Low-wing Piper spars life-limited in new directive

The FAA published two new proposed ADs in recent weeks that continue a series of actions and investigations the FAA began following a 2018 wing separation accident involving a Piper Arrow that killed a commercial pilot applicant and designated pilot examiner. The agency has since noted fatigue cracks of the main spar were also found in two additional accidents involving PA–28 series Pipers, in 1987 and 1993.

The FAA published AD 2020-26-16 a little more than two years later, updating guidance and directives dating back to 2018, and requiring eddy current inspections of thousands of Piper wing spars, including various PA–28 and PA–32 series models.

The results of those inspections informed a new directive, AD 2024-00008-A, published on September 19, followed by AD 2024-00033-A, published on September 23.

The new directives establish or expand a wing spar inspection regime to include all PA–28 and PA–32 series single-engine Piper aircraft. The FAA noted that all of these aircraft have a common spar design, with varying degrees of reinforcement in the original structure.

The FAA noted that a redesigned wing spar assembly is available from Piper, as well as 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—up to 25,000 hours for a reinforced spar, instead of 12,000 hours or 13,499 hours for an unmodified wing spar. 

The FAA also updated the formula for calculating service time that enables risk-based grouping of aircraft (the same formula for calculated service hours, or CSH, that was previously detailed in Piper Service Bulletin 1372 published on April 3) to determine required inspection intervals, and establish life limits for wing spars made with a particular process that Piper has now discontinued: bending the cold spar to create the dihedral.

“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. 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,” the FAA wrote in the AD published September 23 that applies to an estimated 10,665 aircraft previously subject to required spar inspections under the 2020 directive. “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.”

An identical paragraph appears in the AD published on September 19, which applies to an estimated 10,927 PA–28 and PA–32 series aircraft that will now be subject to initial and recurring eddy current spar inspections based on the aircraft’s history, including (calculated) time in service and other factors.

Comments on both directives will be accepted into early November.

“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,” said AOPA Vice President of Regulatory Affairs Murray Huling. “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.”

Cracks found

The FAA and Piper received more than 2,800 bolt-hole eddy current inspection reports as required by the 2020 directive (that was published in 2021, and estimated to apply to 5,440 airplanes), and more than 100 of these “reported a positive eddy current indication, with several including pictures of the bolt hole showing the source of the indication.”

While positive findings included various anomalies other than fatigue cracks, the FAA noted that six fatigue cracks were found in and around bolt holes, three of them verified as fatigue cracks by the NTSB and Piper.

“Other known cracks include those found in an airplane in the same fleet as the 2018 accident airplane, a separately submitted crack finding confirmed with dye penetrant, and a crack located on the lower spar cap surface running alongside the inspection bolt holes,” the FAA wrote. “Given these findings, additional cracks may be present among the other unconfirmed reported indications.”

The FAA noted that fatigue was not the only cause of cracks discovered during the investigations:

“Other cracks have been discovered that may be caused by overload rather than by fatigue. While use of the airplane within its limits should not cause an overload crack, some crack findings have revealed that airplanes have been operated outside their limits. Though cracks due to overload are not the primary source of this corrective action, this emphasizes the need for and importance of inspecting the spar bolt holes for evidence of any cracking.”

Spar modification or replacement

The FAA estimates that replacing the wing spar is less expensive than installation of the reinforcement kit, though the agency acknowledged it has no way to know how many aircraft might require either alteration. Spar replacement is estimated to cost $14,383 per wing (including 40 hours of labor), while reinforcing the wing via installation of the $4,000 kit is estimated to cost $20,150 installed, per wing, including 190 hours of labor. (AOPA contacted the FAA, Piper, and maintenance providers seeking clarification on why the spar reinforcement kit installation requires significantly more shop time than a spar replacement.)

While much of the September 23 AD references the Piper Service Bulletin that details the inspection process, the agency did establish a different inspection schedule for the “Group 1” and “Group 2” aircraft than what Piper previously stipulated.

For Group 1 aircraft (generally newer PA–28s) initial inspections are required at 3,000 CSH, and thereafter at intervals ranging from 1,750 CSH down to 500 CSH, up to 13,499 hours, at which point the spar must be replaced, or a reinforcement kit installed. (Piper’s service bulletin called for an initial inspection at 5,000 CSH.)

Aircraft in this group with kits installed are inspected upon kit installation, and thereafter every 4,800 CSH up to 13,499 hours, and not to exceed every 3,700 hours thereafter, with a 25,000-hour limit on the spar.

The FAA established the same compliance times and inspection intervals for Group 2 aircraft that Piper set forth in Service Bulletin 1372, starting with an initial inspection at 4,500 CSH, and repetitive inspections every 400 CSH thereafter up to 11,999 CSH, when the spar must be replaced or reinforced (if a kit becomes available for Group 2 aircraft).

“Both the FAA and Piper attempted to determine an inspection program that would manage risk to an acceptable level using inspection alone; however, no method could be found that did not eventually require spar replacement,” the FAA noted. The agency, like Piper, also emphasized the importance of following the prescribed procedures precisely to avoid damaging the wing spar assembly during inspection, which would then require spar replacement: “Ensuring further damage is not caused by the inspection itself is important, especially with repetitive inspections; however, inspecting for fatigue cracks as well as other hole anomalies is critical and outweighs the risk associated with repetitive inspections.”