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angle. The easiest and safest way to do this with
a conventional gear aircraft is to dig a hole and place
the aircraft’s tail in it. For a nose gear aircraft, build
a ramp to raise the nose gear to the proper angle.
(2) Make sure the aircraft is tied-down and
chocked. With minimum fuel in the tanks, disconnect
the fuel line to carburetor. The fuel flow with a gravity
flow system should be 150 percent of the fuel
consumption of the engine at full throttle. With a
fuel system that is pressurized, the fuel flow should
be at least 125 percent. When the fuel stops flowing,
the remaining fuel is the ‘‘unusable fuel’’ quantity.
(3) Since the fuel consumption of most
modern engines is approximately .55 pounds per
brake horsepower per hour for a 100 horsepower
engine, the test fuel flow should be 82.5 pounds (13.7
gallons) per hour for gravity feed, or 68.75 pounds
(11.5 gallons) per hour for a pressurized system. The
pounds per hour divided by 60 equals 1.4 pounds
and 1.15 pounds per minute fuel rate respectively.
NOTE: Formula for fuel flow rate gravity
feed is .55 x engine horsepower x 1.50 =
pounds of fuel per hour divided by 60 to
get pounds per minute, divided by 6 to get
gallons per minute. For a pressurized system,
substitute 1.25 for 1.50 to determine
fuel flow rate.
f. Changing Fuel Flow or Pressure: If the
aircraft’s fuel flow rate is less than planned, there
24
AC 90-89A 5/24/95
is a volume or pressure problem. An increase in the
fuel flow volume may necessitate installation of
larger fuel line fittings on the fuel tanks, fuel selector,
and carburetor in addition to larger internal diameter
fuel lines. To increase fuel pressure, install an electrically
driven or engine driven mechanical fuel
pump prior to the first flight.
g. Compression Check: When the engine runin
procedures have been completed, perform an additional
differential compression check on the engine
and record the findings. If a cylinder has less than
60/80 reading on the differential test gauges on a
hot engine, that cylinder is suspect. Have someone
hold the propeller at the weak cylinder’s top dead
center and with compressed air still being applied,
LISTEN. If air is heard coming out of the exhaust
pipe, the exhaust valve is not seating properly. If
air is heard coming out of the air cleaner/carb heat
air box, the intake valve is bad. When the oil dip
stick is removed and air rushes out, the piston rings
are the problem.
h. Last Check: Drain the oil and replace the
oil filter, if applicable. Check the oil and screens
for metal, visually inspect the engine, and do a runup
in preparation for the taxi tests. Do not fly the
aircraft if anything is wrong, no matter how small
or how insignificant. The sky, like the sea, is an
unforgiving and uncompromising environment.
25
5/24/95 AC 90-89A
SECTION 12. PROPELLER INSPECTION
‘‘A tough decision is what a man makes when he cannot form a committee to share the blame’’ George
Lutz, Col. U.S. Air Force, Retired (1994)
1. OBJECTIVE. To help the amateur-builder/
ultralight aircraft owner develop an inspection program
to maintain his/her propeller.
a. There are three kinds of propeller designs:
metal, wood, and composite.
(1) Because of weight considerations,
metal propellers are used more on amateur-built aircraft
than ultralight aircraft. This makes wood and
composite propellers the overwhelming choice for
ultralight aircraft.
(2) Wood propellers are light, reliable, and
inexpensive but require frequent inspections.
(3) Composite carbon-graphite material
props are more expensive than wood, but are stronger
and require less maintenance.
b. All types of propellers have one thing in
common: they are constantly under high levels of
vibration, torque, thrust, bending loads, and rotational
stress. Even small nicks in the leading edge
of the blade can very quickly lead to a crack, followed
by blade separation. Propeller tip failure and
a subsequent violent, out of balance situation can
cause the propeller, engine, and its mounts to be
pulled from the airframe in less than 5 seconds.
c. It is essential that the make and model
propeller is carefully chosen. Always follow the
manufacturer’s recommendations.
d. Exercise caution if experimenting with different
makes and models propellers. A propeller with
the wrong size and pitch will give a poor rate of
climb, cruise, or could cause the engine to ‘‘overrev.’’
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