nuts & bolts
technical counselor
Flight Control Inspection
Pulleys, cables, ball bearings
RI C HARD KOE HL E R , E A A 1 6 1 4 2 7
I
was conducting an annual inspection on a Piper Arrow LPS3. LPS3 has the advantage of more anti-corrosion addi-
recently and found an interesting discrepancy to share tives, but its waxy finish can attract dirt. Do not use WD-40.
with you. This information should be helpful for those Besides it being too thin to provide much lubrication, it
who inspect their own homebuilt aircraft. evaporates in about 40 days.
Part of an annual (condition) inspection is to ensure that Pulleys are usually made of phenolic plastic-reinforced
the flight control system is operating correctly and there cotton fabric. (Phenolic was the first successful “plastic,”
are no signs of abnormal wear or damage. I actuate each with wide use during World War II.) The phenolic pulleys
system, watching each pul-
ley and bell crank for free- Part of an annual (condition) inspection
dom of movement and lack
of binding or wear. Advisory is to ensure that the flight control
Circular 43.14-1B is woefully system is operating correctly and
short on this subject. Every-
thing having to do with pul- there are no signs of abnormal wear or
ley inspection is in five sen-
tences, so I will expand on
damage.
them here.
First, where do you find pulleys? Anytime a control cable often have no bearings or only a plain bronze sleeve, de-
passes through a bulkhead or near one, a fair-lead should signed to bear against a standard AN bolt. They may have
be used to protect both the cable and the structure. A fair- a sealed ball bearing. Pulleys that are somewhat more ex-
lead should never be used to change the direction of a con- pensive are made of aluminum and will usually have a
trol cable and should never deflect a control cable more sealed ball bearing. Of course, in our homebuilts, we might
than 3 degrees. So, pulleys are used where a control cable also find pulleys made of nylon or other plastic materials.
must make a change in direction. They should be inspected Again, avoid WD-40, since it will damage nylon and some
for roughness, sharp edges, and presence of foreign matter plastic bearing surfaces.
embedded in the grooves. The bearing should have proper During the inspection, rotate the pulleys, which usually
lubrication, smooth rotation, and be free from flat spots, only turn through a small arc, to provide a new bearing
dirt, and paint overspray (common after a new paint job). surface for the cable. Make sure the pulley alignment has
The bearings can be just plain phenolic on a bolt, bronze been maintained so that the cable is not riding up on the
inserts, or sealed ball bearings. Obviously, the sealed bear- edge of the pulley flanges and not chafing against guards,
ings do not need further lubrication, but they need to be covers, or adjacent structure. Check all pulley brackets and
checked to ensure the seal is still intact. The plain and guards for damage, alignment, and security.
bronze bearings need to be lubricated regularly. Most main- Each pulley installed in the aircraft must have a cable
tenance manuals call for a few drops of 30-weight oil, but guard installed to prevent the cable from slipping out of
I use any of the newer high-quality spray oils like LPS2 or the pulley groove when cable tension is slacked off. This
EAA Sport Aviation 99
technical counselor
guard can be another bolt, a pin (cotter
or clevis), or a strap.
Besides checking the free rotation
and lubrication of the bearing, you
should evaluate the wear of the pulley
groove. If the cable tension is too great,
the pattern of the cable strands will
be worn in the bottom of the groove.
As already mentioned, if the cable is
misaligned, one or both flanges form-
ing the groove will be worn thin. If too
large of a cable was forced in the pulley,
the flanges will also show significant
thinning. If the cable is too small for
the pulley, then the wear will be only
in a narrow band at the bottom of the
groove. In addition, of course, if the
pulley is not rotating, there will be a
flat spot worn in the groove. Any of
these conditions are cause for replace-
ment of the pulley.
Pulleys for airplanes are not very ex-
pensive. You can expect to pay about $4
for a small plain bearing phenolic pul-
ley up to about $20 for a large 3.5-inch
one with a sealed bearing. Aluminum
pulleys are about twice as expensive as
their sealed bearing phenolic cousins.
Back to the annual on the Arrow:
I rotated the pulleys about a third of
a turn and then wiggled the aileron.
One of the pulleys rotated to the same
position and then stopped rotating. It
had a flat spot. It is a small 1.25-inch
phenolic pulley with a bronze-bearing
insert, and I was able to find the Piper
part at Preferred Airparts for $5.98, plus
shipping. I went ahead and replaced
the bolt, washers, and nut, too, since
the additional cost was less than a dol-
lar. The Arrow is now in a condition
at least as good as new, which is what
a condition inspection is supposed to
ensure.
Richard Koehler is an associate professor
of aerospace sciences at the University of
the District of Columbia and has been an
EAA member since 1980. He is an active
airframe and powerplant mechanic with
inspection authorization, a commercial
pilot with instrument and multiengine
ratings, and a technical counselor and
flight advisor.
100 JANUARY 2008
