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In the AF447 incident, the autopilot system was offline due to the aircraft's pitot tubes being obstructed by ice particles.

Since the physical condition of the plane in air is cold and risky, the same physical obstruction could happen again and again.

Though the pilot is trained to fly the plane in bad weather, I'm wondering why not just use the GPS to aid the plane's computer? The GPS system could be a secondary source to providing the computer with information for flying, like speed and altitude.

If the GPS could've aided AF447 in preventing the autopilot system from going offline, would the pitot's issue not to initialize the system problem then crash a plane?

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3 Answers 3

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GPS speed is already available to the crew, although not in an easy to see place. It isn't designed to cross-check other instruments because it is showing entirely different information. The winds are changing the actual speed of the airplane across the ground (which GPS shows) and altitude affects the air density (which changes the airspeed shown on the airspeed indicator). The difference between the two can be vast, with no easy correlation. Altitude has similar issues, although not as severe.

Even if they could be "synced up", the problem comes in when there is a difference between the two. Which do you trust? GPS receivers can (and do) fail, and could show an incorrect speed as well. When the computer gets conflicting information like it did between the different systems in the AF447 accident, it gives control to the pilot to sort out.

From the time that the autopilot disconnected until the ice blocking the pitot system melted away, only 12 seconds passed. During that time the crew stalled the airplane and then a chain of events ultimately resulted in the crash of a perfectly functioning airplane. The exact same thing had happened numerous times before then without resulting in a crash.

After the fact, the NTSB identified numerous factors and made recommendations to improve crew training (which has been implemented) and hardware/software modifications to the airplane. Aviation very much takes a "continuous improvement" view of things, trying to learn from every accident and to try to prevent the same thing from happening again. Based on what I've seen, I doubt that this exact same scenario will play out again.

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The main issue is that we can only get groundspeed from GPS data. Winds aloft aren't known exactly at a particular place, so they could be rather inaccurate -- certainly not enough to leave to the hands of the autopilot. The decision to use GPS data to verify airspeed would lie in the hands of the crew. Anyway, they were too preoccupied to go and check that their ground speed was sort of accurate.

It's not uncommon for winds aloft to reach speeds between 180 and 200 knots. While previous weather reports can be useful, winds can change quite frequently -- the chances of the wind changing is much more than the chances of all the sources of airspeed becoming unreliable.

GPS data is also not 100% accurate, so keep that in mind, especially if only some of the sattelites can be reached.

What the crash came down to was mainly a loss of situational awareness, combined with their lack of experience with actual stall recovery, was the main contributing factor to the accident.

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and increasing the crew workload by giving them an extra source of information to monitor and verify would have made their problems even worse... –  jwenting Apr 8 at 9:07
    
In the AF447 incident report, the speed of AF447 dropped sharply from 274 knots to 52 knots. And, the pilots lost all of information which can tell them the speed. In such case, GPS can give important information (ground speed) in the situation at which the pitot tubes not function. The tubes can be frozen easily, but GPS will be more reliable in the very cold environment. –  Cray Kao Apr 8 at 11:25
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@CrayKao: the pilots already have GPS information available, but it'd be stupid to use that for automatic control. –  Qantas 94 Heavy Apr 8 at 11:28

The important quantity, measured by pitot probes, indicated airspeed, is not a speed at all. It is a dynamic pressure. Since lift also depends on dynamic pressure and angle of attack, the right backup is angle of attack indicator. In fact it is angle of attack that matters most, but it can't be measured as precisely, so airspeed is used as primary indicator. All transport category aircraft are equipped with angle-of-attack vanes.

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But not all of them have angle of attack indicators in the cockpit. –  Lnafziger Apr 8 at 11:34

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