Reminder that the 737 was far, far, far more than a "software failure": https://news.ycombinator.com/item?id=19414775
The entire point of MCAS was to mask the reality that the plane needed a recertification due to its inadequate aerodynamics. How are they still pinpointing MCAS as the root of the problem rather than the plane's physical instability?
1. Could the 737 MAX be certified without MCAS?
1b. Could the 737 MAX be certified within the existing 737 family without MCAS?
2. Could the 737 MAX be flown safely under normal circumstances without MCAS?
3. Could the 737 MAX be flown safely under exceptional circumstances by a competent pilot without MCAS?
3b. Could the 737 MAX be flown safely under exceptional circumstances by an average pilot without MCAS?
(Note: By competent, I mean a hypothetical "no human errors" pilot. I don't mean to imply that the real pilots were bad or are to be blamed for the crashes; and I am fully aware that systems must be built taking into account that minor operator error will invariably happen.)
As far as I can tell:
1. No, the MAX would not be certifiable without MCAS, due to the requirement that AoA not increase without stick forces increase in certain flight regimes.
1b. No, if the MAX can't be certified, it can't be grandfathered into the existing certificate.
2. Yeah, the MAX would fly perfectly fine in almost all flight regimes without MCAS.
3a. Yeah, even in exceptional flight regimes, a competent pilot could fly the MAX perfectly fine. He'd have to recognise an uncommanded AoA increase and push the stick forward.
3b. Maybe. A mediocre pilot might struggle with the MAX in certain flight regimes, due to its pitch-up tendencies.
Interestingly, a competent pilot could also deal with the erroneously deployed MCAS (recognise the ran away stab trim condition, pull the circuit breaker), as Boeing explicitly assumed.
> mask the reality that the plane needed a recertification due to its inadequate aerodynamics
That's not accurate, I'd say. There was no "masking" going on. I think the aerodynamics are fine, largely, and MCAS was added to fulfil a certification requirement.
***
>The airframe with the engines mounted differently did not have adequately stable handling at high AoA to be certifiable. Boeing decided to create the MCAS system to electronically correct for the aircraft's handling deficiencies.
While accurate, it omits that part of the certification Boeing were chasing was a common type rating with previous 737s on the basis of it having similar handling characteristics. MCAS did not exist because the 737 Max was unsafe, just different.
The reason they were chasing fleet commonality is because it would massively reduce adoption costs for airlines by making expensive training for existing 737 pilots unnecessary.
You're saying the plane is "safe, just different" without MCAS? But Wikipedia says:
"The MCAS flight control law was implemented on the 737 MAX to mitigate the aircraft's tendency to pitch up [...]"
I'm a layman but it sure doesn't sound like safe or normal behavior for a passenger airplane to be fundamentally designed so that it "tends to pitch up"... is it? Are there other airliners that do such a thing?
So the 737Max would not "pitch up" in most normal flying, this is under high AoA situations, like a tight overspeed banking turn I think.
Some people argue that that behavior, without measures to prevent it, would disqualify the plane from certification completely. Others say that those measures are only needed to keep the Max under the same type rating as previous 737 generations. I'm inclined to the former, but I'm no expert so that's certainly not definitive.
> The MCAS is intended to compensate in such cases [where the aircraft pitches up], modeling the pitching behavior of previous models, and meet a certain certification requirement, in order to enhance handling characteristics and thus minimizing the need for significant pilot retraining.
With proper training/re-certification pilots would be trained in the _actual_ characteristics of the plane, thus being able to correct for this behaviour.
That's arguable:
https://www.forbes.com/sites/petercohan/2019/04/02/mit-exper...
https://www.eetimes.com/software-wont-fix-boeings-faulty-air...
The placement of the MAX's engines due to ground clearance requirements lead to a flight behavior at low speeds which would be, if maybe not outright dangerous, at least very finicky if not for the MCAS.
If there is a sensor failure, the aircraft legally should not be dispatched (not meeting MEL), so its on the airline to make sure they don't push pilots into bad situations.
This is a common miss conception, planes can behave quite differently, they already do depending on loading. Then you have common type ratings, e.g. A330, A340 and A350 or 777 and 787 which only requires a short difference training to move between types.
The real reason is that the aircraft does not comply with stick force regulations when approaching stalls.
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.
And like the painted cracked wall, it might be okay in 99 out of 100 cases, but that one case where it isn't might kill a hundred people.
Yaw dampers in general are very common. Most (all?) airliners have them in one form or another. The reason for this is that swept wings inherently tend to increase susceptibility to Dutch roll. And yet we don't conclude from this that it's inherently irresponsible of aircraft manufacturers to make airliners with swept wings.
- Plane was bad, so they band-aided it in factory, but band-aid keeps falling off sometimes. So planes stay grounded.
- Sure, they can make better band-aid but everyone knows planes are bad designed so it won't help much (even if super safe band-aid)
- Correct way is to rebuild the plane to make it good, but this is/was out of the table - it was already way too expensive in the first place.
And here we are.
Boeing management made a decision to sell dual sensors as an option package, cutting the base model down to one single AoA sensor per MCAS. (there are two MCAS systems to provide redundancy)
It was already built adequately, the decision to cut corners on safety to try and leverage a small profit was the problem.
Dual AoA sensors as part of the standard option package and providing adequate training to pilots would have prevented all of this.
One sensor per MCAS means that only one sensor is used at a time, so if it is installed incorrectly or is faulty. The faulty data is fed into the MCAS. This is what caused the crashes.
The more expensive option packages included dual-AoA sensors. Two AoA sensors per MCAS, and then an AoA disagree alarm that would alert the pilot that the AoA sensors are not in agreement.
https://boeing.mediaroom.com/news-releases-statements?item=1...
The AoA sensors on the standard model were meant to be redundant, not work together. Boeings fixes are making the sensors work together, while sacrificing what they originally designed as redundancy.
Boeing is intentionally trying to suggest what you are which frames the problem as a mistake
In reality these systems were meant to be redundant, and not linked together, so they're taking a sensor meant for a failover system and combining it into a single system to create this 'workaround'
It was very disgusting all around.
It is kind of like saying that a person was raped because of wearing revealing clothes. So disgusting.
"Please don't fulminate."