The American National Transportation Safety Board (NTSB) has joined a number of other groups, including AOPA, Piper Aircraft and the Piper Owners Society, in urging the FAA to reconsider its proposed Airworthiness Directive calling for the inspection of Piper PA-28 wing spars for metal fatigue. The AD would be applicable to numerous PA-28 and PA-32 airplanes that have reached a certain threshold of time in service and other factors.
The proposed AD was prompted by a fatal accident last year involving the separation of a wing from an Embry-Riddle Aeronautical University-operated PA-28-R-201 with 7,690 flight hours. According to the FAA, the proposed AD is the result of preliminary investigations that revealed fatigue cracking of a lower main spar cap on the subject aircraft, which is in a visually inaccessible area of the lower main wing spar cap. Should an inspection reveal the need to replace the wing spar, the FAA estimates the cost to be approximately $8,260 (USD) each.
The letter from the NTSB to the FAA stated, in part:
While the affected airplanes all have a similar main spar design in the wing attach-point area where the fatigue cracking was found, engineering data presented by Piper separately to the FAA and the NTSB showed that the localized stress level can vary significantly depending on airplane gross weight, cruising speed, and amount and location of fuel in the wings. The data also showed that the airplanes at greatest risk for fatigue cracking are the PA-28-235 model airplanes, all PA-28R series airplanes, and the PA-32-260 and PA-32-300 model airplanes. The NTSB supports the inspection requirements of the proposed AD for these airplanes.
However, the NTSB notes that the data showed that the risk of fatigue cracking on all affected PA-28 series airplanes other than the PA-28-235 is significantly lower over their assumed useful life. We are concerned that the risks associated with disturbing the joint to complete the inspection may outweigh the risk of fatigue cracking in all affected PA-28 series airplanes other than the PA-28-235 and urge the FAA to re-examine the applicability of the proposed AD.
The complete text of the proposed AD can be found here.