Mandatory SB565 Lycoming fuel pump

I had the pleasure of loosing power on the left engine on my
Aztec at about 30'AGL after takeoff. Because of poor checklist
performace on my part, I had failed to turn on the electical AUX
pumps. Luckily hitting the AUX pump restored power within a few
seconds giving me the option of climbing to altitude to troubleshoot.
As I climbed, above pattern altitude, I monitored the fuel pressure
and turned the AUX pump off to see what happened, and sure enough
the fuel pressure began to fall. I turned the AUX pump back on to keep
fan turning and decided that I had a failed mechanical engine driven pump.

Since there were no repair facilities at the departure airport,
I climbed to cruise altitude and headed for the closest airport
with a mechanic. Once I was leveled off, with cruise power
and mixture set, I again monitored the fuel pressure and turned
off the AUX pump to confirm the mechical pump failure. To my
supprise, the fuel pressure remained up and the engine ran great.
I even increased the throttle and the mixture and everything
continued to work great. Switched between inboard and outboard
fuel tanks on the left side. No problem. Did the problem fix itself?

Decision time again. Should I land at the closest airport with a mechanic
and investigate or press on? I knew that a number of airports lay
along my route of flight and the weather was CAVU, so I had plenty
of options. I decided to continue along my route while carefully
monitoring the left engine's fuel pressure and then get my mechanic
to fix the problem if I made it all the way home, about 2.5 hours of
flying time. (This is one place where having a second engine is nice).

The left engine ran great all the way home with never another glitch and
never a dip in the fuel pressure. However, when I upcapped the left
outboard
tank to refuel, I was supprised to hear a sucking sound of air rushing into
the
tank. Ah ha! A plugged fuel vent.

Sure enough, my mechanic and I found the left outboard tuel tank
vent tubing was almost completely plugged, way up inside, about halfway
between the vent port and the tank. The handy work of a crafty bug with
a flare for sturdy construction work. We cleaned that out, checked the
other
three tanks' vents for the same problem. This is a good argument for
capping
the fuel vent ports when the airplane is parked, just like we cap the pitot
tube.

A full power static run up was done and everything looked good. The theory
was that under full takeoff power, and with a full fuel tank, the vaccum in
the tank
was too great for the engine driven pump to keep the engine feed with fuel,
and that
the extra boost of the AUX pump was enough to overcome this and keep things
working. Once I got to altitude, the engine driven pump alone was able to
keep up
with the reduced fuel flow demands.

Problem solved, right? Wrong! On the very next takeoff, the same probem
happened with loss of left engine fuel pressure. With the mechanic on board
monitoring the fuel pressure, we turned the AUX pump off as we began our
climb as a test. Sure enough, the fuel pressure started dropping. The AUX
was turned back on to restore fuel pressure and to keep the engine running.

The mechanic dicussed the problem with a fuel injection system overhaul
shop and the suggestion was to repace the mechanical fuel pump. We did
and that solved the problem. Out of curiousity, I disassembled the the old
pump to see if I could see a problem, and it was not hard to find. The
rubber
diaphram was split about 3/4s of the way around at one of the creases where
it was designed to flex.

I'm not sure if the plugged vent line caused the diaphram to split, or if
the
diaphram just split from old age / wear and the pump was not able to
overcome
the vaccum caused by the plugged vent. However, after the pump was damaged
and the vent was clear (the second takeoff) the pump was not able to supply
the engine with the necessary fuel flow.

The point is, failure of the mechanical engine driven pump can be
intermittent and
only appear then the load on the pump is greatest (takeoff) even though the
pump
has a major defect. Therefore, I'd not take any risks. If Lycoming is
concerned
about their pumps, I would be too. If the pumps on my Lycoming IO-540s on
the
Aztec fall under this service bulletin, I will replace them.



"BTIZ" wrote in message
news:iU5_d.72049$Tt.31211@fed1read05...

And if you need an engine driven pump then you probably already have a
backup electrical pump. Would there really be any need to replace the
mechanical one before it wears out?


Are people actually flying with this logic?

Key words here are "need" engine driven pump and "backup" electrical.

Is it a matter of the pump failure that could cause other problems besides
fuel pressure, like lack of fuel flow.. do you know why the SB was
suggested?

Like another poster said, it's free.. why NOT do it.

BT