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In the cockpit of the 737 MAX, pilots needing to shut off electric controls of the horizontal tail utilize two cutout switches that perform the same function. On the previous model of the 737, the two switches had separate functions, potentially allowing pilots to shut off automated controls while keeping the functionality of the electric controls on the yoke. (Dimas Ardian / Bloomberg)
May 14 Update : Boeing altered key switches in 737 MAX cockpit, limiting ability to shut off MCAS
May 14 - 2019 - by Mike Baker and Dominic Gates for seattlestimes.com
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...
