in flight system design since the 1960's.

Part of the functional hazard analysis (FHA) for any critical flight system is the need to identify faulty readings, and disregard them. The resolutions become built into the design and programming of the system.

The Autopilot system must be programmed to compare, select, filter, or vote in some way that deals with bad sensors, otherwise the moment your anti-icing system fails you go into a spin. All of the sensors are backed up with at least a duplicate, and wherever possible a completely alternative source that can reconstruct the same information. AoA can be measured directly from AoA sensors (the 737 Max is typical, has two of these, one on each side of the cockpit) and a couple of inertial navigation units (INU) within (probably) more than one attitude and heading reference systems (AHRS). There may be other ways; I'm not a sparky. I do not know what system is on board the 737 max, so I do not know what it actually uses for backup data. But the requirements of FAR 25.1309 speak for themselves, and no single failure may pose a hazard to the aircraft.

I would like to know if sensor de-icing was turned off, or malfunctioning at the time. The flight crew would be aware of either conditions, if they were happening. Since the text of the AD doesn't demand examination of the de-ice, or admonish pilots to keep it turned on, this may have been checked already, and ruled out.

This AD sheds no light on what happened, to me. Raises more questions than it answers.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

RE: Indonesian 737 Max 8

Could differential information collected from GPS receivers in the nose and the tail provide attitude information and/or AoA information?
Maybe not exact, but surely enough to identify a 20 degree error.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Indonesian 737 Max 8

Quote (STF)

...sensor de-icing was turned off, or malfunctioning...

Can icing occur 5000 feet over Indonesia? Also, within such a few minutes?

RE: Indonesian 737 Max 8

GPS is too slow (IIRC) to act as an attitude reference.

With a ground temperature of ~27C, and a dew point ~24C, you are right to be skeptical of icing. However, the atmosphere does not always obey the standard temperature lapse rate.
Since I haven't seen anything yet explicitly ruling out this problem, it remains "on the board" despite the low probability. Especially since it was a contributing factor in the AF447 crash, which bears some similarities.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

RE: Indonesian 737 Max 8

"Could differential information collected from GPS receivers in the nose and the tail provide attitude information and/or AoA information?
Maybe not exact, but surely enough to identify a 20 degree error."

We use GPS sensors at the front and back of the car to measure yaw velocity pitch and accelerations on cars for vehicle dynamics work.

These have replaced conventional accelerometers and gyroscopes so far as we're concerned. Bandwidth >10Hz

Differential GPS should be able to measure nose to tail pitch distance to within 800mm, which, unless you are flying very short aeroplanes, would comfortably indicate to a degree or so.

Not too sure where SparWeb is coming from on this.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Indonesian 737 Max 8

It was phrased as a guess, about the GPS for attitude. If you can get 10Hz and <1m resolution from a GPS board then sure, it can do that if you want it to. And want to add antennas to the tail. May not be necessary if you already have multiple AHRS units with IMU in the aircraft, but a backup to a backup... I didn't mean to get drawn into a comparison of IMU / GPS resolution and data rates. This is far from my specialty, so if you can prove me wrong, touché.

I did feel comfortable asking questions about the seeming lack of redundancy in the AP system. This is more in my wheelhouse. Something there doesn't add up. One faulty AoA sensor should not render the aircraft difficult to control or uncontrollable. The message in the AD (see attached) speaks of a very difficult condition to recover from:

Quote (FAA)

Runaway Stabilizer
Disengage autopilot and control airplane pitch attitude with control column
and main electric trim as required. If relaxing the column causes the trim to
move, set stabilizer trim switches to CUTOUT. If runaway continues, hold
the stabilizer trim wheel against rotation and trim the airplane manually.
Note: The 737-8/-9 uses a Flight Control Computer command of pitch
trim to improve longitudinal handling characteristics. In the event of
erroneous Angle of Attack (AOA) input, the pitch trim system can trim
the stabilizer nose down in increments lasting up to 10 seconds.
In the event an uncommanded nose down stabilizer trim is experienced
on the 737-8/-9, in conjunction with one or more of the indications or
effects listed below, do the existing AFM Runaway Stabilizer
procedure above, ensuring that the STAB TRIM CUTOUT switches
are set to CUTOUT and stay in the CUTOUT position for the
remainder of the flight.
An erroneous AOA input can cause some or all of the following
indications and effects:
• Continuous or intermittent stick shaker on the affected side only.
• Minimum speed bar (red and black) on the affected side only.
• Increasing nose down control forces.
• IAS DISAGREE alert.
• ALT DISAGREE alert.
• AOA DISAGREE alert (if the option is installed).
• FEEL DIFF PRESS light.
• Autopilot may disengage.
• Inability to engage autopilot.
Initially, higher control forces may be needed to overcome any
stabilizer nose down trim already applied. Electric stabilizer trim can be
used to neutralize control column pitch forces before moving the STAB
TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be
used before and after the STAB TRIM CUTOUT switches are moved
to CUTOUT.

The previous AFM instructions were a common "abnormal procedure" for when the elevator trim system suffers a runaway. That kind of procedure is known to pilots. Even light aircraft flight manuals will advise how to respond to failure of the trim system.

This new procedure is more dire, describing the crew's response to "higher control forces" and "relaxing the column causes the trim to move". Makes it sound like an AP gone crazy, not a trim runaway.

RE: Indonesian 737 Max 8

The trick is to use several GPS at the extremity of the vehicle. Most of the noise in a GPS output is due to atmospherics, so if you have more than one they seem the same errors, and so when you subtract the outputs (hence the name differential GPS) the noise disappears.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Indonesian 737 Max 8

this is a bit of a funny one because the trim system that's effected is only functional when the plane is being flown manually.

Quote:

• Continuous or intermittent stick shaker on the affected side only.
• Minimum speed bar (red and black) on the affected side only.
• Increasing nose down control forces.
• IAS DISAGREE alert.
• ALT DISAGREE alert.
• AOA DISAGREE alert (if the option is installed).
• FEEL DIFF PRESS light.
• Autopilot may disengage.
• Inability to engage autopilot.

Now thats some list of items to go wrong and apart from the AoA mismatch possibly Feel diff none of them we would immediately associate with having problems with the Angle of Attack sensor.

With my engineers hat on instead of pilots if the AoA data is getting linked into so many systems i would say it needs to have a third backup vane to allow 2 out of three agreeing for fault tolerance just like we do with the other primary data sources.


RE: Indonesian 737 Max 8

Quote (GregLocock)

The trick is to use several GPS at the extremity of the vehicle. Most of the noise in a GPS output is due to atmospherics, so if you have more than one they seem the same errors, and so when you subtract the outputs (hence the name differential GPS) the noise disappears.

I think that some of the issues here regarding the viability of GPS for use in determining the real-time positioning of aircraft and their orientation are due to the conflation of a few different concepts. Not being an expert, I will be speaking in generalities. But I think I have some idea of what is happening here.

The first is your explanation of the name differential GPS. It's true that comparing the difference between local celestial GPS outputs will be more accurate than looking at a single celestial signal, but only marginally so. That's comparing the absolute location (where am in the world) vs. the relative accuracy (where am I relative to this other local sensor experience the same atmospheric interference).

Differential GPS relies on terrestrial base stations transmitting from fixed, precisely-known locations to broadcast a signal correction to the rovers allowing for very precise real-time location. These fixed terrestrial base stations need to be located within several miles of the rovers for the system to function properly.

Am I correct in my assumption that where you are using the differential GPS to measure vehicle dynamics is at a fixed location like a test track and not out and about anywhere in the world at any given time?

RE: Indonesian 737 Max 8

Aircraft don't use dgps. We use the SBAS system.

Galileo if it ever comes online will also give much better resolution.

Even dgps is a bit pants at vertical position.

RE: Indonesian 737 Max 8

While GPS attitude/heading sensors use the differential signals and phase between antennas to determine angles, they are not technically what is(was) called dGPS. dGPS traditionally refers to using a receiver with surveyed, absolute, GPS location with the same receiving environment to "correct" the GPS at an unknown location. This removes the atmospherics as well as the inherent errors in the GPS satellite outputs to achieve, in the best cases, sub-centimeter positional accuracies.

An attitude GPS with 10-m antenna separation can achieve heading accuracies better than 0.02° rms, which is better than the heading performance of an eTalin from Honeywell. However, during a bad enough war, the US GPS likely will turn back on its Selective Availability, which would possibly cripple a GPS-based attitude/heading sensor. However, modern GPS receivers can receive at least the GLONASS signals, and may still have decent performance.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Indonesian 737 Max 8

It was my understanding that the various flavours of GPS reception based on differential signals were developed to cancel out the effects of selective availability.

There was an urban legend concerning Selective Availability during the Gulf war.
Legend had it that during the Gulf war, the army had a demand for quantities of military grade GPS receivers that could not be met quickly enough. (Apparently there is second set of GPS signals that is available only to the US military).
According to the legend, selective availability was turned off so that civilian GPS receivers could be used by the US military.
Some of us felt that the GPS accuracy was improved during and after the war.
Is there any correlation for this legend?

My concern is that the GPS system may be turned off completely during a war, or at least the civilian service.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Indonesian 737 Max 8

The sa has been turned off for years.

And it is that easy to jam GPS that they have said they won't turn it on again. With Galileo transmitting low Res as well these days it's pretty pointless turning it on again.

RE: Indonesian 737 Max 8

All this talk of using GPS for attitude reference is getting off topic. I am duly impressed by growing capabilities of GPS. It is not used for this function in commercial aircraft. GPS has other functions in commercial aircraft, most especially as a navigation reference. You can even couple your approach to runway landing to a suitable GPS reference. However, attitude is not navigation. Nose pitch up and pitch down is attitude, not navigation.

This is what commercial aircraft use for AHRS (Attitude & Heading Reference System).
Collins
Universal
Honeywell

They contain MEMS gyros. Lots of them. There is no co-ax connector on these boxes. They don't use GPS.

With this context clarified, my question bcomes "How did the autopilot system let the airspeed sensors out-vote the AHRS sensors?"

Now that I'm on this rant, I would be remiss if I didn't spill some more acronyms, such as Air Data and Inertial Reference Unit (ADIRU) and Standby Attitude and Air Data Reference Unit (SAARU), which are similar to the AHRS, except they actually DO HAVE airspeed data inputs. Each of these also depend on inertial references for attitude. The 737 MAX may have multiples or none, depending on the system design. I don't have a 737 MAX Flight Crew Operation Manual here, so I can't tell you exactly which boxes it uses. If it does have these, then inside these specific boxes is an algorithm that evaluates the integrity of all data sources and chooses the best for the attitude and condition of the aircraft. I really want to know if something is going wrong inside these boxes, causing the confusion in the flight director system, because that confusion is supposed to be designed out of them. At a very fundamental level, the design goal in these systems is to minimize this specific hazard.

Here's a summary of the Boeing 777's Fly-By-Wire System for reference. The new 737-Max could be similar.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

RE: Indonesian 737 Max 8

just been speaking to a mate that's done the differences training on it.

Apparently there was no mention of this trimming system feature during the training.

BTW this system is not linked to the Autopilot system its linked to the trim system with an input from the stall protection system apparently. They are getting almost daily updates on the subject. Every time they check in there is updates.

They suspect there will be an airworthiness directive with a mod on the subject.

The 737 max is not fly by wire flight controls. It has fly by wire spoilers and bleed air.

the triple is digital flight controls. ie no direct control runs to the flight controls.

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