FMC Wind Readout Accuracy

[fireflybob]

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.

[hetfield]

Concerning time lag, on A340 it's > 15 s !!!

(wind on nav display)

According AI

[Chris Scott]

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.

[411A]

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)...:yuk:

[galaxy flyer]

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. :cool:

[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. 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.

[misd-agin]

A300 wind display = worthless. Wrong direction, obviously inaccurate velocity.

B757/767 wind display - seems to be highly accurate.

[rcl7700]

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

[Walker Texas Ranger]

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.

[rcl7700]

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

[fireflybob]

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!

[ppppilot]

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

[Chris Scott]

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, :E 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.

[Zorst]

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.

[CJ Driver]

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.

[Chris Scott]

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

[Blip]

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.

[Spitoon]

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.
I am not a professional pilot - but a simple controller. It seems to me, though, that your assertion is slightly flawed with respect to the 'with no delay' bit. I fully agree that the wind velocity can be determined by comparing the HDG/TAS and TRK/GS vectors but these vectors are historical and the resulting WV will reflect the average over which the vectors are determined thus introducing some hysteresis into the measuring system.

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.

[Chris Scott]

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...

[misd-agin]

Chris Scott (http://www.pprune.org/forums/member.php?u=218710)

Join Date: Jan 2008
Location: England (North Downs)
Age: 61
Posts: 180


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.

-----------------------------------------------------------------------

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.

[gmcnutt]

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.

[ppppilot]

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.:}

[Spitoon]

I remain a very simple controller and I may be way wrong here but that doesn't usually stop me.....
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.
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.

[ppppilot]

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

[Chris Scott]

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.

[Old King Coal]

Fwiw, Bill Bulfer's excellent B737 FMC User’s Guide (http://fmcguide.com/Detail.bok?no=1) (section 'Progress 3/3') it says the following:

IRS and FMC CDU winds are relative to True north.

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).

Bulfer's Big Boeing FMC User's Guide (http://fmcguide.com/Detail.bok?no=12) (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. :uhoh:

[tubby linton]

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!