PIPER PA38 — Franklin, AR

What happened

The instructional flight with a flight instructor and a private pilot receiving instruction had been airborne about 2 hours when the accident occurred. During the final portion of the flight, air traffic control had issued an instrument flight rules (IFR) clearance and established radar contact and voice communication with the flight. When the airplane was about 60 nautical miles west of the intended destination, radar contact was lost, and voice communication became intermittent and was eventually lost. The final transmissions received from the airplane were unclear.

The airplane’s flightpath after receiving the IFR clearance was erratic. During the final 5-1/2 minutes of the flight, the airplane made 3-3/4 left turns of varying radius, followed by 1-1/4 right turns. The accident site was located about 0.2 miles from the last recorded position. The airplane was destroyed during the impact with trees and terrain.

The airplane’s cabin heating system used a heat shroud over the exhaust system mufflers to draw heat for the cabin. A postaccident metallurgical examination of the airplane’s exhaust system revealed that one of the mufflers had several fractures present before the accident crash sequence. No other anomalies were detected regarding the airplane, engine or systems that would have precluded normal operation. The compromised muffler allowed combustion gases from the engine to enter the cabin heating system.

The airplane received a 100-hour inspection, and an annual inspection 10 days before the accident. The 100-hour inspection noted that the inspection was performed in accordance with the airplane service manual. The service manual featured a warning that a very thorough inspection of the exhaust system be accomplished at each 100-hour inspection. Based on the condition of the fractured muffler, it is likely that the heat shroud was not removed to facilitate inspection of the mufflers and the degraded condition of the fractured muffler was not detected during the inspections.

The flight instructor’s toxicology testing detected carboxyhemoglobin at 29%, consistent with inhalation of carbon monoxide escaping into the cabin from the cracked muffler. Toxicology testing also detected past use of the stimulant methamphetamine and its metabolite amphetamine, which were detected in his urine but not detected in his cavity blood. Therefore, the identified methamphetamine and its metabolite in the flight instructor were not a factor in this accident. Although a low concentration of ethanol and n-butanol were detected in the flight instructor’s muscle tissue, their absence in liver tissue makes it likely that the identified ethanol was from sources other than ingestion and did not contribute to the accident.

The private pilot’s toxicology testing did not detect carboxyhemoglobin; however, the tests reporting limit for carboxyhemoglobin is 10%. Therefore, although the toxicology results did not report a carboxyhemoglobin level above 10% for the private pilot, it is reasonable based on the operational evidence that the private pilot was also impaired and unable to either take control of the aircraft or communicate with controllers. Toxicology testing also identified the sedating antihistamines diphenhydramine and cetirizine in the private pilot’s specimens. While both could cause drowsiness, given the circumstances of this accident and the presence of the flight instructor, the effect of the private pilot’s use of diphenhydramine and cetirizine were not factors in this accident.

The operational evidence, the erratic flightpath, and the unclear communication with controllers, along with the compromised muffler due to fatigue cracking support both pilots’ impairment due to carbon monoxide exposure.

Furthermore, it is likely that inadequate maintenance inspection failed to detect the compromised muffler, which then allowed carbon monoxide from engine combustion to enter the cabin through the airplane’s cabin heating system during flight.

An editorial "what led to it / how to avoid it" analysis for this accident is generated separately and will appear here.