FMC Wind Readout Accuracy
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It's been nine years since I retired from 747-400 and I remember the wind readout as quite accurate on our aircraft with GPS updating of the IRU position. Indicated winds on final were a good indication as to wether one should suspect low level wind shear.
What makes this thread interesting for me is that I have built and fly a Van's RV-7A homebuilt aircraft with Grand Rapids Sport EFIS. This $3200 (USD) EFIS has wind readout that compares TAS and Magnetic Heading to GPS groundspeed and track. The remote magnetometer has errors of +3 -2 degrees around the quadrant and there is some pitot static error, both affecting the wind calculations but in general it is almost as accurate as your equipment. The Sport EFIS use's averages and is not reliable for a few seconds after a turn such as a base to final turn.
George in Langley B.C.
What makes this thread interesting for me is that I have built and fly a Van's RV-7A homebuilt aircraft with Grand Rapids Sport EFIS. This $3200 (USD) EFIS has wind readout that compares TAS and Magnetic Heading to GPS groundspeed and track. The remote magnetometer has errors of +3 -2 degrees around the quadrant and there is some pitot static error, both affecting the wind calculations but in general it is almost as accurate as your equipment. The Sport EFIS use's averages and is not reliable for a few seconds after a turn such as a base to final turn.
George in Langley B.C.
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I am a simple pilot that agree with Chris Scott affirmation. On the CRJ with flux valves the unusual was to see same wind vector on both ND, and I have flown many of CRJ200 from the first week of its life. The B757 is pretty accuracy, but it is not unusual to see variations from both ND readouts. Same on A340, especially on long haul flights with long periods from alignment. As Scott has very well stated a little variation on the heading may produce a huge deviation on the wind vector. Anyway, strong winds are more reliable because the wind vector gets more relevance on the triangle of velocities and produces not such huge variations. Graphically if within the triangle of v, we decompounds the obtained wind vector in one for real wind and one for error, as the real wind grows and the error remains the same the resulting vector trends to the bigger vector.
At the end the more important wind is what the simple controller reads from the anemometer that it will be the same that you will encounter on the simple touchdown and the simple deceleration roll despite of what your ND is false showing.
Simple flights to all.
At the end the more important wind is what the simple controller reads from the anemometer that it will be the same that you will encounter on the simple touchdown and the simple deceleration roll despite of what your ND is false showing.
Simple flights to all.
Last edited by ppppilot; 28th Mar 2008 at 11:46.
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I remain a very simple controller and I may be way wrong here but that doesn't usually stop me.....
As I understand it, an IRU maintains a position value for the aircraft. (I own up that I know very little about flux valves beyond what I learned from the film Back to the Future but years ago I did the 'How aeroplanes get around for controllers' course that tried to cover the principles of IRS and the like - but I may not have been listening closely enough.) If this assertion is correct other values used in calculations must be obtained from other sources, ADC for example, or derived by comparison with previous position information. Consequently, it seems to me that the TRK and GS vectors must be a historical value representing the average between the two positions used for the calculation. I would assume that for some purposes a longer period is better, (guessing a little here but...) perhaps for fuel calculations its good to know the progress of the aircraft over a longer period whereas when the FMC is determining where to end a turn a much shorter period is better.
If your aircraft are anything like ATC equipment, much of this is transparent to the operator and it works well most of the time. The comparison in the ATC world that immediately comes to mind is of radar tracking systems which, in simple terms, are designed to continue to display a target for a period even if the radar did not receive a response - the position of that target is based in what the tracking system 'expects' the aircraft to do. As I say, it works well most of the time but some systems will do strange things occasionally and knowing what the system is 'thinking' helps to understand why it happens and, maybe, how significant it is to separating aircraft.
Back to wind velocity on the flight deck - is an instantaneous value really what's outside at that vey moment, and if so, do you get any indications of variations? I can't speak for airports outside the UK but I think the wind report that you will get from the controller during the approach really does accurately reflect what is blowing across the TDZ - there's quite a lot of work involved in verifying that it does.
Although I am trying to persuade other posters - with a degree of subtlety, I hope - that reliance on IR W/V on short finals, in preference to ATC surface W/V reports, is generally NOT A GOOD IDEA; it's their accuracy and relevance that seem to me to be the problem, not their currency.
(1) IR HDG has to be instantaneous; most modern airliners do not possess flux valves, their only magnetic heading sensor being the ubiquitous E2B standby compass (or derivative of). IR HDG(T) is converted to HDG(M) by applying the local Variation. [The IRU knows where it is, and has a world map of Isogonals.]
(2) ADC TAS, being IAS-derived, is instantaneous AND not of declining accuracy.
(3) IR TRK is instantaneous for all practical purposes. How would the FMC know when to roll out of a turn otherwise?
(4) IR GS is also instantaneous; the platform constantly knowing the number and magnitude of accelerations it has experienced since last alignement.
(1) IR HDG has to be instantaneous; most modern airliners do not possess flux valves, their only magnetic heading sensor being the ubiquitous E2B standby compass (or derivative of). IR HDG(T) is converted to HDG(M) by applying the local Variation. [The IRU knows where it is, and has a world map of Isogonals.]
(2) ADC TAS, being IAS-derived, is instantaneous AND not of declining accuracy.
(3) IR TRK is instantaneous for all practical purposes. How would the FMC know when to roll out of a turn otherwise?
(4) IR GS is also instantaneous; the platform constantly knowing the number and magnitude of accelerations it has experienced since last alignement.
If your aircraft are anything like ATC equipment, much of this is transparent to the operator and it works well most of the time. The comparison in the ATC world that immediately comes to mind is of radar tracking systems which, in simple terms, are designed to continue to display a target for a period even if the radar did not receive a response - the position of that target is based in what the tracking system 'expects' the aircraft to do. As I say, it works well most of the time but some systems will do strange things occasionally and knowing what the system is 'thinking' helps to understand why it happens and, maybe, how significant it is to separating aircraft.
Back to wind velocity on the flight deck - is an instantaneous value really what's outside at that vey moment, and if so, do you get any indications of variations? I can't speak for airports outside the UK but I think the wind report that you will get from the controller during the approach really does accurately reflect what is blowing across the TDZ - there's quite a lot of work involved in verifying that it does.
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I have realized I am not that simple. I have my wife all the time at my back thinking I am writing to a lover in pprune. Therefore, I am me and my circumstances (=wife + children). That makes me complicated.
So, from a complicated point of view. IRS is composed on Gyros and accelerometers in order to feel the 3D movement and translate it to numbers. Then the first numbers they have are movement=Track and GS. From that and the initial position they obtain your change in position and estimate your new position until their wives (GPS+RNAV) tell them again where they really are because they were wrong again.
Not instant but a lot faster than me thinking.
Tailwinds
So, from a complicated point of view. IRS is composed on Gyros and accelerometers in order to feel the 3D movement and translate it to numbers. Then the first numbers they have are movement=Track and GS. From that and the initial position they obtain your change in position and estimate your new position until their wives (GPS+RNAV) tell them again where they really are because they were wrong again.
Not instant but a lot faster than me thinking.
Tailwinds
Last edited by ppppilot; 28th Mar 2008 at 17:04. Reason: language ignorant
le Canard
Quote from Spitoon:
As I understand it, an IRU maintains a position value for the aircraft.
[Unquote]
I think you are putting the cart before the horse. The inertial platform is initialised while the vehicle (aircraft) is stationary. [Admittedly, it is aware immediately of the local vector (i.e., direction and speed) of the earth's rotation, and uses it to calculate true heading in relation to the poles. But I don't think that has any bearing on my following argument.] Like you, my limited knowledge does not stop me from arguing a case, and any INS references are not to hand - nor easily accessible on this dial-up connection, so I stand to be corrected.
From the moment the aircraft starts to move in relation to the earth, each and every acceleration VECTOR is sensed, and used to update TRK and GS. The IRU computer runs a dead-reckoning "plot" of these multiple tracks and distances. This enables it to calculate the curent position at any moment ("PPOS").
TRK and GS inevitably degrades with time. PPOS degrades as a function of all the miniscule - but rising - errors in TRK and GS.
As I understand it, an IRU maintains a position value for the aircraft.
[Unquote]
I think you are putting the cart before the horse. The inertial platform is initialised while the vehicle (aircraft) is stationary. [Admittedly, it is aware immediately of the local vector (i.e., direction and speed) of the earth's rotation, and uses it to calculate true heading in relation to the poles. But I don't think that has any bearing on my following argument.] Like you, my limited knowledge does not stop me from arguing a case, and any INS references are not to hand - nor easily accessible on this dial-up connection, so I stand to be corrected.
From the moment the aircraft starts to move in relation to the earth, each and every acceleration VECTOR is sensed, and used to update TRK and GS. The IRU computer runs a dead-reckoning "plot" of these multiple tracks and distances. This enables it to calculate the curent position at any moment ("PPOS").
TRK and GS inevitably degrades with time. PPOS degrades as a function of all the miniscule - but rising - errors in TRK and GS.
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Fwiw, Bill Bulfer's excellent B737 FMC User’s Guide (section 'Progress 3/3') it says the following:
Bulfer's Big Boeing FMC User's Guide (which covers the B747, B757, B767, B777) seemingly does not include the same information (least not that I could find it?!) but there's probably a lot of similarities (remembering that the B737 unit is made by Smiths and those of the big Boeing's by Sperry; now Honeywell) between how these units work and ditto wrt their limitations. 
IRS and FMC CDU winds are relative to True north.
The following assumptions are made:
The following assumptions are made:
- The aircraft is flying straight and level with a small angle of attack.
- Sideslip is assumed to be zero.
- Vertical direction is assumed to be level - this can be the source of substantial errors when the aircraft is not in level flight.
- IRS winds will be in error by the velocity error of the IRU. FMC winds, which are derived from radio updating, will be in error as nav radios are tuned to different frequencies due to slightly different noise characteristics and calibration for the navaids.
- If an IRS wind readout (on the ISDU - optional equipment) reads significantly different than the FMC winds, check the IRS positions on POS REF page. This may be the first indication of failure (IRS system drift or FMC failure).
One of the best examples I have seen to illustrate the difference is manged speed on final approach in a mark1 A320.The irs were giving different wind readouts and there was a difference of 10 knots on the managed speed bug between Capt and FO ASIs!
