Actually, this is a well known phenomenon for 250 hp Comanches with
carburetors, and is one reason that the International Comanche Society
strongly suggests that anyone new to flying Comanches get a check out
from a CFI who really knows Comanches.

The pertinent parts of a proper checkout start with the engine start
procedu (1) Before start, electric fuel pump ON and listen to the
sound of the two electric fuel pumps. They should sound like a Latin
rhythm as the two pumps go into and out of phase. If the sound is a
regular beat, one of the pumps is not working. Naturally, you can only
hear the pumps when the engine is not running. (2) Then electric fuel
pump OFF for start, taxi, and runup. This tests the engine driven fuel
pump. (3) Electric fuel pump on just before power up for takeoff. (4)
Electric fuel pump OFF when a safe altitude is reached. Watch the
fuel pressure. If it falls below minimum, electric fuel pump back ON
for ten seconds, then OFF again. Repeat ON and OFF until fuel pressure
does not fall below minimum.

What the procedure does is push a little fuel into the engine driven
fuel pump with each on cycle, eventually breaking the vapor lock. I
have never needed more than three on's. There is no reason to scare the
passengers with a dead engine. The carb bowl has enough fuel in it to
give plenty of time to get the electric pump back on if you watch the
fuel pressure.

Keeping the electric fuel pump off until the last minute before
departure generally alleviates the problem entirely.

Hank
Henry A. Spellman
Comanche N5903P

wrote:
In rec.aviation.owning bk wrote:
: I experienced an engine failure in a single over the Sierras at night
: (the sunset was beautiful). It was in a Commanche, and the engined died
: about 3 seconds after turning off the electric boost pump (10,500'
: after leaving Tahoe.) Happily, the engine recovered as soon as the
: boost pump was turned back on. Also, happily, the owner of the
: Commanchee was in the right seat and is a pretty calm guy. He turned
: the pump off again (to see what the fuel pressure would do.) Sure
: enough, the pressure dropped, the engine faltered and the nose dropped.
: He flipped the pump back on, and I suggested leaving well enough alone
: (I'm not that calm.)

That's allegedly a common problem with PA-24's. The mechanical and
electrical(s) are in parallel. When the electric is on for awhile, no fuel goes
through the mechanical pump and it gets no cooling. When the electric is turned off,
the mechanical pump is vapor locked and can't pump. It can ruin your day if you do it
just after takeoff and don't think fast.

That's just what I've heard.

-Cory

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* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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