Journeyman wrote
A good instructor knows best of all what he can't teach.

You can't teach what you don't know. Paradoxically, you never learn
a subject as well as when you try to teach it to someone else.

Actually, both statements are true - and there is no paradox.

Yes, it is crucially important for an instrutor to know what he can't
teach. Those are the edges of his instruction envelope. They're not
the edges of his flight envelope. You can't teach effectively when
you're pushing your own limits. The corollary is that you can't teach
something you've never done, and neither can you teach something
you've only done once or twice. You need depth of experience.

But just because you know how to do something doesn't mean you know
all the ways it can go wrong, nor all the ways it can be done. Nobody
does. That's where the part about learning it best when you teach it
comes in.

Once upon a time, I was teaching a student to thermal a glider. Now
I've spent many hours thermalling a variety of gliders, and have
reached some minimum standard - I was good enough to have gone off on
XC and reached my destination. In the UK (and pretty much the rest of
the world), that would have been a minimum standard for holding a
glider instructor rating, but in the US we don't consider that
necessary. I was also reasonably proficient with spins - meaning I
had done quite a few, in a variety of aircraft and with various
entries, and could comfortably spin to headings and recover with
minimum altitude loss. Thus I felt myself qualified to teach both
thermalling and spin recovery. What I did not expect was that I would
be doing both that day.

I was in the back seat of a Blanik L-23 training glider - in fact, the
same make and model in which I did spins to headings and got my CFI
spin endorsement. The L-23 is best described as a spin-resistant
glider. With me in the back seat and a very lightweight instructor in
the front, we were almost at the aft limit of cg. Despite this,
getting a consistent spin entry required the nose be raised about 15
degrees above the horizon, and full pro-spin rudder applied as the
stick hit the aft stop and the nose began to drop. Simply bleeding
off the airspeed, even fully cross-controlled, resulted only in a
mushy, sloppy turn. If you wanted a spin entry, you had to work for
it. Full pro-spin controls had to be maintained for the entire
maneuver, and relaxed about 10 degrees ahead of the recovery. The
glider would recover itself as soon as pro-spin rudder was released
and back pressure relaxed. The most challenging part of the maneuver
was timing the re-application of back pressure to minimize speed gain
and altitude loss in the recovery.

I knew all abuot inadvertent spins. I knew people HAD spun out of
thermals. I knew that there had been stall-spin accidents in the L-23
- in fact there had been a stall-spin fatality in an L-23 at the club
where I learned to fly gliders. I knew this meant that with a
ham-handed student (one who had not yet developed the finesse
necessary to thermal well) one ought not to do thermal training at an
altitude lower than necessary for recovery and subsequent pattern
entry, and I abided by the recommendation.

And yet, that day, I learned something - I learned what a REAL
inadvertent spin entry looks like, and how it fools people. There I
was, in the back of the L-23, with a rather heavy student in front.
We were nowhere near the aft cg limit. There was somewhat decent lift
to work, and he was working it somewhat decently. It was Reichmann
(author of Streckensegelflug, the seminal work on soaring - I sure
hope I spelled that right) who said that in smooth air, one should fly
smoothly - and in rough air one should fly roughly. Well, the air was
rough and my student was flying roughly. He was also improving, and
so I cheerfully sat in the back, coaching him occasionally, but
knowing that this was a skill only time in the seat would develop.

He really didn't do anything all that terribly wrong. He applied too
much rudder into the turn - but not a ridiculous amount. He applied
too much backpressure as we entered a stronger area of lift - but some
backpressure was called for. He leveled the wings a bit, applying
aileron to the outside of the turn - but a shallower bank was
appropriate in the stronger lift. And then exactly the wrong gust hit
at exactly the wrong time, and I found myself staring down at the golf
course next to the airport as it began to turn.

As I sat in the back of the glider, I wasn't actually worried or
scared. I had done some aerobatics, so being in this kind of attitude
was no big thing to me. We had plenty of altitude - 2000 ft or so -
and so being able to effect a safe recovery was never in doubt. In
fact, we had altitude to spare, and so I was going to let the student
take a shot at it first. "We're in a spin - recover" I said from the
back. But what I was thinking was "How did you manage to do something
I had to work at without even trying?" My student released the
backpressure, the glider immediately recovered from the spin, and I
talked the student through a reasonable pattern and landing.

I'm not sure how to describe what I learned - but I feel like I
understand inadvertent spin entries a lot better having seen that.
Before, I never quite understood how a spin can just sneak up on you -
and now I've seen it and know what it looks like.

Michael