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FMC Windspeed delay on 737?

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FMC Windspeed delay on 737?

Old 16th Jun 2013, 10:33
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FMC Windspeed delay on 737?

Does anyone know how "old" the information is regarding windspeed and direction on the 73? Is it different with the GPS input on the NG?
The reason for asking is that some people use this figure to decide whether a landing will be, for example, outside tailwind limits. But I think there will be a certain amount of delay while the FMC worlks it all out. I just don't know how much delay, and the tech manual doesn't help.
So you could be using the winds at 100 feet, 200 feet or whatever.
Any techies know about this?
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Old 16th Jun 2013, 12:45
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certain amount of delay while the FMC worlks it all out. I just don't know how much delay, and the tech manual doesn't help
The IR processors compute the wind speed and direction,not the FMC.

You use tower reported wind.

Last edited by de facto; 16th Jun 2013 at 12:54.
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Old 16th Jun 2013, 23:16
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You use tower reported wind
That certainly would not be advisable in certain parts of the world (like many airfields in Africa for instance.

The worst case I have experienced was a tower report of calm with wind on the ND indicating 28 kts tailwind at 200 RA - the runway was wet.

Use all available sources to determine the actual wind conditions. If in doubt err on the side of safety, and certainly do not blindly follow tower reports.
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Old 17th Jun 2013, 02:07
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I'd have to agree with Exeng, in certain SE Asian Capitol cities I have found they often give an incorrect wind. It seems they want to keep the runway in use the same because the other end doesn't have an ILS.
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Old 17th Jun 2013, 04:46
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The worst case I have experienced was a tower report of calm with wind on the ND indicating 28 kts tailwind at 200 RA - the runway was wet.
I have never flown in Africa or dodgy airports in a jet with ND so I can't comment however in 99 perc of airports i have operated to (Europe/ASIA/America),the winds reading are accurate enough,we do all need to be ready for the occasional wind change,increase or decrease,that we are trained and paid for.
A wind of 28 kts at 200 RA could well turn into a mere 5-10 kts TW at threshold Height,the tower wind info may give you a clue of a possible windshear at low level or simply a need to keep a look at your thrust setting/speed when entering this wind change.

What was the wind at 50 ft?
Did you land continue the approach to land?

Use all available sources to determine the actual wind conditions. If in doubt err on the side of safety, and certainly do not blindly follow tower reports.
This I agree especially in limited runway however the company limit of 10 or 15TW is based on tower winds..would you go around because at 200 ft RA you have 5 kts for TW more than you aircraft/SOP ? How would you explain such action?because you dont trust the tower info?

I'd have to agree with Exeng, in certain SE Asian Capitol cities I have found they often give an incorrect wind. It seems they want to keep the runway in use the same because the other end doesn't have an ILS.
Some airports wind readings dont seem to be super accurate but again at airport like beijing,winds direction and speed change all the time from 6000 ft down.
You may have 15 kts tail at 100 ft but 2 to 3 kts cross at 30 ft...

Last edited by de facto; 17th Jun 2013 at 05:22.
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Old 17th Jun 2013, 11:30
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would you go around because at 200 ft RA you have 5 kts for TW more than you aircraft/SOP ?
Exactly my point. Some people sitting next to you may at least draw your attention to it, though. And if you didn't go around and had an incident on the runway for whatever reason, how much credence would/should an investigator place on the fact that the windspeed readout at touchdown was out of limits.
Out of interest, next time out have a look at the windspeed readout on the landing roll. The IRS could not sense drift on the runway (hopefully...!), so it's out of date to a certain degree. I just wonder how much.
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Old 17th Jun 2013, 12:04
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how much credence would/should an investigator place on the fact that the windspeed readout at touchdown was out of limits
I would think the investigator would look at the latest ATIS for weather and the VCR for wind info given by the tower.

I would think that landing on a limited runway,the pilots would pay special attention to the TW component if any and the PNF calling out the TW at 50 at the latest.
The PF must know the difference of landing distance between TW and AB setting,and any room for error must be briefed prior.

On a non limited runway,the tower reported winds is the deciding factor in continuing the landing or not along with other factors of stability criteria(sink rate,speed...)

How do pilot of airliners without Wind display decide to continue their landing or to go around?
Technology is great,use it,but dont over do it either.
Follow your SOP,fly on the safe side but also keep some common sense and commercial thinking.
Out of interest, next time out have a look at the windspeed readout on the landing roll. The IRS could not sense drift on the runway (hopefully...!), so it's out of date to a certain degree. I just wonder how much
Honestly,wind speed and direction is not on my scan during take off nor landing,during take off i use tower winds and wind sock,during landing tower winds and FMC read out from the PNF on request.

I dont think the wind speed and direction is displayed during take off roll,oddly I know it is on Airbus.I will have a look tomorrow and let you know.
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Old 17th Jun 2013, 14:18
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Processed and thus delayed winds (Kalman filtering algorithm) at both ends of the radio.

'Spot wind' request from the tower gives best chance of 'live,' unprocessed conditions in my experience.

Rob
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Old 17th Jun 2013, 14:43
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All down to pot luck and experience, essentially. Those of us with any experience of LGW R26 in the days of 'Laker's Revenge' and other 'local wind effects' will know the value of 'spot winds' which are often measured nowhere near the threshold and thus can resemble chocolate teacups in their usefulness - apart from preserving one's little pink bottom in case it all goes wrong.
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Old 17th Jun 2013, 14:48
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The worst case I have experienced was a tower report of calm with wind on the ND indicating 28 kts tailwind at 200 RA - the runway was wet.
Tower winds are measured at 10 meters
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Old 18th Jun 2013, 10:17
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Whose wind is it anyway?

I agree with those who doubt the accuracy of airfield wind reporting kit.

When experiencing the delights of Keflavik over 2 years, I felt best informed by briefing PM to keep Prog page 2 active and to advise me of the ACTUAL X-wind and/or tailwind, if it looked like it was about to exceed or was already in excess of the promulgated limit.

Try watching the aircraft detected wind values at NOC when the wind is in the SE quadrant and you could be in for an unpleasant surprise? The anemometer is in the lee of a copse of soft topped trees and I feel it's readout is seriously attenuated by these, other fields worldwide will have their own errors.

ATC reported winds bear little relation to the conditions found near the GP aerial/threshold and the board of inquiry may query one's SOLE reliance on such sources?

Last edited by BARKINGMAD; 18th Jun 2013 at 10:19.
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Old 18th Jun 2013, 10:32
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The way I did it for decades was to 'distill' all the information available (including the FMC wind arrow, but not staring at it at 50'), add that invaluable 'experience factor' eg I am at Malaga, the wind is 'calm'......................... and it worked.
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Old 18th Jun 2013, 10:42
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Hmm..
The knowledgeable contributions are much appreciated.
Looks like once again it's experience and "nouse" that will stop the technology from trying to bury us.
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Old 18th Jun 2013, 11:17
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I promise you that you will be hung if you have an accident/incident when using the "tower wind", as most ops manuals state, when the FMC shows something completely different !

For a good example read the Lufthansa Warsaw A320 accident report :

The crew did not turn to account the wind display on EFIS and did not consider the discrepancy between these data and the information on the wind given by air traffic services. They neither did take into account that tailwind component displayed on EFIS exceeds the value defined by OAM as acceptable for this aircraft
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Old 19th Jun 2013, 03:13
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My SOPs state the tower winds are to be used for wind 'limits'.
I have not read the Lufthansa details but i doubt one can expect to check the EFIS at 30 ft during a crosswind landing....by the way max cross wind are not limitations,just demonstrated values.You may have company limits depending on crew experience,runway contamination etc but for these the Airline must provide the pilot with the source to be used,ie FMC or wind tower..
I believe the crosswind and turbulence were more than an FO should be given to handle (i believe the fo was PF,could be wrong)may have contributed to a serious drift off,also Lufthansa SOP may be to use the FMC info if above tower wind,in that case they broke an sop but nothing more.
Obviously,breaking an SOP and ending up in an incident,the pilot is going to be in trouble,however we can see that some sops are difficult to follow in this case as pilots monitor the most important info such as speed,attitude ,thrust setting...winds change constantly...especially in strong gusty winds..monitoring instruments winds should not overload the crews or take over other critical scans we pilots were taught to do from the very beginning....
Now it is up to the pilot judgement to continue or go around until the reversers are selected.

Last edited by de facto; 19th Jun 2013 at 03:18.
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Old 19th Jun 2013, 16:00
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Out of interest, next time out have a look at the windspeed readout on the landing roll. The IRS could not sense drift on the runway (hopefully...!), so it's out of date to a certain degree. I just wonder how much.
That degree being that your are now heading your track. The only wind on your ND then is the difference between airspeed and groundspeed. A quick glance gives you a good idea of the exact headwind (tailwind) component.
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Old 19th Jun 2013, 18:05
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sideslip inaccuracy

Reading the B737 MRG. It says that when the airplane is in a sideslip, it follows a different track for a given heading and thus wind reading are inaccurate.

Something to keep in mind.
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Old 20th Jun 2013, 02:33
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My SOPs state the tower winds are to be used for wind 'limits'.

I know of one airline that has this as an SOP. It has even had tea no biscuits for crews which made a G/A due to FMC winds showing a healthy tailwind that they did not like.
I'm now teaching for another airline which has the 'check the FMC at 50' on finals and if it shows out of limits then go-around'.

They can't both be correct. Surely this is such a safety issue, involving insurance/livres etc. that EASA & FAA should make an industry announcement on this. It should not be left to the opinion of the various DFO's. It is a scientific issue that should be decided by the manufacturers and authorities. It should not be left to the crews of those airlines who do not have a policy to do as they wish, nor should there be diametrical points of view/SOP's on something so technical.
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Old 20th Jun 2013, 03:00
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wow..here we go with winds...

The wind data from the tower is from the station (if they have one) which measures to 5m from ground, from wherever the AWOS or MET equipment is located.. Winds aloft are usually taken 2 times a day (unless the winds are too high to launch a balloon.)

There 'may' be an anemometer on the tower.

Winds from the Tower should be 'advisory' at best to use when setting final approach config., but given the nature of the airport measurement, may have little correlation to the runway end you are approaching.

in regards to sideslip, you are correct, the tube may be more exposed or shielded, depending on the direction of the wind, and the location of the tube...

Last edited by FlightPathOBN; 20th Jun 2013 at 03:02.
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Old 20th Jun 2013, 07:17
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What about gusts? Limitations are normally based on steady wind. The FMC if it is truly accurate (which not everyone accepts) may be within limits at 50 feet and out of limits after touchdown in gusty conditions, or the other way around. I am with BOAC that experience and common sense are key here.

If a crew went around on a 4000 metre runway because the FMC briefly showed 11 knots when the tower was reporting 9 I think the company (and most of us here) might reasonably ask what they were thinking.

However I still subscribe firmly to the view that one go-around too many is better than one too few.

Last edited by lederhosen; 20th Jun 2013 at 07:18.
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