Regulatory Brief -- AOPA questions NTSB Safety Recommendations on control cable attach fittings

Regulatory Brief

AOPA questions NTSB Safety Recommendations on control cable attach fittings

The issue:

On April 16, 2001, the National Transportation Safety Board (NTSB) published three safety recommendations regarding corrosion and cracking of certain stainless steel control cable attach fittings. The recommendations urge the FAA to issue an airworthiness directive (AD) mandating repetitive inspection for and replacement of corroded stainless steel control cable attach fittings installed on Piper Cherokee, Seminole, and Cessna 172 airplanes more than 15 years old. The Board also recommended that the FAA expand the AD to include all aircraft utilizing control cable attach fittings of the same design and material specification. The final recommendation urged the FAA to notify all manufacturers of fixed and rotor wing aircraft of the cracking and corrosion problems currently being experienced with terminals made from certain spec stainless steel.

The importance to our members:

There are approximately 21,706 Piper PA-28s, 304 Piper PA-44s, and 23,968 Cessna 172s more than 15 years old currently on the US registry. Per the NTSB's recommendations, each of these 45,978 airplanes would be required to undergo a detailed inspection of its aileron and elevator/stabilator control cable attach fitting at regular calendar intervals. Further, any attach fittings showing outward signs of corrosion would be required to be replaced, necessitating a replacement of the entire control cable and re-rigging of the airplane's control surfaces. As the control cable in question is of an industry standard design specification and standard material specification, it is highly likely that it is utilized in a very large portion of the piston GA fleet. The NTSB's recommendations urge the FAA to apply any airworthiness action to all aircraft utilizing this part.

Significant provisions:

  • Safety Recommendation A-01-6 recommends that the FAA institute an AD mandating repetitive inspection of control cable attach fittings on Piper PA-28s, Piper PA-44s, and Cessna 172s more than 15 years old.
  • Safety Recommendation A-01-7 recommends that the FAA list all airplanes utilizing control cable attach fittings constructed of SAE-AIAI 303 Se stainless steel and implies that the FAA should expand the applicability of the AD to include these airplanes.
  • Safety Recommendation A-01-8 recommends that the FAA notify all manufacturers of fixed and rotor wing aircraft of the current corrosion and pitting problems with control cable attach fittings constructed of SAE-AISI 303Se stainless steel.
  • NTSB Safety Recommendations A-01-6 through �8 cite 10 instances of control cable attach fitting failures on Piper PA-28, -44, and Cessna 172 airplanes. Nine of the reported failures occurred on Pipers, one on a Cessna 172.
  • The Piper airplanes utilized stainless steel control cable attach fitting (Mil Spec MS21260-S4), while the Cessna utilized a stainless steel fitting manufactured to the AN669 specification (the precursor to the MS21260 terminal).
  • The fittings installed on NTSB referenced Piper airplanes were constructed of SAE-AISI 303Se stainless steel.
  • The fittings installed on NTSB referenced Cessna airplane was manufactured to a specification that provided a relatively greater degree of flexibility in the material of construction. However, NTSB chemical analysis determined that the stainless steel utilized in this fitting most closely matched that of an SAE-AISI 303 Se stainless steel.
  • Analysis of the failed fittings revealed chloride stress-corrosion cracking that had originated at general corrosion pits on the surface of the fittings. Surface pitting was identified as the origin of the cracks that led to part failures.
  • None of the terminal fractures resulted in a serious accident or loss of life. However, three of the cited failures resulted in in-flight loss of stabilator control.
  • FAR Part 43 Appendix D currently requires a thorough inspection of control cable attach fittings for proper installation, apparent defects, and unsatisfactory operation during routine annual and 100-hour inspections.

AOPA Position:

Safety recommendation A-01-8 recommends that the FAA alert manufacturers of the current cracking and corrosion problems with terminals made from SAE-AISI 303 Se stainless steel. Although AOPA suspects that the current cracking and corrosion problems with these fittings are specific to particular airframe applications, we do not oppose this recommendation. However, Safety Recommendations A-01-6 and �7 warrant considerable research before they're considered by the FAA. AOPA's preliminary consultations with owners and operators of affected airplanes indicate that this problem may not be as widespread as the board suspects. Although AOPA recognizes the severity of an in-flight loss of aileron or elevator/stabilator authority, we can support the institution of a mandatory mitigating action only when the perceived problem is confirmed through the real world operational and maintenance experiences of owners and operators of affected airplanes. Thus, AOPA looks forward to working with the FAA, aircraft type-clubs, and owners and operators of affected airplanes to uncover the true extent of this problem and the best means of resolution.

Status:

  • On April 17, 2001 AOPA sent a letter to the NTSB expressing our concern over the extent of the mitigating actions proposed in Safety Recommendations A-01-6 through �8.
  • On April 16, 2001 NTSB published Safety Recommendations A-01-6 through �8.

Related Documents:

AOPA letter to acting NTSB Chairman Carol Carmody, April 17, 2001 (requires Adobe Reader)

NTSB Safety Recommendations A-01-6 though �8 (requires Adobe Reader)