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the bottom of the nick’s V and, in time, will generate
a crack. The crack can migrate across the blade until
the blade fails, producing a massive imbalance
between the propeller and the engine, ultimately
causing structural failure. Cracks in metal blades
CANNOT be repaired. A cracked propeller must be
marked unserviceable and discarded.
c. Warning. Metal propellers are matched/
tuned to the engine and airframe resonant frequency
by being manufactured with a particular diameter to
minimize vibration. DO NOT SHORTEN METAL
BLADES for any reason unless the manufacturer
specifically permits this major alteration.
5. PROPELLER INSPECTION.
a. Wood propellers should be inspected before
and immediately after a flight. Inspect to ensure the
following:
(1) The drain holes are open on metal
edged blade tips
(2) The metal/composite leading edge is
secured and serviceable
(3) The blades, hub, and leading edge have
no scars or bruises
(4) The mounting bolt torque and safety
wire or cotter pins are secure
(5) There are no cracks on the propeller
spinner (if applicable), and the safety wire is secure
(6) There are no small cracks in the protective
coating on the propeller, which are caused by
UV radiation
(7) The charring around the mating surface
of the prop and the engine flange -- both indications
of a loose propeller
b. A word about torque: A new, wooden
propeller should have the mounting bolts checked
28
AC 90-89A 5/24/95
for proper torque within the first hour of flight and
every hour for 10 operational hours thereafter.
(1) After 10 hours, check the bolt torque
every 5 hours thereafter. The mounting bolt torque
also should be checked prior to flight if the aircraft
has been in storage for a long period of time (3 to
6 months).
(2) If the bolts need to be torqued, it is
suggested all the bolts be loosened for an hour to
allow the wood to relax. ‘‘Finger tighten’’ the bolts
until snug and tighten the attaching bolts in small
increments, moving diagonally across the bolt circle.
It is good practice to check the propeller track (see
chapter 1, section 7) as the bolts are torqued down.
The torqued bolts should be safety wired in pairs.
(3) If nylon/fiber insert type nuts are used,
they should be changed every time the propeller bolts
are re-torqued. They should never be used with a
bolt with a cotter key hole in the threaded area
because the sharp edges around the hole will cut
the nylon/fiber insert and reduce the fastener’s
effectiveness. All self-locking nuts should have at
least two bolt threads visible pass the nylon/fiber
insert after torquing.
(4) If any of the following damage is
found, a wood propeller should be removed from
the aircraft and sent back to the manufacturer for
repair. If the propeller cannot be saved, it should
be marked unserviceable.
(i) Any cracks in the blades or hub
(ii) Deep cuts across the wood grain
(iii) Blade track that exceeds 1⁄16’’
limits after attempts to repair
(iv) Any warpage or obvious defect
(v) Extreme wear (leading edge
erosion, bolt hole elongation)
(vi) Any separation between
lamination
NOTE: When parking the aircraft, always
leave the wood propeller in the horizontal
position. This position will allow the wood
to absorb small amounts of moisture evenly
across it’s entire span rather than concentrating
the moisture (weight) in the low
blade and creating a vibration problem.
6. COMPOSITE PROPELLERS
INSPECTION.
a. There are generally two types of composite
propellers: thermo-plastic injection molded propeller
and the carbon/graphite fiber composite propeller.
(1) The thermo-plastic injection molded
propeller is a low cost, thin bladed propeller used
on engines of 80 horsepower or less. Propeller
inspection is straight forward, by examining the
blades and hub for cracks and nicks. If a crack is
found, do not fly until the propeller is replaced. Small
nicks of 3⁄16 of an inch or less can be dressed out
and filled using a two-part epoxy.
(2) Carbon/graphite composite propellers
are primarily used on engines of 40 horsepower and
more. One should inspect for small hair line cracks
in the gel coat. These spider cracks are usually
caused by vibration generated by a mismatch of the
engine and propeller combination. If a crack in the
base material of the propeller other than the gel coat
is found, do not fly until the manufacturer inspects
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Amateur-built Aircraft & Ultralight Flight Testing Handb(19)
