Fatigue may have been behind the fatal April 4 crash in Florida that killed two people – but not pilot fatigue.

An investigative update issued by the National Transportation Safety Board (NTSB) revealed that “fracture features” consistent with metal fatigue were found on more than 80 percent of the lower spar cap and portions of the forward and aft spar web doublers of the left wing main spar on the Piper PA-28R-201. The fatigue features originated at or near the outboard forward wing spar attachment bolt hole.

(In the NTSB photo above, fracture features consistent with metal fatigue can be seen in the aft spar web doubler of the Piper PA-28R-201.)

Left wing came off

The plane experienced an in-flight separation of the airplane’s left wing shortly after takeoff. It went down near Daytona Beach at about 10 a.m., claiming the life of the Embry-Riddle Aeronautical University student who was piloting it and the pilot examiner with the U.S. Federal Aviation Administration who was also onboard. The airplane was registered to and operated by Embry-Riddle Aeronautical University as an instructional flight.

The preliminary report for the accident was published April 16. Parties to this investigation include the FAA, Piper Aircraft and Embry-Riddle Aeronautical University.

Right wing also had fatigue cracks

None of the surfaces exhibited visible evidence of corrosion or other preexisting damage. The right wing also exhibited fatigue cracks in the lower spar cap at the same hole location extending up to 0.047-inch deep. The remainder of the lower spar cap, spar web doublers, and upper spar cap displayed fracture features consistent with overstress.

The structures group of the NTSB’s investigation conducted an inspection of another Piper PA-28R-201 April 18 and 19. The plane inspected had a similar number of total airframe hours and cycles and was used exclusively for flight training of students. That inspection revealed a crack indication at the left lower outboard forward wing spar attachment bolt hole. The crack measured about 0.040-inch long and deep. The airplane’s wings were subsequently reinstalled and examined using new inspection procedures developed by Piper Aircraft. A bolt-hole eddy current inspection probe was used to confirm the location and size of the previously identified crack.

Nine additional PA-28R-201 airplanes have been inspected using ECI techniques under NTSB supervision. No crack indications were detected in these nine inspections.

Investigation is ongoing

The NTSB investigative team is examining corrective actions taken in response to the March 30, 1987, Piper PA-28-181 in-flight wing separation which resulted in three recommendations to the FAA and a subsequent Airworthiness Directive, which has since been rescinded.

The NTSB’s investigation of this crash is ongoing and as such, no conclusions about probable cause should be drawn from this investigative update or the preliminary report. Additional information will be provided as warranted.

The complete investigative update is available at https://goo.gl/GUVLwg