MCAS ALTITUDE V ATTITUDE ??

[Mark in CA]

The Boeing 737 Technical Site

Stumbled upon this site. Posting here in case it may be of use to anyone.

The Boeing 737 MAX

[FCeng84]

Originally Posted by FCeng84

Good idea to pull aside and have a level setting. I concur with everything added to the thread so far. Let me point to one possible source for the confusion about the role of ALTitude in the activation of MCAS. I recall that in the ET accident thread at one point there were several submissions wondering if MCAS could have played a role based on the data suggesting that the airplane did not gain much ALTitude and thus speculating that the flaps would have still been extended. I believe that at least one entry suggested that on climbing out of Addis Ababa it would not be abnormal to be cleaned up to flaps up by the point where the airplane was 1000' above the runway ALTitude. This may have been seen by some as indication that MCAS has logic to prevent activation below 1000'. I am not aware of any direct use of ALTitude in the activation of MCAS. I am checking with a source and will report back once this is confirmed or denied.

Got definitive answer on MCAS activation and altitude. Only inhibit is when within a few feet of the ground. Takeoff with Flaps Up would allow for MCAS activation as soon as combination of autopilot not engaged and sensed AOA above MCAS activation threshold occurs. Similarly, if takeoff involves some flaps out, MCAS is enabled as soon as flaps are fully retracted with no required minimum altitude.

[gums]

Salute FCeng !

Great to have most of our understnding confirmed.

Assume the "few feet of the ground" criteria might even be ground effect that influences AoA/downwash, or #2 radar altimiter.
@JT PLZ let this one stay! Several of the inputs here by Dave, CONSO and Gums were deleted and they included real world examples that the lay person could visualize concerning AoA, as well the fallacy of using pendulums or plumbobs as AoA or other indicators. Many on the main threads are still confusing attitide they reference to siting in a living room chair with aerodynamic pitch and AoA and .....

Gums sends...

[infrequentflyer789]

Originally Posted by Ian W

I am surprised that after the Lion Air crash and the AD that any pilot on the Max would be unaware of the MCAS issue and have briefed accordingly before any flight about stab trim cutout if trimming became difficult - it is part of the standard runaway trip checklist. The fact that another crew had crashed and could have survived if they had switched stab trim to cutout should have concentrated minds.

ATC comms from both accident flights, and reports from the flight that survived, indicate that the unreliable airspeed was the first focus - I don't know if that is right or wrong or if the prioritization should have been different, but that is what all three crews appear to have done.

According to the report the crew that survived MCAS ran 3 NNCs - IAS disagree, ALT disagree, and then Stab Trim Runaway. From the FDR traces they had around four minutes of fighting MCAS, after flaps up, before they turned it off.

Ethiopian was in the ground after six minutes of flight.

[kiwi grey]

Originally Posted by Ian W

I would have thought that the first check on unreliable airspeed in the Max would be set stab trim switches to CUTOUT that means manual trim but MCAS is now out of the loop.

But that would put the aircraft into a known unsafe condition.
The MCAS was created solely because the B737MAX is sufficiently unstable without it (under some circumstances) to not be certifiable, and the stab trim switches are required to be on for MCAS to work.
Boeing have their "gentleman's parts" in a vice on this one

[futurama]

Originally Posted by infrequentflyer789

ATC comms from both accident flights, and reports from the flight that survived, indicate that the unreliable airspeed was the first focus

Actually the "unreliable airspeed" report has never been confirmed for the Ethiopian flight. There was a rumor that a crew on another flight heard ET302 reporting "unreliable airspeed" to ATC, but no one can seem to locate this other flight. Leaks from those who have listened to the ATC tapes (e.g., via Reuters, etc.) state that the pilot reported a "flight control" problem, but not "unreliable airspeed" specifically.

From AvHerald: "..., that there had been no PAN PAN or MAYDAY call, there was also no change in transponder code away from the assigned code 2000, there had been no indication of unreliable airspeed by the flight crew, there had been no distress in the voices or abnormal background noises during transmissions from the aircraft that would have alerted air traffic control to assume a serious problem on board of the aircraft."

[Ian W]

Originally Posted by kiwi grey

But that would put the aircraft into a known unsafe condition.
The MCAS was created solely because the B737MAX is sufficiently unstable without it (under some circumstances) to not be certifiable, and the stab trim switches are required to be on for MCAS to work.
Boeing have their "gentleman's parts" in a vice on this one

My understanding is not that the Max is unstable without MCAS but that the pull force on the control column is not linear close to stall AoA. This is why MCAS does not operate with AP engaged. So a regulatory 'protection' was put there for linear control forces. Or as in the case of the Max pilots with little feel for manual flying could to pull through into a stall as the column pull force got lighter. This is probably more likely in a high speed stall such as in a steep turn.

[gary f]

Right on, whether it's communication breakdown or crew members not knowing where to look up important data.

I wouldn't discount the pressure for on time performance from the airlines and flight crews racing to get to the plane from a previous flight and getting into a hurry up situation.

[Vilters]

Back to the original question : Altitude versus Attitude.
It does not matter; when sensors:systems feed wrong information, the outcome is predictable.

About the pilots and pressure on them;
Most pilots are trained using the autopilot and protection systems. But => Most are "scared as hell" to turn protection systems OFF.