Wind gradient occurs due to wind friction with the ground; it's effect is
only apparent in the last 200' or so of the final approach.
It can not occur just prior to the base leg turn (unless you are doing a
VERY low circuit :) )
So this is the point of the whole debate; why add half the wind speed to
your safe speed at the FUST check, when that extra speed is not required
until late final?
You have already added half the stall speed to achieve safe speed at the
FUST check; this provides the margin you need for turbulence, gusts,
thermals, etc. on circuit.
If you add the allowance for wind gradient at the FUST check, the advantages
are that it is done and out of the way.
I can't think of any other advantage? < please insert here >
The disadvantages are that, on windy days:-
- your downwind ground speed will be unnecessarily high; with 15-20 knots on
the ground, it can easily be 30 knots plus at a 1000'. So 50 knots plus 10
equals 60 knots indicated, plus a tail wind of 30 knots gives a ground speed
of 90 knots. You are moving - the pilots downwind will be over in almost
half the time he or she is used to. This time is used to make a downwind
call, check the landing area is clear, and plan the circuit and when to turn
base. In my experience students who have decent downwind legs tend to do
much better circuits and landings than when their downwind is rushed or
busy. They have time to plan out the rest of the circuit.
- high ground speeds on down wind can cause some pilots to subconsciously
believe they are flying too fast - and tend to slow down so their base turn
is slower than intended.
- the base leg turn is going to cover a lot of ground - the pilot can more
easily overshoot the base leg turn, possibly leading to a serious undershoot
situation.
Any others? < please insert here >
Does anyone disagree that that the allowance for wind gradient is not
actually required until late final (where the wind gradient is)? If so, why
is that?
So, if we conclude that the allowance for wind gradient is not actually
required until late final, it all comes down to when it is most practical
and/or convenient to add it.
I think the pro's and cons (from my viewpoint, at least!) suggest that the
after the turn on to base leg argument has the advantage.
Also the GFA do not have to amend the instructors manual and BGK's.
The GFA ops panel has decided at their last meeting that the standard
procedure would be that the allowance for wind gradient is added at the FUST
check; I'm not sure how they planned to promulgate this through the system,
as they don't intend to issue an ops directive.
However with both the instructors manual and the BGK requiring the allowance
to be added after the base turn, I can't see how they can without an ops
directive, otherwise the standard practice and documentation are in
conflict.
cheers
David
>-----Original Message-----
>From: [EMAIL PROTECTED]
>[mailto:[EMAIL PROTECTED]]On Behalf Of Peter
>Rundle
>Sent: Tuesday, 11 September 2001 8:15 AM
>To: [EMAIL PROTECTED]
>Subject: Re: [aus-soaring] Downwind leg airspeed
>
>
>Two things that bother me about this thread,
>
>All of the discussion has centred around the extra work load
>of the poor student pilot. As if the laws of physics don't
>apply to experienced pilots. Maybe that is why so many
>experienced pilots have accidents? No don't start! an
>experienced pilot returning after a long flight is just as
>suceptible to increased work load as any student fresh off
>the ground on a ten minute circuit.
>
>Only wind gradient near the ground has been discussed. In
>strong (25-30kt) wind conditions it is quite easy to get a 15kt
>wind sheer (flying through rotating turbulence where a head
>wind component of say 7kts becomes a tail wind for example would
>do it) Our "standard" GFA glider doing a "standard 50kts" circuit
>would suddenly find itself doing only 35kts, thats very very
>close to a stall. If this happened just as the glider started
>it's turn onto base leg then this is a stall and probably an
>incipient spin. If the pilot doesn't actually crash into the
>ground but pulls off a miraculous recovery then they'll probably
>still crash as there is bugger all chance of making it to the
>airfield now with such a strong head wind to contend with.
>
>So thanks but no thanks I'll continue to add my extra knots at
>the fust check.
>
>rgds
>
>Pete
>
>
>
>
>
>
>
>
>
>
>
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