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WilloW_737

Headwind component Vref calculation clarification ?

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Greetings;

 

A simple subject but I wish clarification.

 

If the auto-throttle is disengaged or is planned to be disengaged prior to landing, the recommended method of 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.

 

Do we add the headwind component to the Vref or to Vref + 5?

 

As an example, let us assume: Vref is 140 knots, headwind 20 knots and half the headwind is10 knots.

 

Therefore, is the correct configuration: Vref 140 + 5 + 10 = Vref 155, OR Vref 140 +10 = 150

 

Thank you, WilloW


WilloW (YMHB)
http://www.Flaps2Approach
B737-800 simulator

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Here's what I found:

 

"With autothrottles disengaged an approach speed wind correction (max 20knots) of 1/2 steady headwind component + gust increment above steadywind is recommended. Do not apply a wind correction for tailwinds. The maximum command speed should not exceed landing flap placard speedminus 5 knots."

 

From this I understand that the corrections apply to Vref without the +5 correction which is more like a generic rule in low wind conditions.


George Golas

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

I hate gravity!

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The way I interpret the rule is that the VREF+5 recommendation assumes calm or average headwinds at most airports. Applying the 1/2 steady headwind rule, that means any headwind up to 10 kts, so 1/2 of steady "normal" winds is +5. But once the headwind component exceeds 10kts, you no longer use +5 but rather +6, or +7, etc... up to a limit of +20.

And if you have a gust, you add the whole amount of that gust speed to the computed 1/2 headwind component.

 

Example: you are landing on runway 27, and the winds are 8 kts gusting to 12 kts.

Half of runway component is technically +4, but any headwind of 10kts or less is automatically +5, so you will use +5. You will then add +4 kts (12-8=4) to that +5 headwind which results in VREF+9.

 

Hope that makes sense.


A.J. Domingo

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Thank you... To summarize:

 

Always Vref+5 if headwind less than10 knots.

 

If headwind greater than 10 knots, divide headwind by 2.

 

Formula (wind < 10 knots): Vref for headwind less than 10 knots = 5 + gust - headwind.

 

Formula (wind > 10 knots): Vref for headwind greater than 10 knots = headwind/2 + gust - headwind.

 

If Vref calculation is over 20 knots, then Vref+20 (as per guide)

 

Cheers, Willow


WilloW (YMHB)
http://www.Flaps2Approach
B737-800 simulator

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Formula (wind > 10 knots): Vref for headwind greater than 10 knots = headwind/2 + gust - headwind.

.

 

This is correct formula for all winds.

Eg. wind 0kt, gust 10kt, then Vap=Vref+10kt. It;s between +5 and +20 so you don't have to add additional 5.

5kt is also additive for A/T use.

 

If Vref calculation is over 20 knots, then Vref+20 (as per guide)

 

Or flap placard speed minus 5kt.

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