738 Vfly with x wind

[pilotho]

Heres the scenario:

Wind 90 degrees from the left 20G30 i.e. no headwind component with F30

vfly speed is half the headwind + all gust component with min of +5 and max +20 knots

Question is this, as it's all crosswind technically vfly would be Vref30+5 or should it be Verf30+10 due to the gust factor?

I personally would fly +10 but I'm not sure. Any opinions?

[flyburg]

Vref+10, your fctm should have some example calculations.

edit: fcom to read fctm.

[dhardesthard]

If the reported wind is 90 degrees to the R/W then there will be a headwind as verified by your G/S. Engage the auto-throttle and see what speed the auto-throttle flies.....that's your answer.

[PEI_3721]

Vref+10 seems very sensible.
Remember that in gusty conditions the wind direction can swing as well as change strength, thus although the additive helps with the gust (control / stall margin) it could contribute to a higher GS if there is a tailwind component at touchdown; don’t forget to check the landing distance calculations for the worst case. 15 deg direction change at max gust?

dhardesthard don’t give the problem to automation; at best it might only give an out of date answer – what the wind was or averaged. Automation doesn’t think, pilots can and must think about the problem (threats) and consider a range of outcomes – think ahead.

[flyburg]

Oh well, if you don't take my word for this, simply look at the boeing FCTM! In the first chapter under command speed it will give you some sample calculations.

DHARDESTHARD, Use the automatics to come up with the answer?? Seriously dude, if you are a simmer or made a joke, you had me! If not, get out of the flying business!

PEI 3721, very sensible, also to be applied to take off calculations, but you knew this :ok:

[in my last airline]

You add half the steady headwind component to a minimum of 5kts plus all the gust. So you should be flying Vref+15 in this case and hold the gust until touchdown, landing at Vref+10. Don't want to lose the gust in the flare! If autolanding Vref+5 is adequate according to Boeing.

[flyburg]

Not correct,

It is half the HWC plus the entire gust, with a minimum of five!!

Not, half the HWC with a minimum of 5 plus the entire gust, or 5 plus half the HWC plus the entire gust.

[dhardesthard]

The autothrust does a much better job than any human can. It's inputs are much more current and accurate and it frees you up for other tasks. A pilot can only assimilate so much info and at a much slower rate. Where is he getting the info from? The tower wind is not at the threshold and is several seconds late plus it is rounded of to the nearest 10 degrees and nearest 5 knots. The INS or GPS data is much more accurate and this is what the automation uses so unless there is a malfunction I can see no cons to using the automation.

[RAT 5]

The A/T will attempt to fly the speed selected in the MCP. Thus there first has to be human input; hence the question.

For max X w/v calculations your use steady wind + 1/2 gust. The minimum Vfly additive is +5 plus all gust. If the wind is really all X, but gusting I can't believe it will suddenly become all headwind. If it is from the right it might even become a little tail. Adding ALL the gust, with little headwind to reduce ground speed, could cause floating and all the associated consiquences. It is not an exact science, even though people make it out to be so. A modicum and airmanship is always a good idea.

[Denti]

Airmanship is always a good idea. However only using half the gust factor is nowhere suggested in the boeing manuals. With the last revision we even got new wind limits which explicitly include the whole tower reported gust value for all wind limit calculation including crosswind.

Anyway, trusting the automatics is not completely bad, but especially in gusty conditions it is a very good idea to disconnect both autopilot and autothrust (remember: boeing recommends manual thrust in manual flight) and just fly it manually. A 30kt crosswind is out of limits by 5 kts for autoland anyway so you have to fly it manually at some point anyway. Additionally is the NG autothrust pretty slow and not the best thing to use in bad conditions.

[in my last airline]

Thanks Flyburg, 100% correct. I think it was like I said in the old days but the new FCTM is as you said. Thanks.

[flyburg]

You're welcome :ok:

[Checkboard]

The autothrust does a much better job than any human can.

Rubbish.

Automation systems have advantages and limits and humans have advantages and limits.

[Capt Chambo]

From the Boeing 737NG FCTM dated June 30th 2010, pages 1.11 & 1.12

...the recommended method for approach speed correction is to add one half of the reported steady headwind component plus the full gust increment above the steady wind to the reference speed. The minimum command speed setting is VREF + 5 knots.....
When making adjustments for winds, the maximum approach speed should not exceed VREF + 20 knots or landing flap placard speed minus 5 knots, whichever is lower.
And
Note: Do not apply wind additives for tailwinds. Set command speed at VREF+ 5 knots (autothrottle engaged or disconnected).