Why don't airliners have a navigation GPS as "back up" in case of a pitot tube malfunction? Aeropero 603 and Air France 447 plus many other im sure, could have been saved if these aircrafts would have had a simple Navigation GPS as a plan B. The GPS could have given the pilot the basic information needed to keep flying instead of leaving them confused about what the instruments were showing due to the blocked pitot tubes. having reliable information of ground speed, heading and altitude would have helped the pilots recognize the situation they were in, which was caused by Pitot tubes blockage. Most private pilots now use GPS why not the big airliners? There is many blogs about why airliners do not have GPS tracking so the planes could be found after a crash, but none talking about why GPS is not use to help prevent some of those crashes.
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.
The short answer to this question is that commercial airliners do have GPS and they have for a long time. AF447 almost certainly would have been equipped with GPS. While what Terry said about the difference between airspeed and ground speed is very true, AF447 could have seen from their GPS unit that they were moving much slower than what they should have been.
Unfortunately, judging from the cockpit voice recorder transcripts, it seems that they were panicking and failing to think logically or communicate well with each other. It was almost entirely a case of pilot error, not lack of accurate information. The pilot in the right seat was almost continually pulling back on his side stick. This caused the aircraft to climb and bleed off airspeed until the wings stalled. Even after the aircraft entered a stall condition, the pilots never lowered the nose, which would have recovered from the stall. Instead, the pilot in the right seat actively held the nose high for the entire descent to the ocean.
Also, the pitot tubes thawed several minutes before the actual crash. All of the instrument indications were correct for quite a long time before the aircraft hit the ocean.
For more information on AF447, see Popular Mechanics' annotated cockpit voice recorder transcript of the incident.
Airliners do have GPS units. As the other answers and comments have indicated, GPS does not provide airspeed (or angle of attack) information.
It would seem reasonable to use the GPS as a backup in the following way: Under normal operation, the difference between TAS and the GPS estimate (allowing for some computational latency) should be fairly constant (the difference being the wind speed). When the difference exceeds some noise threshold for a period of time it issues a warning.
This wouldn't help in the case of Aeropero 603 but may have helped in the Air France 447 incident. (To clarify what I mean by help, I mean to reduce cockpit confusion.)
In the former case, a similar tracking system between the radar altimeter and the barometric altimeter may have helped.