Head Wind drop off on Landing
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Head Wind drop off on Landing
Was with some glider pilots who say you will land further down the runway if the wind drops off on final approach, I been saying will land short
So what's the answer ? ( cause if in glider, PE ( potential energy ) has to be converted in to KE
Thx
So what's the answer ? ( cause if in glider, PE ( potential energy ) has to be converted in to KE
Thx
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The answer is the exact same for gliders and powered airplanes. Assuming no other change in configuration, less head wind means higher ground speed, or more forward distance at the same descent rate. Hence, you land further down the runway. To compensate in a powered plane, you reduce power; in a glider you pull more spoiler.
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Except to maintain your correct approach speed you’ll have to increase your rate of descent. The answer isn’t as simple as it seems.
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Gliders usually do not do a constant angle approach, so the approach angle will necessarily be steeper with more headwind at the same airspeed. Therefore a dropping off headwind means a landing further down the runway as the approach angle flattens out, unless you pull more airbrakes, which are extremely effective. Or do a slip, which serves the same purpose.
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A bit of quick and dirty analysis.
Say you are on a stabilised 3° approach at 100 KTAS into a 10 knot headwind with 450 ft/min rate of descent (90 knots ground speed).
At 500' the headwind disappears, so your TAS instantly drops by 10 knots.
To regain that 10 knots you lose some extra height by the formula H=(V1^2-V2^2)/2g. For our case that equates to about 78' excess height loss to be restabilised at 100 KTAS.
So you are now at 422' and 100 knots ground speed and still descending at 450 ft/min so your glide angle has reduced due to the increased ground speed to about 2.7°.
The touchdown point has now moved from 500/Tan(3) to 422/tan(2.7) or 9540' to 8950'.
So the reduced glide angle has not compensated for the height loss required to regain the approach airspeed.
So the answer is 'it depends'. The higher the shear layer, the more likely you are to recover from the height loss without adding power.
Say you are on a stabilised 3° approach at 100 KTAS into a 10 knot headwind with 450 ft/min rate of descent (90 knots ground speed).
At 500' the headwind disappears, so your TAS instantly drops by 10 knots.
To regain that 10 knots you lose some extra height by the formula H=(V1^2-V2^2)/2g. For our case that equates to about 78' excess height loss to be restabilised at 100 KTAS.
So you are now at 422' and 100 knots ground speed and still descending at 450 ft/min so your glide angle has reduced due to the increased ground speed to about 2.7°.
The touchdown point has now moved from 500/Tan(3) to 422/tan(2.7) or 9540' to 8950'.
So the reduced glide angle has not compensated for the height loss required to regain the approach airspeed.
So the answer is 'it depends'. The higher the shear layer, the more likely you are to recover from the height loss without adding power.
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Sure it is. You either accept the long landing or compensate. The compensation takes the form of increasing rate of descent via either power or spoilers.
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The answer is the exact same for gliders and powered airplanes. Assuming no other change in configuration, less head wind means higher ground speed, or more forward distance at the same descent rate. Hence, you land further down the runway. To compensate in a powered plane, you reduce power; in a glider you pull more spoiler.
starting point - just turned final , maybe 500ft agl / 55kt IAS and all is stable with ie 20kt head wind - so in theory about 35kt groubd speed
as you sink w 1/2 air brakes - the wind drops to 0kts - remember the AC has the Kinetic energy of 35 kts - this would be stall already in a twin Puchacz w 2 big guys
as such would need to convert Potential energy into kinetic to bring speed up to 55 kts again = dive !!!
remember also that old formulas about acceleration of a mass ( takes time )
...... I calculated if started at 500ft where wind drops out , you would need to drop to 400 ft just to regain speed !
practically speaking - I noticed all the guys landing short on the strong wind day, and the curved slope of the runway made the wind drop off at about 300ft
my good luck was I saw them, and on the next flight aimed further down and surprising just barely passed the threshold
—— so something is not adding up for me
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Seldom does the wind drop 20 knots instantly. Air has mass, just like the airplane does.
Glider starts sinking, push in the spoilers. Adjust pitch to regain airspeed if necessary.
If you KNOW there's a wind shear, plan for it with a bit of extra speed above the shear!
Glider starts sinking, push in the spoilers. Adjust pitch to regain airspeed if necessary.
If you KNOW there's a wind shear, plan for it with a bit of extra speed above the shear!
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Seldom does the wind drop 20 knots instantly. Air has mass, just like the airplane does.
Glider starts sinking, push in the spoilers. Adjust pitch to regain airspeed if necessary.
If you KNOW there's a wind shear, plan for it with a bit of extra speed above the shear!
The runway in the country side had a strong curve ( hill ) with trees at the far end. It was very obvious the wind was passing over and there was a wind protected zone at the bottom of the hill where the TDZ was- no wind
On approach on every flight the wind would almost disappear a few hundred feet AGL before landing.
The thing which puzzled me were the experienced guys said “no wind - will glide further” but the opposite was true. In a steady state I would agree with them, but not in a wind shear
it’s like in flare in a powered AC - in the flare, if a 20kt wind would disappear -> will you glide further ( not ). If in flare get strong wind all of a sudden - you land longer ( as I experienced in the jump seat on jets in HK and MEL )
Last edited by skippybangkok; 31st Oct 2018 at 01:19.
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Did the maths behind it based on the Puchacz and assumed 10:1 approach ( probably steeper in real life )
At 250 ft AGL, of the wind dropped from 20 knits to almost zero ( which it was doing on this runway cause it was shielded form the wind ), you would drop 67 meters shot of your aiming point,
Of the sheer happens huger up, you could actually land long
( Again- wrong assumption ur not going to manipulate the AB in real life )
At 250 ft AGL, of the wind dropped from 20 knits to almost zero ( which it was doing on this runway cause it was shielded form the wind ), you would drop 67 meters shot of your aiming point,
Of the sheer happens huger up, you could actually land long
( Again- wrong assumption ur not going to manipulate the AB in real life )
