Kevin, do you have any known examples of this happening? Flying off the
winch is very similar to a glider high scenario, yet I've never had a
problem releasing even under tension.

Jim,

Several years ago, the PIC pulled the release on an L13 and started his
right turn. The problem was that he was still on tow behind the Bird
Dog. A second later, both tow plane and glider were pointed at the
ground - still attached. The tow pilot reported that he pulled his
release and nothing happened. Several, much more "assertive" attempts
ensued and finally the 2 aircraft parted company - having lost 1000ft.
Had this been a pattern tow, there would have been fatalities.
Interestingly, both aircraft had problems releasing. The Birddog has a
Schweizer release. As I was chief instructor at the time, I asked for a
written statement from the (rated) glider pilot. The names have been
removed.

.................................................. ................

Glider pilot report

Saturday November 6th, being towed by the birddog L-19 in the Blanik
L-13, I had a serious release incident.

On preflight I often noticed that the release mechanism is not as clean
as on the Schweizer models. I had never given any attention to that
point and that was already the first mistake.

Reaching 5000', I over emphasized on the climbing turn after the
release. I pulled the release and made my right turn. Being busy
clearing the right turn And getting to the right airspeed, I did not
notice the rope was still attached until the glider violently yawed to
the left and caused the birddog to start to go into a spin.

Over emphasizing on the turn without verifying that the rope actually
released after pulling the release is obviously a terrible mistake.
Unlike the 2-33 in which the release is very sensitive, the blanik
requires you to pull the handle all the way back and will not release
with a side load on the jaws!

Having pulled the handle twice and noticing that this was hopeless, I
saw that the towplane could not release either. Indeed, the birddog
release also locked when its the tail got pulled up by a glider. To make
matter worse, the tow plane with its tail pulled up was forced in to a
dive by the glider. It could not level off even with full aft elevators.
From there the only hope was a release or a rope break before hitting
the ground (another reason for not using a too strong rope).

Trying to get back on normal tow position I dove. I quickly noticed that
this would not be possible. Thanks to the streamlined shape of the
Blanik I could dive fast enough to remove some of the load on towplane
tail. It allowed the tow pilot to release the rope with a strong yank on
his release handle.

Fortunately, this incident did not aggravate into a tragic accident.
This incident showed me how quickly things can deteriorate if I don't
use the proper procedures. Everybody knows how important verifying the
correct effect of one's input is. I did not know it enough on that flight.