PPRuNe Forums Using the RAT to estimate airspeed.

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 17th Jul 2012, 20:49 #1 (permalink) Join Date: Jul 2009 Location: Brisbane Age: 27 Posts: 2 Using the RAT to estimate airspeed. After reading the the final report on AF447, where unreliable airspeeds (among a myriad of other problems) contributed to the accident, I was wondering whether the Ram Air Turbine could be used to determine airspeed (roughly or accurately) as an emergency backup? RAT uses airflow to power critical systems on an ACFT. The kW generated would increase roughly with the square of the airspeed (or would it be linearly?). If the electrical power generated is known (which the flight computers I assume WOULD have a value for), and also possibly the altitude, wouldn't it then be a (relatively) simple step to calculate curves for airspeed with respect to power? I am going by some simple equations from wind air turbine power generation, which would apply to a RAT. A flight crew could use a simple (perhaps memory item) relationship between power from the RAT and airspeed to estimate airspeed when other instruments are proving unreliable/confusing. Could the crew of AF447 have used this idea (perhaps if it were in the FCOM/AFM for non-normal conditions, and/or the QRH) to make a decision?
 18th Jul 2012, 11:45 #2 (permalink) Join Date: Jul 2010 Location: Freedom Sound Posts: 24 using the RAT to estimate airspeed. Interesting theory, unfortunately the RAT's fitted on many large passenger carrying aircraft these days do not power the electrical systems on the aircraft but power an hydraulic pump which is fitted on the back of the RAT strut. The idea being that this pump provides sufficient hydraulic power to enable the aircraft's flying control actuators to keep control of the aircraft. Certainly the case on A320/A330 and B757/B767 aircraft.
 22nd Jul 2012, 22:50 #3 (permalink) Join Date: Apr 2009 Location: UK Posts: 180 Just use the gps monitor page for an indication of speed and altitude to get you started and then dive into the qrh for pitch and power settings.

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