Amateur-built LANCAIR N846PM accident near Firebaugh, California

Probable cause

A failure in the windshield, frame, or windshield-to-frame bond, which resulted in a rapid decompression during cruise flight.

NTSB analysis

The pilot reported that, while in cruise flight at 25,000 ft, the airplane's windshield shattered, immediately followed by a rapid decompression of the cabin. The pilot shut down the engine, entered an emergency descent, and navigated to a nearby airport. During the approach for landing, the pilot received an anomalous landing gear indication and chose to perform a gear-up landing. The airplane touched down with an 8-knot tailwind and slid along the runway until it impacted a fence, continued across a road, and came to rest in dirt. Both wings were substantially damaged.

Postaccident testing showed that the relay modules responsible for opening and closing circuits to the left main landing gear were dislodged when the windshield shattered, resulting in the anomalous gear indication. The landing gear system functioned normally once the modules were reinstalled.

A portion of the windshield that remained attached to the frame exhibited a fracture consistent with pure tension loading, likely from internal pressurization. Due to the curvature at that location, the fracture area was likely one of high stress on the windshield. Examination of the windshield fragments and frame did not show any evidence of impact from a foreign object, but showed significant structural flexing of the fuselage due to delaminated wet-layup plies during construction, as evidenced by extensive cracking at the aft engine cowling. However, the absence of an airplane structural analysis from the manufacturer precluded further review of the structural flexing of the fuselage. A material and bond evaluation showed that the windshield frame material did not exceed its glass transition temperature, indicating that the acrylic windshield was likely intact at the time of the failure; however, further analysis of the bond between the windshield and window frame could not be performed, as the manufacturer did not provide bond strength design values for either section.

Several other factors in the construction of the windshield may have contributed to increased stresses at the location of a key fracture, which may have caused cracks to initiate and grow. In particular, parallel grinding marks around the edges of the windshield were inconsistent with the use of a dual-action grinder with 40-grit sandpaper as prescribed by the windshield installation instructions. These grinding marks may have introduced stress concentrations at the edge of the windshield and may have contributed to one of the primary windshield fractures. The overall appearance and mechanical behavior of the adhesive during the examination suggests that the bond strength between the windshield and frame was relatively low. Thus, the presence of disbonded areas may have affected the stress state on the windshield and contributed to increased stress at the primary fracture area. Finally, the overall composite layup varied from the specified layup schedule for the fuselage skin at two locations on the windshield frame. Alterations in the material used in the fuselage skin can change the elastic response to a given load. The change in the layup schedule and elastic response may have increased the stress at the location of the primary fracture area.

Text from the NTSB. Open the NTSB docket for WPR17LA104 for the full record.