When Air France flight 447 was reported missing, news reports blamed storms over the Atlantic. As a frequent traveler, this didn’t make sense to me. Then reports started coming out blaming the pitot tubes that measure an airplane’s speed. It turns out that both Airbus and Air France already knew of problems with the pitot tubes and were in the process of replacing them.
My outrage is this: why wasn’t GPS technology used to determine the airspeed? The on-board computer could have compared a calculated speed using GPS data to the speed readings from the pitot tubes, and automatically warned the pilot. The Airbus A330 has state of the art avionics including GPS according to their site. If free or really inexpensive iPhone apps like V-Cockpit GPS and MotionX-GPS can tell Jane and Joe how fast they are going, why can’t Airbus do so?
Further, why hasn’t America’s FAA or Europe’s EASA aviation authorities required that GPS information be used as a backup to pitot tubes? Commercial airplanes have several pitot tubes for redundancy. But if one stops working due to freezing weather over the Atlantic, then all of them are likely to stop working for the same reason. An airplane would need an alternate method of measuring speed, not more tubes. It is a crying shame that a proven technology like GPS was not used.
We programmers are used to being blamed for failures. Sometimes the blame is well deserved, like the spectacular explosion of the Ariane 5 rocket (see video). Other times, the fault is with management, like the failure of the FBI’s $170 million Virtual Case File (VCF) project. In this case, our profession could have easily saved 228 lives and hundreds of millions of euros.
Airspeed is not the same as ground-speed. GPS is only useful for the latter. For example: The speed at which Air France 447 was “passing” through the *air* while being tossed around during heavy turbulence would have been significantly different to the speed at which it was passing over the ground. That is why airflow is used to measure aircraft speed and not GPS.
Perhaps this is the best way to think about it: If a plane is climbing at a 45 angle, it will take twice as long to cover the same ground as an aircraft flying horizontally (perpendicular to the ground) at the exact same speed. GPS would only show an accurate* speed reading for the latter.
*Actually, not that accurate at all because GPS has a 5-10% margin for error when measuring speed for non-airborne objects. Even if it was possible for GPS to measure air-speed, that margin would be very significant to an object travelling at 500-600mph.
But the plane’s altimeter can be used to extrapolate/calculate the airspeed, no? As for a 5-10% margin of error, I think that it would still be better than nothing when pitot tubes fail.
The 5-10% margin for error with GPS land-speed calculation is absolutely irrelevant because, as already stated, GPS *cannot* measure air-speed.
Even if GPS could (and I emphasise: It cannot), a 5-10% margin for error would be *more* dangerous than having to contend with faulty pitot tubes. The ACARS messages sent while the plane was going down indicate a discrepancy in speed readings no more than 30 knots (approx. 15mph) apart. Ie. Less than 5%, but still enough to tear apart a commercial jetliner travelling at such high speed.
I mean really, what does the pilot do with this added layer of (pointless) complexity? Get a passenger to draw straws to decide whether to trust pitot tube #1, #2, #3 or the GPS which is about as much use as a banana when calculating air-speed?
As for extrapolating air-speed from other instruments when you haven’t even got time to radio an SOS message… well, don’t get me started….
Granted, it’s more difficult than I realized, but… When GPS data is combined with altimeter reading, engine thrust, plane orientation, and other info, I find it hard to believe that it’s impossible to calculate the airspeed. It may require artificial neural networks to accomplish the task — don’t know for sure what would work best. But my educated guess is that it’s feasible given some thought and research.
I think your article shows some reasonable doubt on aviation security…
Basically: You use the altimeter to calculate the vertical velocity, you use GPS to calculate the horizontal velocity, you use Pythagoras to calculate “diagonal” velocity. It really is a simple task and you will end up with that small error of GPS (which gets smaller as you travel at high speeds).
There are already personal GPS-devices out there that do this and with enough GPS-satellites in reach you can even use GPS itself as altimeter, so you would have a completely independent system to double check all your measurements.
To improve accuracy you could additionally use Russian Glonass and later European Galileo, which is especially designed to be usable at high latitudes and as an altimeter.
Even it’s not perfect, huge margins between the systems would make every pilot realize that something is wrong.