Air France 447. Why not use GPS on jumbo jets?

[Waldo]

Just watched the Air France 447 documentary. Due to frozen air-speed measurement devices on the outside of the jet, pilots were presented with false air speed data, and this kicked off a chain of events which ended with the jet crashing in to the ocean.

 

I'm curious, why do they not use GPS on jets, to determine location and air-speed? Wouldn't that be a more fool-proof mechanism (can't freeze up etc)?

[iansheff]

There are websites you can track where a plane is. Not sure what they are now I lost the links out my favourites but you should be able to find them.

Here is one of them http://www.flightradar24.com/

[Mantaspook]

This sounds a good idea in principle but you have to bear in mind the data provided by the satellite system would provide the aircrafts speed over the ground (GROUNDSPEED)

The aircraft systems would also need to know the airplanes speed through the air (AIRSPEED) which as a direct bearing on the STALLING SPEED, below which the aircraft will fall out of the air.

 

Lets say an aircraft has a landing speed of 100MPH - If it is battling against a 50MPH wind when it coming into land the AIRSPEED=100MPH whilst the GROUNDSPEED=50MPH.

 

A stand-alone GPS based system would be telling the pilot he is travelling at 50MPH & needs to speed up to the landing speed of 100MPH, if he did this he would be landing at double the speed needed to land into that wind.

 

Large airliners like Jumbo Jets are equipped with Inertial navigation systems (INS) and the pilots can ask for a display of the aircrafts GROUNDSPEED so there would be no need to rely on the satellite navigation system which still drops out occasionally as the satellites drop below the horizon, but as stated earlier, it is the relative movement through the air that is more critical to the safe handling of the aircraft.

[Obelix]

Just watched the Air France 447 documentary. Due to frozen air-speed measurement devices on the outside of the jet, pilots were presented with false air speed data, and this kicked off a chain of events which ended with the jet crashing in to the ocean.

 

I'm curious, why do they not use GPS on jets, to determine location and air-speed? Wouldn't that be a more fool-proof mechanism (can't freeze up etc)?

 

They can and do use GPS on jets for position (although an inertial gyrocompass is the gold standard)

 

Airspeed doesn't relate to ground speed though - as others have said there is the effect of jet streams to consider for a start. There is also something that a pitot tube or similar device show indicated airspeed, known as KIAS (knots indicated air speed) and this is the effective airspeed not the real airspeed. As you climb high, the air gets thin and the effective airspeed drops due to the lack of pressure. Wings stall at a particular effective airspeed, not an actual "real" airspeed so that's what they measure to indicate to the pilots. A commercial aircraft with say a minimum safe speed with no flaps of perhaps 240 KIAS needs to do 240 knots at sea level to have that speed - climb to 12km and you might have to do 500 knots to get that same KIAS.

[Waldo]

Yes. That makes perfect sense. Thank you for your replies.

 

I imagine it's a very complex area; computer systems deciding exactly which data to present to the pilots, given any particular set of circumstances and available data (some sensors stop working etc).

 

As the pitot tubes froze up, and stopped reporting data; would it have been a good idea if the computer systems then, reported ground speed from GPS or inertial gyrocompass? They knew altitude, so could have used altitude as the basis of computing a rough estimate of ground speed required to keep flying? (not taking in to account wind speed).

 

I wonder if that would have been more helpful to the pilots, than having no indication of speed at all?

 

The final moments: How the jet crashed

 

10.29 (GMT) Air France Flight 447 departs from Rio de Janeiro in Brazil for Paris.

 

2.10am (local time) Pitot tubes freeze, auto-pilot disconnects and co-pilot Pierre-Cédric Bonin is flying the aircraft. He takes the fatal decision to pull up the nose of the jet. It climbs rapidly, causing it to stall. He receives the first stall warning.

 

2.11am The aircraft stalls and begins falling at a reported rate of 10,000ft per minute.

 

2.12am Bonin, in a panic, asks if the plane is going up or down.

 

2.14am An automated voice calls for the pilot to pull up. Seconds later, the plane hits the water.

 

Source: http://www.independent.co.uk/news/world/europe/final-verdict-on-air-france-447-sensors-left-pilots-helpless-7917949.html

[Obelix]

Not really. If you lose ASI then you don't pull the nose up, it's that simple really. If you were in level stable flight, then you will remain in it - trust the artificial horizon, check that the VSI isn't going bonkers (showing unplanned descent) and then work the checklists. The problem was, as I understand it that a high speed high altitude stall was never considered as likely and wasn't practiced as a cockpit drill, hence them holding a stall all the way to the ocean. If they'd kept the aircraft level there was more than sufficient engine power to keep flying regardless.

[alchresearch]

Whats the current situation on GPS usage? Years ago it was classed as a US military owned system that could be turned off at any time. I remember my first GPS unit coming with a disclaimer stating this, and that they'd accept no responsibility for the unit becoming useless.