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FMS - Approach Wind Correction


JamesLannister
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  • 2 weeks later...
On 11/27/2022 at 4:54 AM, JamesLannister said:

What do you put in here? Say the wind is 10knots do you put in a value of 10? If the wind is gusting do you put in the gust speed or is it used for a different purpose

 

This is simply a speed additive to VAPP. VAPP is computed by the FMS to be VREF + an assumed 10kt headwind. The FM recommends a speed additive of 1/2 the steady state wind or all of the gust, whichever is greater, up to 15 kts. 
Hence, 1/2 of 10kts is 5kts +VREF = VAPP default

Let's say you have a 15 kt headwind, half of that is 7.5, so round up and add 8 to VREF (calculated from perf software or QRH/manuals). In this situation, you would only add 3 kts to VAPP. 

Winds are 10 gusting 25: 15 kt gust factor. Add the entire 15 to VREF. Which would be 10kts in wind correction. (You've already got the 5 kts added)

To summarize, VREF is 1.3Vso, VAPP is actually what you're going to fly on final, and its value with respect to VREF will change depending on wind and configuration corrections (stuck slats/flaps, floating spoiler, etc.)

An important note: the hook, the amber line above the red Stick Shaker bar (zipper) on the PFD, displays 1.2Vs in actual configuration for takeoff and it displays 1.3Vs of actual landing configuration. It does this based off of AOA, and in normal circumstances, this will always be what you base your approach speed on. The FMC computes approach speed on its own, but it does so based off of weight and configuration input. It gets pretty close (if your weight inputs are right), but for predictable handling during landing and the flare, the hook is what is used as a VAPP reference. So, assuming calm winds, fly hook +5kts. 15 kts wind, fly hook plus 8, etc. Under normal circumstances (no flight control malfunctions), even if you are allowing the FMS to control speed using the VAPP window (during an RNAV), you still honor hook. Add to the wind correction window, or take away, so that your VAPP is referencing AOA. 

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