We finally know fuller details* about the as proposed MCAS fix not-a-fix >
There are four main changes to the B737 MAX flight control system software that have been developed to prevent future accidents like the ones that happened with the Lion Air and Ethiopian Air flights. They include the following:
1. Angle of Attack (AoA) comparison – an addition to MCAS that will now compare readings from both angle of attack sensors on the aircraft. If there is a difference of more than 5.5 degrees the speed trim system will be disabled. Also included in this change is something known as a “midvalue select” which uses data from both sensors together to create a third input that will help to filter out any AOA signal oscillatory failures or spurious sensor failures. This modification will prevent MCAS from commanding nose down trim when a single AoA sensor reports a false AoA as it happened in the two accident flights.
2. MCAS resynchronization – this change will account for manual electric trim inputs made by the pilot while MCAS is activating. It will track whatever input the pilot makes and return the pitch trim to that setting when MCAS retrims back to normal.
3. Stab trim command limit – is an addition that will limit the maximum nose down trim that the
automatic flight control system can command to prevent the pitch trim from reaching an
uncontrollable situation.
4. FCC monitors – software monitors have been added to the flight control computers that will
cross check pitch trim commands against each other. If a difference is detected by these
monitors the automatic trim functions are disabled. This protection helps prevent erroneous
trim commands from a myriad of causes that could occur in the automatic flight control system.
These design changes in the software that controls the automatic pitch trim features including MCAS
should prevent angle of attack sensor failures from causing the pitch trim to operate when it should not.
Further, they should prevent the trim from activating erroneously for other reasons as well.
Unfortunately, we don't know if flying the plane without MCAS is even safe. MCAS was required for a reason, and disabling it at an inopportune time might be disastrous.
MCAS was required to keep a linear relationship between the force applied to the flight stick and the pitch-up control moment.
There is nothing magical about this linear relationship; it is an intuitive configuration for pilots, but many other aircraft do not follow it. The requirement makes sense for single-certification, but we must be clear in understanding what is actually happening with this system.
The system counters the hazard of pilots experienced in 'regular' 737s getting close to stalling without realizing, due to lighter stick inputs not having the intended effect. Any MCAS malfunction would direct their attention to this issue.
Actual anti-stall systems (MCAS is not anti-stall, nevermind some shoddy reporting) would still function if a pilot were to approach this flight envelope. This includes cabin alerts, stick shakers, etc.
The scenario where MCAS cuts out, and it's in the envelope of conditions where it actually functions, and the pilots fail to notice this, and the MCAS inputs were needed to avoid approaching a stall, and the pilots fail to correct and avoid the stall .. it's a contrived hypothetical.
MCAS is not a system that activates on a normal flight. Only in relatively extreme circumstances does it even function, and then it only seeks to make intuitive pilot behavior less likely to approach stall conditions. A good pilot monitoring airspeed, trim angle, AoA, etc. will be able to avoid a stall just as well without the system.
The scenario where MCAS cuts out, and it's in the envelope of conditions where it actually functions, and the pilots fail to notice this, and the MCAS inputs were needed to avoid approaching a stall, and the pilots fail to correct and avoid the stall .. it's a contrived hypothetical.
On a 737 they are retracted early in the climb, typically between 1000 and 1250 feet. If the slight stick movement the pilot is accustomed to to bring the elevation down 2-3* fails to do so cause MCAS does not engage, there's not a whole lot of distance to recover from a stall then.
If the slight stick movement the pilot is accustomed to to bring the elevation down 2-3* fails to do so
This is completely unrelated to MCAS, though? Since the goal of MCAS wasn't "bring the nose down" but instead "increase the pressure on the stick required to maintain a certain nose-up attitude", I'd be really flabbergasted if it was supposed to operate in a normal takeoff environment.
MCAS was required because without it the control stick feedback is incorrect in some high-power, high-AOA scenarios - scenarios not within the normal flight envelope. That's all it was for - stick feel. And that's important! But not something that'll knock a plane out of the sky.
Comments
We finally know fuller details* about the as proposed MCAS fix not-a-fix >
There are four main changes to the B737 MAX flight control system software that have been developed to prevent future accidents like the ones that happened with the Lion Air and Ethiopian Air flights. They include the following:
1. Angle of Attack (AoA) comparison – an addition to MCAS that will now compare readings from both angle of attack sensors on the aircraft. If there is a difference of more than 5.5 degrees the speed trim system will be disabled. Also included in this change is something known as a “midvalue select” which uses data from both sensors together to create a third input that will help to filter out any AOA signal oscillatory failures or spurious sensor failures. This modification will prevent MCAS from commanding nose down trim when a single AoA sensor reports a false AoA as it happened in the two accident flights.
2. MCAS resynchronization – this change will account for manual electric trim inputs made by the pilot while MCAS is activating. It will track whatever input the pilot makes and return the pitch trim to that setting when MCAS retrims back to normal.
3. Stab trim command limit – is an addition that will limit the maximum nose down trim that the automatic flight control system can command to prevent the pitch trim from reaching an uncontrollable situation.
4. FCC monitors – software monitors have been added to the flight control computers that will cross check pitch trim commands against each other. If a difference is detected by these monitors the automatic trim functions are disabled. This protection helps prevent erroneous trim commands from a myriad of causes that could occur in the automatic flight control system.
These design changes in the software that controls the automatic pitch trim features including MCAS should prevent angle of attack sensor failures from causing the pitch trim to operate when it should not. Further, they should prevent the trim from activating erroneously for other reasons as well.
* https://transportation.house.gov/download/kiefer-testimony
Unfortunately, we don't know if flying the plane without MCAS is even safe. MCAS was required for a reason, and disabling it at an inopportune time might be disastrous.
MCAS was required to keep a linear relationship between the force applied to the flight stick and the pitch-up control moment.
There is nothing magical about this linear relationship; it is an intuitive configuration for pilots, but many other aircraft do not follow it. The requirement makes sense for single-certification, but we must be clear in understanding what is actually happening with this system.
The system counters the hazard of pilots experienced in 'regular' 737s getting close to stalling without realizing, due to lighter stick inputs not having the intended effect. Any MCAS malfunction would direct their attention to this issue.
Actual anti-stall systems (MCAS is not anti-stall, nevermind some shoddy reporting) would still function if a pilot were to approach this flight envelope. This includes cabin alerts, stick shakers, etc.
The scenario where MCAS cuts out, and it's in the envelope of conditions where it actually functions, and the pilots fail to notice this, and the MCAS inputs were needed to avoid approaching a stall, and the pilots fail to correct and avoid the stall .. it's a contrived hypothetical.
MCAS is not a system that activates on a normal flight. Only in relatively extreme circumstances does it even function, and then it only seeks to make intuitive pilot behavior less likely to approach stall conditions. A good pilot monitoring airspeed, trim angle, AoA, etc. will be able to avoid a stall just as well without the system.
Literally a take-off where one AOA sensor fails.
But MCAS is disabled when flaps are extended, such as on takeoff?
On a 737 they are retracted early in the climb, typically between 1000 and 1250 feet. If the slight stick movement the pilot is accustomed to to bring the elevation down 2-3* fails to do so cause MCAS does not engage, there's not a whole lot of distance to recover from a stall then.
This is completely unrelated to MCAS, though? Since the goal of MCAS wasn't "bring the nose down" but instead "increase the pressure on the stick required to maintain a certain nose-up attitude", I'd be really flabbergasted if it was supposed to operate in a normal takeoff environment.
The goal was always bring the nose down, stick input not required.
MCAS was required because without it the control stick feedback is incorrect in some high-power, high-AOA scenarios - scenarios not within the normal flight envelope. That's all it was for - stick feel. And that's important! But not something that'll knock a plane out of the sky.
I thought MCAS was so the flying characteristics were the same as the previous 737 model to avoid recertifying?