Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

seattletimes.com

Post reply on HN
Print view View original

241–250 of 341 posts

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#241

Post by magicalhippo »

Xixi wrote: post #176
CydeWeys wrote: post #102

Earlier quoted context omitted.

You need three redundant sensors for safety-critical systems, so that if one fails you know which one is failing. Airbus has three AoA sensors (and three of others as well) on their planes for this reason. And it's worth pointing out that 737s already have two of these sensors, but don't use them both for MCAS (?!?!).

The A350 has 4 AoA sensors: I suspect it is a lesson learned from an A320 that had 2 AoA sensors froze in the same position, outvoting the only AoA sensor that was still working. In other words: failures are not always independent.

I still don't see how a frozen AoA sensor cannot be detected by the reduction in noise or correlation with other sensors (such as gyroscope or accelerometer).

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#242

Post by codedokode »

manigandham wrote: post #204
JudgeWapner wrote: post #111

Earlier quoted context omitted.

> The engine mounting certainly does not make the plane unsafe. I don't think this is a very...'precise' statement. what is "safe" in the context of aviation? If you had no MCAS, then this plane has a very bad propensity to stall . I hope we can both agree that's an unsafe airplane. You can't just say "don't do that [pull up too much] and then you're fine". The aircraft "wants" to pitch up too high and stall, which p…

The aircraft flies fine. MCAS is a stability control system that applies in certain cases (full power, high angle of attack) where the nose pitches up. It may lead to a stall but that's not what it's correcting for and it doesn't just happen during level flight. If the MCAS system was disabled then pilots handle it manually. Trim is not an exotic concept and just takes training and understanding of the operating char…

To me MCAS looks like an ugly software hack implemented to save money on redesigning a plane and re-training pilots. It brings several problems:

- it is difficult for pilots to understand how MCAS works, and how to diagnose failures. They are not programmers or engineers. And MCAS doesn't provide any debugging information about its work anyway. And pilots don't have time to analyze it.

- it adds new points of failure

- sometimes there are situations when automatic systems shut down and pilots have to control everything manually. With MCAS, they would have more things to look after. Pilots who got used to flying with autopilot are usually not comfortable with manual flight, and systems like MCAS only make things more complicated for them. An example is a recent crash of SSJ100 in Moscow where experienced pilots failed to land a plane manually after autopilot has turned off.

So I think that plane designers should be very careful with adding more automatic controls. Especially if their purpose is only to save money at cost of making place less safer.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#243

Post by eCa »

erentz wrote: post #43
djsumdog wrote: post #28

What is the endgame now for 737 MAX planes? Every airline using them has had several weeks to totally reorganize to deal with these huge missing parts of their fleets. Boeing keeps talking about fixing them and putting them back in the air, but I dunno. At this point, it should be agreed the fundamental design flaws are serious, and Boeing should have simply made a brand new plane, with much more modern controls and…

> What is the endgame now for 737 MAX planes? The bugs will be addressed and they'll still fly. Airlines have no choice, there's a limited capacity in the world for airplane construction, several thousands of orders on the books for the 737 and 320 waiting to be filled. What can an airline do? They can cut back some orders but not all orders. > I'd honestly like to see the 737 MAX taken out of service and Boeing simp…

> Airlines have no choice

Anecdotal: I know of at least one family that booked a trip on a non-Boeing airline, at additional cost, to avoid risking being on a Max.

Imagine that after the Max is back in service, that there is a serious incident in the first few months - it doesn't have to be fatal, just a close call of some kind - how many more customers will avoid the Max at all costs?

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#244

Post by codedokode »

_ph_ wrote: post #222
bronson wrote: post #218

Earlier quoted context omitted.

The aircraft flies fine in most flight profiles, yes. At high angle of attack, the MAX-8 is more unstable than pilots were trained to expect. And the MCAS has done an absolutely miserable job of fixing this. Worst case scenario bad: it caused the loss of two airplanes that, if the MCAS had never been added to the MAX-8, would have reached their destinations just fine. Until the it flies predictably in every flight pr…

Alternatively just train the pilots on the behavior in the domain where MCAS would activate. MCAS main purpose is to lower the training requirements.

There are already lot of things that pilots have to keep in mind and look after. Learning how MCAS changes the behaviour of the plane makes everything more complicated.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#245

Post by manigandham »

codedokode wrote: post #242
manigandham wrote: post #204

Earlier quoted context omitted.

The aircraft flies fine. MCAS is a stability control system that applies in certain cases (full power, high angle of attack) where the nose pitches up. It may lead to a stall but that's not what it's correcting for and it doesn't just happen during level flight. If the MCAS system was disabled then pilots handle it manually. Trim is not an exotic concept and just takes training and understanding of the operating char…

To me MCAS looks like an ugly software hack implemented to save money on redesigning a plane and re-training pilots. It brings several problems: - it is difficult for pilots to understand how MCAS works, and how to diagnose failures. They are not programmers or engineers. And MCAS doesn't provide any debugging information about its work anyway. And pilots don't have time to analyze it. - it adds new points of failure…

Yes, that's what I said. The artificial behavior would be fine if the pilots actually knew it was artificial and how it was being done. That lack of understanding and training is the critical error, made worse by a poorly designed system that failed too easily.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#246

Post by tempestn »

magicalhippo wrote: post #241
Xixi wrote: post #176

Earlier quoted context omitted.

The A350 has 4 AoA sensors: I suspect it is a lesson learned from an A320 that had 2 AoA sensors froze in the same position, outvoting the only AoA sensor that was still working. In other words: failures are not always independent.

I still don't see how a frozen AoA sensor cannot be detected by the reduction in noise or correlation with other sensors (such as gyroscope or accelerometer).

It probably could, but you'd have to code for that, rather than expecting the majority of 3 sensors to always be right.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#247

Post by mcsoft »

cesarb wrote: post #125
mcsoft wrote: post #44

Earlier quoted context omitted.

They had to install MCAS because the plane was unstable in high AoA mode, i.e. during takeoff. And it was unstable because larger CFM engines could not fit under the wing anymore so they had to put it further forward and a bit up. This increased the pitch up moment when a full power is applied. Also, can't really see how the height of the fuselage above the ground affects fuel efficiency.

> They had to install MCAS because the plane was unstable in high AoA mode, i.e. during takeoff. Forgive me if I'm missing something, but isn't MCAS disabled while the flaps are extended, which is the case during takeoff and landing?

You are right though pilots prefer to retract flaps soon after liftoff since extended flaps mean more drag and less efficiency. And the danger comes from the low altitude thus no margin for error.

In the doomed ET302 flight they retracted flaps 62 seconds after liftoff at only about 1200ft above the ground. MCAS kicked in 5 seconds later. Had autopilot been engaged it'd also had saved them from MCAS trim down commands.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#248

Post by nicoburns »

the_mitsuhiko wrote: post #235
jwr wrote: post #155

Earlier quoted context omitted.

I think the 737 MAX should be properly re-certified as a new airplane once the "fixes" are implemented. The whole idea that you can significantly alter an existing airplane by mounting larger engines more forward, add new automatic systems, and still pretend that it's the same airplane, is flawed.

The 737 MAX cannot be certified as a new plane so that is not an option.

Why not?

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#249

Post by mcsoft »

cletus wrote: post #118

So a lot is known at this point: 1. The A320neo caught Boeing completely off-guard; 2. The threat of the likes of American Airlines (already a mixed Airbus/Boeing customer and the largest airline in the world) placing a large A320neo order for regional aircraft operations scared the bejesus out of Boeing management; 3. For airlines like Southwest that are pure 737, the prospect of adding a plane that didn't share a c…

> There was a safer configuration as an optional extra purchase;

Forgive me if I'm missing something but these extra options are AoA cockpit indication and AoA discrepancy alert when two sensors disagree.

They don't alter software, MCAS would continue relying on just one sensor regardless of this indication. It's just an extra bit of information for pilot's decision making.

Sounds to me like another hack over the hack hardly adding much extra safety.

Re: Boeing altered key switches in 737 MAX cockpit limiting ability to shut off MCAS

#250

Post by gmueckl »

bushido wrote: post #219
TomVDB wrote: post #168

Earlier quoted context omitted.

Here’s my thought experiment on that: if I booked a flight to my vacation destination and, at the last moment, I discovered that I’d be flying in a 737 MAX, would I cancel my trip or pay a considerable amount of moment to get all our tickets changed? I very likely would not. Would you? Would others? It’s one thing to answer a question of a pollster, it’s another to take concrete action.

As others have stated, I will not be booking a ticket on a 737 max, period. Granted I might not be a typical discount shopper, I pick flights based on comfort, not price and have checked the aircraft/airframe I will be in for years. However, I have a tough time seeing anyone I know taking a risk with flying the 737 max for a 10% discount in fares. Typically, fuel costs to the airline can be estimated at 10-20% of the…

You are not guaranteed to fly on the model of airplane stated when you buy the ticket. This guarantee would be highly impractical because the airline would only have a limited ability to supply a replacement plane when the scheduled one is unavailable. So if push comes to shove, airlines will stop telling you what model of airplane they plan to use at all.
Post reply on HN