Such a tragedy... Forgive my ignorance, maybe some pilots can shed some light on this. In the article it mentions that even though the plane was flying in "alternate" mode, my understanding is that it still has limits to what inputs can be given. That said, even if autopilot is off, why wouldn't the computer have emergency functions to negate strange situations like this? When would pulling completely back on the sti…
There is a famous incident with an A330 test flight where the auto pilot system happily flew into the ground precisely because, unlike Bonin, it did not panic and give in to the instinct to pull the nose back up. I'm guessing this incident made the engineers who designed the plane a little more eager to provide for a true unhampered manual override.
What really happened aboard Air France 447
251–260 of 374 posts
Re: What really happened aboard Air France 447
#252Earlier quoted context omitted.
Yes, but when groundspeed with a slight lag is all that's available is it really that much worse than nothing, which seems to be the failure mode at present? I stress I'm not a pilot and I'm sure you'd want to have a Big Flashing Warning Light telling you that ground speed air speed, but there'd seem to be some benefit over nothing at all.
The ground speed is not useful information. You can be moving backwards relative to the ground and still be overspeed. The only thing that matters to the airframe is the airspeed, and the only way to know the airspeed is to sample the air around the plane.
(asking here because your other reply is nested too deeply)
> The margin between stall and overspeed is something like 20 knots at that altitude.
Whaaa? I thought aircraft have a much wider margin.
How does it change with altitude?
If the margin is so low, then wouldn't a sudden wind gust simply knock the plane off the sky?
Re: What really happened aboard Air France 447
#253Earlier quoted context omitted.
This already happens. I have not been trained to fly an airliner, but I believe that the priority is something like stall, gpws (flying into the ground), tcas (flying into other traffic), master warning, and master caution. These all have their own lights and sounds in the cockpit, and if I know them, so does the flight crew. Presumably the 150 messages that needed to be looked at were all "caution" messages. Too man…
The plane tried to physically force the controls to nose down, I'm glad you mentioned that. I thought that's what a plane should do at first, and then on second consideration wondered if the psychology of panicking pilots would be to fight the controls in order to wrest the plane from going down.
It does seem like that happens. Wikipedia links to a few incidents involving the stick pusher:
Re: What really happened aboard Air France 447
#254Earlier quoted context omitted.
The margin between stall and overspeed is something like 20 knots at that altitude. You're not going to be able to calculate your airspeed from the groundspeed within that margin. (FWIW, I'm pretty sure this GPS speed information is available in the cockpit already. If not, you can get it off your iPhone if you really care.) Finally, the flaps-down speed range of a Cessna 152 is 35-85 kts. So if you're facing into 85…
True about the small margin between stall and overspeed, I didn't mean you could regularly fly like that. But according to the article, their air speed decreased from > 220knots to 90knots. What I meant was that regardless of wind, 90knots air speed should give you a ground speed so low that it should leave no doubt that you are stalled. Apparently the PF was even thinking they were in overspeed at that point. So a g…
That's an interesting point. But if you're so panicked that you're ignoring the voice saying "stall", are you really going to check your GPS and then do a back-of-the-envelope to verify? Probably not. Because if you weren't panicked, you'd be out of the stall by following the stall recovery procedure.
Re: What really happened aboard Air France 447
#255Earlier quoted context omitted.
A commenter below (cellularmitosis) makes a very interesting point: Bonin may not have realized he was in "alternate law" (let's call it "mode" instead of "law"). In "normal mode" the computer will not let pilots stall the plane, whatever they do; it will accept the commands up to what it considers dangerous. There's an "envelope" of acceptable plane movements; pilots can move inside this envelope but not outside of…
yeah, I agree with that. Also there's another factor in that stall warnings may have been seen as an ADIRU malfunction rather than an actual stall. In those cases, I suppose pulling back might make some sense if you think you have bad ADIRU input and the plane is in normal law.
I think the whole point is that in almost all circumstances an ADIRU failure will lead directly or indirectly to alternate law. Hence the correct procedure should be to assume alternate law and cross check.
Once your instruments start failing left, right, and centre you should go into what I call "advanced free fall" mode, check horizon (true, false or otherwise), check altitude, check parachute, repeat... If you hit gimbal lock (or similar INS failure) in the dark, well just bend over and kiss it goodbye.
Re: What really happened aboard Air France 447
#256Earlier quoted context omitted.
Ah, that bit about being trained to ignore their body is interesting. I was not aware of that, and it answers my question pretty well.
when instruments were first introduced to planes, most people thought that something caused all instruments to malfunction like crazy as soon as they entered clouds due to the ghost spatial effects the pilots assumed were true ("I can't be turning, I feel like I'm going in a straight line!" or "I can't be going straight, I feel like I'm turning"). If I remember correctly, the average lifetime of a non-instrument trai…
There was a thread on here recently about this:
http://news.ycombinator.com/item?id=3144099
I think my comment on the subject is near the top: to summarize, it's not as dramatic as all that. :)
Re: What really happened aboard Air France 447
#257Earlier quoted context omitted.
The ground speed is not useful information. You can be moving backwards relative to the ground and still be overspeed. The only thing that matters to the airframe is the airspeed, and the only way to know the airspeed is to sample the air around the plane.
(asking here because your other reply is nested too deeply) > The margin between stall and overspeed is something like 20 knots at that altitude. Whaaa? I thought aircraft have a much wider margin. How does it change with altitude? If the margin is so low, then wouldn't a sudden wind gust simply knock the plane off the sky?
http://en.wikipedia.org/wiki/Coffin_corner_%28aviation%29
(Perhaps a better explanation than the article: http://de.wikipedia.org/wiki/Datei:CoffinCorner.png)
And to answer your question about wind gusts: yes. That is why you don't fly into thunderstorms.
Re: What really happened aboard Air France 447
#258Earlier quoted context omitted.
There is other debris that can clog pitot tubes. Birgenair Flight 301 crashed because some wasps built a nest in a pitot tube during an extended layover: http://en.wikipedia.org/wiki/Birgenair_Flight_301 189 people died. (Also note the failure to react properly to the stall warnings. This comes up again and again.)
I wonder if in a couple of decades we'll look back at pitot tubes and wonder what the hell we were thinking using such a fragile solution. (Of course, in a couple of decades we'll probably have a much more accurate external method of finding out speed/airspeed.)
The real WTF is that planes are kept aloft by aerodynamics. One day we'll look upon that the way nowadays we think of zeppelins.
Re: What really happened aboard Air France 447
#259Earlier quoted context omitted.
Accelerometers measure acceleration. You won't feel any unusual G forces for any constant velocity, whether it's climbing or descending.
Yes you're right...Stall is not a free fall but a sudden reduction in lift which can occur for instance when the angle of attack increases beyond a certain value.
It's actually not a sudden reduction in lift either. Even in a stall, lift equals gravity. It's just that the normal relationship between angle of attack and lift is reversed to that higher angle of attack lowers lift rather than increasing it. This makes it inherently unstable and difficult to control, but there is in principle nothing preventing you from flying a stalled airplane all day.
Re: What really happened aboard Air France 447
#260Earlier quoted context omitted.
On the other hand, it's not obvious to me that the simulator would behave correctly in this regime either. Having the simulator work correctly in a deep stall is probably not high on their list of priorities. It's possible that this was even the first time ever that an Airbus had been stalled like that, so maybe no one actually knew how it would behave.
Yes, that's possible. I remember reading an article by the author of X-Plane about how stalls are mostly simulated by trying it in the real aircraft and programming something similar, because the airflow dynamics are too difficult to model in real time. (Incidentally, the Cessna 172 is nearly impossible to stall in X-Plane, but somewhat easier to stall in FlightGear. I wonder who's closer :)
By easier, do you mean that you have to exert more back pressure to have it stall, or that it quickly gets out of the stall? I think it can vary a bit depending on weight and the exact model you're flying. In some models it's difficult to maintain the stall because the nose will just drop.
You should get an introductory flight lesson, then you could ask do have a stall demonstrated and see which flight simulator matches best. There are a lot of signs you don't get outside a full motion simulator, such as wing buffeting and "mushy" controls.