Gentlemen:

This incident happened a couple of months ago and I have not reported it yet
as I just finished determining the real cause.

Climbing thru 17,000 feet south of Lake Tahoe I was sitting in the right seat
flying with a friend in the left seat.  He has 30 hrs IVP time as PIC and
everything was going just fine ... I was feeling fat, dumb and happy.

The cabin altitude light came on and he asked "whats that mean" I started to
tell him it ment the cabin was at 10,000 feet and I looked down at the
pressure controller  to see what was up when WHUMPHHH  the cabin dumped
pressure.

At this point while still Fat and Dumb...I was no longer Happy !

Don the Oxugen Masks, Declare a Maday, reverse course to Tahoe, get on the
ground and trouble shoot.

We were able to pressureize the cabin on the ground to 2PSI with run up power
and it seemed to hold.   Flew home, checked all potential leak points,
replaced a bunch of parts in the door seal pump/dump valve etc.

Discovered the problem was human error.   The door on this IVP and for that
matter the only other one I ever flew required that the first latch be well
over center and in order to do so, you would hear two distinct clicks.  One
when first closed and the second when a little more pressure was applied to
the handle.

What happened is the first latch was not fully seated to the second click and
at altitude, the door seal could not stay large enough to bridge the gap.  
It was leaking (thus accounting for the slow leak and the rising cabin) and
finally the size of the gap became large enough and the building pressure
strong enough to vent the cabin entirely.

Moral to the story:  Make SURE the cabin door is FULLY latched.    Check it
twice.  And imagine how this would have played out if the failure had
happened at FL 250 and not at 17,000 MSL

Hope this saves somebody some drama some day


Dave Riggs