3 ms·

The 737 Max is not aerodynamically unstable to the point of causing these crashes. Any airplane can stall in any attitude or at any airspeed. The Max is not i

by mulmen 5y ago

The 737 Max is not aerodynamically unstable to the point of causing these crashes. Any airplane can stall in any attitude or at any airspeed. The Max is not inherently unstable or aerodynamically unsuitable for flight. The Max could fly without MCAS just fine, but pilots would have to be re-trained. Airlines didn't want to foot the bill for such an expense so Boeing applied the Maneuvering Characteristics Augmentation System (MCAS) to literally augment the maneuvering characteristics of the Max to match the 737-NG, which pilots are already familiar with. Bugs in the system then led to runaway trim which caused a stall and crashes.

WalterBright 5y ago
The runaway trim was driving the nose down into a dive, not up into a stall.
- mulmen 5y ago
  Ah, right. MCAS thought there was an impending stall and took action to prevent it.
  - salawat 5y ago
    Correct, because the AoA sensor was indicating the plane was busy belly flopping into the airstream according to one flight or another's Flight Data Recorder.
sofixa 5y ago
> Bugs in the system And most of all, and the truly inexcusable part, zero redundancy.
lamontcg 5y ago
Then the correct and simple solution is to rip out the MCAS and re-train all the pilots.
- VBprogrammer 5y ago
  As several comments here have stated, it's unlikely that the 737 Max without MCAS would be a certifiable airplane. In a way MCAS makes the Max fly more like the NG but only in the sense that it makes the Max fly more like any other certifiable airframe. There are plenty of aircraft with common type ratings which are more different than the Max and the NG. As an example the 757 and the 767 share a common type rating.
  - lamontcg 5y ago
    Okay so then the stated objection that I quoted is technically incorrectly, but the spirit is the same. This is not a well-engineered plane and it requires the MCAS to fly it, and without the electronic band-aid the flight envelope would never have been acceptable. So it might not exactly be a fighter jet but its outside of the bounds of passenger airlines.
- adgjlsfhk1 5y ago
  this just isn't true. Airbus has used tricks like this for decades without problems. the problem was just a totally incompetent implementation.
ummonk 5y ago
Well that's not true either. Yes it's true that it is aerodynamically stable, and could fly without MCAS just fine. But it isn't just an issue of retraining. To prevent inadvertent stalls, FAA regulations require increasing control stick force feedback - a requirement which the Boeing 737 Max did not meet without MCAS.
- VBprogrammer 5y ago
  I absolutely accept this, in a world were deep technical explanations aren't exactly in fashion, it's the only explanation which fits the available evidence. However, it also runs contrary to the official / media friendly version of what Boeing say happened. I wish we had a good source, the best I've found is the 737 technical site, I wish it was available in an official report.
  - salawat 5y ago
    It actually was if I recall. https://www.faa.gov/foia/electronic_reading_room/boeing_reading_room/media/737_RTS_Summary.pdf https://www.faa.gov/foia/electronic_reading_room/boeing_read... Page 10: >The 737 MAX was designed to handle and feel the same to the pilot as the 737 NG. Without the MCAS function, in some small areas of the flight envelope — such as approaching a stall and during higher g-force maneuvering — the new engines contribute to the control column feeling lighter in the 737 MAX than the regulations allow. These are not areas of the flight envelope in which the airplane normally operates. However, FAA regulations - specifically 14 CFR 25.143, 25.201, 25.203, 25.251, and 25.255 - still require the control column to have a higher pull-force feel in these flight regimes than would exist on the 737 MAX without the added stability from the STS and MCAS function.
    - VBprogrammer 5y ago
      Good find! Interestingly I remember that being a relatively well accepted explanation here before the Ethiopian crash which is before that was published. Though I'm sure similar wording appeared elsewhere.
Gwypaas 5y ago
> MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation. > The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up. http://www.b737.org.uk/mcas.htm#background http://www.b737.org.uk/mcas.htm#background The following debacle of only relying on one input, trying (and still trying) to blame the pilots, a 1960's designed system, not declaring it as a safety critical piece to reduce review and so on is well documented.

by mulmen 5y ago

WalterBright 5y ago

The runaway trim was driving the nose down into a dive, not up into a stall.

mulmen 5y ago
Ah, right. MCAS thought there was an impending stall and took action to prevent it.
- salawat 5y ago
  Correct, because the AoA sensor was indicating the plane was busy belly flopping into the airstream according to one flight or another's Flight Data Recorder.

sofixa 5y ago

> Bugs in the system And most of all, and the truly inexcusable part, zero redundancy.

lamontcg 5y ago

Then the correct and simple solution is to rip out the MCAS and re-train all the pilots.

VBprogrammer 5y ago
As several comments here have stated, it's unlikely that the 737 Max without MCAS would be a certifiable airplane. In a way MCAS makes the Max fly more like the NG but only in the sense that it makes the Max fly more like any other certifiable airframe. There are plenty of aircraft with common type ratings which are more different than the Max and the NG. As an example the 757 and the 767 share a common type rating.
- lamontcg 5y ago
  Okay so then the stated objection that I quoted is technically incorrectly, but the spirit is the same. This is not a well-engineered plane and it requires the MCAS to fly it, and without the electronic band-aid the flight envelope would never have been acceptable. So it might not exactly be a fighter jet but its outside of the bounds of passenger airlines.
adgjlsfhk1 5y ago
this just isn't true. Airbus has used tricks like this for decades without problems. the problem was just a totally incompetent implementation.

ummonk 5y ago

Well that's not true either. Yes it's true that it is aerodynamically stable, and could fly without MCAS just fine. But it isn't just an issue of retraining. To prevent inadvertent stalls, FAA regulations require increasing control stick force feedback - a requirement which the Boeing 737 Max did not meet without MCAS.

VBprogrammer 5y ago
I absolutely accept this, in a world were deep technical explanations aren't exactly in fashion, it's the only explanation which fits the available evidence. However, it also runs contrary to the official / media friendly version of what Boeing say happened. I wish we had a good source, the best I've found is the 737 technical site, I wish it was available in an official report.
- salawat 5y ago
  It actually was if I recall. https://www.faa.gov/foia/electronic_reading_room/boeing_reading_room/media/737_RTS_Summary.pdf https://www.faa.gov/foia/electronic_reading_room/boeing_read... Page 10: >The 737 MAX was designed to handle and feel the same to the pilot as the 737 NG. Without the MCAS function, in some small areas of the flight envelope — such as approaching a stall and during higher g-force maneuvering — the new engines contribute to the control column feeling lighter in the 737 MAX than the regulations allow. These are not areas of the flight envelope in which the airplane normally operates. However, FAA regulations - specifically 14 CFR 25.143, 25.201, 25.203, 25.251, and 25.255 - still require the control column to have a higher pull-force feel in these flight regimes than would exist on the 737 MAX without the added stability from the STS and MCAS function.
  - VBprogrammer 5y ago
    Good find! Interestingly I remember that being a relatively well accepted explanation here before the Ethiopian crash which is before that was published. Though I'm sure similar wording appeared elsewhere.

Gwypaas 5y ago

> MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation. > The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up. http://www.b737.org.uk/mcas.htm#background http://www.b737.org.uk/mcas.htm#background The following debacle of only relying on one input, trying (and still trying) to blame the pilots, a 1960's designed system, not declaring it as a safety critical piece to reduce review and so on is well documented.