Approach Speed Adjustment for Xwind Landings

[ThunderSkye]

The formular for approach bug speed in crosswind landings (1/2 Headwind component + the full gust) has lead to some confusion among some of the crew at our airline.
The first half of the formular is self explanatary, but the second half needs some explanation. Please can someone verify the meaning i.e. does it mean that only the headwind component of the gust is taken into consideration, or does it mean that the total gust is taken into consideration immaterial if it is a direct headwind component or a full 90 degree component giving 0 knots headwind effect (in theory)
Thanks

[Zoner]

1/2 the steady and all the gust up to a max of 20 at our zone. This is for windshear protection and headwind or crosswind makes no difference.

[ThunderSkye]

Thanks for the reply, but surly it is only the headwind portion of the gust that effects flight, and could carrying bug +20 with a reduced flap setting due to gusty conditions not cause a dangerous situation i.e. floating and drift close to the ground during flare?

[Mad (Flt) Scientist]

Xwind components will certainly affect flight.

Take an extreme case - if you had 100kt pure Xwind gusts, I think you'd want a bit of extra approach speed so that the sideslip induced by the gusts was minimized, no? Even at more reasonable values, it makes sense to increase speed - every proportion by which you increase approach speed correspondingly decreases the effect of a gust in ALL axes.

[mbcxharm]

From the Boeing 757 FCTM: (1.12)

Wind additives are half headwind component of steady wind + full gust (irrespective of whether the gust is a headwind or crosswind).

In addition:

"The gust correction should be maintained to touchdown while the steady headwind correction should be bled off as the airplane approaches touchdown".

But what happens when you have a steady but strong crosswind? According to the FCTM you would fly at Vref + 5, but I've just had a quite interesting OPC exercise on crosswind landings where extra speed helped a great deal with controllability in the flare especially when the spoilers etc. started popping...

[john_tullamarine]

I suspect that the main intention of the procedure is to address a reasonable balance of control and potential for landing distance overrun.

Gust is carried through to the flare/touchdown as the phenomenon is random so far as the pilot is concerned.

The steady wind additive is bled off to match the typical ground boundary layer reduction in (head)wind during the final stages of the approach and landing. There was a related thread (http://www.pprune.org/forums/showthread.php?s=&threadid=10456) on this topic several years ago which might give some further insight.

While I agree that one needs to be cautious with gusting crosswinds, carrying extra speed MAY cause some sweaty moments toward the end of the landing roll .. especially as that extra speed makes a float more likely in most, if not all, aircraft .... which makes getting rid of the extra energy all the harder before the end of the seal ....

At day's end .. you pays your money and you takes your chances ... but have a good story at the enquiry if you're making it up as you go ...

[ThunderSkye]

Gentleman thank you for your response, and John thanks for the thread, it made interesting reading.
My initial inquiry was in response to an incident at Narita in the past month, in which a crew experienced a 90 degree crosswind 15kts gusting 30kts and landed bug plus 20kts. During the flair the aircradt started to drift towards the edge of the runway and the crew applied aileron to try and correct the drift which resulted in a pod stike.
As mentioned in one of the previouse responses, it is a reccomendation and conditions should be assesed on the day.
Thank You

[john_tullamarine]

.. which, perhaps, was associated with longer than expected float prior to touchdown ?

I might have my own views on whether accepting that sort of crosswind condition makes good sense or not ....

[Centaurus]

Thunderskye. I wrote to Boeing on this subject a few years ago as like you I could not fathom why add all the gust at 90 degrees.
The reply was short and said that is our recommendation and that's it. Personally, I think that recommendation has led to long floats, hot brakes, and over-runs.

[john_tullamarine]

I think, to be fair in this ever-increasing litigious world, the reported response from Boeing makes a lot of sense from Boeing's viewpoint .. throwing the decision back on to the pilot .. whether literally or via operator flight standards policy.

As they say ... making decisions is what the big bucks are for ...

[ThunderSkye]

Personally I feel that Half the headwind component plus the full headwind component of the gust value is sufficient, but then we don't write the rules, we just try and apply them, when they make sence. Hopefully I never have to answer to this theory, but it has alway's worked for me.
Thanking all of you for the response, I always thought that the formula was applied as I interpreted it, but it appears not, and I still do not understand why?

[Intruder]

Up to our posted crosswind limits (742/744), there is no required approach speed additive for direct crosswinds -- only for headwind component.