Approach Tips

[Motor Plane Driver]

Hi All,

I was looking for "Rules of Thumb" regarding corrections for wind in the outbound legs for an approach or holding pattern.
One that is stated is "for every knot add or subtract 2 seconds depending on whether head or tailwind" That is fine up until you have a tailwind of more than 20 or 30kts outbound. For example in a holding pattern having a 40kt tailwind would mean a 60 second outbound leg would reduced to -20 seconds.
Some advice would be helpful.
Cheers

[404 Titan]

MPD,

From what I remember I use to add or subtract one second for every one knot of wind and half the crosswind as a drift angle in the outbound leg which use to work pretty well. Hope this is of assistance.
:)

[Arm out the window]

It makes more sense to me to adjust timing with respect to the head or tailwind component as a percentage of your TAS.
For example, say your TAS is 120 KT, in one minute in still air you'd go 2 nm.
If the aim of adjusting your timing is to fly the same ground track for the holding pattern or approach outbound leg in all wind conditions, then if there was a 10 kt headwind (roughly 10 % of your TAS), you should extend into wind for 10 % of the nil wind timing, ie go for 6 seconds more.
To check if it's going to work: 120 TAS - 10 kt headwind = 110 groundspeed. Flying for 66 seconds, you should go 2.01 nm, which is close enough for me.
So in the example given above about the 40 kt tailwind, at 120 TAS, the wind would be about 30 - 35 % of TAS, so I'd decrease the timing by that much, ie a third of 60 = 20 seconds.
GS would be 160 kt, which if you flew for 40 seconds, would take you about 1.8 nm, again close enough for me compared to other ways of working it out.
I think this percentage method is about the easiest way of doing it, and it works out pretty well most times. It's not exact, but accurate enough, and easy to do in your head when the pressure's on.

[Hugh Jarse]

After estimating the wind velocity (or reading it off your RNAV) you simply estimate 2 factors:

1. Head/Tailwind component on the outbound leg;
2. Drift angle " " " " "

General rule of thumb for Outbound Timing is as per 404Titan: Increase/Decrease outbound timing by 1 second-per-knot of Headwind/Tailwind component.

The theory behind drift angle correction is, after allowing for wind effects on your track and radius of turn (inbound), that the A/C should roll out of the inbound turn on, or very near to track.

General rule of thumb for Drift Angle Correction is to apply:
a. Three times the drift angle to a maximum 30deg angle for a one minute pattern or;
b. Two times the drift angle to a maximum 30deg angle for a two minute pattern.

All performed using rate 1 turns.

Hope this helps...............

[Arm out the window]

Doubling or tripling the drift angle can be a bone of contention, in that by doing so you're deliberately flying off the nil wind outbound track rather than just compensating for the effects of known wind to fly that track.
Obviously if you just pick a heading adjusting the track for the drift only you would have to use a greater than rate 1 turn to intercept the inbound track in the case of a crosswind blowing you into the pattern, so I guess what it boils down to is a choice between either not flying the track accurately or busting rate 1 - you can't really win!