Airborne_Again wrote:
They may conceptually be different systems, but it’s all done in the Flight Control System computers is it not?
It´s been a long time since I flew the A330, and I can´t remember, nor do I have easy access to any manuals, but I´m sure you can find an Airbus manual online and see the schematics.
I don´t recall if the Airbus system is called Flight Control System (FCS) (I know it is on the Gulfsteam G650/GIV), but it seems likely, and yes the FCS, via the Flight Control Computer(s) (FCC(s)) would be controlling the FBW flight control surfaces, as well as receiving input from the flight deck controls (pilot and copilot side stick on the Airbus). It´s still not the FBW that makes the magic.
Airborne_Again wrote:
Aren’t Airbus pilots taught to do just that in a windshear/microburst situation, relying on the FBW system to get maximum performance without stalling the aircraft?
The answer to you post remains that it´s the flight envelope protection system (which lives within the FCS) that via the FCC commands the FBW control surfaces to only allow pitch up to just below Crit AoA (Airbus Alpha Floor Protection) – and this is when the aircraft is operating in “normal” mode.
I´m don´t think you get max performance (in wind shear conditions = you´d prob. want best Rate of Climb) while pulling the side stick full aft, since the Alpha Floor protection “simply” keep you away from the critical AoA (Stall), which would be a lower speed (higher pitch/(AoA) that that of best RoC).
Airborne_Again wrote:
Even the MCAS system can’t prevent the pilot from stalling the aircraft if the yoke is pulled back fast enough.
It’s doing its “best” trying to do exactly that by trimming forward with a ferocious speed. And you will run out of elevator authority really quick.
MCAS is totally different however to Alpha Floor on the Airbusses, as Alpha Floor limits the pitch directly, not via trim. MCAS uses elevator trim.
A further problem with MCAS was that you could order the plane with ONE AOA sensor only. And if that one showed b.s. then MCAS would follow it literally to it’s and everyone elses demise. Those planes which have 2 AOA sensors (which afaik has become compulsory now) would have reacted differently to a single failure, even though I never fully understood what it would do if the 2 sensors do not show similar values.
Yeager wrote:
and this is when the aircraft is operating in “normal” mode.
I´m don´t think you get max performance (in wind shear conditions = you´d prob. want best Rate of Climb) while pulling the side stick full aft, since the Alpha Floor protection “simply” keep you away from the critical AoA (Stall), which would be a lower speed (higher pitch/(AoA) that that of best RoC).
Correct. In the case of Air France the protections were not available as the system reverted to alternate law when the airspeeds disagreed. Hence Alpha Floor could not prevent the pilots from stalling the airplane.
A further problem with MCAS was that you could order the plane with ONE AOA sensor only. And if that one showed b.s. then MCAS would follow it literally to it’s and everyone elses demise.
I think perhaps you are misremembering this issue. I don’t believe a single AoA sensor was an option. However, the implementation of MCAS was such that it picked a single AoA sensor as reference when the avionics powered on and stuck with it. An AoA disagree warning was an option, though the tabloid take that it was optional for cost reasons is just silly, it has much more to do with airline SOPs.
Part of the package delivered to get the MAX flying again was that the input is discarded if there is a disagreement. This is somewhat dubious IMO since they are mechanical sensors of the same design stuck out in the wind, ice, rain and “snarge” out on the nose of the plane. The chances of simultaneous failure aren’t negligible. None the less, the changes to limit the authority of MCAS are the real fix.
LondonMike wrote:
None the less, the changes to limit the authority of MCAS are the real fix.
Likely, without knowing it, what´s problematic for Boeing is certification of the MAX without having the current authority of MCAS, as per design. I´d imagine that a limited MCAS authority would not be acceptable to the certifying authority (FAA).
LondonMike wrote:
I think perhaps you are misremembering this issue. I don’t believe a single AoA sensor was an option.
No, the 2nd AOA sensor was the option lots of carriers did not take. At least that is how I recall it and that is how it was communicated by several organisations such as the NTSB if memory serves right.
LondonMike wrote:
None the less, the changes to limit the authority of MCAS are the real fix.
That and proper training how to get around it. Setting flaps is one way to disable MCAS.
LondonMike wrote:
Part of the package delivered to get the MAX flying again was that the input is discarded if there is a disagreement. This is somewhat dubious IMO since they are mechanical sensors of the same design stuck out in the wind, ice, rain and “snarge” out on the nose of the plane.
If you remember that also Airbus had to change a full batch of pitot tubes this certainly is an argument. After all, the old style pitot was blamed for the onset of the troubles of AF447. And not to forget, it was also AOA faults (though computer, not sensor) which caused the massive A330 upsets Qantas faced, which did a very similar thing, put the airplane in an uncontrolled descent.
Mooney_Driver wrote:
No, the 2nd AOA sensor was the option lots of carriers did not take. At least that is how I recall it and that is how it was communicated by several organisations such as the NTSB if memory serves right.
As I said, I think you are misremembering that. From the final report on Ethiopian Airlines ET-302:
The takeoff roll and lift-off was normal, including normal values of left and right angle-of-attack (AOA). During
takeoff roll, the engines stabilized at about 94% N1. Shortly after liftoff, the left Angle of Attack sensor
recorded value became erroneous and the left stick shaker activated and remained active until near the end of
the recording. In addition, the airspeed and altitude values from the left air data system began deviating from
the corresponding right side values. The left and right recorded AOA values began deviating
I can’t access the final report at the moment but the Lion Air LNI043 preliminary report said:
The DFDR recorded a difference between left and
right AoA of about 20° and continued until the end of recording.
Of course there is a chance that there was an option to only take a single AoA sensor but either that option was negatively correlated with crashing the thing (in the same way is you are twice as likely to have an engine failure in a twin) or they were extremely unlucky.
LondonMike wrote:
As I said, I think you are misremembering that.
Indeed. It has been misstated in many places. Thanks for setting the record straight.
I never understood how a plane could be certified with only one AOA indicator anyhow, so I should have caught this earlier.
So it is the AOA comparator which was optional or is the whole “story” that a vital piece of equipment regarding AOA was optional bogus?
So it is the AOA comparator which was optional or is the whole “story” that a vital piece of equipment regarding AOA was optional bogus?
From what I understand the AoA display was an option. If you didn’t have that option then you also didn’t get an AoA disagree annunciation.
The impact of not having warning light going off at a time when the stick shaker was going off constantly, the trim wheels clattering about in typical 737 style and the aircraft trying to fly itself into the ground? I don’t see how anyone could see it as significant (though, for what it’s worth it was enabled as part of the change package to get them back into service).
Something I only read about today is that they promised EASA that they’d come up with a “synthetic” AoA sensor, presumably with the aircraft performance data as input to some a model which weighs the inputs and comes up with an estimated AoA. Pretty interesting.
The AoA indicator on the PFD is a simple software addition, which apparently was a $1MM option. Unlike the Airbus green dot, the AoA indicator makes the PFD quite busy. Not sure how it would help with an MCAS abnormal condition, as the AofA may have a common fault as the MCAS abnormal condition? Would you then have an AoA mis compare abnormal condition? You are now in the realm of a reductio ad absurdum. Perhaps that’s why the good folk in Boeing made it an uneconomic option?
Also not sure how the Boeing/AoA stick shaker/pusher system deals with compressibility? Through an ADC or does it switch off above say FL200?
