More technology is not always the solution
If it weren’t about the AF447 disaster, some comments of readers in some of the forums were actually funny. Especially those ones writing that Air France just had to install GPS in addition to pitots to indicate the speed of the aircraft when the pitot tubes got unreliable due to icing. After all the el-cheapo navigation system for the car shows the speed quite well. Well, while the speed on the GPS display is pretty precise, it has almost no value for keeping the aircraft in the air. As you you may from school aircrafts are keep in the air by a force called lift. And aircraft has to create at least enough lift force to neutralize the force of gravity. The lift is dependent on the speed you move the airfoil through the air. Okay, that’s vastly simplified but sufficient for my explanation. Dependent on the aircraft design the needed speed to create such an amount of lift is different: For example the Fieseler Fi156 has an incredible low minimum speed: 27 knots. Out of this reason this aircraft is capable to fly backwards in really strong winds. But this backwards flying (negative speed if you want to say so) is just in relation to ground … the airspeed is still positive. The difference between the speed of the wind and the speed of the aircraft have just to be larger than the minimal speed of the aircraft and the wind speed must be larger than the aircraft speed. Don’t believe me … just go into the next harbor and look at the gulls. Of course you won’t see a Boeing or an Airbus backwards … at least not with usual winds. Another nice example are aircraft carriers: Those have really large engines to accelerate to and to keep top speed. Out of a reason: When an aircraft carrier moves with 30 knots, the air on the flight deck on the aircraft carrier already moves along the airfoil with 30 knots even on calm day. Let’s assume a fresh breeze (6 beaufort) or 30kn. When the carrier steers in the direction with the wind, the air would move along the airfoil … well … not at all. When the carrier goes in the direction against the wind, the air would flow along the airfoil with 60 kn. That’s the reason why an aircraft carrier turns into the direction against the wind to start aircrafts. It gives the aircraft a headstart in order to generate lift. And to get an aircraft to 30kn you need really big engines. To get back to the difference between airspeed and speed over ground? What’s the speed over ground? 30 knots. In all cases. But you have three different air speeds. The GPS only measures the speed over ground. However as mentioned above the lift - the force keeping the aircraft in the air - is dependent of the air speed. At 0 airspeed no lift. At 60 kn a lot of lift. At 30 kn airspeed, something in between. And then you have the jet engines and the catapult. Speed over ground is something different than air speed. And a GPS can’t measure air speed. The air speed can only be measured by measuring the speed of the air flowing along the aircraft. To do so, modern aircrafts use so called pitot tubes in conjunction with measuring equipment to measure the static pressure (the ambient air pressure). The pitot tube measures the ram air pressure into the pitot tube. If there is something blocking the air from pushing into the probe, it can’t measure the speed. So … a GPS receiver would have helped the pilots of AF447 not a bit. There are other means like the Backup Speed Scale but that is working completely different.