Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
CE-07-45
August 24, 2007
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin (SAIB) advises you
of an airworthiness concern for airplanes built by Aeronca, Bellanca,
Champion, or American Champion Aircraft Corporation (ACAC) where
possible flight control cable fraying, if undiscovered/unchecked by
maintenance, could lead to loss of flight control.
This airworthiness action has been taken after consideration of the
responses from ACAC as well as airplane owners/ operators through
relevant associations and type clubs, using the procedures found in the
Small Airplane Directorate Airworthiness Directives Manual Supplement
(Airworthiness Concern Process Guide). At this time, this airworthiness
concern is not considered an unsafe condition that would warrant an
airworthiness directive (AD) action under Title 14 of the Code of
Federal Regulations (14 CFR part 39).
Background
There are several service difficulties for frayed elevator and
rudder control cables in the areas where the cables pass over pulleys
in the empennage of the airplane. Recently a fixed based operator
reported that a pilot performing aerobatics in a Decathlon (8KCAB)
noticed a change in elevator control stick pressure and position. Upon
landing, it was discovered that the up elevator control cable had
stretched as a result of three of seven strands failing. The fraying
had occurred where the up elevator cable passes over a pulley just
below the vertical stabilizer.
Discussion
Metallurgical examination of the above-mentioned cable by a
laboratory has not discovered any abnormalities in the wire rope used
in the fabrication of the cable. However, the wire rope was not
manufactured by a company listed on the military Qualified Products
List (QPL) of Products Qualified Under Detail Specification
MIL-DTL-83420 Wire Rope, Flexible, For Aircraft Control, General
Specification For; document number QPL-83420-14. Wire rope that is not
manufactured by a company that is listed in the QPL does not mean the
rope is not adequate for the application; however, it raises the
possibility that the rope may not meet all the performance requirements
required by the MIL-DTL-83420 specification, including a reduction in
the wire rope fatigue life.
Cable wear and degradation is expected, especially in the vicinity of
the pulleys. Section 8, “Inspection and Repair of Control Cables and
Turnbuckles” of Chapter 7, “Aircraft Hardware,
Control Cables, and Turnbuckles” in Advisor Circular AC 43.13-1B,
“Acceptable Techniques, Methods, and Practices – Aircraft Inspection
and Repair” includes recommended procedures for
inspection and care of control cables. Specifically, paragraphs 7-149.d
and 7-152 respectively cover close inspection in critical fatigue areas
and cable corrosion protection/lubrication (for carbon steel cables).
The ACAC service manual does call for a visual inspection of the
control cables at the 100-hour or annual inspection. It is assumed that
any initial/minor fraying of control cables will be detected at the
inspection before it can progress to cable stretch or failure.
Airplanes flown routinely for aerobatics will also incur more cable
wear than those flown with much lighter loads imposed on the flight
controls.
The pulleys used with the flight control cables for elevator, rudder,
and aileron are self lubricating and sealed so lubrication is not
required. However, most of the pulleys used for the elevator trim and
flaps (for those airplanes that have flaps) are constructed of aluminum
riding on a steel insert. These pulleys require lubrication with
general purpose oil at intervals not to exceed 100 hours to keep the
pulleys from binding and causing undue control cable wear (See the
Airplane Service Manual for details).
Recommendations
We recommend that the airplane flight control cables be thoroughly
inspected every 100 hours. This is especially important in the 2- to
3-inch areas where the cables travel over pulleys. If possible, the
inspector should have another person move the control being inspected
over its full travel, while the cable is being inspected. Fraying of a
few wires may be difficult to detect unless the cable is bent such that
broken wires protrude from the cable. Some cable bending occurs where
the cable passes over the pulleys, however, the inspector may wish to
remove the cables from the airplane for a complete and thorough
inspection. If cable replacement is required, we suggest the wire rope
used in the replacement cable be manufactured by a QPL qualified
company. As of the writing of the SAIB, any cable manufactured by ACAC
will contain QPL qualified wire rope.
In addition , we highly recommend lubrication of the elevator trim and
flap pulleys (for airplanes with flaps – 5 of 7 pulleys require
lubrication) at intervals not to exceed the recommended intervals in
the airplane service manual.
For Further Information Contact
Wesley Rouse, Aerospace Engineer, FAA Chicago Aircraft
Certification, 2300 E. Devon, Des Plaines, IL 60018; phone: (847)
294-8113; email: wess.rouse@faa.gov