Gentlemen,
I’d like to weigh in with Grayhawk on the “when to
refuel” issue. I have a TSIO-550 powered Lancair IV-P that has 6+ hours
of fuel aboard when fully filled; more if I slow it down a bit. After 400+
flight hours in the past 1 ½ years, I am very comfortable with the fuel consumption
rate, and the amount remaining at any given time. I installed an Electronics
International MVP-50 engine/aircraft monitoring system, along with EI’s standard
capacitance fuel probes. The probes measure actual fuel in the tanks, and the
MVP-50 separately integrates measured fuel flow over time to calculate total fuel
burned/total fuel remaining. (oth the MVP-50 and Chelton EFIS systems use this latter
information to project distance and time to fuel exhaustion, as do other EFIS.)
As standard procedure, I check the fuel added to each tank each
time I fill up completely, and compare it to the tank probe-measured fuel
remaining. Over time, I have confirmed that on level ground the fuel
calibration is accurate within 2 gallons at all fuel amounts for each tank. This
is not true in flight, however. The displayed fuel quantity varies only
slightly (estimated at less than 10% for moderate climbs or descents—using
pitch angles the airplane can sustain for the time needed for the fuel quantity
measurement to stabilize), however even a slight amount of yaw causes the measured
quantity to vary significantly. I can generate fuel measurements of half to
twice what’s actually in each wing --when they are partially filled--by sustained
excessive out of trim conditions in yaw. When the aircraft is returned to a
level pitch and a yaw-trimmed position, the fuel quantity measures within 2
gallons of actual for all amounts—as on the ground. I tested this extensively
during initial flight testing, and possibly could improve it with a
re-calibration? I have resisted doing this because it is well within my allowable
tolerance now, and as someone else on the list pointed out, there’s always
a chance to “dis-improve” what I now have.
The reason I am very confident in my fuel measurement/management
system is that I cross-check the quantity required to refill each tank with the
corresponding fuel quantity indicator, and then with the total computed fuel
burn since last fill up. Unlike the quantity probes, the fuel burned
calculation performed by the MVP-50 is very easy to re-calibrate, and over time
I have refined it such that it is consistently within 2% of the actual fuel
burned, no matter what the flight conditions or power setting.
Both the quantity and totalizer values are alarmed on my MVP-50 to
indicate low fuel in each tank and low total fuel remaining, and I tested this
function during flight test as well. The result is that I have two very reliable,
independent instrumentation systems to measure fuel burn/fuel remaining, and the
ability to check the calibration/accuracy with each fill-up. I also know that I
have 6+ hours fuel aboard after fill-up WITH MY NORMAL OPERATING METHODS. I
also alarmed the fuel flow above 45 GPH to indicate a possible fuel leak, but
this is a reliable indicator for only some of the failure modes—still, another
useful feature of the MVP-50.
I think each pilot should use a system he/she is comfortable
with, and dealing very conservatively with fuel and icing will provide a longer
life expectancy! Still, I believe the instrumentation and monitoring systems
now available actually allow us to fly safely without filling the tanks at
every stop—as I did faithfully 30 years ago… Also, if you’re
still building, I can recommend, without reservation, EI’s MVP-50
engine/aircraft monitoring system. It’s been one of the most reliable and
useful instrumentation systems on my airplane…for this and other
functions.
Bob