[2Ccltvcm]

I want to know why the Boeing flight computer needs pitot tube input at all. Modern ublox GPSes can easily obtain 3D lock on multiple satellite constellations within a minute of booting. Several of these in parallel for redundancy if you are paranoid. Flight controllers on fixed wings don't even need a magnetometer to stabilize. Just GPS path heading. If all else fails, solid state accelerometers are very reliable. Accelerometer only based dead reckoning works great. If all else fails, a single accelerometer should be sufficient to get the plane relatively stable. A barometer can help too, but doesn't seem necessary. These systems can be easily combined with fallback logic to keep the plane in the sky. I just don't understand what is so hard about this for Boeing. I understand airspeed is not the same as ground speed, but this should provide enough information to the flight computer to keep the plane in the air or at least stable.

[mnw21cam]

If all you are using is ground-based position/speed, then you are ignoring the very real possibility that the air you are flying through is not stationary relative to the ground. In actual fact, especially at high altitudes, the air can be moving very fast, and the difference between ground speed and airspeed can be the difference between flying and stalling.

Also, your GPS measurements give you position, direction, and speed, but they don't give you orientation. You would have to have another instrument to feed that into the system (such systems exist).

But yes, it would be a sanity check.

[2Ccltvcm]

Can a near-stall condition be detected solely with some combination of GPS, accelerometer, and barometer?

[mgsouth]

[Disclaimer: not an aeronautical enginner or pilot.]

Unfortunately, no. Stalling is a function of the wing's _angle_ relative to the flow of air, not of speed. If the angle is too sharp the air can't follow the curve of the wing. The critical angle is (pretty much) independent of speed. For example: if you stick your hand out of the window of a car traveling at 60 MPH, and hold it almost flat to the wind (say 80 deg.), then the air can't follow down the back of your hand. All of the "push" is backwards, and there's no push up. If you hold it at 30 deg. then the air flows around your hand, which deflects the air down and your hand up, very strongly.

Even if you're only traveling at 5 MPH, if you hold your hand at 30 deg. the air will flow around your hand and deflect it upward; it will just be a very weak effect.

The angle between the wing and the air flow is what is called the "angle of attack", and what the AoA sensors measure. The only other instrument that comes close is the Attitude gauge (the globe thing). However, it measures the plane's angle relative to the horizon, and air moving relative to the plane usually isn't parallel to the ground in conditions where the AoA matters.

Wikipedia article, with much detail, pictures, etc.: https://en.m.wikipedia.org/wiki/Angle_of_attack

[burfog]

You'll need the air speed and direction.

Normally, speed is from a tube aimed into the air. Normally, direction is from a little fin that can spin.

There are lots of alternatives:

Direction can be via multiple tubes aimed into the air, each with slightly different direction.

Speed can be from a hot wire. Weather stations sometimes use this.

You can get both via lidar. You just need to make it sensitive enough to pick up a response from minute particles of dust or ice.

I think I just invented a new way: do a short-duration high-power pulse of an electron source or an EUV laser, causing the air to fluoresce at enough distance from the aircraft to be clear of the boundary layer. Track the motion of the fluorescing air with multiple cameras.

[mnw21cam]

Yay, yet another way to accidentally fry people on the ground if you accidentally switch on the wrong system. Radar already provides a way to do that.

[Merad]

Unless you limit yourself to flying very near the ground and very near sea level, the speed of an aircraft is more complex than a single number. In fact four different speed numbers are commonly used: indicated airspeed, calibrated airspeed, true airspeed, and ground speed.

* IAS is the raw airspeed reading from the pitot tube.

* CAS is IAS corrected for instrument errors, e.g. if the plane is at an angle that disrupts air flow around the pitot tube.

* TAS is basically CAS adjusted for altitude and air pressure. It’s the aircraft’s speed relative to the air around it.

* Ground speed (or speed over the ground) is TAS adjusted for the wind. This is the number that GPS is going to give you.

IAS and CAS are particularly important for describing performance characteristics - if an aircraft stalls at 100 knots CAS, then it always stalls at that CAS. If you try to describe the stall speed in terms of TAS you go from a single data point to a graph of speed and altitude.

[nroets]

"Why (does) the Boeing flight computer needs pitot tube input at all ?" If there is a strong tailwind, the plane needs a much higher ground speed to avoid stalls.

If these accidents prove anything, it's that we need a computer that takes many different inputs (GPS from the tail and the nose, pitot, barometer, AoA indicator, input from the pilot, engine RPM, etc) and put them into a mathematical model of the airplane before overriding the pilot.

[julik]

Additionally the AOA sensor - which is basically a weather vane - does not output usable data before the airflow around the airplane has reached certain velocity (it needs air flowing around it). Which is reported... by the pitot tubes.