FMC Wind Readout Accuracy
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FMC Wind Readout Accuracy
Following on from another thread can anyone comment on the accuracy of FMC wind readouts particularly during final approach? Does IRS drift rate affect the accuracy? How is the FMC wind derived?
If possible any technical references would be appreciated.
Thanks for any help.
If possible any technical references would be appreciated.
Thanks for any help.
Phew...
No tech refs available in my present location, Bob, but am fairly confident that the IRS wind deteriorates in accuracy as a function of how long it is since you aligned the IRUs. [For the uninitiated, this is nothing to do with position updating; the only time you can re-align an IRS is when the aircraft is stationary.]
This can be illustrated by the growing disparity between the instant W/Vs provided by the different FMCs, which typically use differing IRUs for W/V.
IRS PPOS error is proportional, roughly speaking, to time, and - in pre-laser INS days, used to be recorded in the Tech Log for each INS on arrival at destination (retrofitted B707s, et al). It can be expressed in knots.
For what it's worth, I'd never base a decision to land or G/A on the basis of an IRS W/V readout at 100ft or 50 ft - unless, of course, the decision was to go-around. In addition to the above, there can be considerable shear between 100ft and the surface. Airfields with local W/V effects near a Rwy threshold should have nearby anemometers, although in the real world...
Thanks for the opportunity to avoid contributing to a messy discussion involving speculative justice on Ryanair pilots.
This can be illustrated by the growing disparity between the instant W/Vs provided by the different FMCs, which typically use differing IRUs for W/V.
IRS PPOS error is proportional, roughly speaking, to time, and - in pre-laser INS days, used to be recorded in the Tech Log for each INS on arrival at destination (retrofitted B707s, et al). It can be expressed in knots.
For what it's worth, I'd never base a decision to land or G/A on the basis of an IRS W/V readout at 100ft or 50 ft - unless, of course, the decision was to go-around. In addition to the above, there can be considerable shear between 100ft and the surface. Airfields with local W/V effects near a Rwy threshold should have nearby anemometers, although in the real world...
Thanks for the opportunity to avoid contributing to a messy discussion involving speculative justice on Ryanair pilots.
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Stand alone GPS units (Honeywell HT9100, for example) can provide quite accurate winds aloft data, however, as has been mentioned, to use that data to make a decision about whether to land (or not) would be quite foolish....I personally have noticed wind data to show a tailwind of 100 knots (approximately) at 200 feet, when in actual fact, there was no such wind....otherwise we would have been truly in the weeds at the far end (not a happy thought)...
It is good information, but just that more info not a decision-making tool. However, my experience is that more often than not, pretty good information. In the C-5, the jumpseater would watch the winds on final and call-out the drift , which made hand-flown ILSs easier. On a PAR, it could be real handy for the controller, esp in Germany with a front approaching at the end of 24-hour duty day. 
My experience with the B737 has convinced me that the FMC's in that aircraft indicate the wind accurately and with no delay.
The FMC simply calculates the wind by comparing the two vectors Heading/TAS and Track/GS. Now the assumption here is that there is no sideslip!
I have experimented in the simulator where I ask the sim instructor to make the wind calm while I fly straight and level. I then introduce rudder while keeping the wings level with opposite aileron. This results in a sideslip which means the heading is now difference to the track (remembering that the wind is calm).
It is amazing just how much the rudder can be displaced and still be able to maintain a constant heading with opposite aileron (and a few degrees of roll).
Anyway if you have the FMC wind displayed on the CDU, you will notice that the moment the rudder displacement is introduced, the FMC wind immediately changes to reflect the difference between heading and track. There is NO delay in this change! (And no I do NOT believe this is a simulator flaw. As far as I know, the FMC in the simulator is the same as those found in the real aircraft, and is simply fed information by the simulator as it would be by the real aircraft. In other words, it operates just as those found in the aircraft.)
If there is any delay in the display of the actual wind, it would simply be due to the aircraft's inertia, nothing more.
By the way, have you ever noticed when you make an approach to land with an engine inoperative, that there is always around 6 kt of crosswind? I'm certain that the instructor has set the wind to calm and that this apparent wind is due to the asymmetrical sideslip.
The FMC simply calculates the wind by comparing the two vectors Heading/TAS and Track/GS. Now the assumption here is that there is no sideslip!
I have experimented in the simulator where I ask the sim instructor to make the wind calm while I fly straight and level. I then introduce rudder while keeping the wings level with opposite aileron. This results in a sideslip which means the heading is now difference to the track (remembering that the wind is calm).
It is amazing just how much the rudder can be displaced and still be able to maintain a constant heading with opposite aileron (and a few degrees of roll).
Anyway if you have the FMC wind displayed on the CDU, you will notice that the moment the rudder displacement is introduced, the FMC wind immediately changes to reflect the difference between heading and track. There is NO delay in this change! (And no I do NOT believe this is a simulator flaw. As far as I know, the FMC in the simulator is the same as those found in the real aircraft, and is simply fed information by the simulator as it would be by the real aircraft. In other words, it operates just as those found in the aircraft.)
If there is any delay in the display of the actual wind, it would simply be due to the aircraft's inertia, nothing more.
By the way, have you ever noticed when you make an approach to land with an engine inoperative, that there is always around 6 kt of crosswind? I'm certain that the instructor has set the wind to calm and that this apparent wind is due to the asymmetrical sideslip.
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Glad someone created this post. I fly a CRJ2 and have noticed many times that when on final approach the wind seems have nothing to do with what the tower reported. I have sometimes been expecting serious windshear or turbulence when looking at my wind vector and comparing it to the tower report, only to find a smooth ride all the way to the ground. It can vary from 30 to 180 degrees from the reported, with wind speeds often similar to the tower reports. I always thought it was just because the FMC needed at least a few unaccelerated seconds to spit out the correct wind. All other times the wind vector seems accurate.
rcl
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rcl7700,
The tower reports surface winds. Not the winds aloft during your approach. Yes there will be a difference as the surface winds have well the surface to change the winds direction and velocity.
I fly the CRJ as well and find it to be highly accurate.
The tower reports surface winds. Not the winds aloft during your approach. Yes there will be a difference as the surface winds have well the surface to change the winds direction and velocity.
I fly the CRJ as well and find it to be highly accurate.
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Yes the tower calls out surface winds. With differences in wind direction, sometimes 90-120 degrees, and a smooth ride from 1,500 ft agl all the way to touchdown, you sometimes wonder about the accuracy.
We have airports that due to terrain are not allowed to use certain runways at night. This sometimes leaves us having to land with tailwinds. As PNF I have had the time to keep an eye on the vector. I have seen predominantly headwinds until close to the runway, when it disappears, only to see the windsock showing indeed a clear tailwind, and all this with a stable smooth ride all the way. My experience is that on approach it doesn't always give accurate info, not that it really matters at all in that phase of flight. Again this doesn't happen always, but it has surprised me how far off it can be sometimes, hence the comment.
rcl
We have airports that due to terrain are not allowed to use certain runways at night. This sometimes leaves us having to land with tailwinds. As PNF I have had the time to keep an eye on the vector. I have seen predominantly headwinds until close to the runway, when it disappears, only to see the windsock showing indeed a clear tailwind, and all this with a stable smooth ride all the way. My experience is that on approach it doesn't always give accurate info, not that it really matters at all in that phase of flight. Again this doesn't happen always, but it has surprised me how far off it can be sometimes, hence the comment.
rcl
Thread Starter
Thanks for all the interesting replies.
Most are based on experience but there doesn't seem to be any hard technical information on FMC accuracy but I thought the posting concerning sideslip was interesting - will take a look next time I have an engine out!
Most are based on experience but there doesn't seem to be any hard technical information on FMC accuracy but I thought the posting concerning sideslip was interesting - will take a look next time I have an engine out!
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When I flew the CRJ200 there was usually a problem with the heading reference. The heading for the cm1 was obtained from the right flux valve located near the end of the right wing. The cm2 was the opposite flux valve. There was always a small difference between both hdgs. A very curious fact was, every day, when taxiing near the same building, one of the hdgs deviated more than 8 degrees to the other, with the correspondent caution msg. It was corrected by itself as the plane were far away from the building, and by the TO time the difference was good enough to continuation of the flight. The wind shown on each display after the TO was very different, becoming the same as the difference on the hdg disappeared. If you want to see that very clear on the CRJ, try to vary during the fly, the hdg of the PNF with the small lever at the center pedestal for the gyro correction,. You will see that a pair degrees variation in hdg produces a huge variation in the calculated wind. Moving the gyrocompass further away, you can obtain amazing winds on the ND.
As far as I know, airplanes not having ADIRUs obtains wind from a continuous computation HDG vs TRK and TAS vs GS.
I believe the ADIRUs calculates the wind from track, drift and GS obtained with 3D gyros and accelerometers and the TAS obtained by the ADC, but this could not be exact.
Both systems are very accurate, except during ground operations. In that cases drift is forced to be 0, so the only real data is the head/tail wind component which is the same as the TAS/GS difference.
Tailwinds
As far as I know, airplanes not having ADIRUs obtains wind from a continuous computation HDG vs TRK and TAS vs GS.
I believe the ADIRUs calculates the wind from track, drift and GS obtained with 3D gyros and accelerometers and the TAS obtained by the ADC, but this could not be exact.
Both systems are very accurate, except during ground operations. In that cases drift is forced to be 0, so the only real data is the head/tail wind component which is the same as the TAS/GS difference.
Tailwinds
Instant, but accuracy declining with time
Blip's example is spot-on. As he says, the only way of finding the W/V you are experiencing in flight is to complete the triangle of velocities; the first vector being the HDG/TAS, and the second the TRK/GS. IRS supplies instantaneous values of W/V.
If the aircraft is not travelling precisely forward in relation to the air-mass, however, its heading no longer represents the first vector, and an artificial wind vector corrupts the reading.
So, in the absence of sideslip, what about the accuracy of an IRS W/V? No doubt it appears to be impeccable in the simulator, which is going nowhere (literally). It is unlikely that its psuedo-IRUs are programmed to simulate the "drift" of position-accuracy that real IRUs demonstrate on a long flight.
The accuracy of the W/V rests on those 4 parameters. Remember that, on most current aircraft, the only one that is not IRS-dependent is the TAS, which is calculated by the Air-Data Computer (or the AD part of an ADIRU). The accuracy of this TAS does not deteriorate as the flight progresses. The accuracy of the IRS HDG (Heading) is, I believe, also maintained. [I'm not going to show my ignorance by trying to explain why...] The calculations of TRK (Track) and GS Ground-speed), however, are affected by the decaying accuracy of IR Position - its gradual loss of spacial orientation, if you like. Unlike a GPS or Radio (DME/DME) Position, an IR Position is calculated by old-fashioned dead-reckoning; not position-fixing. IR Position cannot be updated while the vehicle is in motion. [All that an FMC can do, in the absence of GPS or Radio positioning, is to use the last-known IR Position error to apply a corrective bias to the IR Position.] As the position accuracy deteriorates, so do the Track and GS.
In summary; the IRS W/V is instantaneous, but its accuracy declines with the passage of time since last IRS alignement. My guess is that short-haul aircraft on ever-shorter turnrounds may not always be re-aligning their IRUs. This could lead to an error of long-haul magnitude at the end of a day's flying.
Bob,
In your privileged position as current flight crew,
perhaps you or some other contributor to this thread would be so kind as to test the assertion that I made (in my earlier post) re the disparity between the W/Vs displayed by the different IRSs, please?
For example, if FMC2 uses IRU2 to display W/V normally on the captain's ND (HSI), and FMC1 uses IRU1 to display W/V normally on copilot's ND (HSI); my understanding and recollection is that the disparity between the two W/Vs increases with time, as their positions drift apart.
If the aircraft is not travelling precisely forward in relation to the air-mass, however, its heading no longer represents the first vector, and an artificial wind vector corrupts the reading.
So, in the absence of sideslip, what about the accuracy of an IRS W/V? No doubt it appears to be impeccable in the simulator, which is going nowhere (literally). It is unlikely that its psuedo-IRUs are programmed to simulate the "drift" of position-accuracy that real IRUs demonstrate on a long flight.
The accuracy of the W/V rests on those 4 parameters. Remember that, on most current aircraft, the only one that is not IRS-dependent is the TAS, which is calculated by the Air-Data Computer (or the AD part of an ADIRU). The accuracy of this TAS does not deteriorate as the flight progresses. The accuracy of the IRS HDG (Heading) is, I believe, also maintained. [I'm not going to show my ignorance by trying to explain why...] The calculations of TRK (Track) and GS Ground-speed), however, are affected by the decaying accuracy of IR Position - its gradual loss of spacial orientation, if you like. Unlike a GPS or Radio (DME/DME) Position, an IR Position is calculated by old-fashioned dead-reckoning; not position-fixing. IR Position cannot be updated while the vehicle is in motion. [All that an FMC can do, in the absence of GPS or Radio positioning, is to use the last-known IR Position error to apply a corrective bias to the IR Position.] As the position accuracy deteriorates, so do the Track and GS.
In summary; the IRS W/V is instantaneous, but its accuracy declines with the passage of time since last IRS alignement. My guess is that short-haul aircraft on ever-shorter turnrounds may not always be re-aligning their IRUs. This could lead to an error of long-haul magnitude at the end of a day's flying.
Bob,
In your privileged position as current flight crew,
perhaps you or some other contributor to this thread would be so kind as to test the assertion that I made (in my earlier post) re the disparity between the W/Vs displayed by the different IRSs, please?For example, if FMC2 uses IRU2 to display W/V normally on the captain's ND (HSI), and FMC1 uses IRU1 to display W/V normally on copilot's ND (HSI); my understanding and recollection is that the disparity between the two W/Vs increases with time, as their positions drift apart.
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http://en.wikipedia.org/wiki/Kalman_filter
...and I wouldn't take note of Blip's simulator experiments - the simulator does not synthesise 'real' FMC function.
...and I wouldn't take note of Blip's simulator experiments - the simulator does not synthesise 'real' FMC function.
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I am dubious of Chris Scott's assertion that the W/V calculations in a pair of uncorrected Inertial systems would diverge. Although the absolute positions diverge, they do so only because they have integrated tiny differences in acceleration vectors. Thus if one inertial platform believes I am heading 342 degrees at 423 knots, and the other thinks I am heading 343 degrees at 422 knots, then after an hour or so the two positions will differ by a significant amount - but the instant wind vector calculation should still be within a degree and a knot between the two boxes.
Hi CJ Driver,
Your examples are realistic, but I would presume that − when you say “HDG” − you mean “TRK” (track)? If a given IRU made the same error in heading and track (e.g., +1 degree) and the errors in TAS and GS were also the same (+1 knot, say); then the difference in the 2 W/Vs would be negligible: 1 degree and 0 knots.
Unfortunately, it’s not that simple. Forgive me for going right back to the basics... There is no simple relationship, as far as I know, between IR HDG error and IR TRK error; although the latter is likely to be greater. The errors in TAS and GS are entirely unrelated, as the former can only be calculated from the current indicated airspeed (IAS), density altitude, and other factors; whereas the latter is calculated, as you know, by the IRU itself.
In the absence of a suitable protractor, I’m going to have to choose a very simple case. Developing your example, can I propose that the two IR headings are equal, but that the IR tracks differ by the 1 degree you have stated? Also, can I propose that the TASs calculated by the two ADCs are equal; and that (unlike your example) the IR GSs are also equal? And that the GSs and TASs are the same?
Continuing, let the (common) HDG/TAS vector be 342°/422kt
Let the IR1 TRK/GS vector be 342°/422kt
and the IR2 TRK/GS vector be 343°/422kt
IR1 wind vector is NIL
IR2 wind vector is 072°/007
So IR1 W/V (as displayed) is CALM
and IR2 W/V (as displayed) is 252°/007
Your examples are realistic, but I would presume that − when you say “HDG” − you mean “TRK” (track)? If a given IRU made the same error in heading and track (e.g., +1 degree) and the errors in TAS and GS were also the same (+1 knot, say); then the difference in the 2 W/Vs would be negligible: 1 degree and 0 knots.
Unfortunately, it’s not that simple. Forgive me for going right back to the basics... There is no simple relationship, as far as I know, between IR HDG error and IR TRK error; although the latter is likely to be greater. The errors in TAS and GS are entirely unrelated, as the former can only be calculated from the current indicated airspeed (IAS), density altitude, and other factors; whereas the latter is calculated, as you know, by the IRU itself.
In the absence of a suitable protractor, I’m going to have to choose a very simple case. Developing your example, can I propose that the two IR headings are equal, but that the IR tracks differ by the 1 degree you have stated? Also, can I propose that the TASs calculated by the two ADCs are equal; and that (unlike your example) the IR GSs are also equal? And that the GSs and TASs are the same?
Continuing, let the (common) HDG/TAS vector be 342°/422kt
Let the IR1 TRK/GS vector be 342°/422kt
and the IR2 TRK/GS vector be 343°/422kt
IR1 wind vector is NIL
IR2 wind vector is 072°/007
So IR1 W/V (as displayed) is CALM
and IR2 W/V (as displayed) is 252°/007
Last edited by Chris Scott; 27th Mar 2008 at 11:06. Reason: Recognition that wind direction is conventionally expressed as a “FROM” vector, not a “TO” vector.
Zorst.
Please describe to us what you beleive we would observe with regards to wind indications in the real aircraft if we were to fly straight and level in actual nil wind conditions with crossed controls. Please also explain the reasons why.
What about the IRS wind data on the overhead panel? Would it too be misrepresented in the simulator?
What other aspects of the FMC operations does the simulator not represent accurately?
Thank you.
Please describe to us what you beleive we would observe with regards to wind indications in the real aircraft if we were to fly straight and level in actual nil wind conditions with crossed controls. Please also explain the reasons why.
What about the IRS wind data on the overhead panel? Would it too be misrepresented in the simulator?
What other aspects of the FMC operations does the simulator not represent accurately?
Thank you.
Last edited by Blip; 27th Mar 2008 at 03:40.
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Originally Posted by Blip
My experience with the B737 has convinced me that the FMC's in that aircraft indicate the wind accurately and with no delay.
The FMC simply calculates the wind by comparing the two vectors Heading/TAS and Track/GS.
The FMC simply calculates the wind by comparing the two vectors Heading/TAS and Track/GS.
As has been pointed out, comparing the aircraft calculated WV with a surface wind will almost inevitably produce differences, particularly if the aircraft has been descending toward the runway during the period considered for the calculation. Bear in mind, also, that many surface wind reports are generated by equipment that averages the surface wind measurements (usually over 2 minute period). Even an 'instant' wind will be averaged over a short period, commonly 5 seconds.
Spitoon, that has to be the most disingenuous remark ("a simple controller")... 
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.
Items (3) and (4) are, I believe, the weakest links. They are instantaneous, but their accuracy declines as a function of time since last IRU alignement. The latter can only be performed when the aircraft is stationary.
The relevance of an IR W/V reading at 100 ft agl on a night approach, accurate or otherwise, is another matter...
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.
Items (3) and (4) are, I believe, the weakest links. They are instantaneous, but their accuracy declines as a function of time since last IRU alignement. The latter can only be performed when the aircraft is stationary.
The relevance of an IR W/V reading at 100 ft agl on a night approach, accurate or otherwise, is another matter...
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Chris Scott
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Spitoon, that has to be the most disingenuous remark ("a simple controller")...
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.
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I have more confidence in 757/767 IRS generated wind data than field reported winds(if there's a disagreement). If a runway approach end windsock indicates a different wind than is reported I can't remember the IRS generated wind being in disagreement with the windsock. It's fairly simple to glance at the wind display arrow just prior to flare/touchdown.
Reported winds are the legal answer, but all available data should be used.
A300 wind display on Nav display was worthless. Wind velocity/direction on FMC page was much better.
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Spitoon, that has to be the most disingenuous remark ("a simple controller")...
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.
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I have more confidence in 757/767 IRS generated wind data than field reported winds(if there's a disagreement). If a runway approach end windsock indicates a different wind than is reported I can't remember the IRS generated wind being in disagreement with the windsock. It's fairly simple to glance at the wind display arrow just prior to flare/touchdown.
Reported winds are the legal answer, but all available data should be used.
A300 wind display on Nav display was worthless. Wind velocity/direction on FMC page was much better.