Boeing altered key switches in 737 Max cockpit

[Yo_Mama_Been_Loggin]

In the middle of Boeing 737 cockpits, sitting between the pilot seats, are two toggle switches that can immediately shut off power to the systems that control the angle of the plane’s horizontal tail.

Those switches are critical in the event a malfunction causes movements that the pilots don’t want. And Boeing sees the toggles as a vital backstop to a new safety system on the 737 MAX — the Maneuvering Characteristics Augmentation System (MCAS) — which is suspected of repeatedly moving the horizontal tails on the Lion Air and Ethiopian Airlines flights that crashed and killed a total of 346 people.

But as Boeing was transitioning from its 737 NG model to the 737 MAX, the company altered the labeling and the purpose of those two switches. The functionality of the switches became more restrictive on the MAX than on previous models, closing out an option that could conceivably have helped the pilots in the Ethiopian Airlines flight regain control.

Boeing declined to detail the specific functionality of the two switches. But after obtaining and reviewing flight manual documents, The Seattle Times found that the left switch on the 737 NG model is capable of deactivating the buttons on the yoke that pilots regularly press with their thumb to control the horizontal stabilizer. The right switch on the 737 NG was labeled “AUTO PILOT” and is capable of deactivating just the automated controls of the stabilizer.

On the newer 737 MAX, according to documents reviewed by The Times, those two switches were changed to perform the same function — flipping either one of them would turn off all electric controls of the stabilizer. That means there is no longer an option to turn off automated functions — such as MCAS — without also turning off the thumb buttons the pilots would normally use to control the stabilizer.

Peter Lemme, a former Boeing flight-controls engineer who has been closely scrutinizing the MAX design and first raised questions about the switches on his blog, said he doesn’t understand why Boeing abandoned the old setup. He said if the company had maintained the switch design from the 737 NG, Boeing could have instructed pilots after the Lion Air crash last year to simply flip the “AUTO PILOT” switch to deactivate MCAS and continue flying with the normal trim buttons on the control wheel. He said that would have saved the Ethiopian Airlines plane and the 157 people on board.

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