C172 fuel cap

Well, during the climb at about 8000 ft the engine quit just as if the
mixture control had been pulled. Went through all the normal checks but
could not get a restart. Notified approach and set up a glide for the
nearest airport. At around 4-5K the engine restarted on it's own. I
completed a normal landing and thoroughly check the engine over. Everything
checked ok and after power checks took off completed the flight.

To try to make this short let me just say that in the next couple of months
I had 5 or 6 more engine outs all while in a climb at around 6-9 thousand
feet. It got so that it didn't even scare me anymore because it would
always restart after descending 2-3 thousand feet.

It was obvious that it was fuel starvation but why? I went through the
service manual and checked everything I could think of. Fuel cap vent, main
tank vent behind strut, fuel lines, carb and main fuel strainers, etc. I
did the vent checks call out in the manual plus a few more that Cessna
engineers wanted after consulting with them. Everything check ok, or so I
though.

I was at a loss. But one day while I was doing a preflight I happen to pull
a little harder than I normally do on the main vent behind the left strut
and the vent line popped down. This is the line that makes a 90 degree turn
forward to catch ram air to slightly pressurize the tank in flight. I then
notice that the paint line where it comes out of the wing was now even with
the bottom of the wing. This is where the line was when the aircraft was
originally painted. Sure enough the line had been pushed up in the wing,
only about a half inch from its original position. I had checked this
dimension before but it's very difficult to get an accurate measurement on
the centerline at the bend.

The moral to the story is that this dimension is critical. The vent is
placed behind the strut to minimize the chance of impact icing since it's
not heated.
Turns out that the strut is somewhat of a airfoil and when the vent is too
high above the strut it puts it on the low pressure side during a climb. As
I'm sure you know you have to keep increasing the angle of climb at higher
altitude to maintain a climb and this puts the vent more into that low
pressure region. You reach a point where this actually will start pulling
air from the tank and when it reaches a flow that the fuel cap vent cannot
match you get negative pressure and engine stoppage. You then drop the nose
during the descent putting the vent back in positive pressure and eventually
the engine restarts.


This is fantastic! And it makes so much sense. An instructor friend gave me an
article with detailed drawings on how to position the vent. Mine was correct
but I didn't understand (until now) why it was so critical. I would imagine
that as the angle of attack would increase, as you say at high altitude, there
would be a tendency for a lower pressure area to form above the right cap as
well. What do you think?

Jim

You have a very interesting and plausible explanation. Ours was a
172H and it never did it again as far as I knew (over several years).
Didn't the single-tank-above-~8000 fuel placard AD apply only to 172s?

I think the concept of a vent problem is better than non-turbulent
air. Thanks for the long post!

"Ron Natalie" wrote in message om...
wrote in message ...
Thanks for the note. I've checked my vent tube and think it's in the
right place. My fuel caps are not original and I get a small amount of
blue streaking on the right wing aft of the cap.

The right cap on a 172 should be vented so that it only vents inward.

Those silicone rubber vent valves get old and leak, and will suck
fuel out of the tank. You'll see the leaky tank stay full longer than
the good one, as the lower pressure holds back the fuel.
Cessna won't sell the rubber valve. Got to buy the whole cap.
Cheaper than Piper, though.

Dan

Jim,

You're right, there is always a low pressure on the top side of the wing.
This is why a tank will drain so quickly if a fuel cap is left off or lose.
That is why the cap is vented in one direction. You want a vacuum is pull
air into the tank but don't want air pulled from the tank.

Bill


This is fantastic! And it makes so much sense. An instructor friend gave
me an
article with detailed drawings on how to position the vent. Mine was
correct
but I didn't understand (until now) why it was so critical. I would
imagine
that as the angle of attack would increase, as you say at high altitude,
there
would be a tendency for a lower pressure area to form above the right cap
as
well. What do you think?

Jim