Hey everyone!

As everyone is all over the MCAS System on the 737 MAX now, can anyone give me a detailed technical description to the system, preferably by Boeing?

As I am an Airbus pilot, I want to understand the principle here and compare it to the Airbus protections and stabilities and I am glad to explain those a little as well. What exactly are the inputs to the system? If I recall correctly from over 10 years ago when I was a ramp agent, the 737 only has 2 AoA sensors. Is that correct or am I forgetting something?

And just so I can get my head around the matter a bit more: There was no mentioning of the system in the initial FCOMs to the flight crews? What can you find in the manuals now?

Thanks everyone for a qualified answer!

Best regards!

flyinTLow


P.S.: Sorry if this has been discussed, the search function did not give me any useful returns.

I don't know MCAS beyond the generalities but using a directed Google search tends to work much better than the internal search function.

E..g.: "airliners.net:MCAS 737 MAX"

And just so I can get my head around the matter a bit more: There was no mentioning of the system in the initial FCOMs to the flight crews? What can you find in the manuals now?

Guess what? Nothing! "MCAS" appeared in the abbreviations index of the FCOM and... that's it ! Nothing new in FCOM nor AFM. The only official publication about it was Boeing bulletin NLH-15, in which "MCAS" has never been mentionned, just a remainder of the trim runaway NNC memo items...

I am also a mere Airbus pilot. But I’m led to believe that a key difference is that Boeing’s MCAS system is linked only to the capt AOA. So, a single faulty/frozen AOA vane could cause a nose down input from MCAS. Whereas, on an Airbus the same would have to be a double fault. As many will be aware it did happen to Lufthansa in the A320, they recovered by switching 2 ADRs off. Which is now the recognised procedure.

I expect at some point the argument will need to be raised that a single failure should not lead to such a drastic and confusing situation as what happened to Lion air.

I wonder what will happen next. Lots of money spent on the MAX by large and powerful corporations.

greg85 wrote:

I am also a mere Airbus pilot. But I’m led to believe that a key difference is that Boeing’s MCAS system is linked only to the capt AOA. So, a single faulty/frozen AOA vane could cause a nose down input from MCAS. Whereas, on an Airbus the same would have to be a double fault. As many will be aware it did happen to Lufthansa in the A320, they recovered by switching 2 ADRs off. Which is now the recognised procedure.

I expect at some point the argument will need to be raised that a single failure should not lead to such a drastic and confusing situation as what happened to Lion air.

I wonder what will happen next. Lots of money spent on the MAX by large and powerful corporations.

Those are exactly my thoughts. I thought after the TK accident in AMS Boeing realized critical systems should not be linked to only a single source. I am not saying Airbus is doing everything better, and the LH Bilbao incident showed that Airbus is immune to problems. But as a reaction to the above mentioned incident the A350 is now equipped 4 independant AoA probes. So why would Boeing decide to go a different way?

Maybe my question belongs here: as a simple ppl i understood that MCAS is not operative if: autopilot on/ flaps out/ speed high, but is there any relation with GPWS or is my thinking irelevant ?

Pluto707 wrote:

Maybe my question belongs here: as a simple ppl i understood that MCAS is not operative if: autopilot on/ flaps out/ speed high, but is there any relation with GPWS or is my thinking irelevant ?

AFAIK there is no relation to GPWS. MCAS would seem more closely related to stall warnings.

The warning hierarchy in aircraft is as follows: stall > GPWS > TCAS.

Thats what i ment, so MCAS can act even at ground proximity, when GPWS became active ?? Jeez...

Well, falling out of the sky is probably the most important thing that needs immediate action. Not a pilot, but I would imagine all 3 systems would notify the crew immediately irregardless of what any one system is doing. In any situation, above anything else, the most important thing to do is to fly the aircraft. If I recall correctly, "Fly The Airplanes" is printed at the top of emergency check lists which means above anything else you might be doing, no matter how important, always fly the airplane first.

Please expand on your Jeez statement.................

flyinTLow wrote:

Hey everyone!

As everyone is all over the MCAS System on the 737 MAX now, can anyone give me a detailed technical description to the system, preferably by Boeing?

Some technical details here, slightly incorrect in that MCAS trims at 2.7 deg per sec where the page says 2.5..

http://www.b737.org.uk/mcas.htm

MCAS can be thought of like a high speed stick pusher, it is designed to work like in a high speed spiral dive outside the normal flight attitudes. Only is activated with flaps up. When activated MCAS trims nose down for 10 seconds, then it trims nose up. If the timer is interrupted by pressing the manual trim switches, the timer resets and starts from zero again. The electric trim moves at 2.5 deg per sec, which means to counter the MCAS trim a pilot would need to trim about 10% longer.

The 737 does have two AOA sensors, each with two resolvers (effectively say the left side generates Left 1, and Left 2, the right side Right 1 and Right 2, these are not Boeing terms, they are just to illustrate what is being generated). So each AOA sensor can feed both sides. MCAS AOA source changes each sector.

MCAS is an autopilot function which works if the autopilot is on or off. When manually flying the MCAS function in the FCC will pitch the stab nose down by commanding the electric stab motors directly. There is no way to isolate MCAS, the “fix” is to turn off the power to the stab motors, with the motors off the MCAS function is still commanding nose down however the motors are not powered.

There is no EICAS or FMA to say the MCAS functionality is active. MCAS can be a single point of failure as the FCCs do not check to see if the active AOA resolver is generating a reasonable AOA. This could be fixed by comparing each AOA resolver AOA against each other and the aircraft performance and voting out bad AOA from the MCAS system input side.

flyinTLow wrote:

There was no mentioning of the system in the initial FCOMs to the flight crews? What can you find in the manuals now?.

Next to nothing in the pilots manuals, and only a little in the maintenance training.

zeke wrote:

Next to nothing in the pilots manuals, and only a little in the maintenance training.

Hmm.
Did one US airline use the maintenance info to backtrack to Boeing for info?
Or how did that single airline include it in pilot info?*
( * rather tenuous remembrance of having read this somewhere.
another path would have been via Brazil, where MCAS info provision was required? )

Delete

zeke wrote:

flyinTLow wrote:

Hey everyone!

As everyone is all over the MCAS System on the 737 MAX now, can anyone give me a detailed technical description to the system, preferably by Boeing?

Some technical details here, slightly incorrect in that MCAS trims at 2.7 deg per sec where the page says 2.5..

http://www.b737.org.uk/mcas.htm

MCAS can be thought of like a high speed stick pusher, it is designed to work like in a high speed spiral dive outside the normal flight attitudes. Only is activated with flaps up. When activated MCAS trims nose down for 10 seconds, then it trims nose up. If the timer is interrupted by pressing the manual trim switches, the timer resets and starts from zero again. The electric trim moves at 2.5 deg per sec, which means to counter the MCAS trim a pilot would need to trim about 10% longer.

The 737 does have two AOA sensors, each with two resolvers (effectively say the left side generates Left 1, and Left 2, the right side Right 1 and Right 2, these are not Boeing terms, they are just to illustrate what is being generated). So each AOA sensor can feed both sides. MCAS AOA source changes each sector.

MCAS is an autopilot function which works if the autopilot is on or off. When manually flying the MCAS function in the FCC will pitch the stab nose down by commanding the electric stab motors directly. There is no way to isolate MCAS, the “fix” is to turn off the power to the stab motors, with the motors off the MCAS function is still commanding nose down however the motors are not powered.

There is no EICAS or FMA to say the MCAS functionality is active. MCAS can be a single point of failure as the FCCs do not check to see if the active AOA resolver is generating a reasonable AOA. This could be fixed by comparing each AOA resolver AOA against each other and the aircraft performance and voting out bad AOA from the MCAS system input side.

flyinTLow wrote:

There was no mentioning of the system in the initial FCOMs to the flight crews? What can you find in the manuals now?.

Next to nothing in the pilots manuals, and only a little in the maintenance training.

Almost right, but not quite...

Faults that affect MCAS also affect Speed Trim and are annunciated with the Speed Trim Fail indication on the
flight controls panel.

BravoOne wrote:

Faults that affect MCAS also affect Speed Trim and are annunciated with the Speed Trim Fail indication on the
flight controls panel.

And... did they see the light ( perished Lion Air crew )?

zeke wrote:

flyinTLow wrote:

MCAS AOA source changes each sector.

As far as I understood AoA source depends on the active FCC. There are two independent FCCs installed on the B737 series, one of which is active, the other one is on standby. The active FCC alternates with each flight sector.
Assuming that FCC A reports false AoA values, is there a way to manually deactivate FCC A so that FCC B becomes active? How would pilots do that? Is there a switch to choose between FCC A and FCC B manually?

BravoOne wrote:

Almost right, but not quite...

Faults that affect MCAS also affect Speed Trim and are annunciated with the Speed Trim Fail indication on the
flight controls panel.

I think we are talking cross purposes here, I was saying there is no FMA or EICAS to inform pilots that MCAS is active, I was not talking about a fault in MCAS.

Another one... No fault in MCAS, but for some reason, acft encountered power/climbing problems, was maneuvering at +- 1500 ft agl, some high turn to avoid terrain, AoA, speed...and....MCAS lowered nose, direction ground

Pluto707 wrote:

Another one... No fault in MCAS, but for some reason, acft encountered power/climbing problems, was maneuvering at +- 1500 ft agl, some high turn to avoid terrain, AoA, speed...and....MCAS lowered nose, direction ground

which is the thing it is supposed to do. ... But ..
This is kind of a Scylla and Charybdis thing: either nose down or pancake into the ground due to a persistent stall.
( not clear how wide the path is in between S and C.)

zeke wrote:

slightly incorrect in that MCAS trims at 2.7 deg per sec where the page says 2.5..

My reading is that it trims at 0.27 deg per second, up to a max of 2.5 deg.

Rexus wrote:

zeke wrote:

flyinTLow wrote:

MCAS AOA source changes each sector.

As far as I understood AoA source depends on the active FCC. There are two independent FCCs installed on the B737 series, one of which is active, the other one is on standby. The active FCC alternates with each flight sector.
Assuming that FCC A reports false AoA values, is there a way to manually deactivate FCC A so that FCC B becomes active? How would pilots do that? Is there a switch to choose between FCC A and FCC B manually?

I would like to push attention to these questions, which have not been answered yet. I am really curious to learn more about how pilots can manage which FCC is active and how they can deal with faulty FCCs.