On Jun 28, 11:56*am, danlj wrote:
A recent post, in response to a stall-spin fatal crash, was the sort
of things folks usually say:
":Generally it is OK to make 180 or 360 degree turns in a modern
gliders, as long as you start at at least 300ft and keep the airspeed
up to a safe manoeuvring speed. The failure to do the latter seems to
have been the probably cause of this accident."

My experience is that it's safe to manoeuver very close to the ground,
in airplanes or gliders, but that it's NOT safe to do this without
practice.

I have long pondered why so many pilots get into trouble with low-
altitude manoeuvers. Here's a little analysis.

1: Motion seems different down low.

Remember the "pivot altitude"?
When we learned to do 'turns about a point', we were told that this is
the altitude at which the down wing can be kept on one apparent ground
point. Above this altitude, the wing seems to move backward over the
ground as we turn; below this altitude, the down wing seems to sweep
forward across the ground as we turn.

The pivot alttitude for a typical slow airplane or glider is well
below 1000 ft agl. Most of us don't "live" there while flying. The
FARs oppose it, the desire to soar conflicts with it, good sense
discourages it.

But we all have to descend below this altitude in order to land.

In the pattern, usually only one maneuver, the turn from base to
final, occurs below the pivot altitude.
At this point, our attention is almost always on the runway ahead, so
we don't notice the low wing sweeping forward over the ground.

Guess what? Most stall-spin accidents occur in the turn from base to
final.

2: At all speeds we FEEL we're going faster when we're low than when
we're high, even in straight flight.

3: Our 'instinctive' responses to the aircraft are based on our habits
of perception.

Unfortunately for safe low-altitude flying, our habits of perception
are all wrong below the pivot altitude.

A: We misjudge the turn because the wing sweeps forward across the
ground, making us feel as though we are turning much less effectively
than we really are.

B: We misjudge our speed because the scenery zooms past, making us
feel as though we are moving at a much higher airspeed than we really
are.

So... If we 'instinctively' follow our habits of perception when we're
below the pivot point, we will (A) rudder the turn to make the wing
stop moving over the ground, and (B) slow down to make the scenery
pass a the 'right' rate. Presto! We've stalled and are spinning. Just
because we tried to correct that excessive speed and that ineffective
turn 'better'.

If we're going to reduce low-altitude stall-spin accidents, I believe,
we're going to have to train and practice low-altitude maneuvering.

This is *not* dangerous if we plan it intelligently -- If we follow
the same precautions we follow when high: speed protects from stall;
and always have a safe landing option available. (Therefore it can't
be practiced at every airport!)

Consider: at an altitude of 300 ft agl, an ancient 20:1 glider can
expect, at best L/D, to go nearly a mile before it touches the ground
in level flight, and once it gets into ground effect, drag is about
halved.
For example, once upon a time, years ago, I experienced a rope break
in a Blanik L-13 at about 300 ft agl. I was a bit more than 20 kt
above stall, almost halfway down the runway. *I did a 180, slowing
just a bit to 'save' altitude. Then I flew at approximately minimum L/
D downwind past the beginning of the runway, and now, below 200 ft
agl, I did another 180. I finished this turn was well below 100 ft
agl, just over the touchdown zone. I accidentally extended the Fowler
flaps instead of deploying the spoilers. I realized the mistake
immediately, because the glider's descent slowed. Because I had 4000
ft of runway still ahead, I just started laughing at my mistake and
left the flaps down. We drifted in ground effect for more than 2500
feet before landing.

Ironically, I felt safe doing this because at this field we are
blessed with having landable terrain in every direction, so if any
manoeuver didn't work as I expected, I had an an off-runway
alternative. Even when we're in a less favorable situation, this will
work - but we can know it will only if we've practiced it in
safe,planned and understood conditions. And the anxiety of uncertainty
and of the unknown seriously distracts from judgement and creates a
sense of emergency when none actually exists.

In a glider with a glide ratio of 40:1, from 300 ft agl we can expect
to go 2 miles, plus whatever altitude we can get from the excess
kinetic energy we have kept, to be at a safely above stall sped, plus
whatever we gain from ground effect when *really* low.

So... let's define, at airfields where we can do so safely,
maneuvering routes and altitudes in which pilots can become skilled at
manoeuvering below the pivot altitude.

I realize that I've written heresy. Get out the tar and feathers.

Dan Johnson

Forget the heresy, tar and feathers.... Good perspective and comments
Dan.

Regardless of where you are, or what kind of aircraft you are flying,
the cardinal rule of "FLY THE PLANE FIRST" and airspeed, airspeed,
airspeed needs to be paramont in a pilot's head. It sure worked out
over the Hudson River.

Unfortunately, we seem to be able to talk / write about safety til we
are blue in the face, and pilots, including some VERY experienced
pilots, seem to make some terrible mistakes when faced with unexpected
situations. Regardless, it is important to keep on talking / writing
about safety concerns. Every time we lose another pilot (or
passenger), that is one more too many.

Bob T.