"HL Falbaum" wrote in message
...
One answer is based on understanding what the "reported wind" really
means. 15kt gusting to 25 kt means that the average speed over time (I
have forgotten the exact time interval but about 30 to 60 sec) is 15kt. If
peak gusts are 25kt, then that means it can fall as low as 5kt on the
average, and for some moments can be 0kt. The gusts come from turbulence,
and can therefore come from *any* direction. The danger is not so much the
25kt gust but the 5kt lull. Hence the advice to add the gust differential
to the approach speed. Some advise "half the wind plus half the gust
differential"--more or less the same result. What is needed is to remain
above stall speed until touchdown! Thankfully, the wind is much less very
close to the ground---3-4 ft (1 meter). My glider stalls at 38kt --1.3 Vs
is therefore ~ 50kt. So, with a "negative" (opposite direction) gust, I
would be very close to stall without the extra velocity.

As you said, I totally agree that "The gusts come from turbulence, and can
therefore come from *any* direction". So, the aircraft may also come
"close to stall without the extra velocity" with a tailwind situation
on the final leg!

But the rule says to add nothing in a tailwind situation? Probalbly because
the decreasing windspeed when closer to the ground will effectively produce
an increasing airspeed to the aircraft. But this doesn't looks right to me,
to fly just 1.3 Vstall on the base leg and the final leg, just because I'm
going
to land tailwind!

Am I right to say that extra speed stands for a security reason. It is not
simply to compensate the facing wind component. Otherwise, a pilot
may simply use less airbrake to get the same glidepath on the final leg!..

Thought, why not adding the extra knots to the 1.3 Vstall, whatever the
wind direction? Even tailwind?

As the glider come closer to the runway, still plenty of time to reduce
the airspeed before landing!


As to the tailwind question---the wing only knows airspeed. But the
landing field does not move with the wind, so the ground speed is
eventually important. The pilot will be tempted to slow at this point as
the ground speed seems high. Then there is the loss of control issue then
the glider is rolling on the ground, but has little aerodynamic control
due to low airspeed. Finally there is a wind shear issue at about 200 ft.
If you are landing downwind, you will descend into a decreasing tailwind,
and inertial forces will give you an increase in airspeed. It is best to
be on final approach before descending into this shear, which BTW, always
exists. It is just the magnitude that varies. This is so speed adjustments
can be made without the complications of turning--i.e. raised stall speed,
increased workload, etc.

Sooner or later, you will have to slow down in order to land and stop. So
fly the "no wind" approach speed and add the gust differential, until it
is time to roundout and land. Use the brakes if the tailwind is leading to
a imminent loss of control.

Sure!

Happy Landings
Hartley Falbaum
ASW27B KF USA