Using the RAT to estimate airspeed in an emergency.
 

Sometimes, pitot tubes in ACFT become unreliable (with corresponding unreliable airspeed values), as was the case for AF447. Would it be possible to deploy the Ram Air Turbine and gauge a rudimentary airspeed (e.g. stalled or flying) by reading how many kW are being generated?

In other words, could the RAT be used as part of a redundant (or emergency backup) system of measuring airspeed?

Apologies if this seems way out in left field.

I make a few assumptions here: That the FMC would require some reading on the power generated by the RAT and; that there would be a value in kW (or some other unit) able to be displayed, with which the airspeed may be reverse calculated using a rough (or otherwise) relationship of kW generated to airspeed.

Gleaming some equations from windpower, there are some moderately complex formulae relating airspeed to power generated. After factoring in the area of the blades, the temperature and air pressure (which for this matter have corresponding values for altitude etc.), curves may be created which relate power to airflow rate.

If pitot data becomes unreliable, (not addressing PFD clutter in an emergency) would it be possible to refer to curves relating airspeed to kW made by the deployed RAT? Perhaps in the FCOM, AFM or the QRH? At the very least, could this help aircrew identify/confirm a stalled condition?

Thankyou for reading this query. Any ideas? :O

Is a RAT generator frequency wild or is RAT RPM controlled to maintain a constant generator RPM? If RPM isn't controlled then I'd hazard a guess that AC frequency would correlate to airspeed. If it is controlled then the issue might be allowing for the current electrical load

With respect, I think you should lie down in a dark room.

I'm not certain of the details of the A330 RAT. But most of these have governors controlling variable pitch props to maintain a uniform speed (or at least prevent overspeed). In the event that the RAT is a constant pitch (variable speed) unit, its speed would also be a function of its electrical and hydraulic loads at any moment. And oil temperature/viscosity.

I doubt a manufacturer would bother to determine the necessary performance curves to make this of any use for such a low probability event.

As others have stated, GPS can provide a ground speed that, with some performance curves can be used to limp along in the event of a pitot static system failure.:8

Isn't that what they all said to Edison? ;)

I thought it was a good example of using one's imagination for possible solutions. I don't think it would work without substantial re-design/modification of the RAT/Emergency Electrical generating system but someone's thinking.

Deployment of the RAT, at least on the A330, automatically sheds most electrical busses due to the greatly reduced generating capability. The blade pitch is fixed and the generator is kept at constant speed by other means.

It is an idea born of not knowing how a RAT works.

Not really. The other way around, perhaps. It is also a two blade variable pitch propeller driving a hydraulic pump.

I agree a good start - so try this - a deployable paddle, a tiny wing, whose stall angle would provide a good airspeed figure. The onset of stall would be measured by moment arm on the paddle axis.

Backup airspeed indications
 

Just show the raw angle of attack data to the pilots?

deSitter:
I had a related thought a while ago, however using a small paddle that
already exists - the AOA sensor.

Seems one should be able to get a rough air speed reading by using the existing (at least in some designs) damping/test coils to deflect the AOA sensor vane up and and then down a couple of degrees.

Airspeed would be (nonlinearly,very I suspect ) proportional to the current required to achieve the target deflection. Would also need altitude etc for density.
(Do this to one at a time of course)

One nice feature of this is that at no new sensors and possibly even existing HW could be used.

I have no knowledge of where the damping coils sit in the system, I have just seen them mentioned.
If not suitable it would be possible to create a new "fit compatible" sensor with dedicated drive coils.

If you have AoA then why would you need IAS?

Yes, that is why I would think of a wing with a known stall angle per air speed and density. This angle would be found by turning the little wing-paddle until it lost lift. It would probably get you within the 15 knots you need even in tight spots. You'd have to know the AoA from other means to get the actual stall angle.

You would of course have to apply more and more torque to the axis to counteract the moment arm from impinging air. Maybe a strange cylindrical wing could be used.

Why bother with the RAT? Put out the speed brakes. Either you are thrown forward a bit or ....nothing happens. Thrown forward equals fast. Nothing happens equals slow.

AF447's crew did the experiment, but they didn't draw the necessary conclusions. They didn't realize that they had received a no effect result from the speed brakes.:(

Speed brakes can be stowed. Many RATS cannot.

KBPsen;

Thanks - I should know better than to go by memory!

Machinbird gives the common sense answer, to stick your finger out the window :)

-drl