Like the Lion Air 737 MAX crash in Indonesia, the crash of an A320 acceptance test flight in France in 2008 involved the extreme trim of the horizontal stabilizer that the pilots could not overcome. See Chapter 18: Not All Pilots can carry out Test Flights.
The A320 pilots attempted to check the automatic stall protection system at too low an altitude with little height to recover should something go wrong.
Unfortunately, the angle of attack sensors due to ingress of water used to clean the fuselage had frozen in the level-flight position making the computer believe the aircraft was not going to stall.
When it eventually did stall, the extreme upward trim of of the horizontal stabilizer made it impossible to push the nose down to regain speed, for with the low-slung engines at full thrust also pushing the nose up, the elevators could not overcome the two. The stalled aircraft (not filled with fare-paying passengers) plunged into the sea.
In the Lion Air case, the downward trim came about because a new safety system (which no pilots had been told about) pushed the nose down because of a faulty angle of attack sensor indicated that the aircraft was was nose-up and in danger of stalling.
WHAT THE TWO ACCIDENTS HAVE IN COMMON is failure of the pilots to deal with the extreme trim of the horizontal stabilizer and therefore being unable to recover.
In the A320 case, there was a warning to USE MANUAL TRIM, which the pilots either did not notice or ignored–they anyway had little time.
In the 737 case, it was not so simple as the computer overrode the trim system even in manual, and the trim had to be completely disabled. The pilots did not even know that they should do that, though on when problems had occurred with that very aircraft on a previous flight the pilots had managed to do so and recovered. Unfortunately, the the pilots of the crashed flight were not told about that how those pilots had disabled the trim.
We highly recommend the reader click HERE to see the fascinating article by Bjorn Fehrm on Leeham News explaining why Boeing introduced their new anti-stall system on the 737 MAX to cope with the forward positioning of the larger engine nacelles covering the higher bypass LEAP-1B engines–the 737 conceived many years ago was designed to be very low on the ground, hence the odd (not round) shaped nacelles.
Boeing’s just issued warning to users of the 737 MAX that was not in any of the manuals.
“This bulletin directs flight crews to existing procedures to address this condition. In the event of erroneous Angle of Attack (AOA) data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds. The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 seconds after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down stabilizer continue to occur unless the stabilizer trim system is deactivated through use of both STAB TRIM CUTOUT switches in accordance with the existing procedures in the Runaway Stabilizer NNC. It is possible for the stabilizer to reach the nose down limit unless the system inputs are counteracted completely by pilot trim inputs and both STAB TRIM CUTOUT switches are moved to CUTOUT.
Additionally, pilots are reminded that an erroneous AOA 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.
– Inability to engage autopilot.
– Automatic disengagement of autopilot.
– IAS DISAGREE alert.
– ALT DISAGREE alert.
– AOA DISAGREE alert (if the AOA indicator option is installed)
– FEEL DIFF PRESS light.
In the event an uncommanded nose down stabilizer trim is experienced on the 737 – 8 / – 9, in conjunction with one or more of the above indications or effects, do the Runaway Stabilizer NNC ensuring that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight.”