5 ms·

The design of this system, from what I've heard so far sounds insane. For some background, systems intended to automatically override pilot input upon detectin

by Zak 8y ago

The design of this system, from what I've heard so far sounds insane. For some background, systems intended to automatically override pilot input upon detecting an impending stall have been used on large, complex aircraft for a long time. The most common mechanism is a stick pusher, which mechanically pushes forward on the stick to mimic what a pilot should do about an impending stall. The force of a typical stick pusher is low enough that a pilot can overcome it with arm strength. A stick pusher, or a pilot pushing forward on the stick is not enough for the aerodynamics of the 737 MAX in certain circumstances. The position of the engines means the thrust tends to push the nose up, and at low speed, the elevator controlled by the stick alone is not enough to push the nose down when nearing a stall. Instead, the trim must be used, which moves the whole horizontal stabilizer rather than just the elevator. That's not part of the procedure on most aircraft, including other 737s, so regulators would not allow pilots rated for 737s to easily transfer their rating to the new aircraft, if they were willing to certify such an aircraft at all. MCAS is the solution to this problem. It automatically uses the trim to force the nose down with greater authority than the elevator alone could provide. While the concept is sound, the implementation strikes me as reckless. First, the system uses only one of the two angle of attack sensors to determine that the aircraft is nearing a stall. Such sensors have a fairly high rate of failure relative to the safe operation of a commercial aircraft. As far as I can tell, causing the system to use both sensors is a software change and all models of the aircraft had the required hardware. I can't imagine a good reason the system was designed this way, especially since Boeing has included MCAS on a previous aircraft, the tanker version of the 767, and it did use two sensors in that application. Second, activating the system automatically, and allowing it to contradict the pilot's attempt to pitch up using the elevator seems like a mistake. While stalls are almost always the result of pilot error, I'm inclined to think automated systems should let humans have the last word. A better design might have been to automatically trim nose-down in response to full forward pressure on the stick at low speed. That could be in combination with a stick pusher providing full forward pressure on the stick when an impending stall is detected in the absence of pilot input to the contrary.

linuxftw 8y ago
A compounding problem is, if the MCAS system is disable, now the flight characteristics are completely changed. Without additional training, there's no way for the pilots to know they have to trim down.
- Zak 8y ago
  There's no way for pilots to know they have to trim down under what conditions?
  - linuxftw 8y ago
    During otherwise normal flight conditions, the nose might unexpectedly pitch up. MCAS is designed to prevent that when AoA > somevalue to prevent stalling.
    - Zak 8y ago
      During normal flight conditions, the nose does not pitch up beyond the pilot's ability to control with the elevator, and the pilot doesn't fly especially close to the critical angle of attack. MCAS is for an unusual combination of conditions where both elevator and trim might be needed to reduce the angle of attack. An example would be if the pilot set a fairly high nose-up trim to maintain the slow speed of final approach, then decided to go around (abort the landing), applied full thrust, and fully raised the flaps too early. A pilot would definitely know that using trim is an option if the elevator was not producing enough nose-down pitch, but a pilot who has put the aircraft into the situation where a stall is imminent during a passenger flight has already made a series of errors.
      - linuxftw 8y ago
        > An example would be if the pilot set a fairly high nose-up trim to maintain the slow speed of final approach, then decided to go around (abort the landing), applied full thrust, and fully raised the flaps too early. This scenario seems it would apply to previous generation 737s as well. But for some reason, MCAS was required for the MAX 800. Everything I have read so far indicates the MAX 800 has some 'pitch up on it's own' type problem that can lead to a stall. So while I'm sure it's possible to crash the plane in a variety of ways, none of that has to do with MCAS.
crimsonalucard 8y ago
Perhaps a beeping red indicator light/sound asking the pilot if he needs to override MCAS rather than a silent correction.
darkpuma 8y ago
> "A stick pusher, or a pilot pushing forward on the stick is not enough for the aerodynamics of the 737 MAX in certain circumstances. The position of the engines means the thrust tends to push the nose up, and at low speed, the elevator controlled by the stick alone is not enough to push the nose down when nearing a stall. Instead, the trim must be used, which moves the whole horizontal stabilizer rather than just the elevator. That's not part of the procedure on most aircraft, including other 737s, so regulators would not allow pilots rated for 737s to easily transfer their rating to the new aircraft, if they were willing to certify such an aircraft at all." I believe this is incorrect. From what I understand, the 737 NGs and 737 MAXes both pitch up when thrust is increased and both may need to be trimmed back to counteract this. The reason MCAS is there is because the characteristics of the MAX are different from NG (that is, the airspeeds and angles of attack at which particular trims are required have changed), making the MCAS system regulatorily necessary, not mechanically necessary. Because either the MAX needs a system that emulates the handling of the NG, or pilots need to be trained specifically for the MAX, and the former is cheaper. In other words the MAX is not objectively mechanically worse, but rather objectively mechanically different.
- Zak 8y ago
  All jets with low wings and underslung engines (most modern airliners) experience a pitch-up moment from engine thrust, and trim is used to compensate. When the plane is being flown in some steady state, pilots adjust the trim until no force on the controls is required to maintain that state - they're not getting a specific setting from a table or memory. What's different is that at low speed, high thrust, and high angle of attack, the elevator of the MAX does not always have enough pitch moment to reduce the angle of attack. I'm inclined to say having an elevator that's not effective enough to recover from all the stall conditions regulators test for without help from the trim is probably a flaw. Whether there's a good technical reason the elevator couldn't be more effective (probably just bigger), a regulatory reason (some rule says it's not a 737 anymore if they change it), or a non-technical reason (e.g. more shared parts to make it cheaper), I can't guess.
  - darkpuma 8y ago
Moru 8y ago
The design is this way due to a long number of decisions going back 50 years. All this due to a staircase that not even exists any more[0]. [0] https://www.latimes.com/local/california/la-fi-boeing-max-design-20190315-story.html https://www.latimes.com/local/california/la-fi-boeing-max-de...

by Zak 8y ago

linuxftw 8y ago

A compounding problem is, if the MCAS system is disable, now the flight characteristics are completely changed. Without additional training, there's no way for the pilots to know they have to trim down.

Zak 8y ago
There's no way for pilots to know they have to trim down under what conditions?
- linuxftw 8y ago
  During otherwise normal flight conditions, the nose might unexpectedly pitch up. MCAS is designed to prevent that when AoA > somevalue to prevent stalling.
  - Zak 8y ago
    During normal flight conditions, the nose does not pitch up beyond the pilot's ability to control with the elevator, and the pilot doesn't fly especially close to the critical angle of attack. MCAS is for an unusual combination of conditions where both elevator and trim might be needed to reduce the angle of attack. An example would be if the pilot set a fairly high nose-up trim to maintain the slow speed of final approach, then decided to go around (abort the landing), applied full thrust, and fully raised the flaps too early. A pilot would definitely know that using trim is an option if the elevator was not producing enough nose-down pitch, but a pilot who has put the aircraft into the situation where a stall is imminent during a passenger flight has already made a series of errors.
    - linuxftw 8y ago
      > An example would be if the pilot set a fairly high nose-up trim to maintain the slow speed of final approach, then decided to go around (abort the landing), applied full thrust, and fully raised the flaps too early. This scenario seems it would apply to previous generation 737s as well. But for some reason, MCAS was required for the MAX 800. Everything I have read so far indicates the MAX 800 has some 'pitch up on it's own' type problem that can lead to a stall. So while I'm sure it's possible to crash the plane in a variety of ways, none of that has to do with MCAS.

crimsonalucard 8y ago

Perhaps a beeping red indicator light/sound asking the pilot if he needs to override MCAS rather than a silent correction.

darkpuma 8y ago

> "A stick pusher, or a pilot pushing forward on the stick is not enough for the aerodynamics of the 737 MAX in certain circumstances. The position of the engines means the thrust tends to push the nose up, and at low speed, the elevator controlled by the stick alone is not enough to push the nose down when nearing a stall. Instead, the trim must be used, which moves the whole horizontal stabilizer rather than just the elevator. That's not part of the procedure on most aircraft, including other 737s, so regulators would not allow pilots rated for 737s to easily transfer their rating to the new aircraft, if they were willing to certify such an aircraft at all." I believe this is incorrect. From what I understand, the 737 NGs and 737 MAXes both pitch up when thrust is increased and both may need to be trimmed back to counteract this. The reason MCAS is there is because the characteristics of the MAX are different from NG (that is, the airspeeds and angles of attack at which particular trims are required have changed), making the MCAS system regulatorily necessary, not mechanically necessary. Because either the MAX needs a system that emulates the handling of the NG, or pilots need to be trained specifically for the MAX, and the former is cheaper. In other words the MAX is not objectively mechanically worse, but rather objectively mechanically different.

Zak 8y ago
All jets with low wings and underslung engines (most modern airliners) experience a pitch-up moment from engine thrust, and trim is used to compensate. When the plane is being flown in some steady state, pilots adjust the trim until no force on the controls is required to maintain that state - they're not getting a specific setting from a table or memory. What's different is that at low speed, high thrust, and high angle of attack, the elevator of the MAX does not always have enough pitch moment to reduce the angle of attack. I'm inclined to say having an elevator that's not effective enough to recover from all the stall conditions regulators test for without help from the trim is probably a flaw. Whether there's a good technical reason the elevator couldn't be more effective (probably just bigger), a regulatory reason (some rule says it's not a 737 anymore if they change it), or a non-technical reason (e.g. more shared parts to make it cheaper), I can't guess.
- darkpuma 8y ago

Moru 8y ago

The design is this way due to a long number of decisions going back 50 years. All this due to a staircase that not even exists any more[0]. [0] https://www.latimes.com/local/california/la-fi-boeing-max-design-20190315-story.html https://www.latimes.com/local/california/la-fi-boeing-max-de...