Boeing reveals it disconnected crucial Sensor failure alarm on MAX8

Boeing is bracing for another bit of bad news after it was revealed that Southwest Airlines first learned about the crash of a Lion Air MAX 8 that a special alert warning of malfunctioning sensors was no longer a standard feature on the latest 737s.S

According to Federal Aviation Administration officials, they were not aware of this change either.

Boeing 737 MAX8 aircraft have two “Angle-of-Attack” or AOA sensors on the left and right of the plane’s nose. Initial reports on the two MAX8 crashes indicate that in both the right sensor appeared to have been mis-reading the flow of air by up to 60 percent.

Boeing said initially that airlines would have to pay more for the sensor alarm, but what it did not say was that the sensor-failure alarm had been disconnected entirely. But there’s more folks.

Both the Lion Air and Ethiopian Airlines Boeing’s that crashed killing 346 people did not have the alarm failure system which is known as AOA disagree alerts. The big problem is that Boeing and the FAA certified a plane that had no redundant fail-safe.

That is like reading about aviation in 1956. Just after the war, when men were men and pilots were gods of the air, afraid of nothing.

Passengers are much more discerning these days than the early pax – they now have social media to keep them abreast of the latest machine faux pax.

That hasn’t stopped Boeing from acting like it is the middle of 20th Century when it comes to crisis management. What is making things worse for Boeing is the publication of its Emergency Airworthiness Directive in November 2018 after the Lion Air Crash that for some reason, was not escalated properly through the industry.

All Boeing MAX jets, both the 8 and 9, are grounded pending a software fix followed by pilot retraining. Or maybe no pilot retraining, because already Boeing has mobilised its useful twit brigade of aviation bravados to suggest possibly that this is not required.

Yes, dear reader, even in 2019 a large international company based in the U.S.A. is trying to convince reasonable people that there’s no danger in cutting another corner.

Boeing has not indicated why it made the decision to remove the sensor failure alarm. However, the manufacturer was aware of error reading problems.

IN November 2018 it released an airworthiness directive indicating that:

“This emergency 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. This condition, if not addressed, 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.”

The airworthiness directive called for aircrews to be provided with runaway horizontal stabiliser trim procedures under certain conditions. But the Ethiopian crew flying the plane in the March crash were not provided with these new procedures.

The Directive also advised that:

“In the event of an uncommanded horizontal stabilizer trim movement,
combined with any of the following potential effects or indications
resulting from an erroneous Angle of Attack (AOA) input, the flight crew
must comply with the Runaway Stabilizer procedure in the Operating
Procedures outlined the following would happen:


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.
• IAS DISAGREE alert.
• ALT DISAGREE alert.
• AOA DISAGREE alert (if the option is installed).
• FEEL DIFF PRESS light.
• Autopilot may disengage.
• Inability to engage autopilot.

If any aircrew experienced these conditions, then Boeing issued the following procedure to recover :

Disengage autopilot and control airplane pitch attitude with control column and main electric trim as required. If relaxing the column causes the trim to move, set stabilizer trim switches to CUTOUT.

If runaway continues, hold the stabilizer trim wheel against rotation and trim the airplane manually.
Note: The 737-8/-9 uses a Flight Control Computer command of pitch
trim to improve longitudinal handling characteristics. In the event of
erroneous Angle of Attack (AOA) input, the pitch trim system can trim
the stabilizer nose down in increments lasting up to 10 seconds.

In the event an uncommanded nose down stabilizer trim is experienced
on the 737-8/-9, in conjunction with one or more of the indications or
effects listed below, do the existing AFM Runaway Stabilizer
procedure above, ensuring that the STAB TRIM CUTOUT switches
are set to CUTOUT and stay in the CUTOUT position for the
remainder of the flight.

An erroneous AOA input 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.
• IAS DISAGREE alert.
• ALT DISAGREE alert.
• AOA DISAGREE alert (if the option is installed).
• FEEL DIFF PRESS light.
• Autopilot may disengage.
• Inability to engage autopilot.

Initially, higher control forces may be needed to overcome any
stabilizer nose down trim already applied
. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB
TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be
used before and after the STAB TRIM CUTOUT switches are moved
to CUTOUT.

The higher control forces part of this directive are very important, as in both accidents it appears the pilots could not overcome the stabiliser because of air speed.

That is an incredible directive, pointing to the probable causes of both accidents according to both Boeing itself, and initial accident reports.

What we have to ask is this:

Why did the Ethiopian Airline officials not institute this Directive as per the Emergency documentation as soon as possible after the November release?

When both accidents end up in court, which they will, this is going to be difficult for aviation officials to explain. The manufacturer may be to blame for releasing an aircraft to service with a single fail-safe which as we all know is probably criminal.

Using the phrase “hazardous” instead of “catastrophic” to describe the effect of a failure of the system is also going to be quite hard to explain to judges and juries.

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