What happened
On March 2, 1996, a Cessna A 185 F crashed into a coniferous forest near Trossingen, Germany, resulting in three fatalities (the pilot and two passengers). The aircraft was engaged in a series of flights to drop skydivers. After performing several jumps near Leutkirch and Augsburg, the pilot intended to drop the remaining two passengers near Trossingen.
Witnesses near Trossingen observed the aircraft flying at a low altitude in an uncoordinated, undulating manner, accompanied by fluctuating engine noise. The aircraft then performed a steep climb followed by a sudden bank to the left, plunging into the trees. The impact caused the aircraft to be destroyed and immediately catch fire.
The investigation
The BFU investigation focused on determining the cause of the aircraft's erratic flight path. While the wreckage was heavily damaged, investigators found no defects in the flight control surfaces that could account for the loss of control.
Toxicological analysis revealed a carboxyhemoglobin (CO-Hb) level of approximately 52% in the pilot, with similarly high levels found in the two passengers. Investigators ruled out the post-crash fire as the cause of the high CO levels.
Technical examinations of the exhaust system, conducted in coordination with the police and the Technical University of Braunschweig, identified cracks in the exhaust collector (muffler). These cracks were located on the weld of the outlet pipe, hidden beneath reinforcement plates. The presence of vibration patterns and soot deposits confirmed these were fatigue cracks that had developed over time.
Findings
- The primary cause of the accident was the incapacitation of the pilot due to severe carbon monoxide poisoning.
- Exhaust gases leaked from the muffler into the cabin heating system, which used air warmed by the exhaust collector to provide heat to the cockpit.
- Fatigue cracks had formed in the exhaust collector, allowing gases to escape into the heating ducts.
- The cracks were located beneath reinforcement plates, making them impossible to detect during standard visual inspections.
- The installation of an additional silencer increased exhaust backpressure, which likely promoted the escape of gases through the existing defects.
Safety action
Following the investigation, the BFU issued safety recommendation 03/96, suggesting that for aircraft equipped with additional silencers, the monitoring of cabin air quality should be mandated in addition to standard maintenance interval inspections.