When I was a student pilot, during the long hot summer days of
Sacramento, we always kept the doors open in the Cessna 140. Every once
in a while I'd give the door a shove with my elbow and refresh the hot
cabin air. On my private checkride, the examiner spent the entire ride
trying to get the door to close, saying he was going to fall out. After
the ride he chewed the FBO out up and down for having a door that
wouldn't stay closed. I never thought to try to close the door, it was
hot!
A year later, with a fresh IFR ticket in my pocket I flew the family
down to Monterey. At about 11pm over the Salinas mountains IFR the door
on the Bonanza popped open. Charts flew everywhere, including out the
window. I tried slipping, etc but couldn't get it closed. Since it was
dark I didn't want to try some small airport I'd never been to before
so I diverted to Modesto (a larger airport). I just remember thinking
to myself that if there was ever a time I was going to forget the gear,
this was it. On landing, it is important to grab and hold the door
though. About 1/2 through the roll out the door sprung full open and
then back again. It almost came off the hinges. I think the roundedness
of the Bonanza door made it different than the flat Cessna door. The
Bonanza door trailed about 4". You could pull really hard to hold it to
only 3" but the last bit wasn't going to happen.
My wife and kids probably have a good 600 hours sitting in the plane
now and are all very execellent door closers. The Bonanza just had a
strange door closing mechanism. You turned the handle past two clicks
to grab both latches, something that was missed that night. I now drive
a Mooney and the door is much more obvious. The door handle doesn't
like up with the arm rest unless it is fully closed.

-Robert, CFI