I could not disagree more.
The heavy landing was not caused by wind gradient.
The initial loss of speed might have been caused by the wind gradient, but
the heavy landing was caused by the pilot at the controls with HUA (Head Up
Ars..). Pilots need to understand that they must have Safe Speed near the
Ground when they start to flare or expensive crunching noises will follow
shortly!
This loss of airspeed is very rarely a sudden problem, but a gradual process
with the pilot paying insufficient attention to small changes in speed and
glide path until they sufficiently into the wind gradient. Then the nasty
stuff bits and the pilot's safety margins are very small.
Any time your stable platform is displaying a loss of speed coupled with a
developing undershoot, you are experiencing wind gradient.
Notice the symptons
1. Loss of airspeed
2. Undershoot
If you apply standard approach procedures to both problems, you will have no
problems.
You must control the undershoot with a reduction of airbrake
You must control the airspeed by lowering the nose
If the pilot is paying attention and catches it early, it is a small
adjustment.
The glider requires an excess of airspeed to provide a safety margin of
angle of attack for landing. That margin of angle of attack is used when we
raise the nose (increase the angle of attack/lift) to reduce the rate of
descent. If you have insufficient margin (speed/AoA) then it is physically
impossible to prevent the glider from crashing into the ground. This saftey
margin of AoA is set by safe speed near the ground. If you do not have safe
speed near the ground at Check One/Round-out, then you cannot prevent the
glider from thumping into the ground as you raise the nose to check the
descent.
The heavy landing was caused by a lack of airspeed at Check One and that is
the pilot's fault.
(some aircraft have very effective airbrakes that can seriously change the
amount of lift required with full airbrake ie ASTIR)
The real issue here is a lack of understanding.
Power people tend to roll any wind phenomena into 'Wind Shear'
I beleive that this approach does not provide pilots with enough knowledge
of:
* Wind Gradient
* Curl-over
* Wind Shear
Wind Gradient
-------------
Wind gradient is the reduction of speed in layers of air as you approach the
ground.
As a glider in a steady state descends into the wind gradient, the momentum
of the glider remains constant. The component of airspeed produced by the
wind is deceased as the wind speed reduces. This is visible as a reduction
in the ASI. Two aerodynamic effects will now occur. Approach speed is very
close to best L/D. As the glider slows, the sink rate of the glider
increases. The glide is no longer in a balanced state. Lift plus drag no
longer equeal weight. To get back to the balanced state, the glider must be
accelerated, therefore the sink rate will be increased even further as the
glider aerodymanics trades height for speed. This is observed as a
delevoping undershoot. if somebody is sub-conciously tweaking the nose up
to maintain the aiming point, the airspeed will decay rapidly at the most
dangerous position.
If there was a single layer of wind gradient, the glider would then increase
speed and return to a balanced steady state. However, the glider is sinking
into additional layers of wind gradient.
The process is accerated as the slower layers of air have more and more
effect. The stronger the wind, the greater the overall loss of speed and
the faster that loss can occur.
If you are paying attention, the solution is simple:
* Lower the nose to fix the airspeed problem
* Close the brakes to fix the undershoot
The closer to the ground you are, more judgement is required as to how low
to push the nose. You simply must regain sufficient airspeed to flare the
aircraft, to reduce the rate of descent (Safe Speed near the Ground).
Once you allow the glider to undershoot in the wind gradient, recovering
safe speed near the ground will require immediate and postive action. You
will need to accelerate down with the wind gradient trying to take speed
off as fast as you are accelerating. This needs you will chew lots of
height and the undershoot will get worse before the performance of the
glider improves. If you get caught badly, it may not be safe to get back to
the airfield.
Curl-over
---------
Almost always, when the wind gradient effect extends to ground level, there
has been a significant component of curl-over. Examples that come to mind
are:
* Landing down the hill at Bathurst were the approach end of the strip is
on the top of a 20' hill
* Landing to the west at Harden were the top of the hill is 100 yds down
the strip and the touchdown is 50 yds back toward the fence/ drop-off
* The old strip at Wagga were the western approach was over a line of 60'
Gum trees
All of these situations produce a curl-over effect which is a significant
downwash of air. The effect is different from Wind graident in that there
may not be a large decrease in airspeed, just a savage sink rate/Undershoot.
A pilot dragging a glider in on a slow appraoch is going to get a nasty
suprise as the glider is dragged from the sky at low altitude/low airspeed
with little or no chance of recovery. This loss of safety margins is part
of the reason for an increase Safe Speed near the ground during windy
conditions
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Peter
White
Sent: Sunday, 9 September 2001 8:24 PM
To: [EMAIL PROTECTED]
Subject: Re: [aus-soaring] Downwind leg airspeed
Gents and Ladies,
The complex and thought out answers you guys provide always seem to miss
the basic points.
Why do we add half the wind to our circuit speed?
I don't think it is because of gusts, although they are a factor which
needs to be considered.
I don't think it is because the MOSP says so.
It is my long held understanding that it is because of WIND GRADIENT and
that about a third of the wind speed is lost in those last few feet to the
ground, due to friction close to the earth's surface. The rest of the
compensatory "half the wind speed" is to allow for gusty conditions.
It is also my belief that a lot of the heavy and short landings we see at
our clubs is a result of WIND GRADIENT not being allowed for .
Five knots for "mum and the kids" has killed more glider pilots than has
claimed those that "forgot" to add half the wind, because the top strand of
wire at the upwind end of a short paddock only takes a fraction of a second
to decapitate those that have floated the extra 100m into the paddock due
to their extra PENETRATION at five knots too fast. "Mum and the kids"
always thought hubby was a good pilot, little did they know that his
favourite safety expression......"five knots for mum and the kids" was a
fatal error just waiting to put his head in the back seat.
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