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Old 25th May 2011, 19:22
  #2381 (permalink)  
Machinbird
 
Join Date: Jul 2009
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Airspeed indicators & diversity

Pitot tubes have been around for a long time, but they are not the end-all solution for airspeed measurement. Other technology even predates that. Take this example from WWI:




Diversification has posted links to two companies doing work with Laser measurement of windspeed. Previously we noted another set of patents related to use of lasers for airspeed measurement.

FBW aircraft have unique requirements for airspeed information that is not obvious to the casual observer. They need to know the airspeed in order to properly set system gains. This is critical for proper performance of the control servo loops.

The increasing prevalence of FBW technology is exacerbating the pre-existing problems with pitot icing.

Used to be, you iced up the pitot enroute, no big deal. Leave the trim and power set and pretty soon it will come back. And if it doesn't, it still flies like an aircraft and you can keep in a safe speed range by proper use of power, speed brakes, attitude, AOA, or whatever else works on that aircraft for that purpose.

On a FBW aircraft that trims itself, the situation is different. The aircraft suddenly has no idea if the power is set correctly, no idea if the control system gain is set correctly, and once it tries to change trim, has no idea if that is set correctly. The designers wisely pass control to a higher level computer that can figure this out.
Unfortunately, the higher level computer may have been processing other tasks, and has not been involved with actual control of the aircraft since 2 landings ago.

Add to this an array of messages sent by the computer relinquishing control trying to describe its problems and advising corrective actions, but initially delivered out of priority sequence. Further add to this what must be a cacophony of bells, clicks, alert tones, and short musical sequences in the audio spectrum.

If the FBW aircraft flew like an aircraft at this point, and I had flown it before in that configuration, probably no big deal, but the computer is still trying to help me out and I am lulled by its help. Four of five critical tasks are still being handled by the computer.

All I have to do is remember to handle the last critical task, wait for the display to settle, and push buttons, turn knobs in a carefully considered sequence.

When you consider the implications of loss of airspeed information on a FBW aircraft in full detail, it is an unacceptable risk.
It is time to provide more diversity in airspeed sources.
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