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2. RECOMMENDATIONS FOR ALL
PROPELLERS.
a. Never use a propeller for a tow bar when
moving the aircraft.
b. Never stand in front of or in-line of a rotating
propeller.
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AC 90-89A 5/24/95
c. Never ‘‘PROP’’ an engine on uneven or
wet/snow covered ground.
d. Always inspect the propeller before and
after a flight.
e. When working on a propeller, make sure
the ignition is off first.
f. Always maintain the propeller to manufacturer’s
instructions.
g. To avoid nicks and cuts, do not perform
run-ups near gravel/loose stones.
h. Apply a coat of automotive wax once a
month to protect the finish and keep out moisture.
i. Assume a propeller is unairworthy if it has
suffered any kind of impact or ground strike.
j. After any repair or repainting, or if vibration
or roughness is noted, re-balance the propeller.
k. Propeller blades should be balanced within
1 gram of each other to avoid over stressing the gear
reduction system and propeller shaft.
l. Check the bolt torque on all newly installed
propellers every hour of operation for the first 10
hours and once every 5 hours thereafter.
m. After torquing the propeller, track the
blades.
FIGURE 5 - Propeller Tracking
3. PROPELLER TRACKING CHECK.
a. Ensuring good powerplant operation first
starts with a properly installed propeller. Each
propeller should be checked for proper tracking
(blades rotating in the same plane of rotation). The
following procedure is simple and takes less than
30 minutes:
(1) Chock the aircraft so it cannot be
moved. Remove one sparkplug from each cylinder.
This will make the propeller easier and safer
to turn.
(2) Rotate the blade so it is pointing
straight down.
(3) Place a solid object (e.g., a heavy
wooden block that is at least a couple inches higher
off the ground than the distance between the propeller
tip and the ground) next to the propeller tip so
it just touches.
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5/24/95 AC 90-89A
(4) Rotate the propeller slowly to see if the
next blade ‘‘tracks’’ through the same point (touches
the block, see figure 2). Each blade should be within
1⁄16’’ from one another.
b. If the propeller is out of track, it may be
due to one or more propeller blades that are bent,
a bent propeller flange, or propeller mounting bolts
that are over or under torqued. An out-of-track
propeller will cause vibration and stress to the engine
and airframe and may cause premature propeller failure.
4. METAL PROPELLER INSPECTION Perhaps
the two biggest problems affecting the airworthiness
of metal propellers are corrosion and
nicks on the leading edge.
a. Identifying Corrosion.
(1) Surface corrosion can occur on the surface
of metal blades due to a chemical or electrochemical
action. The oxidation product usually
appears on the surface of the metal as a white powder.
(2) Pitting corrosion causes small cavities
or pits extending into the metal surface. This is an
advanced form of corrosion, appearing as small dark
holes that usually form under decals or blade overlays.
(3) Inter-granular corrosion, rare and difficult
to detect in propellers, is the most dangerous
form of corrosion. It attacks the boundary layers of
the metal, creating patches of lifted metal and white/
gray exfoliation on the surface of the propeller. It
is sometimes found in propellers that had a ground
strike and have been straightened.
(4) If any of these signs of corrosion are
found, do NOT fly the aircraft. Refer to the manufacturer’s
maintenance manual for corrosion limits and
repairs or AC 43.4, ‘‘Corrosion Control for Aircraft,’’
and AC 20-37D, ‘‘Aircraft, Metal Propeller
Maintenance,’’ for additional maintenance information
and corrective actions.
b. Nicks and Metal Blades.
(1) Nicks in the leading and trailing edge
of a metal blade are usually V-shaped. They are
caused by high speed impact between the propeller
and a stone or piece of gravel. Properly trained
individuals can ‘‘dress out’’ the crack if the nick
is not too wide and/or deep. Before each nick is
dressed out, each nick and surrounding area should
be inspected with a 10-power magnifying glass for
cracks. If an area looks suspicious, inspect the area
again using the propeller manufacturer’s approved
dye penetrant or fluorescent penetrant method.
(2) If the nick is left unattended, the high
propeller operational stresses will be concentrated at
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