I always
like to share happy little "incidents" with others on the list in
hopes we all might benefit – not just me {:>). I landed at Shady Bend
none-stop from NC with the left tank empty and 14.7 gallons in the right
tank.
On the way
returning from the Rotary Round up at Shady bend; I stopped to refuel at WayCross, GA
as I normally do. Everything thing has been work outstandingly on the
trip thus far.
I took off
and got up to my intended cruise of 5500 MSL (30 kt headwind there, but bumpy
as heck any lower). With such a headwind, I knew the flight would be
uneventful (the incidents happen when I have a tail wind).
After about 30 minutes I decided to switch from my right wing tank to my
left. So I turned on the boost pump and both high pressure pumps and
turned the fuel selector switch to the left tank.
Normally
there is a non-event, there is no hiccup or hesitation – normally!
Well this time things were different, the engine started to surge - big
time – as did my heart. The fuel pressure is wiggling a small amount but
clearly with pressure above 35 psi it is adequate.
My
mind races back to that incident a long time ago where I glided 12 miles engine
out – and I decided there was no benefit to be gained by repeating that
experience. So as I rapidly raced cause and solution hypothesis through
my head, I reached over and turned on the cold start switch. The engine
began to run almost normally, I switched cold start off and the surge
returned. I hit "nearest" on my Garmin and turned toward the
nearest airport (17 miles). Then back to the engine. It is still
surging although not quite as badly as it had been – but still way too much to
be comfortable with – and that fuel pressure is still nominally around 40
psi. What goes?
I then
turned off my secondary injectors - the engine began to run smoothly (although
I don't know if this was more than a coincident of timing). So
recalling my lesson learned from that earlier incident (switch tanks, dummy!),
I switched back to the right tank – happily every thing settled down (including
my heart) and the engine ran normally.
After
getting a bit closer to the target airport, I gathered my nerves (what remained
of them) and switched back to the left tank – there was a momentary burble, but
then the engine started running normally and did so for the rest of the flight
burning 10 gallons out of that tank.
Naturally, I
am trying to figure out what could possibility have cause this momentary
hiccup. Here is the best I can come up with.
I landed at
Shady bend with the left tank with approx ¾ gallon in it. It so happens
the gas cap on that tank is in need of an O ring replacement as I noticed a
small amount of fuel streaming from the cap area. Well, the aircraft set
outside through a few light rain showers while at Shady Bend. I believe
that water may have seeped through the cap and collected in the tank.
(No, I did not do a drain of my tanks checking for water as I should
have). I used to do it religiously, but after 8 years of so of never
finding a single drop of water, I had lapsed in that check. No excuses –
just the facts.
So if a ½
cup or water or so had gotten into that empty tank, it would naturally have
settled at the bottom – right next to the fuel pick up. I refueled at WayCross putting fuel
into both tanks – no problem with the right tank (with a non-leaking
cap). So I believe that I took off with some amount of water in the left
tank. When I switched fuel tanks that slug of water was sitting right
there waiting to be sucked into the system. This is why (I believe) the
fuel pressure did not go down as it would have if there had been no liquid
being pumped through the system. So it basically took a few hair
raising moments to suck all the water through the system and into the engine –
then all was well.
I am not
certain why hitting the "cold Start" switch would have made any
difference, but I did turn it on twice and each time the engine settled
down. Also when the secondary injectors were momentarily turned off the
engine smoothed out. It may have to do with injecting water into the
secondaries (which are positioned further away from the engine) and perhaps the
evaporative effect was sufficient to freeze the water vapor??? Don't
know, but I am convinced that water in the fuel was the most likely cause.
I think the
principal point is that had I done the fuel drain according to Hoyle I would
have likely discovered the water and drained it. So once again, a bit
red-faced to admit to failing to follow long established check-list procedures
(I know none of the rest of you do that), I found an interesting "incident"
to relate to the list.
Fly Safely out there – Ya' hear!
It is so nice of you to investigate every known
failure mode, so the rest of us lessor pilots don't have to. Thank
you............
The water injected, is displacing a like volume of
fuel. It is not actually the water that causes the stumble, but the fact that
the engine is starved for fuel (over lean mixture). So the cold start adds fuel
and the engine runs again.
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com