Beech C90 N3688P accident near Wikieup, Arizona

Probable cause

The failure and separation of the left wing’s outboard section due to a fatigue crack in the lower spar cap. Contributing to the accident was the operator’s decision to repair the wing spar instead of replacing it as recommended by the aircraft manufacturer. Also contributing to the accident was the failure of the Non-Destructive Testing inspector to detect the fatigue crack during inspection.

NTSB analysis

The pilot was conducting a firefighting support flight, with an Air Tactical Group Supervisor on board the airplane. Automatic Dependent Surveillance Broadcast (ADS-B) data indicated that the airplane was on station for about 45 minutes completing multiple orbits over the area of the fire. The last ADS-B data point showed the airplane in a descent, and its airspeed was about 151 knots at an altitude of about 2,300 ft agl.

According to a witness, the airplane descended in a steep dive and impacted the side of a ridgeline in mountainous desert terrain. No distress call from the airplane was overheard on the radio. Another witness observed the outboard left wing falling to the ground after the aircraft had impacted the terrain. The outboard left wing, which had separated outboard of the engine nacelle, was located about 0.79 miles northeast of the main wreckage and did not sustain thermal damage.

During a scheduled maintenance inspection several months before the accident, eddy current (EC) non-destructive testing (NDT) of a left wing’s lower forward spar cap detected a crack in a fastener hole. The hole was then oversized/reamed to a larger size, but the EC reinspection still produced a crack indication. The operator then submitted a structural damage report and service request detailing the crack indication to the aircraft manufacturer. The aircraft manufacturer responded to the operator that the crack indication necessitated the replacement of “the center section forward spar cap, center section forward lower fittings and both outboard main spar assemblies.”

However, the operator and their maintenance provider elected to repair the wing spar instead of replacing the spars, as indicated by the aircraft manufacturer. The maintenance facility owner contacted a Federal Aviation Administration (FAA) Designated Engineering Representative (DER) for the design of the repair. The repair involved oversizing the affected fastener hole and installing an external doubler around the hole location. The repair was installed and signed off several months before the accident. The DER claimed no knowledge of the communication between the aircraft manufacturer and the maintenance provider about the crack indication, though the maintenance provider claimed otherwise.

After the repair, an eddy current inspection conducted by a commercial NDT inspector showed the wing spar repair to be successful and did not reveal a crack indication. Further, there were no other crack indications on the airplane.

A postaccident examination of the spar fracture surface revealed that the left wing’s lower spar cap fractured from a fatigue crack that initiated at the aft inboard fastener hole. The fatigue crack measured 2.484 inches in length and exhibited striations consistent with crack propagation. A study comparing the crack length. striations, flight hours, and number of cycles suggests the crack was large enough to have been seen visually at the last inspection. Therefore, it is likely that the NDT inspector omitted the EC inspection of the fastener hole or missed the fatigue crack indication given its length.

The fatigue separation of the lower spar cap was not in the same area where the repair was accomplished. The repaired area was inboard of where the left wing separated. The DER-approved repaired area was not identified in the recovered wreckage and therefore could not be examined. Nevertheless, the area of the wing spar fatigue crack would have been removed from the airplane if the airplane’s operator and maintenance provider had followed the procedure outlined by the aircraft manufacturer, which noted that the crack indication necessitated the replacement of the spars.

Text from the NTSB. Open the NTSB docket for WPR21FA266 for the full record.