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Old 26th Apr 2019, 15:13
  #4372 (permalink)  
737 Driver
Join Date: Apr 2019
Location: USA
Posts: 217
Originally Posted by HundredPercentPlease View Post
When I was trained on the 737 (classic and NG), the stab trim runaway was a runaway - where the trim wheel ran continuously. We would identify it as a runaway, because it was continuously running, and flick the switches.

1. MCAS activation is not continuous, it's bursts.
2. The 737 trim system likes to provide bursts of random trim at certain times (STS) so bursts are considered normal.
Honest question: How long does a trim wheel have to spin before you deem it "continuous" and thus subject to intervention by the pilot?

In the case of Ethiopian, the MCAS input was not like the normal Speed Trim you see during takeoff. If you take a look at the DFDR traces of the automatic trim before the flaps were retracted, you can see what these inputs look like. Short and seemingly random inputs.

When MCAS activated, it ran the trim nose down for 9 continuous seconds. Please count that out to yourself. MCAS moved the stab about 2.5 degrees. That would be about 37 spins of the flight deck trim wheel. Please imagine that white stripe on the trim wheel making 37 trips passed the Captain's knee. All this time, the Captain was holding the yoke and must have felt the changing trim pressures. This happened TWICE (one other MCAS input was interrupted by the pilot trimming nose up). Ultimately, it wasn't even the 8122-hour Captain who suggested that the trim cutout switches be used - it was the 361-hour First Officer.

So honestly, just how long does the stab have to continuously run before a fully qualified 737 type-rated Captain determines that he has runaway stab trim?
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