Lion Air and its pilots were aware of new 737 flight system

November 27, 2018

Lion Air and its pilots were advised of the new 737 MAX stall prevention system which is in the Boeing manual, claims an Australian Boeing 737 check and training captain.

Speaking exclusively to, the captain said the new system, – the Maneuvering Characteristics Augmentation System (MCAS) – is “not a system as such” but simply computer code, a control law, that compensates for the MAX’s greater power.

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“Quite frankly it is incidental and only operational when the MAX is being flown manually,” he said.

“All it does is compensate for the MAX’s extra power over the previous NG model and applies with a little more downward force if in a stall.”

In the loss of Lion Air Flight 610 on October 29, erroneous data from a faulty Angle of Attack sensor caused the MCAS to think the plane was about to stall, which activated the downward force on the Stabilizer Trim to get the nose down.

Instead of switching off the Stabilizer Trim the pilots appear to have battled the system.

According to the 737 Check Captain, reports that MCAS is not in the manual “are not correct.”

“It is absolutely there. It’s not a Checklist Memory Item but nor are the other causes of a Runaway Stabilizer Trim.”

“What is a Checklist Memory Item is how to switch off a Runaway Stabilizer Trim.”

“It’s very, very simple.”

Lion Air
Boeing 737 Stabilizer Trim cutoff switches

“We don’t want our pilots troubleshooting which issue is causing the problem – we want them to turn it off.”

“So, on the 737NG, there are, I think four causes for a Runaway Stabilizer Trim now there are five.”

According to the check captain, all airlines were briefed on the new feature and the detail is in the pilot manuals.

Clearly, the Lion Air pilots knew how to deal with such a problem because on the three previous flights of that aircraft the same problem of incorrect Angle of Attack data was causing the system to activate as it thought the plane was about to stall.

“Those pilots switched the Stabilizer Trim off.”

“What is an issue is why the Lion Air plane was allowed to fly in the first place,” he said.

“In Australia, it would have been grounded.”

“You fix a problem once and if it reoccurs you ground the plane for detailed examination,” said the check captain.




  1. Sounds like the flightcrew was using the old Boeing procedure of engaging the stab-trim brake by inputting opposite elevator with the yoke. “Those pilots switched the Stabilizer Trim off.” We need to find the pedestal and see the switch positions. Also, I'm not sure if the FDR will record the positions of the stab trim cutout switches when they are moved. Should record any electric trim input by either side?