Indonesian aircraft missing off Jakarta
Is the MAX somehow more tricky in stall behavior that it needs more protection from coming close to one?
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And without going into detail, the correction from AoA for the static pressure is minimal (other corrections like ground effect, reverse thrust etc are more pronounced and still small).
The stabilizer is an immense “secondary” flight control surface.
You can not fly a typical jet transport without intensively using the stabilizer trim, you could probably fly it without using the elevator and just using the stabilizer trim. It is an accepted backup system for the elevator (e.g. on A320, A330/340 in case of total FBW failure)
The elevator is for maneuvres, the stabilizer trim is for all long term pitch changes.
There have been several instances where the stabilizer ended up at the stops and the elevator lacked authority to maintain attitude.
It is a somehow inevitable for most modern aircraft flying over a wide speed range, having a sophisticated high lift system, powerful engines mounted above or below C/G and being optimized for low drag. You would need a much larger stabilizer and more powerful elevator hydraulics to prevent this situation, or an all flying stabilizer like on the L1011.
Once again unfortunately there is a lot of different longitundinal trim systems (especially in smaller aircraft, but also for example on the BAe 146 which does not have a stabilizer trim), which prevents pilots to be trained consistently when learning to fly. Hence there is no consistent policy how to use trim, it is type specific which does not help. Also the amount and type of autotrim is differing significantly between aircraft types, as already mentioned for the STS, which actually is the opposite of "normal" autotrim und just usd as an "automatic in manual flight".
I seriously doubt that all pilots have a full understanding of the longitudinal trim system of their aircraft.
Originally Posted by Defred
Despite the outcry from some posters above in the light of the Boeing bulletin and FAA directive, the fact remains that existing procedures should have brought this aircraft safely back to ground.
Given the normal operation of the STS, pilots could be excused for thinking stab trim motion was "normal", especially given everything that was going on...
Originally Posted by Volume
Looking at a typical flight, the ammount of elevator position changes is probably less than the ammount of stabilizer trim changes.
FAA:
This 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. We are issuing this AD to address this potential resulting nose-down trim, which 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....
This 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. We are issuing this AD to address this potential resulting nose-down trim, which 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....
........do the Runaway Stabilizer NNC ensuring that the STAB TRIM CUTOUT switches are set to CUTOUT
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Interesting and highly plausible scenario ExSp33db1rd.
Just to be clear, your thinking would be that the trim had run all the way forward, the crew were holding full backstick, THEN realised they need to used the stab trim cutout switches? In your scenario the aircraft would now have full nose down trim and no electric pitch trim. Their only option left was the manual pitch trim, but you can't wind that quickly enough to recover with a high rate of descent from 5000'.
Have I got your thinking correct based on the previous incident?
Just to be clear, your thinking would be that the trim had run all the way forward, the crew were holding full backstick, THEN realised they need to used the stab trim cutout switches? In your scenario the aircraft would now have full nose down trim and no electric pitch trim. Their only option left was the manual pitch trim, but you can't wind that quickly enough to recover with a high rate of descent from 5000'.
Have I got your thinking correct based on the previous incident?
N4790P
Despite the outcry from some posters above in the light of the Boeing bulletin and FAA directive, the fact remains that existing procedures should have brought this aircraft safely back to ground.
The bulletin and the AD both say this.
It appears that maintenance and training are the major contributors to this tragic accident.
Maintenance: a maintenance culture that allows an aircraft to fly 4 successive sectors with a known issue affecting air data is completely unacceptable, and it certainly wouldn’t happen in any airline with a proper attitude towards maintenance and safety.
Training: given the above was allowed to occur, a well trained crew should still have been able to handle it. In the absence of well trained crew, an accident was simply a matter of time.
There is no such thing as pilot-error in an airline, only a poor training system which allows poorly trained pilots to operate.
The bulletin and the AD both say this.
It appears that maintenance and training are the major contributors to this tragic accident.
Maintenance: a maintenance culture that allows an aircraft to fly 4 successive sectors with a known issue affecting air data is completely unacceptable, and it certainly wouldn’t happen in any airline with a proper attitude towards maintenance and safety.
Training: given the above was allowed to occur, a well trained crew should still have been able to handle it. In the absence of well trained crew, an accident was simply a matter of time.
There is no such thing as pilot-error in an airline, only a poor training system which allows poorly trained pilots to operate.
Just a thought as I have no knowledge of the Max data pack
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If you have trimmed for a speed and configuration, then you shouldn't need to trim again and again. Don't fly it on the trim. If you're using the trim more than the elevator, you're doing something fundamentally wrong.
Basically if you have to do the same elevator correction again and again, you should better use the trim...
Even if you have trimmed for a speed and configuration you are burning fuel and therefore need to apply corrections from time to time, unless the autopilot does this for you. These corrections should be done by trim.
In modern aircraft trim is less and less used by the pilots actively, autotrim has widely replaced it. Another skill eroding...
Interesting and highly plausible scenario ExSp33db1rd.
If it takes the force of 4 hands on the wheels to prevent the aircraft from nosediving, which hand do you use to operate the trim cutout switches ?
Originally Posted by Volume
Even if you have trimmed for a speed and configuration you are burning fuel and therefore need to apply corrections from time to time, unless the autopilot does this for you. These corrections should be done by trim.
In modern aircraft trim is less and less used by the pilots actively, autotrim has widely replaced it. Another skill eroding...
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Can someone explain to me why anyone in their right mind would design a system whereby information from one sensor (in this case, the AoA sensor) corrupts the information coming from other independent systems (airspeed, altitude, and indeed even seemingly groundspeed)?
The equivalent in a car would be to allow the external temperature sensor to mess with the speedometer reading and the fuel gauge reading. Oh, and also allowing it to floor the throttle!!!
If airspeed and altitude readings remain unaffected, the pilot is surely going to have a much better view of what the plane is doing and what has gone wrong.
The equivalent in a car would be to allow the external temperature sensor to mess with the speedometer reading and the fuel gauge reading. Oh, and also allowing it to floor the throttle!!!
If airspeed and altitude readings remain unaffected, the pilot is surely going to have a much better view of what the plane is doing and what has gone wrong.
What it may cause is the Air Data Computer informing other systems of this AoA disagree, and other systems acting on this. I.e., a flight augmentation system, which heavily depends on AoA for e.g. envelope protection, would most likely switch to a degraded mode and inform the pilots of this. Depending on the aircraft the auto pilot may disconnect as well. So while there are effects and a possible increased workload for the pilots, there is no reason think that a broken AoA sensor presents invalid airspeed and altitude as well.
Interestingly, as far as I know the failure of one or both AoA sensors is not often part of the simulator training curriculum (pilots, please correct me if I am wrong). At least not to the extent that unreliable airspeed is trained these days (in part because of the AF447 accident, which put more emphasis on this in the simulator).
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With the Max being the first 737 simulator data pack delivered to the TDMs as binaries I'm wondering whether anomalies with the AoA vane and associated systems are within the MDD as the operators no longer have any ability to pick and choose nor introduce malfunctions into their training programs. The OEM decides.
Just a thought as I have no knowledge of the Max data pack
Just a thought as I have no knowledge of the Max data pack
- MDD is Malfunction Description Document (describing all malfunctions which can be inserted on the device)
- TDM is Training Device Manufacturer
Looking in this 737 QRH, there is no instruction for pilots on affected systems when AoA disagrees and what actions to take, neither STS or autopilot is mentioned. Could be an outdated one [edit: it IS an outdated one, 2009 and NG only].
Last edited by Flutter speed; 8th Nov 2018 at 11:04. Reason: remark
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Can be viewed at: http://rgl.faa.gov/Regulatory_and_Gu..._Emergency.pdf
https://ad.easa.europa.eu/ad/US-2018-23-51
I guess that means it applies to European operators, as well.
Bernd
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Is it conceivable that if both pilots were making a coordinated maximum effort to control the runaway stabiliser trim, a miscoordination of simultaneous push and pull in a highly stressful situation could have triggered the Elevator Control Column Overide Mechanism? This would cause an immediate and significant reduction in elevator authority and increased control forces greater than those experienced in manual reversion.
Last edited by OVER THE TOP; 8th Nov 2018 at 12:25.
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As I mentioned in another post, the AoA data is used to calibrate the measured static pressure and therefore not fully independent as in your example. As such there is a direct link between AoA sensor data and airspeed / altitude. However, this calibration is small and a miscompare between the two AoA sensors is unlikely to invalidate airspeed (though as said, it may influence the displayed airspeed and trigger a disagree between IAS in extreme scenario's).
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So, for a 737, what would be the largest difference between IAS, uncorrected for AoA, and CAS, within the range of AoA that could be reasonably expected in flight? The large airplanes I've flown didn't correct IAS for AoA, and without looking them up, I don't recall that the airspeed calibration charts having a correction larger than 5 ish knots. Seems if it came down to a choice between an IAS that differed from CAS about 5 kt at low airspeeds/high AoA and no usable airspeed indication at all if the AoA fails, I know what I'd prefer. Especially if my takeoff and landing charts were referenced to IAS, whcih makes CAS a relatively uninteresting number. (except in cruise, where IAS and CAS are usually fairly close anyway)
Here is another point of view. There is evidence the aircraft was diving steeply before impact with the water. Regardless what caused the stabilizer to have moved fully forward, there is no doubt the pilots would have been trying to pull out of the dive by pulling hard on the elevator. Extreme air loads caused by high speed would have been working against them. The significant forward position of the stabilizer would be in direct opposition to the elevator position during the attempted recovery; thereby severely limiting the effectiveness of the elevator.
Under the heading Manual Stabilizer Trim, (Boeing 737 FCTM Non Normal Section), the following advice is offered:
"Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the airloads to allow manual trimming."
To relieve airloads, the crew must momentarily release all backward pressure on the elevator then rapidly wind the stabilizer trim backwards manually or electrically. In turn, this allows more effective elevator control. In other words, a yo-yo manoeuvre. The crew needed to react instantly and correctly to relieve air loads in this manner. Unfortunately, the Lion Air crew did not have the altitude to successfully recover before impact.
Under the heading Manual Stabilizer Trim, (Boeing 737 FCTM Non Normal Section), the following advice is offered:
"Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the airloads to allow manual trimming."
To relieve airloads, the crew must momentarily release all backward pressure on the elevator then rapidly wind the stabilizer trim backwards manually or electrically. In turn, this allows more effective elevator control. In other words, a yo-yo manoeuvre. The crew needed to react instantly and correctly to relieve air loads in this manner. Unfortunately, the Lion Air crew did not have the altitude to successfully recover before impact.
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So, for a 737, what would be the largest difference between IAS, uncorrected for AoA, and CAS, within the range of AoA that could be reasonably expected in flight? The large airplanes I've flown didn't correct IAS for AoA, and without looking them up, I don't recall that the airspeed calibration charts having a correction larger than 5 ish knots. Seems if it came down to a choice between an IAS that differed from CAS about 5 kt at low airspeeds/high AoA and no usable airspeed indication at all if the AoA fails, I know what I'd prefer. Especially if my takeoff and landing charts were referenced to IAS, whcih makes CAS a relatively uninteresting number. (except in cruise, where IAS and CAS are usually fairly close anyway)
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I agree this note in the manual is likely not related to this accident. All the data available indicates the accident aircraft was well above stall airspeed for the pressure altitude they were operating in and thus there was plenty of elevator authority available.
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the failure of one or both AoA sensors is not often part of the simulator training curriculum
AoA is not directly indicated, AoA is not directly used by the pilot during normal or ermengy procedures.
AoA failure is implicitly trained as some of the failures which can be caused by AoA failure are trained.
If you only have 2 AoA, failure of one or failure of both should not make much difference, if both do no longer agree, yo do not know which one is faulty, so you have to assume both have failed.
We even already had the case that 2 out of 3 did fail (freeze at a safe value), so the computers decided that they must be correct, and the single different one (indicating an unsafe value) must be at fault...