Reply to post: Re: Hey software, get the fuck out of the way!

Boeing 737 pilots battled confused safety system that plunged aircraft to their deaths – black box

sanmigueelbeer Silver badge

Re: Hey software, get the fuck out of the way!

It was the designer who allowed a single faulty sensor to put the software into a dangerous state.


Look at the two lines for the Angle of Attack (AoA) sensors. One is for the left and one is for the right. Notice that the lines are separated? At any given time, the AoA sensors should never be separate. They're meant to be redundant so the readings are meant to be the same.

There are actually two faults to this unfortunate events, they are:

1. One of the Angle of Attack sensor was faulty. The AoA sensor feeds information to the FMS and one of the systems, MCAS, reads from this.

2. The MCAS was an "undocumented feature" introduced by Boeing which a lot of airlines were not familiar of.

For unknown reason, after take-off the MCAS was reading to input from the AoA sensor and immediately took action for the one that had the worst reading. For the next few minutes, the both pilots were battling with the MCAS to keep the plane from crashing until it was no longer possible.

It's a very bad idea to have software making decisions based on the output of a single sensor.

Imagine you flying at night across ocean and into a thunderstorm. It is very hard to determine which way is up or down or what angle is the plane's nose pointed. This is what the MCAS, with the help of the AoA, is meant to do.

But the root cause to the crash was a faulty AoA sensor. The fault of the two pilots was that they were unaware of the MCAS and they were unable to disable it. With previous flight, one of the flight crew disabled the MCAS and was able to gain control of the a/c.

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