What happened
On November 9, 2018, a Piper PA28-236, registration N91770, was involved in a fatal accident near Guthrie Center, Iowa. The flight was a personal trip from Le Mars Municipal Airport to Osceola Municipal Airport.
Witnesses at the departure airport noted that during the initial engine start, the engine experienced multiple attempts characterized by loud popping and backfiring. On the final attempt, the engine started and rapidly accelerated to a high speed. One witness observed that the interior of the aircraft windows appeared covered in mist prior to takeoff.
During the flight, a student pilot passenger contacted air traffic control, reporting that the pilot was experiencing a heart attack and that he intended to divert the aircraft to Perry Municipal Airport. The student later communicated via a common traffic advisory frequency that he would instead attempt to land at Guthrie County Regional Airport. GPS data showed the aircraft flying in an erratic manner, performing climbs and descents over the area of the diversion airport, before ultimately colliding with terrain in a wings-level attitude.
All four occupants sustained 4 fatal injuries.
The investigation
Investigators examined the aircraft's exhaust system, which utilized a heat exchanger shroud around three mufflers to provide cabin heat. The examination of the aft muffler revealed a longitudinal crack at a seam weld and a circumferential crack. The wall thickness at the site of the longitudinal crack was measured at only 0.018 inches, significantly thinner than the 0.109 inches found near the middle of the muffler.
Analysis of the heat exchanger shroud and cabin heat supply tube revealed tan and grey particulate deposits. Laboratory analysis confirmed these deposits contained high levels of lead and bromine, which are consistent with engine exhaust.
Maintenance records indicated the aircraft had undergone an annual inspection on November 3, 2018. While the mechanic reported inspecting the mufflers for cracks or discoloration, the investigation noted that the crack and perforations might have been obscured by oxides and deposits.
Findings
- Toxicology testing revealed high levels of carboxyhemoglobin in all occupants, with levels reaching 58% and 48% in the pilots and 41% and 20% in the passengers.
- The pilot's reported symptoms, initially believed to be a heart attack, were consistent with carbon monoxide poisoning.
- The engine's aft muffler had a crack that allowed exhaust gases to enter the cabin through the heat system.
- The crack in the muffler was likely the result of corrosion and thinning of the muffler wall.
- The aircraft was not equipped with a carbon monoxide detector.