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This recent aviation.SE question asks why isn't GPS used as a backup speed indicator. The answers given mention that GPS cannot indicate airspeed. However, there are other methods to indicate airspeed. At the very least, a small anemometer and vane might be deployed in a fashion similar to how a ram air turbine is deployed. Or at the very least, deploy the ram air turbine and measure the RPM or voltage.

Another creative method might be to release a small red sponge ball, and have a camera/sensor mounted in the top of the tail (where the A380 already has a camera) determine the vector that it flys away. Image processing for this type of work has been available for years already.

Of course, these solutions would only be deployed when necessary. I wager that using the ram air turbine to determine the airspeed could be done in software with no added weight or mechanical components added to the vehicle. Assuming that the vehicle already has a camera in the tail (such as the A380), adding the ability to deploy a small sponge ball would be less of a retrofit than adding cellular telephone service.

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    $\begingroup$ Both of the methods you propose have significant difficulties. They would need more information from other systems. For example, the RAT will rotate at different speeds for the same airspeed and different altitudes and on different days. $\endgroup$
    – Simon
    Feb 1, 2015 at 18:20
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    $\begingroup$ we already have backup procedures for a broken ASI, which is to fly pitch and power. the problem is not the lack of a airspeed reading, but poor airmanship. $\endgroup$
    – rbp
    Feb 1, 2015 at 18:43
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    $\begingroup$ I repeat. If it was flying safely before the ASI failed, why cannot it continue to do so? Same pitch and power = same airspeed. I am of course assuming cruise, otherwise, this and your methods all fail. $\endgroup$
    – Simon
    Feb 1, 2015 at 18:45
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    $\begingroup$ That is precisely the case study of what happens when pilots do not use good airmanship and correct decision making. If they had left pitch and power where they were, they would still be here today. There is no reason that AF447 could not have been safely recovered. $\endgroup$
    – Simon
    Feb 1, 2015 at 18:58
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    $\begingroup$ They need to know right now what their true airspeed is this second.. Why? It was mach 0.82 the second before the failure. It is mach 0.82 the second after the failure. Your ball tells them that in the second in between, it is mach 0.82. What next? $\endgroup$
    – Simon
    Feb 1, 2015 at 19:02

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In order to answer your question first we need to figure out what we're trying to accomplish with airspeed. For the most part the goal is "envelope protection" - keeping the pilots from either stalling the aircraft or overspeeding (and overstressing) it.

So the first thing to know is thaton modern transport category aircraft there are already backup systems to compensate for a failed airspeed indicator.

For starters there are usually at TWO (visible) airspeed indicators (one for each flight control station - pilot & co-pilot). The airspeed indicators are usually using two different pitot (ram air pressure) sources, so that if one pitot system or indicator fails the aircraft can still be flown on the others.
In addition to those two indicators there is often a third "tie-breaker" system on modern aircraft which is used to determine which system has failed (if two systems agree the third is declared insane and ignored).

In the unlikely event that all of the pitot-based airspeed indicators fail they aren't the only system providing envelope protection: Angle-of-Attack vanes are used to determine when the wing is approaching the critical angle of attack (and is in danger of stalling). These operate independently of the pitot system as a redundant form of stall protection, and as with the pitot system aircraft usually have more than one AoA vane installed.

Both the pitot-based airspeed indicators and the AoA vanes are critical systems, and are heated (to prevent ice from accumulating and interfering with their proper operation) and monitored (by onboard computers and/or the pilots).

Finally in the face of a failure of all of those systems there is the ultimate fallback method that others have described: Pitch + Power = Performance.
A pilot should be able to at the very least maintain level flight, descend, climb, and make standard-rate turns within a safe flight envelope by reference to engine power and aircraft attitude.
While that's far from an ideal situation it is one pilots are trained for.

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  • $\begingroup$ Thank you! This answer does seem to address the concerns, both my own and those of others, in the comments on the OP. $\endgroup$
    – dotancohen
    Feb 1, 2015 at 21:57
  • $\begingroup$ Is it right that for getting a fairly good landing speed, if you declare emergency, the tower should be able to estimate your KIAS based on radar data and whether? $\endgroup$
    – yo'
    Feb 2, 2015 at 15:14
  • $\begingroup$ @yo' It's possible, but it would be the same quality estimate you'd get from a GPS and a wind report/forecast: If you know the wind you can work out your airspeed given your groundspeed. $\endgroup$
    – voretaq7
    Feb 2, 2015 at 20:13
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To expand on voretaq7's answer -- airspeed is measured by aircraft and used by pilots as a proxy for Angle of Attack because early aircraft designers found it easier to measure airspeed (using pitot and static pressures in a calibrated differential pressure gauge aka airspeed indicator) than they did measuring the Angle of Attack (alpha) of the wing directly (this requires an AoA vane with its associated electrical transducer).

The US Navy has a long-standing practice of fitting AoA gauges into the cockpit of its aircraft, and naval aviators are trained to use the AoA gauge as a primary flight instrument; however, civil aviation retains the legacy of airspeed as a proxy for AoA, despite the near-universal fitment of AoA vanes to transport class aircraft to provide stall warnings. This has started to change, though: Boeing glass cockpits with recent firmware provide a mini-AoA gauge as an option. While it's not accurate enough (primarily due to annoying Mach effects) to be used as a primary flight instrument in the style a naval aviator uses their Hornet's AoA gauge when landing on a carrier deck, it's still useful as a backup/crosscheck for airspeed trends (if your ASI is showing 50 knots with the AoA indicator reading low, you have a broken ASI, not a stall).

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  • $\begingroup$ This is a very good observation. I think it would help with learning as well. I used to fly an aircraft with automatic slats and I think the subconscious feedback when they deployed helped me a lot. A angle of attack indicator would be much nicer. $\endgroup$
    – copper.hat
    Feb 1, 2015 at 22:38
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Possible approaches for a backup Airspeed Indicator

Why not use GPS?

  • Does not indicate airspeed, only ground speed.

Why not use the RAT?

  • The RAT will rotate at different speeds for the same airspeed and different altitudes and on different days. (Simon)

Why not use a coloured ball?

  • The ball would be straight back during cruise unless weighted down significantly. (ratchet freak)
  • The speed of the ball would help determine airspeed. For a second or two. Now what? And there is a much simpler method. If you are flying safely, and you lose airspeed indication, leave the pitch and power where they are, and you will continue to fly safely. (Simon)

Why use anything at all?

  • We already have backup procedures for a broken ASI, which is to fly pitch and power. the problem is not the lack of a airspeed reading, but poor airmanship. (rbp)
  • If the plane was flying safely before the ASI failed, why cannot it continue to do so? Same pitch and power = same airspeed. I am of course assuming cruise, otherwise, this and your methods all fail. (Simon)

Other approaches:

  • Some experimental aircraft use a venturi to measure airspeed. (rbp)
  • The Tiger Moth has a hinged plate on the wing struts that rotates as the air pushes against it, and you can read that angle against a track marked in mph. (Dan Hulme)
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  • $\begingroup$ I should add that it is also possible to fly pitch and power in regimes other than cruise, it's just a lot more complex and requires knowledge of the numbers. In cruise, leave it all alone and it will continue to cruise. $\endgroup$
    – Simon
    Feb 1, 2015 at 20:14

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