Shadows on the MAX

The Boeing 737 MAX is one of the most exciting new aircrafts in the aviation panorama, but it’s quickly becoming a nightmare due to two incindents in a few months, maybe caused by the same failure.

I guess you already understood that I am dealing with the Lion Air flight 610 and the Ethiopian Airlines’s 737 which crashed in the sorroundings of Addis Ababa.

The incident happened in Indonesia showed how sophisticated the latest member of the 737 series is, but what is supposed to be a life-saver feature is becoming, through media articles, the opposite.
Indeed China has decided to ground immediately all the 737 MAX regisered in its territory, for at least 9 hours until further talks with Boeing and FAA.

In order to understand how the stall recovery system (runaway stabilizer) works you should have a clue about how the aircraft stalls, as we discussed in the “Adventure of a flight time” course.
The stall is an unusual situation that occurs when the airflow over the wing is not smooth enough to generate lift.

As you may have understood the stall doesn’t happen all of a sudden, there is a phase called pre-stall during which the recovery systems and the alerts are triggered.
The pre-stall phase is recognized as the Angle of Attack (AoA in the follwing) sensor measures that the wing has exceeded its maximum angle of attack, and the aircraft is soon starting to lose speed and stall.

The sensor basically works as a little wing on the nose of the plane, measuring the angle formed by the airflow and the sensor itself (copyright: Aviation Stock Exchange)

As a result of this inputs from the sensors, the aircraft automatically excludes the yoke commands and pitch itself down to recover the stall.
That behaviour leads to an unsteady vertical path as shown by the following graph

ET302’s path from flightradar24

This is, as you understand, a critical feature that, potentially, can save many souls on board of the new generation of the 737 series.
But you just saw what may happen if the system fails.

To be fair that feature wasn’t implemented only to avoid incidents, but it’s a result of the relocation of the engines. Indeed the new engines of the 737 MAX were repositioned in order to fit better under the wings, and this is generating a pitch-up effect which should be contrasted by the stabilizers.

Without this system (MCAS) the 737 MAX couldn’t even fly. That’s why it’s always active throughout the flight.

The Lion Air’s flight experienced a failure of the AoA sensor which gave inputs as the aircraft was about to stall when it was perfectly fine and under control. The pilots, then, fought against the recovery system, and it ended up badly.

Following that incident the Boeing Company released a bulletin to “enphasize the procedures provided in the Runaway Stabilizer Non-Normal Checkilist”.
In fact the pilots are supposed to be trained enough to exclude the stabilizer inputs and proceed the flight.

Well, Lion Air is one of the worst airlines for security and maintenance, so I wasn’t really surprised that the flight, that couldn’t even take-off, ended up in the ocean.
But the incident of yesterday suggest that the 737 MAX might have a serious problem, and a non-normal checklist is probably not enough to avoid other fatalities.

For example the Turkish Airlines flight 1951 crashed shortly before landing at Amsterdam due to a radio altimeter failure that gave the autopilot the “impression” to be on the ground, it was a 737NG.
This particoular incident was also fault of the crew that was partially distracted and didn’t notice that the airspeed was decreasing.

Official report stated the ditraction of the crew as partially responsible of the crash. (copyright CBC)

In this case the engineers didn’t find anything wrong with the radio altimeter, and it was too expensive to, eventually, fix every single 737 flying.
So a non-normal procedure leading to the disengagement of autopilot and autothrottle was enough to ensure a safe landing for every flight.

But the issue regarding the MAX is way more critical (it happens in manual flight, so you can’t just disconnect the autopilot) and there aren’t many aircraft flying, so it would be less expensive to retire and fix the MAXes and, since Boeing is full of orders for this airplane, they could still earn a lot.

Last but not least I would like to specify that this is only an hypotesis of what happened, as the two incidents are really similar, but no official report was released with the reasons of the accident.


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