Failsafe airspeed indicator

[baxtrom]

Ciao fellow engineers!

With Air France flight 447 in mind (and assuming faulty pitot tubes caused the tragedy) could one imagine a robust, down-to-earth stone age backup technology which would give pilots rough indication of airspeed if all else fails? Perhaps a small membrane or similar in view of the cockpit window which would deform elastically from the air flow?

[TonyMcC]

How about a similar system using satellites like a car's satnav? I feel my satnav gives a more accurate indication of speed than my speedometer. Just realised that this would indicate speed over the ground and not airspeed!

Edited May 24, 2011 by TonyMcC

[baxtrom]

How about a similar system using satellites like a car's satnav? I feel my satnav gives a more accurate indication of speed than my speedometer. Just realised that this would indicate speed over the ground and not airspeed!

 

Perhaps a GPS based system is the best backup? But, like you say, it will give you ground speed.

Also, I imagine something brutally low tech! Something that wouldn't stop working after power loss, thunderstorms, EMPs, mega solar flares et c

[CaptainPanic]

Something really low-tech would still require an interface in the cockpit, and that would complicate things a lot. It means you have your measurement, a wire (hydraulic, electric (analog/digital), or even just a piece of string), possibly something to translate that signal into something understandable, and a readout itself. You can assume that something that takes measurements on the outside of the airplane will often not be directly visible to pilots...

 

You could also mount that low-tech device in the cockpit, which means no (digital) interface is needed because it is directly visible. An experienced pilot can then estimate airspeed from a single observation. But building things into the cockpit itself, or in front of the windows will have its own disadvantages in aerodynamics and safety (structural integrity of the airplane).

 

Note that I don't say it's impossible to come up with low-tech failsafe solutions to make flying safer... I only say that it's a little more complicated than suggested until now in this thread.

[baxtrom]

Thanks for your input Captain Panic!

Actually, the device I imagine could be a small (decimeter scale) plate made from plastic or similar. It could be mounted to the fuselage in front of the cockpit window. With increasing airspeed, the stagnation pressure on the plate would bend it, producing a visual indication of airspeed. Depending on the material, the influence of temperature on the elastic modulus could be negligible. If it could work practically I have no idea.

 

However, it's just a first draft of a preliminary embryo of an idea.

[CaptainPanic]

It should be only airspeed related, but definitely not pressure, otherwise it will show different airspeeds at different altitudes (where the pressure is different)!

 

Hmm... I guess that the stall speed however is a combination of airspeed, angle of attack and pressure... and that's exactly what the pilots want to know (are they close to stall, or not). At the same time, they want to know if they are getting close to dangerous air speeds (much too fast).

 

I guess it might be possible to find empirical relations which combine those parameters into a single "stall warning", or "you're going much too fast" warning. But I don't know how to obtain such combination of parameters with a single low-tech measurement which can be directly visible to pilots at all times.

[baxtrom]

It should be only airspeed related, but definitely not pressure, otherwise it will show different airspeeds at different altitudes (where the pressure is different)!

 

Hmm... I guess that the stall speed however is a combination of airspeed, angle of attack and pressure... and that's exactly what the pilots want to know (are they close to stall, or not). At the same time, they want to know if they are getting close to dangerous air speeds (much too fast).

 

I guess it might be possible to find empirical relations which combine those parameters into a single "stall warning", or "you're going much too fast" warning. But I don't know how to obtain such combination of parameters with a single low-tech measurement which can be directly visible to pilots at all times.

 

 

True, stagnation pressure is lower at higher altitude. Back to the drawing board.. perhaps vortex-induced vibration of a string or similar could indicate airspeed acoustically. Would be annoying on long distance flights though

[ewmon]

a robust, down-to-earth stone age backup technology which would give pilots rough indication of airspeed if all else fails?

Ram air turbines.

 

[baxtrom]

Ram air turbines.

 

 

When I wrote "robust, down-to-earth stone age", that was what I meant. Doh yourself.

[ewmon]

When I wrote "robust, down-to-earth stone age", that was what I meant. Doh yourself.

I said "doh" because the RAT already exists (or used to exist), is proven, is made for emergencies, and can easily be retrofitted --- and there I was trying to re-invent it in my mind. The German V1 buzz bomb used a propeller to measure distance. Speed is simply the derivative of distance. A pinwheel would be a stone age RAT.

 

So, it seems a "stone age" device needs to be a simple device using simple physics principles, and have a simple, accurate, precise, and robust operation -- and also one that's easy to "read", that is, one that requires little/no "instrumentation". By "robust", I mean one that performs in all sorts of situations: daylight/midnight, all-weather, all-altitude, all-temperature, all-attitudes (RPY), etc.

 

How about a multi-string aeolian harp that produces different tunes at different air velocities.

[baxtrom]

How about a multi-string aeolian harp that produces different tunes at different air velocities.

 

 

That's more like it, similar to my second suggestion. However, it will probably drive pilots nuts..

Perhaps one could imagine a resonant device tuned so that it will be over-critical at normal speed and produce tones only at close-to-stall speeds (say at horizontal angle of attack to simplify things). Basically a mass-spring system.

[InigoMontoya]

Perhaps a small membrane or similar in view of the cockpit window which would deform elastically from the air flow?

And when your aircraft encounters icing conditions/rain/etc. how much of your membrane's deflection is due to airspeed and how much is due to impacting water droplets (or whatever).

 

I'd think a more accurate/reliable approach would be to simply have redundant pitot tubes... Say 3 or 4 of them. If they start to give significantly different numbers, then one or more of them has just iced up, an alarm sounds and the pilot is implement emergency procedures. You'd want them in different locations so they don't all ice up at the exact same moment and there'd have to be some calibration due to localized Mach number on different parts of the airframe, but one would think that wouldn't be too terribly hard to do.

Edited May 26, 2011 by InigoMontoya

[baxtrom]

I'd think a more accurate/reliable approach would be to simply have redundant pitot tubes... Say 3 or 4 of them.

 

I guess there is an unfortunate strong correlation between icing up of one pitot tube and the icing up all of them.. If one does, probably others follow very soon. Then again it could be possible to have different types of pitot tubes from different producers.

 

Still I'm interested to know what would be the simplest, down-to-earth way of determining approximate airspeed and avoiding stall if all other systems fail. A force majeure device.

[InigoMontoya]

I guess there is an unfortunate strong correlation between icing up of one pitot tube and the icing up all of them.. If one does, probably others follow very soon.

Agree. That was the point of having them in different locations (so that they would see slightly different environments thereby maximizing the time lag from first freeze to last freeze) and instituting emergency procedures the moment they start behaving differently from each other.

 

I work in a field where sensors taking a crap is a common occurrence. In my field, at least, the easiest way to identify sensor failure is to have multiple sensors in different locations. They may all go south, but somebody has to go first so to speak. The moment one deviates from the others... You know you have a problem. In the case of pitot tubes, you may only have a couple minutes before the others freeze up as well, but those are valuable minutes wherein you can drop altitude so that your airspeed will more closely match ground speed (read: get out of the jetstream or similar high velocity, high altitude airflow) so that GPS can be used in a more meaningful way AND the velocity delta separating stall speed from Mach buffeting is much larger so there's a lot more room for error. So... Maybe you only get a couple minutes warning, but that could make a huge difference.

Edited May 26, 2011 by InigoMontoya