Calculating your app speed.

[Lump Jockey]

Non pilot here, so would appreciate the help! How do you calculate the approach speed if you know the direction and stregth of the wind? (1)Eg. wind at 230/8 knots, landing r/w23. (2)What if you're landing r/w20? Please explain your thinking! Cheers!
LJ

[Flyin'Dutch']

Hi Lumpjockey,

Can I refer you to the thread you started a few weeks ago yourself which seems to contain all the answers you need.

http://www.pprune.org/forums/showthread.php?s=&threadid=79202

The wind has actually not a lot to do with your approach speed. Small adjustments are made for gusty conditions, and on a very calm day you can safely nibble a bit of the 1.3 without coming to any harm.

You may enjoy getting one of the TT books or book a trial lesson.

FD

[Lump Jockey]

FD, cheers, but it didn't satisfy my desire!;) Is there no maths involved, ie, tan/sin/cosine? Thought there would be!
Cheers again!
LJ

[Flyin'Dutch']

LJ wrote:

Is there no maths involved, ie, tan/sin/cosine? Thought there would be!

Nope..................

Speed is calculated following the formulat that I gave before.

You just steer the course that keeps you going to your touchdown point either by the crab method or the winglow method.

Have fun!

FD

[Whipping Boy's SATCO]

Approach speed remains largely unchanged. However, if you want to get into tan/sin/cos etc, try and figure out the crosswind and headwind components.

On the other hand, as a rough rule of thumb try the following:

If the wind is 30deg off rw hdg then the crosswind component is 30 minutes around the clock - ie half.

If the wind is 45 deg off head it is 45 minutes around the clock - 3/4

finally, if the wind is 60 deg off rw hdg it is 60 minutes around the clock - whole.

The maths isn't quite right but it sure is good enough for government work:)

[bookworm]

The rule of thumb for speed over the threshold is 1.3 times the stalling speed, as FD says. One issue frequently overlooked is that you should do this calculation in CAS not IAS. Calibration errors can make a significant difference.

[Lump Jockey]

Oh, s#¤*! I really dropped a clanger here! Really sorry guys, but what I meant to say, honestly, was how do you calculate TO speed if you have a 30 deg x-wind at so and so knots? You can take my original figures for arguments sake. Damn, sorry guys! :O
So, basically, would like to know, wind at 230/8 knots, taking off r/w23. (2)What if you're landing r/w20? Please explain your workings.......!

[Tinstaafl]

The T/O speed is weight dependent (because the amount of lift needed is weight dependent).

I think you're confusing groundspeed with airspeed.

It's airspeed that is important ie the speed of the a/c related to the airmass in which it located and is shown on the airspeed indicator.

Groundspeed is the airspeed plus or minus any tail- or headwind effect. Note that groundspeed is irrelevent WRT the aerodynamics ie flying the thing.

You don't calculate the required speed from the h/wind &/or crosswind. You extract the required speed from the a/c's runway performance charts then wait for that airsspeed to be reached.

Using a standard atmosphere (ie the pressure/temp/density for which the airspeed indicator is calibrated) and ignoring instrument inaccuracies etc then in nil wind Airspeed = Groundspeed.

If (say) 60 kts is the t/o speed then in the above conditions Airspeed will need to equal 60kts. Groundspeed would also happen to equal 60kts but that irrelevent as far as flying is concerned.

Lined up on the runway with a 20 kt headwind then as far as the a/c's wings are concerned it's already moving at 20 kts even though the wheels aren't rolling. When the a/c eventually accelerates along the runway to 60 kts airspeed & gets airborne the groundspeed would happen to be 40 kts.

If there was a 60kt headwind then the a/c would already be doing 60kts (as far as the wings are concerned) and could get airborne immediately ie without accelerating down the runway. The g/s in this case would be zero.

If there was a tailwind during the takeoff roll eg 20kts then the a/c would have to accelerate to this groundspeed for the wings to consider themselves at a zero speed. It would then have to gain another 60kts for the wings to be at 60 kts airspeed. In doing so, the a/c will have accelerated to 80 kts groundspeed. It's possible for a tyre speed limitation to come into effect here since the wheels will be rolling along at the groundspeed and not whatever the airspeed is.

This works similarly for landing.

The crosswind component is largely irrelevent *except for* mainly some handling/controllability/sideload on the undercarriage issues. This imposes a limit to how much x/wind component is acceptable. There are skills that you will be taught to deal with these issues.

It's the headwind/tailwind component that is important re the take off and landing distance.

Considering the constantly varying strength & direction of the wind then approximations are accurate enough to derive the h/wind, t/wind & x/wind components.