Le Bourget 2017 - Foto: Per Gram
In a move likely linked to the ongoing investigation into the fatal crash in October of a Lion Air Boeing 737 MAX 8, the aircraft manufacturer has issued a bulletin to operators explaining a system unique to the MAX that automatically pushes the nose down to help prevent stalls in certain flight profiles. Boeing on Nov. 10 issued a multi-operator message (MOM) explaining the MAX’s maneuvering characteristics augmentation system (MCAS) “commands nose-down stabilizer” in certain flight profiles using “input data and other airplane systems.” MCAS is operated by the flight control computer and “activated without pilot input and only operates in manual, flaps-up flight,” Boeing explained. MCAS is not part of previous 737 designs, Boeing’s MOM confirms. The system also was not covered in MAX flight crew operations manual (FCOM) or difference training for 737NG pilots. “This is the first description you, as 737 pilots, have seen,” the Allied Pilots Association (APA) told American Airlines (AA) pilots in a message Inside November 13, 2018,
on its website.
“It is not in the AA 737 Flight Manual Part 2, nor is there a description in the Boeing FCOM. It will be soon.” Aviation Week has reviewed the 737 MAX-family flight crew operations manual for another large MAX-family operator. It does not reference MCAS. A multi-page document issued by the airline’s flight operations department that highlights the differences between the MAX and 737 NG does not mention MCAS or any other changes to the auto-trim system. While the Nov. 10 Boeing message does not reference any inservice incidents or accidents, its timing, combined with an earlier MOM and related regulatory mandate, suggests that MCAS could be suspected of contributing to the Oct. 29 loss of Indonesian LCC Lion Air flight JT610. Angle Of Attack Readings Investigators are looking at several anomalies, including faulty angle of attack (AOA) readings, as part of the probe. JT610, a MAX 8, crashed into the Java Sea about 13 minutes after departure from Jakarta in good weather, killing all 189 onboard. Preliminary analysis of the flight-recorder data suggests the crew struggled to control the aircraft. Investigators also confirmed that the airframe involved, PK-LQP, experienced faulty airspeed and AOA readings on previous flights. A Nov. 6 MOM from Boeing cautioned that “an erroneous AOA” can trigger automatic nose-down pitch-trim.
Boeing referenced a documented procedure for runaway pitch trim that includes switching the system off, as a remedy. FAA in a Nov. 7 emergency directive— which other regulators adopted—sheds more light on the issue. “The [directive] was prompted by analysis performed by the manufacturer showing that if an erroneously high single AOA sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer,” FAA said. The directive requires operators to update their flight manuals with Boeing’s information. The 737 has two independent AOA sensors. FAA’s explanation suggests that if one sensor tells the aircraft that the AOA is too high, the auto-trim system could be triggered. Neither the Nov. 6 MOM nor FAA’s directive mentions MCAS. Boeing’s Nov. 6 MOM does not reference any new analysis, nor does it specify that a single piece of bad data could trigger the system. American has “found no instances of AOA anomalies with our 737 MAX 8 aircraft,” the APA message said. “That is positive news, but it is no assurance that the system will not fail. It is mechanical and software-driven. That is why pilots are at the controls.” In response to an Aviation Daily query, Boeing said the company is “taking every measure to fully understand all aspects of this incident, working closely with the investigating team and all regulatory authorities involved. We are confident in the safety of the 737 MAX.”
—Sean Broderick, sean.broderick@aviationweek.com,
—Bill Carey, bill.carey@aviationweek.com,
—Guy Norris, guy.norris@aviationweek.com agreement.
Flight control feature of Boeing 737 MAX under scrutiny after Lion Air accident
13 November 2018
The U.S. Allied Pilots Association (APA), which represents American
Airlines Group Inc. pilots, alerted its members of a new flight control feature
of the Boeing 737 MAX models. This feature, called MCAS (Maneuvering
Characteristics Augmentation System), applies nose down stabilizer in specific
conditions when the aircraft nears a stall. The APA quoted a Boeing message as
follows:
MCAS (Maneuvering Characteristics Augmentation System) is implemented on
the 737 MAX to enhance pitch characteristics with flaps UP and at elevated
angles of attack. The MCAS function commands nose down stabilizer to enhance
pitch characteristics during steep turns with elevated load factors and during
flaps up flight at airspeeds approaching stall. MCAS is activated without pilot
input and only operates in manual, flaps up flight. The system is designed to
allow the flight crew to use column trim switch or stabilizer aislestand cutout
switches to override MCAS input. The function is commanded by the Flight Control
computer using input data from sensors and other airplane systems.
A line of inquiry into the October 29 accident involving
a Lion Air Boeing 737 MAX 8 focusses on a possible issue with one of the
aircraft's AOA (Angle of Attack) sensors. A subsequent Emergency Airworthiness
Directive issued by the FAA stated that erroneously high single angle of attack
(AOA) sensor input could result in "repeated nose-down trim commands of the
horizontal stabilizer". Which would be the MCAS commanding these nose-down trim
commands.
APA said the logic behind MCAS was not mentioned in training or in any
other manuals or materials. Safety Committee Chairman Capt. Michaelis stated:
"It's pretty asinine for them [Boeing] to put a system on an airplane and not
tell the pilots who are operating the airplane, especially when it deals with
flight controls."
Operational Evaluation Report
The MCAS system was briefly mentioned in the Operational Evaluation Report,
prepared by the Brazilian aviation authorities, ANAC. These reports are prepared
by aviation authorities, among others, to determine pilot qualification and type
rating requirements including training, checking, and currency
requirements.
The Brazilian ANAC report, dated January 10, 2018 contains Operator Difference
Requirements (ODR) Tables, listing design differences between the Boeing 737-800
and the Boeing 737-8 (MAX), as proposed by The Boeing Company and validated by
ANAC.
One of these pertained to MCAS:
Level B Training is defined by the FAA as 'Level B Training. Level B
training is applicable to related aircraft with system or procedure differences
that can adequately be addressed through aided instruction. At level B,aided
instruction is appropriate to ensure pilot understanding, emphasize issues,
provide astandardized method of presenting material, or aid retention of
material following training.Level B aided instruction can utilize slide/tape
presentations, computer based tutorial instruction,stand-up lectures, or video
tapes.'
The Operational Evaluation Report prepared by Transport Canada,
dated 30 November 2017, contained the same table, without the line referring to
MCAS.
WHAT WE CAN LEARN FROM LION
AIR 610
By Captain Shem
Malmquist
A FSI
Commentary
Editors Note: While the Lion Air 610 investigation continues, it's
important to remember that hindsight bias is no substitute for understanding
potential combination system failure events. We welcome reader response to
Captain Malmquist's commentary.
On October 29, 2018, Lion Air flight 610, a Boeing 737 Max 8, crashed
shortly after takeoff from Soekarno-Hatta International Airport in Jakarta,
enroute to Depati Amir airport. The aircraft was brand new, only having been in
service for two months.
The day prior to the accident flight maintenance had
been performed due to an airspeed and altitude indication problem and an
elevator feel differential pressure light illuminated. There were also reported
problems with the angle of attack sensing system. In signing the items off, a
term familiar to most pilots was used, terms such as "test on ground ok" and
"test on ground satisfied".
In the immediate aftermath of the accident, Boeing sent out a
bulletin which was quickly adopted by FAA as an airworthiness directive to
follow the Runaway Stabilizer non-normal checklist. The bulletin stated that in
the event of erroneous angle of attack data the stabilizer may trim nose down in
increments lasting up to ten seconds.
The runway trim exercise is commonly taught in simulators. It is a
simple matter, in the Boeing, to counter the trim movement with control column
input, and then shut off the stabilizer trim switches thus removing electrical
power from the system. In these simulator events it is just one system, the
stabilizer trim, that has gone wrong. The motor has gotten "stuck"
in the "on" position, and that is driving the stab trim motor. Just remove the
power. However, the Boeing bulletin points to something a bit different. Here
we can now have a faulty sensor input into a computer system that misleads the
system to do the wrong thing. This was not something that could affect earlier
models of the B-737.
It appears that the feature to add nose-down trim was new to the Max
8 version. It also now appears, based on recent reports, that few pilots flying
the aircraft were aware that the aircraft had an angle of attack protection
feature built into its trim system.
We do not yet know what other systems might be impacted here. Many
systems are connected to the angle of attack sensor, including flight
instruments and warning systems. If, as reported, the pilots were not aware of
the system functionality, it would not be surprising to find that they reacted
incorrectly to it.
We will learn more as the investigation progresses. In the meantime,
there have already been multiple articles and comments indicating that the issue
here was pilot competency. I would like to remind all pilots that these types
of accidents are rarely due to lack of pilot skill. It is all too easy, as
pilots, to think "I would not have done that." The response seems obvious with
the clarity of hindsight bias. Part of this arrogance stems from the way we
train.
In the simulator the events we see are fairly simplistic. They
rarely involve one system failing in a subtle way, cascading into multiple other
systems. In 1974 a Northwest Airlines Boeing 727 crashed after stalling due to
blocked pitot tubes due to ice.
Many wondered how an experienced flight crew might not recognize that
they were stalling, after all, it was well known that one outcome of iced pitot
tubes could be that the airspeed increases with climb, much like an altimeter.
The higher indicated speeds led to the crew pulling the nose up to try to
prevent an overspeed, but shouldn't that have been obvious?
As outlined in the NTSB report, the Board suspected that
the elevator feel pitot tubes were also blocked. This creates a confusing
situation where the airspeed indication is increasing, and that can be coupled
with the control forces acting as if the plane is flying faster as well. This
then mixes with overspeed warnings as well as stall warnings. A very confusing
situation.
We do not know, yet, all that the Lion Air crew had to contend with.
We do know that these combination failure events can lead to a situation that
can be very difficult to sort out. We also know it is hard for a pilot to
handle a situation where the airplane is doing something outside of what they
know to be possible.
Instead of this being another "I would not have done that"
conversation, use it instead to start delving into your aircraft systems.
Knowledge is power. In this case, knowledge can keep you alive.
Captain Shem Malmquist is a veteran 777 captain and accident
investigator. He is coauthor of Angle of Attack: Air France 447
and The Future of Aviation Safety and teaches an online high
altitude flying course with Beyond Risk Management and Flight Safety
Information. He can be reached at shem.malmquist@gmail.com
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