Quickmarch 488 Posted January 30, 2015 Report Share Posted January 30, 2015 I'm not up on modern passenger aircraft avionics and instrumentation. However, I find it hard to believe that something as important as airspeed isn't cross-checked - analog to digital somewhere in the programming. If this were the case, then iced up pitots might be backed up by GPS or SatNav. Link to post Share on other sites
allardjd 1,853 Posted January 30, 2015 Author Report Share Posted January 30, 2015 The problem with GPS and satnav is that they give groundspeed, not airspeed, and particularly when there's heavy WX lurking about, that is most definitely not the same thing. Airspeed is so critical in the "coffin corner" of high altitude flight that a good deal of precision is called for. Also, there is a fair amount of lag with those methods, since several data points have to be collected and speed derived as a time vs. distance thing. For airspeed, pitots are about the only truly useful game in town, though I expect Doppler flow detectors might be developed at some point as an icing-conditions backup for pitots, even if it meant pumping a slight flow of a glycol solution to provide a medium to measure. I don't think Doppler flowmeters can see gas (air). John Link to post Share on other sites
allardjd 1,853 Posted January 31, 2015 Author Report Share Posted January 31, 2015 There's now a report that the captain was "out of his seat" attempting to pull the circuit breakers on some of the computer components (allegedly the FACs - Flight Augmentation Computers). It seems that the source is specifically alleging that he was NOT attempting to reach the "Reset" buttons on the overhead panel but was trying to pull circuit breakers that were somewhere behind him. The implication is that the crew believed that the automation was interfering with their ability to return to controlled flight.Not sure at this point how solid this information is, but it is interesting. John you can turn off each or all of the flight control computers on the rhs of the overhead panel... @ Nigel - if the guys at PPRUNE are correct, what's on the overhead are the Resets, i.e. equivalent of a re-boot button. To actually turn them off, the circuit breakers must be pulled and those are not on the overhead. John Link to post Share on other sites
Quickmarch 488 Posted January 31, 2015 Report Share Posted January 31, 2015 @JA, I'm probably one of a small community who truly understand GPS. On the boat we use the boatspeed (little propeller under the boat in the waterflow) that gives our speed through the water - aka the pitot. We use the GPS to determine speed over the bottom (GS) and constantly compare the two. This lets us know what the current is doing to us. There are a thousand things that can effect boatspeed, only by comparison can you make informed decisions as to which one is the culprit. Sorry if I gave the impression that I thought using these sattellite based instruments as PRIMARY instruments. I really meant just as a cross-check. Hey, if they both read zero, put the nose down. Link to post Share on other sites
allardjd 1,853 Posted January 31, 2015 Author Report Share Posted January 31, 2015 Hey, if they both read zero, put the nose down. Amen to that, brother. I guess my only real point was that GS via GPS will not be of much use in the coffin corner where the margin between low speed stall and high speed stall gets pretty narrow and the difference between airspeed and ground speed can be large. All the wing knows is airspeed, air density and angle of attack (and ice or movable surfaces changing its shape). It will respond to those things and pretty much ignore anything else. ...(little propeller under the boat in the waterflow) that gives our speed through the water... Is that what they used to call a pit log? John Link to post Share on other sites
Quickmarch 488 Posted February 1, 2015 Report Share Posted February 1, 2015 Absolutely! Short for pitometer log. I may be mis-leading you, again. Modern pit logs are a small paddlewheel mounted in a plug that can be extracted into the hull for cleaning. The mount has a flap valve inside that closes off the opening as the log is pulled. The old (original) ones - you had to be real quick with the plug and always got, at least, a faceful of water. These gadgets feed data directly to the nav system as well as to a cockpit or nav station gauge, some are analog, some are digital. Link to post Share on other sites
Captain Coffee 2,030 Posted February 1, 2015 Report Share Posted February 1, 2015 Absolutely! Short for pitometer log. I may be mis-leading you, again. Modern pit logs are a small paddlewheel mounted in a plug that can be extracted into the hull for cleaning. The mount has a flap valve inside that closes off the opening as the log is pulled. The old (original) ones - you had to be real quick with the plug and always got, at least, a faceful of water. My '81 boat has the "traditional" style Link to post Share on other sites
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