17 October 2024
The long-running saga of Piper aircraft wing spars is back with two new Airworthiness Directives proposed by the US regulator, the FAA. Up to 21,000 aircraft could be affected.
The new directives are: AD 2024-00008-A, published on 19 September, followed by AD 2024-00033-A, published on 23 September. The FAA is accepting comments on the proposed ADs until 7 November 2024.
The new ADs are the result of eddy current inspections of thousands of Piper wing spars made mandatory in a previous directive.
The proposed AD 2024-00033-A says, “The FAA evaluated the reports required by AD 2020–26–16 and determined that repetitive inspections of the lower main wing spar bolt holes for crack(s) and non-crack damage (including deep scratches, gouges, and thread marks) and replacement or modification of the main wing spar should be required, calculated service hours (CSH) should be used instead of FSH (factored service hours) to determine times for required actions for each main wing spar, and that certain airplanes should be removed from the applicability and a new airplane model added to the applicability.
“This proposed AD would require calculating the CSH for each main wing spar; repetitively inspecting the lower main wing spar bolt holes for crack(s) and non-crack damage and taking corrective actions as needed; and replacing or modifying main wing spars at a specified time.”
Piper has been involved throughout and conducted more detailed inspections in a study of 24 main wing spars with 20 having positive eddy current* indications.
“In an attempt to support less onerous inspections and to understand the causal factors, Piper investigated the residual stresses in the critical bolt-hole area,” said the FAA.
“That investigation showed that the residual stress due to the spar cold bending process is a significant contributing factor in reducing the fatigue life of the spar bolt holes. An additional outcome of this investigation is a change to all new manufactured spars having machined dihedral bends to eliminate the residual stresses in the critical area.”
Piper has made available a redesigned wing spar assembly and also a reinforcing kit that can be installed on many newer PA–28s including the Cherokee, Warrior, Archer, and Piper Pilot models.
The spar reinforcement kit enables longer inspection intervals, and a longer life limit of up to 25,000 hours for a reinforced spar, instead of 12,000 hours or 13,499 hours for an unmodified wing spar.
The Aircraft Owners and Pilots Association (AOPA) in the US has been following the process. Murray Huling, Vice President of Regulatory Affairs, said, “We’re relieved to see that Piper has designed a reinforcing kit that will help ease the burden on owners and mitigate the safety concern for many aircraft.
“We’re also glad to see a spar replacement option now exists that eliminates the need for recurring inspections and corrects the original defect. That said, these directives pertain to a very large number of aircraft and implement a complex formula for assessing risk that we’ll need to look at closely before making our formal comments on these proposed directives.”
The proposed Airworthiness Directives can be downloaded here.
*Eddy current testing is an electrical non-destructive testing (NDT) inspection method used for a variety of purposes, including for flaw detection, material and coating thickness measurements, material identification and establishing the heat treatment condition of certain materials.
