|737 MAX throttle quadrant showing trim wheel and stab trim cutout switches (lower right).|
The Wall Street Journal is reporting that, after preliminary analysis of flight data from the downed Ethiopian 737 (ET302), investigators now believe the controversial Maneuvering Characteristics Augmentation System (MCAS) activated and may have played a part in the accident. This finding is significant because the MCAS system has now been implicated in both this crash and the Lion Air crash which occurred last year.
MCAS - What is It?
To recap, the MCAS system was installed on the 737 Max aircraft to mitigate some unique handling characteristics of the new model which differed from older 737s. Boeing originally chose not to document this new system in the aircraft flight manual, but has since briefed all Max operators on the existence and function of the system in the aftermath of the Lion Air crash.
Among the reasons Boeing engineers may have had for not including the system in the flight manual are that the system was only supposed to ever activate during aerodynamic stall conditions in manually controlled flight, which in normal operations would never be seen. Entire careers are flown without ever seeing an actual stall, so this rationale might have been thought sound.
The problem for the MCAS system wasn't necessarily its intended operation, which was to be rarely if ever seen, but rather any potential failure modes. Unintended activation of the system due to a mechanical fault has now been suggested as a factor in both Max crashes. Flight data from the Lion Air crash show the pilots repeatedly fighting the inputs from a misfiring MCAS system, and according to latest reports, the MCAS system also activated on the mishap Ethiopian airliner.
Adding to the controversy of the existence of an undocumented system is the revelation that the system can be activated by a single angle of attack (AOA) sensor. Angle of attack sensors measure the angle of the relative wind over the wings. Too great of an angle between the wing and the airflow over it will result in an aerodynamic stall wherein the wing stops producing lift.
The questions being asked involve the engineering decision to use the input of a single AOA sensor to trigger the MCAS system to operate. There are two (or more) AOA sensors installed on all airliners which among other things are used to provide "stick shaker" stall warning to pilots if they get too slow or approach a stall. Again, a stall is something that most airline pilots will never see outside of a training simulator where stall recovery is practiced routinely.
What Did the ET302 Pilots Know about the MCAS System?
Lion Air JT-610 crashed on October 29, 2018. The investigation of that crash first brought the MCAS system and a malfunctioning AOA sensor to light. On November 7th, Boeing released an Operations Manual Bulletin (OMB) to all 737 Max operators. This bulletin mentioned that erroneous AOA signals can cause the trim to run uncommanded by the pilot. The directed remedy is to apply the runaway stabilizer trim checklist which directs the use of the center pedestal mounted stabilizer trim cutout switches. The text of the bulletin is as follows:
The Indonesian National Transportation Safety Committee has indicated that Lion Air flight 610 experienced erroneous AOA data. Boeing would like to call attention to an AOA failure condition that can occur during manual flight only.
This bulletin directs flight crews to existing procedures to address this condition. In the event of erroneous 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.
A subsequent Emergency Airworthiness Directive (EAD) directed this information to be included in the flight manual of all Max aircraft within three days.
In my view, it is reasonable to assume that the ET302 pilots were well aware of the MCAS system, its possible failure mode due to an erroneous AOA sensor, and the steps to be taken to remedy the malfunction.
Why Didn't They Just Turn it Off?
The investigation of the Lion Air crash has revealed that on the flight immediately preceding the mishap flight, an off-duty 737 qualified pilot was occupying the jumpseat. That aircraft also suffered the same malfunction of the AOA sensor resulting in uncommanded nose down trim. On that flight, however, the guest pilot recommended that the operating pilots use the stabilizer trim cutout switches, which they did. That flight landed uneventfully.
The pilots on the subsequent Lion Air flight fought against the nose down trim commands continually, but never did deactivate the electric stabilizer trim with the cutout switches. The errant automated trim commands eventually trimmed the aircraft into an unflyable condition.
It would seem easy to Monday morning QB the actions of the Lion Air mishap pilots, but it must be remembered that there were many other things happening at the same time. One important thing to note is that the stick shaker activated right at liftoff and continued for the entire flight. The stick shaker is a device that literally vibrates the control yoke when an aircraft approaches an actual stall. It is loud and disconcerting when activated. The pilots were no doubt startled and distracted.
Another point to note is that the MCAS inputs would not "present" like a traditional runaway trim situation. Typically, a runaway trim malfunction in a simulator would simulate a stuck switch where the trim wheel would run continuously in one direction. During the mishap Lion flight, the flight data recorder showed the pilot actively trimming back against the MCAS inputs followed by a few seconds delay when the MCAS system would reactivate and start trimming forward again.
Another system called "speed trim" installed on earlier and subsequent 737 models can also run the electric trim with the autopilot disengaged, so it is not completely unusual to see the trim wheel spinning by itself with the autopilot off. This "negative training" may have contributed to the pilots not focusing on the uncommanded movement of the trim wheel even though speed trim only functions with flaps extended while the MCAS system only functions with the flaps retracted.
What Happened Then on ET302?
The flight data recorder and cockpit voice recorders from ET302 have been recovered and sent to France where they were downloaded and decoded by the BEA, the French equivalent of the NTSB. The data from the recorders have not been released to the public, however investigators have an "emerging consensus" that the MCAS system activated and contributed to the accident. The story also noted that this preliminary finding is subject to revision.
The pilots of ET302, however, had something that the Lion pilots did not, and that is a detailed description and knowledge of the MCAS system and the procedure to disable it by throwing two easily reached switches. Without more information from the accident investigation, it is simply too early to reach any definitive conclusions about the fate of that airliner.