Beowulf wrote: ↑2019-03-21 10:50am
Broomstick wrote: ↑2019-03-19 09:04am
The airframe of the Boeing Max is less stable than any other 737.
It's less stable, but not unstable. The column stick forces reduce once you hit a certain angle of attack, but they don't reverse. If you keep pulling back, it will go into a stall, but if you let go of the yoke before then, it will go back into normal flight.
Yes, I said it was less stable. At a certain point decreasing stability becomes a bad thing, particular in an aircraft that is supposed to be delivering boring, uneventful transportation.
Based on what I've managed to glean, if MCAS is engaged then it may not
go back into normal flight, or at least not immediately, The MCAS system is designed to push the nose down, which yes, is the way to avoid/recover from a stall, but when you're low to the ground that can become a problem. In both crashes, based on preliminary information, it looks like the MCAS repeatedly pushed the nose down resulting in rapidly fluctuating vertical and eventually a crash.
Broomstick wrote:This was "solved" not by actually fixing the weight-and-balance problem in the hardware but by attempting to write software to fix the problems by having a computer modify the pilot's control inputs. This is something we have been doing routinely with fighter aircraft which are arguably inherently unstable in current designs and require a functioning computer to be flown.
Almost every other modern airliner is Fly By Wire. Every Airbus. Every B777. Every B787. If those lose their flight computer, they are done. Every FBW airliner modifies the pilot's control inputs. If the B737 was FBW, it would not have to have MCAS. It would be able to just increase the stick forces as appropriate, as the plane computer creates them in the first place.
You still need a well designed airplane for a FBW system to work well. Also, while the original 737's were not FBW (that not existing in the late 1960's) the latest designs are incorporating FBW into the design, although not quite as thoroughly as the A320 forward with Airbus.
The MCAS is also different a FBW because it can, in fact, be turned off in case there is a problem with it. Unfortunately, this does not seem to have been effectively communicated to pilots.
So... instead of a hardware solution to a hardware problem in this instance Boeing decided to use a software fix for a hardware problem.
Every FBW airliner does this.
And changing the W&B requires testing and reprogramming, but unlike the B737 Max, when they put the new LEAP engine on the A320 they did not have to modify the nose strut to ensure ground clearance, and they could mount the new engine where the old one was. With the B737 Max they had to do both of those things and that will alter the flight characteristics of the airframe. Yes, you can disguise some bad traits with software, but that doesn't mean you should
. Sometimes, changing the hardware is a better solution - except of course for time, expense, bean-counters...
In addition, instead of the standard "triple-redundancy" of the aviation industry the MCAS system that is supposed to compensate for the hardware problem relies on not three but only two sensors.
Nit: MCAS is activated by whichever flight computer is primary, and only reads from the sensors on that side. It's no redundancy, as the theory was that the pilot should be able to determine the trim is being driven wrong by something, and do the runaway trim memory checklist.
So if your primary flight computer is toast you're fucked? If that's what you're saying that's not acceptable. That's why triple redundancy in critical systems became an industry standard.
And who came up with that theory, anyhow? It doesn't seem to be working - we've had two separate flight crews crash, and there is now a report of a third that avoided a crash only because a pilot sitting in a jump seat happened
to have a little more knowledge than, apparently, the average pilot of these aircraft. So... how is that theory working out in the real world?
If pilots are missing the fact the trim is running away then it may be a human factors issue. That requires better communication. It probably also means that 1-2 hours is NOT sufficient training to transition to these aircraft despite Boeing's claims that it is.
The result of all this is that you have an airplane with a nose-up-down tendency to an unusual degree (which in aviation usually means unsafe, or at least less stable), and a software system that masks this... until it disengages or is turned off or simply can no longer adequately compensate, at which point you have a human pilot called in (because humans are there to handle situations the machines aren't programmed to handle) to deal with an airplane of flight characteristics of such unusual nature as to be dangerous.
All FBW airplanes mask oddities in the stability of the aircraft. And the B737 Max when not flown near the edges of the envelope has essentially the same flight characteristics as the B737 NGs. We have had crashes of FBW airliners where the pilots were dumped into control when they didn't understand the flight characteristics had changed. Specifically, AF447.
"Normal climb-out" should NOT be an "edge-of-the-envelope" maneuver.
Also, AF447 involve failure of the triple-redundancy system due to ice-over pitot-tubes, and the fact that on a dark and stormy night over the Atlantic if the instruments aren't giving the pilots real information they are fucked because in those conditions you can NOT fly using Eyeball Mark I. If you think AF447 was solely due to a lack of understanding on the part of the pilots you don't know what you're talking about. There is evidence that the pilots were getting multiple airspeed readings and had no way to know which one was accurate. At 2 hours, 12 minutes, and two seconds into the flight the CVR captured the pilots saying "I don't have any more indications" and "we have no valid indications". That means they don't know what the fuck was going on, in IFR conditions where you can NOT rely on bare human senses to keep you alive. That is an airplane problem, not a pilot problem, except of course it definitely became a problem for everyone aboard.
In this thread I go into detail about Air France 447 based on the document produced on the accident by the BEA (French equivalent to the US NTSB)
Broomstick wrote:There are some software problems here. Inputs that result in the anti-stall system engaging doesn't just result in a nose-down input (which is what you do to prevent a stall) but a very strong nose-down input that is hard for the pilot(s) to fight. In it's first iteration this system would keep re-engaging multiple times which could (and apparently did) result in the up-down-up-down climb/dive seen in both crashes. If the system is turned off - meaning turned off so it won't be automatically re-engaged - then you have the pilot flying a plane of different than typical flight characteristics which can be hazardous. I don't know if this something that can be addressed by specifically training pilots for 737 Max emergency procedures, but Boeing advertised and sold this aircraft as something that didn't require specific training for already certified 737 pilots. Which would only compound any other problems involved with this mess.
After the Lion Air crash, Boeing put out a notice on emergency procedures relating to MCAS, and how to disable it.
Apparently that communication was not effective. Now, I don't know if that was a Boeing problem or an Ethiopian Air problem or a these-two-particular-pilots problem, but definitely there was a problem there.
Broomstick wrote:It's not "pilot error" if the pilot was told he didn't need any additional training and wasn't informed of unusual flight characteristics.
Is it a pilot or training error if the pilot isn't able to identify the trim appears to be running away downward, and doesn't implement the memory checklist to resolve it?
If more than one cockpit crew screws this up in a fairly short period of time I begin to suspect it is NOT pilot error, although it certainly could be a human factors problem. We have had not one, not two, but THREE flight crews get tripped up by this since October of last year, and two of those crews have crashed. So yeah, maybe it's a "training error" - meaning there's something wrong with the training. Not enough, perhaps, or something else. Maybe there's some issue with this information being imparted to the flight crew. It's not sufficient to shrug and say "human error" when the same error keeps happening.
This is going to be patched by software, and I doubt there will be another MCAS induced crash during the life of the MAX.
I'm not convinced a patch is going to cut it for this airplane.