Is it safe to assume then that the fuel
return line isn’t able to return all the fuel that isn’t being
used? Maybe there are restrictions in the return line that prevents fuel
from retuning at a flow rate that would prevent it from also bypassing the
mechanical pump?
Or does the return line start after the
bypass value for the mechanical pump?
Kevin
From: Lancair Mailing
List [mailto:lml@lancaironline.net] On Behalf
Of Dennis Johnson
Sent: Tuesday, June 12, 2007 8:47
PM
To: lml@lancaironline.net
Subject: [LML] Boost Pump
*WITH NO SPECIFIC KNOWLEDGE*, it does seem
that because the Dukes pump is *pre* the engine, that too much fuel really
can't happen, it's metered by the fuel divider and if the throttle body is not
open, then the fuel is returned via the return line. So in effect you are
just pumping fuel back into the tanks via the return line....
I think that assumption is not totally correct. I have an IO-550
and I have tried the "hot start" technique of running the boost pump
on high with the mixture at cut-off and the throttle closed. The idea is
to circulate cool fuel through the mechanical fuel pump and then return it to
the fuel tank. I get a substantial amount of fuel coming out the
cylinder drains when I do that. So at least in my case, high boost has
enough pressure to overcome the relief valve in the mechanical fuel pump and to
provide fuel to the injectors. Another data point is that shutting
down my engine at the end of a flight by pulling my mixture control to
cut-off won't work if the boost pump is on low; the engine keeps idling.
According to Flying Magazine's
September 2004 Aftermath
column, a Bonanza with a TCM TSIO-520 crashed shortly after takeoff probably because
the pilot inadvertently had the boost pump in the high position.
According to the article, full power operation with the boost pump on high
would not cause an appreciable loss of power, but (according to the NTSB) when
manifold pressure is reduced below 24 inches, the engine will quit.
Fuel system problems are a common cause of experimental crashes.
I would be extraordinarily wary about making any change to my fuel system.
Major discovery: Flying is more fun than building.