Wind Correction

[Yaqatari]

Greetings everyone

Why manufacturers determine wind correction speed in approach based on headwinds and gust only and not take into account the X-wind component?

It would be great if someone can provide me with a good reference.

Thank you

[john_tullamarine]

Probably because

(a) the prime concern relates to maintaining energy for the approach .. ie to cover the consideration of an airspeed reduction. One needs to note that the maximum energy is a concern for runway overrun also so there has to be both upper and lower speed limits in the energy management plan

(b) crosswind doesn't have a great effect on the approach energy .. rather it is a challenge to handling management

I presume that the various heavy OEMs will have internal papers relating to their strategies .. can't say that I have ever seen anything definitive. There may well be an article in one of the Boeing Aero magazines.

[ImbracableCrunk]

You do take in to account the gust factor, be it headwind or crosswind, and you do have crosswind limitations. (Boeing 737)

[oceancrosser]

Yes but you base wind corrections on headwind component. Boeing in it´s FCTM says that gusts mainly increase pilot workload, but have otherwise limited effect.

[ImbracableCrunk]

And all of the gust.

"The recommended method for approach speed correction is to add one half of the reported steady headwind component plus the full gust increment . . ." (B737NG FCTM)

If you have a RWY36 and wind 090/15G25, your wind additive is Vref+10 (0 HW + 10 gust)

[Mad (Flt) Scientist]

Could the people citing adding half the headwind perhaps identify the types these see this on. I don't believe every OEM does this.

[Yaqatari]

john_tullamarine (http://www.pprune.org/members/31647-john_tullamarine) thank you for your reply.

I believe crosswind will affect approach energy. For example if I only have a direct crosswind during approach although I am approaching at Vref ( Vref + 5 for B737 ) I actually approach with a lower ground speed.

Also if I just approach at Vref whine having a headwind I will touch down with a lower energy ( or ground speed ) which is better for stopping.

I know that there is a concern that the wind will die near touch down and thus this additive was put to cover that in case it happens and there was no time counter act with throttle but I don't see why the cross wind is not taken into account if this was the reason for the wind correction. My only explanation is that you can gain that speed lost from a dying crosswind by reducing the crab angle ( which you will do anyway to keep the center line )which can be done quickly but I am not sure.

again thank you for explanation.

[Yaqatari]

ImbracableCrunk (http://www.pprune.org/members/156810-imbracablecrunk)

the gust exceeding the relative wind is always added because the gust can't be trusted :)
it is by nature and definition with no direction ( all over the place )

I hope this answers you.

[Yaqatari]

Mad (Flt) Scientist (http://www.pprune.org/members/58150-mad-flt-scientist)

not sure

I think the Airbus for example adds 1/3 of the steady winds and this changes when using auto throttle but I'm not an Airbus guy yet

[Yaqatari]

oceancrosser (http://www.pprune.org/members/76081-oceancrosser)

in the B737 all gust exceeding the steady winds are added. the correction limit is 20 kts.

[Yaqatari]

Thank you all for passing by.

[FE Hoppy]

Embraer have a document called Understanding Vref and Approach speeds.
Google will find it for you.

[Yaqatari]

FE Hoppy (http://www.pprune.org/members/2880-fe-hoppy)

thank you

will check it out

[exobis]

not always 20 kts
The maximum command speed should not exceed landing flap placard speed
minus 5 knots.

[Centaurus]

"The recommended method for approach speed correction is to add one half of the reported steady headwind component

Not forgetting of course, the additional note in the FCTM which is to bleed off the half the steady HW component approaching touch down. It does not define the term "approaching touch down" This could mean after the flare or could also mean passing the outer marker, or passing 500 feet. Interpret as you wish.

Observations in the simulator reveal that 90% of pilots make no attempt to deliberately bleed off this specific 1/2 HW component because they assume it means during the flare. This inevitably means excess float unless the pilot spikes the aircraft on to the runway during the float. A potentially dangerous practice.

[Goldenrivett]

This inevitably means excess float unless the pilot spikes the aircraft on to the runway during the float. A potentially dangerous practice.
Why does there have to be "excess float"?
If you are carrying more speed than normal, simply touchdown within the touch down zone with a slightly lower attitude. The amount of flare needed is relative to the pitch attitude on the glide path - it is not some fixed pitch attitude.

[PEI_3721]

Golden, the problem may not be as simple as suggested.
Any additional energy due to speed additive before the threshold has to be dissipated during the flare and landing rollout. A general view may be that it is easier to slow down on the runway with brakes etc, than via drag in the air.

However some aircraft may encounter problems with nose-wheel first landings, others with delayed spoiler, braking, or reverse operation due to the higher speed-related lift delaying air ground switching. A mishandled attempt to achieve a firm touchdown could invoke more pitch than intended, even more float, or a bounce.
Another factor is that the flying technique might oppose the norm, the feel of the aircraft does not matched what little experience may be available, particularly in windy, gusting conditions which might require more attention to lateral-directional handling than is available for deviating from the normal pitch manoeuvre. Then you might get all of the axis wrong during the flare.

Aim to reduce speed at or before the threshold, how much to reduce, to carry into the landing, will depend on aircraft type and experience. Always use the higher speeds in landing distance calculation, a safety margin for what if; and remember the approach speed is a datum, plus or minus some value.
Aircraft certification requires a landing demonstration from Vref - 5, at the threshold and with fixed power (flight idle?), so there are more margins to be aware of, but should not be planned to be used.