FlightAware Discussions

Professional Passenger Has Basic Question

Hello everyone and a very pleasant 2021 to all.

Last night I was watching a program concerning Air France flight 447 from Rio de Janeiro to Paris. They discussed the cause for the crash and reported that the pitot tubes became frozen and thus the pilots, without any visual references, could not save the aircraft. I do not blame them for that because I cannot judge what I do not know.

What I do know is that GPS signals have been available to aircraft for many years. I have had an application on my iPhone for a few years now that tracks my location, altitude above mean sea level, and velocity from satellites. It updates as I drive down the road. We will drive on to an overpass that could have a 10 story building underneath it in my altitude changes as I go.

While I am not sure if this application was available for iPhones in 2009 it certainly makes sense that it should have been available to aircraft. Why do not aircraft use GPS for backup navigational data?

There must certainly be some eloquent and reasonable explanation. I just cannot think of what it is. Please enlighten me.

Thank you!

The problem was that the aircraft stalled. GPS information will not help you finding out your airspeed. It will only tell you your position and ground speed.

Thank you Mr. Hill… However, from the report I read provided by the French equivalent of the NTSB, that aircraft was still airborne for about 6 minutes after it got into trouble. It did not stall immediately. The pilots were confused by the erroneous readings from the frozen pitot tubes and spent 5 or 6 minutes trying to sort it out. By the time they discovered the truth, it was too late. GPS will give you velocity, heading, and altitude. My last trip to Atlanta was clocked at 525mph @ 28,000 feet. Even in 2009 I cannot imagine why some sort of back up GPS system could not have provided them with enough data to save the aircraft. There must be a good reason for that. Plenty of people smarter than I am flying around up there and flying desks down here. So, why? Thanks again!

Well… I found a conversation about this subject from 2015… " Airliners do generally have GPS receivers, but GPS doesn’t give you the information that’s needed in this case.

In AF 447, the primary consequence of the blocked pitot tubes was that the flight envelope protection system couldn’t do its job properly and therefore automatically disengaged together with the autopilot. The subsequent crash was caused by the pilots’ confusion and failure to realize which consequences this had for the aircraft’s reaction to their control inputs.

However, what this system needs to function is not something a GPS system can possibly provide. GPS knows how fast the aircraft is moving relative to the earth , but for flight envelope protection you need to know how fast the aircraft is moving relative to the surrounding air , which GPS doesn’t have any way of knowing. The differences between these values is caused by wind and can be pretty large. It would be a recipe for disaster to let the computer limit what the pilot can do based on an arbitrary assumption that there’s no wind. (And, as Terry comments, there’s an additional difference caused by air pressure).

If I remember correctly, the AF 447 investigation concluded that the pitot tubes had thawed and began functioning properly well before impact; it was not the loss of airspeed indication that prevented the pilots from salvaging the situation. By this time the pilots were confused and thought the airspeed indication was still malfunctioning because it showed a much slower speed than they thought they ought to have – whereas in fact it was working perfectly well."

But you know what? I still don’t buy it. In a day when we can shoot a missile through a window 1000 miles away, when we can send submarines around the world, when Google use its Sensor Vault algorithms to accurately predict human behavior, there has got to be a way to back up all of us flying around up there. Let’s just figure it out.