New Bern, NC, USA
N30521
Piper PA-44-180
During initial climb, the certified flight instructor (CFI) noticed very little resistance from the stabilator; however, he was able to land the airplane uneventfully using stabilator trim. Examination of the airplane revealed that the stabilator control cable turnbuckle terminal fitting had failed due to corrosion. The National Transportation Safety Board had issued a recommendation to the Federal Aviation Administration (FAA), more than 6 years prior to the incident. The Safety Board recommended that the FAA issue an airworthiness directive (AD) to require recurrent visual inspections for evidence of corrosion on control cable terminals, and the inspections should be performed after removal of any safety wire or safety clips on the terminals. At that time, the airplane manufacturer had committed to revising its maintenance manuals to include similar instruction. At the time of the incident, the airplane manufacturer had issued a service letter, but had not yet updated their maintenance manuals; and the FAA had issued a non-mandatory Special Airworthiness Information Bulletin in lieu of an AD. A mechanic performed an annual inspection of the incident airplane, and returned it to service, 21 days prior to the incident. The mechanic stated that he inspected the incident airplane's control cables for damage during its recent annual inspection; however, he believed he missed the corrosion on the turnbuckle terminal fitting due to a safety wire that covered it. The mechanic added that when he performed the annual inspection he was not aware of a manufacturer service letter or an FAA SAIB regarding the inspection for corrosion.
On August 16, 2007, about 1830 eastern daylight time, a Piper PA-44-180, N30521, sustained minor damage during the initial climb from Craven County Regional Airport (EWN), New Bern, North Carolina. The certificated flight instructor (CFI) and a student pilot were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local instructional flight conducted under 14 Code of Federal Regulations (CFR) Part 91. According to the CFI, the student pilot had just completed a touch-and-go on runway 22 at EWN. The CFI took control of the airplane to demonstrate a maneuver, and noticed that there was very little resistance from the stabilator. He then checked all trim settings, trimmed the airplane for climb, and continued to fly the traffic pattern for runway 22. There was still little resistance from the stabilator, but the CFI was able to land the airplane without incident. Subsequent examination of the airplane by maintenance personnel revealed that the "cable assembly - stabilator control, aft, lower," (turnbuckle terminal fitting, part No. 62701-153) was fractured. The turnbuckle terminal fitting was forwarded to the National Transportation Safety Board Materials Laboratory, Washington, D.C., for further examination. The examination revealed that most portions of the fracture area were covered by corrosion deposits, and similar corrosion deposits were noted on the exterior surface of the terminal, in the radius adjacent to the fracture. The airplane was manufactured in 1978. On April 16, 2001, the Safety Board issued Safety Recommendation Letter A-01-6 to the Federal Aviation Administration (FAA). The safety recommendation letter stated, in part: "Since March 1998, the National Transportation Safety Board has learned of six airplanes on which flight control cable terminals have fractured. Additional terminals from some of these six airplanes and terminals from four other airplanes were also examined and found to be cracked...The examination identified chloride stress-corrosion cracking...Airplanes operating in a warm, humid, salty air environment are most likely to develop these stress corrosion cracks..." The recommendation letter also stated that New Piper Aircraft committed to revise each of its maintenance manuals to specifically require a magnified visual inspection of all cable fittings for corrosion during scheduled maintenance checks. The letter further stated, "Inclusion of these inspections in the service manuals, however, will not make them mandatory, and years may elapse before Piper updates each of its maintenance manuals." Safety Recommendation A-01-6 stated: "Therefore, the National Transportation Safety Board recommends that the Federal Aviation Administration: Issue appropriate airworthiness directives, applicable to Piper PA-28, Piper PA-44, and Cessna 172 series airplanes older than 15 years, to require recurrent visual inspections, on an appropriate, calendar-based interval, for evidence of corrosion pits or cracking on control cable terminals that were or may have been constructed from SAE-AISI 303 Se stainless steel. The inspections should be performed after removal of any safety wire or safety clips on the terminals. Require that any terminals with signs of corrosion or cracking be removed from service immediately." According to the FAA's response, available on the Safety Board's website: "The Federal Aviation Administration (FAA) has received the information from aircraft manufacturers and type clubs on their review of this safety recommendation. All aircraft manufacturers have confirmed that most aircraft use SAE AISI 303 Se stainless steel material for flight control systems. The review revealed factors like water, exhaust gases, dissimilar metals, and battery off-gasses may contribute to corrosion problems. The review found no evidence of corrosion or pitting in the aircraft fleet, but found corrosion in isolated specific airframe applications in some Piper models where the battery is located very close to the area in which control cables enter the bottom of the fuselage. The Piper PA-12, -14, -18, -28 and -28R have batteries installed in the tail cone and have reported 18 cases of corrosion in a fleet of 14,564 aircraft. There were no other reports of cable problems on any other Piper models. Cessna 172 reported 11 cases of broken/frayed cables in a fleet of 24,925 aircraft. All aircraft manufacturers mandate an inspection of control cable attach fittings at annual or l00-hour inspection intervals. None of the occurrences resulted in any serious accidents, and most of the occurrences were discovered during maintenance activity. Based on the results of the evaluation, the FAA does not believe airworthiness directive action is warranted. The Aircraft Owners and Pilots Association (AOPA), based on input from its aircraft owners/operators, suggested that the FAA issue an Alert Service Bulletin or a Special Airworthiness Information Bulletin (SAIB) as an alternate solution. The FAA concurs with AOPA and on November 1, 2001, issued SAIB CE-02-05 recommending that owners or operators of rotary and fixed-wing aircraft inspect the flight control cables at 100 flight hour intervals or at each annual inspection. If corrosion is found, it is recommended that the control cable attach fittings be replaced even if the manufacturer's maintenance manual does not recommend replacement of corroded fittings. I have enclosed a copy of the SAIB for the Board's information. I consider the FAA's action to be completed on this safety recommendation." The Safety Board noted the recommendation closed with an acceptable response. According to a representative from Piper Aircraft Inc., the company issued service letter (SL) 1069 on March 31, 2003. The SL stated: "...New Piper is at this time in the process of revising our Service/Maintenance Manuals. When each manual comes up for revision, we are adding in the scheduled maintenance checks notes the following: 'For airplanes 15 years or older, using a 10X magnifier, visually inspect the entire surface of each cable terminal, turnbuckle, or other cable fitting for corrosion or cracking. Any evidence of corrosion or cracking, however minute, is cause for replacement...'." At the time of the incident, the incident airplane's maintenance manual did contain instructions for checking control cables and fittings for slippage, distortion, broken strands, chafing, deterioration, etc.; however, there was no specific mention of or guidance about corrosion. The maintenance manual had not yet been revised to contain the information in SL 1096. Additionally, CFR Part 91 operators were not required by the Federal Aviation Regulations (FARs) to comply with an SL or SAIB. Review of the incident airplane's maintenance records revealed that its most recent annual inspection was completed 21 days prior to the incident. The mechanic that performed the annual inspection, and returned the airplane to service, was interviewed via telephone. The mechanic stated that all of the operator's maintenance was accomplished "in-house." The mechanic inspected the incident airplane's control cables for damage during its recent annual inspection; however, he believed he missed the corrosion on the turnbuckle terminal fitting due to a safety wire that covered it. The mechanic was asked if he was familiar with a Piper Aircraft SL or an FAA SAIB regarding the inspection for corrosion. He stated no, but that he has heard mention of them subsequent to the incident.
A failure of the stabilator control cable turnbuckle terminal fitting due to corrosion. Contributing to the accident were the mechanic's inadequate annual inspection, and the aircraft manufacturer's failure to provide adequate information in its maintenance manuals regarding the inspection of control cable terminals for corrosion.
Source: NTSB Aviation Accident Database
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