Flight Training - August 2019 - 50

AOPA

PILOT
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ADVANCED PILOT OPINION

high attitude. Dutifully, MCAS started
adding nose-down trim to counter what
it thought was a dangerously nose-up
condition. Unlike light airplanes that use
small trim tabs to trim the elevator, trim
on jet airliners is achieved by moving the
entire stabilizer. Moving that massive
surface makes the trim extremely powerful-more powerful than two pilots
can overcome. That's what apparently
doomed both the Lion Air and Ethiopian
Airlines flights after a similar series of
up/down gyrations.
The baffling part, however, is why the
pilots didn't use the stabilizer trim cutout

will stop it. Holding the red autopilot
disconnect button should also do the
trick. Or turn off the autopilot's main
power switch. It's also a good idea to
know where the pitch trim and autopilot
circuit breakers are, if all else fails. Place
red or yellow collars on them so they're
easy to find in the heat of battle. There
are many ways to disable automation in
aircraft, and pilots should be very familiar with all of them.
While the Lion Air and Ethiopian
Airlines investigations are still ongoing,
I have to question the background and
flight time of the pilots involved. It was

PILOTS OF ALL AIRPLANES WITH AUTOPILOTS
UTILIZING ELECTRIC PITCH TRIM SHOULD KNOW HOW
TO DISABLE THE TRIM IN CASE OF A RUNAWAY.

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switches, which would disable the electric trim and allow manual trimming of
the stabilizer. This is the crux of Boeing's
defense for not mentioning the existence
of MCAS.
Whether trim is running away
because of a malfunction or because of
MCAS, the fix is the same: turn off the
electric power to the stabilizer. The
Ethiopian crew eventually got to those
switches but, by the time they did, it's
speculated that the airplane was so far
out of trim that aerodynamic forces on
the tail made manual trimming impossible. While this investigation is ongoing,
there are already important lessons
learned from these accidents.
Pilots of all airplanes with autopilots
utilizing electric pitch trim should know
how to disable the trim in case of a runaway. Typically, holding the trim switch
in the direction opposite to the runaway

JARGON
MCAS

50  FLIGHT TRAINING AUGUST 2019

revealed that the first officer in the Ethiopian crash had only a few hundred hours
total time. If true, it doesn't take much
imagination to realize that he was likely
little to no help in attempting to figure
out the problem. And the captain, now
essentially flying solo, would have been
wrestling the controls while attempting
to sort out the problem, leading to task
saturation.
In the end, it's apparent that Boeing
should have at least made pilots aware
of MCAS' existence and that the fix for
a runaway MCAS is the same as for a
runaway stabilizer trim. Boeing also is
well on its way to recertifying MCAS to
rely on two angle of attack sources with
comparator logic and limiting MCAS'
authority on stabilizer movement. FT
PETER A. BEDELL is a pilot for a major airline and

flies the Boeing 737NG and Max models.

Boeing's Maneuvering Characteristics Augmentation System (MCAS) is a flight control
law designed to enhance the pitch stability
of the 737 Max to make it fly like other 737s.
The system activates in manual flight-not
on autopilot-when the flaps are up and the
airplane is at a high angle of attack.
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Flight Training - August 2019

Table of Contents for the Digital Edition of Flight Training - August 2019
