Performance Data at Indicated Airspeed
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Join Date: Apr 2002
Location: UK
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Dear Idlestop and Shawn Coyle.
And to everyone also.
The answer to the 'problem' is that there was/is a blockage in the Static system at the point before the static pipework splits to the Altimeter and ASI. Changing to the alternative source had no effect. This provides the error which manifests itself as an incorrect indicated airspeed and lag on thr altimeter due to the incorrect static pressure delivered. Hence the over-reading of airspeed during descent and under-reading during the climb. This isn't too greater a problem when flying VFR but the dangers are there if flying on instruments. However it would be most helpful if PEC correction data were available in cercumstances other than S&L. During my time flying Gazelles IFR and on approaches there was the Aircraft Allowance to put into the computation for IFR arrivals to make allowances for altimeter lag and pitot position errors at that angle of approach. It is always worth checking the relationship of data given by the ASI and that by the GPS. There should be the W/V in there somewhere.
Unless somebody knows differently.
And to everyone also.
The answer to the 'problem' is that there was/is a blockage in the Static system at the point before the static pipework splits to the Altimeter and ASI. Changing to the alternative source had no effect. This provides the error which manifests itself as an incorrect indicated airspeed and lag on thr altimeter due to the incorrect static pressure delivered. Hence the over-reading of airspeed during descent and under-reading during the climb. This isn't too greater a problem when flying VFR but the dangers are there if flying on instruments. However it would be most helpful if PEC correction data were available in cercumstances other than S&L. During my time flying Gazelles IFR and on approaches there was the Aircraft Allowance to put into the computation for IFR arrivals to make allowances for altimeter lag and pitot position errors at that angle of approach. It is always worth checking the relationship of data given by the ASI and that by the GPS. There should be the W/V in there somewhere.
Unless somebody knows differently.
Join Date: Dec 2001
Location: Philadelphia PA
Age: 74
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Glad to be of assistance in troubleshooting the problem.
You are right about the lack of information on pitot static errors. The FARs take care of it, sort of, by wording that requires minimal errors, unlike the FW which requires even the minimal errors to be put in a graph in the Flight Manual.
By the way, only the UK military uses Aircraft Allowance - everyone else works on the assumption that the manufacturer had better make a pitot static system that has such small errors that you can take the aircraft to the published Decision Height without worrying about hitting the ground.
But that could lead to a whole discussion about QFE and other things...
You are right about the lack of information on pitot static errors. The FARs take care of it, sort of, by wording that requires minimal errors, unlike the FW which requires even the minimal errors to be put in a graph in the Flight Manual.
By the way, only the UK military uses Aircraft Allowance - everyone else works on the assumption that the manufacturer had better make a pitot static system that has such small errors that you can take the aircraft to the published Decision Height without worrying about hitting the ground.
But that could lead to a whole discussion about QFE and other things...
Join Date: Dec 2001
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Genghis
QFE is something that only works well in the UK - no airfields higher than 400' AMSL.
It wouldn't work nearly anywhere else - Leadville Colorado is 9,975' AMSL - so you'd be at an altimeter setting of somewhere around 20" of mercury if my math is right. They don't make altimeters with that much adjustment. And even then it's only good for the immediate airport vicinity.
In the long run, we'll be using GPS for altitude - it's already as accurate as any baro system, requires no transmission or read back of altimeter settings, introduces no significant errors after the spheroid is corrected, etc.
And when we look back on using barometric pressure for height measurement we'll be saying - who ever dreamed that up?
QFE is something that only works well in the UK - no airfields higher than 400' AMSL.
It wouldn't work nearly anywhere else - Leadville Colorado is 9,975' AMSL - so you'd be at an altimeter setting of somewhere around 20" of mercury if my math is right. They don't make altimeters with that much adjustment. And even then it's only good for the immediate airport vicinity.
In the long run, we'll be using GPS for altitude - it's already as accurate as any baro system, requires no transmission or read back of altimeter settings, introduces no significant errors after the spheroid is corrected, etc.
And when we look back on using barometric pressure for height measurement we'll be saying - who ever dreamed that up?
To answer Shawn, and off thread, it was surely the engineering student given an aneroid barometer and asked to measure the height of a tower block. Came up with about six solutions, bar (no pun intended) the obvious!
I once spent 2 weeks on an aircraft carrier. After a week of morning met-briefs, I finally worked out by the Royal Navy didn't give QFE. I suppose I was young and innocent then.
I'm perfectly acquainted with the QFE/QNH debate, it's one of those old chestnuts that'll never go away - but causes mild amusement with visiting North Americans. But concerning barometric pressure - what else could you use? Radalt is too expensive for simple aircraft and doesn't work at altitude or whilst manoeuvring, temperature is too coarse and varies too much at S/L. Just as Churchill said about democracy, pressure altitude is perfectly awful, but better than all the alternatives.
I suppose now we have the geometric altitude given to us by GPS, but even that we'd have to be careful about since the world just aint that round, and the update rate on GPS isn't really up to aircraft manoeuvring anyway.
G
(N.B. Idle stop, I think the correct answer is go to the Janitor and say "if I give you this nice barometer, will you tell me how tall the building is").
I'm perfectly acquainted with the QFE/QNH debate, it's one of those old chestnuts that'll never go away - but causes mild amusement with visiting North Americans. But concerning barometric pressure - what else could you use? Radalt is too expensive for simple aircraft and doesn't work at altitude or whilst manoeuvring, temperature is too coarse and varies too much at S/L. Just as Churchill said about democracy, pressure altitude is perfectly awful, but better than all the alternatives.
I suppose now we have the geometric altitude given to us by GPS, but even that we'd have to be careful about since the world just aint that round, and the update rate on GPS isn't really up to aircraft manoeuvring anyway.
G
(N.B. Idle stop, I think the correct answer is go to the Janitor and say "if I give you this nice barometer, will you tell me how tall the building is").
