Overview
On 12 May 2001, a Piper PA-24-250 Comanche, registration G-ARIE, crashed at Osea Island, Essex. The aircraft was destroyed, and both occupants died in the impact. The two crew members were the pilot and a qualified flying instructor, who were conducting a flight to revalidate the pilot's Private Pilot Licence.
History of Flight
The pilot, a part owner of G-ARIE since 1995, had arranged a one-hour dual flight with a flying instructor. An acquaintance observed the pilots before departure, noting no unusual events except a backfiring sound during engine start. Takeoff occurred around 1515 hrs under CAVOK conditions with a northeast wind of 10-12 kt.
Numerous witnesses on the ground near Osea Island saw the aircraft descending in a nose-down spiral or spin. A pilot flying a Yak 11 in the area observed the Comanche spinning to the right at a nose-down pitch of about 60°. He counted four to six turns before impact and broadcast a Mayday call at 1527 hrs. Radar tracking showed the aircraft's groundspeed varied, and it did not indicate extreme maneuvering before loss of contact. No emergency call from G-ARIE was recorded.
Medical and Operational Findings
Post-mortem examinations revealed elevated carboxyhemoglobin levels in both pilots, consistent with carbon monoxide inhalation during life. The symptoms of carbon monoxide exposure include confusion and impaired judgment, which can affect pilot performance. The aircraft had flown on 6 May 2001 without issues, and no carbon monoxide symptoms were reported then.
The flight was intended for PPL revalidation, which includes exercises such as stalls and incipient spin recovery. The Comanche is not cleared for intentional spinning, and its stall speed is approximately 60 kt clean. The aircraft was within normal weight and center of gravity limits. Post-accident calculations indicated the propeller was rotating with very little power at impact, and the aircraft struck the ground in a steep nose-down attitude consistent with a spin to the right.
Investigation and Cause
Examination of the wreckage revealed a fatigue crack in one of the exhaust stub pipes near its attachment flange to the cylinder head. The crack allowed exhaust gases to enter the cockpit, causing carbon monoxide contamination. The formation of the crack was influenced by a reinforcing doubler welded to the pipe. The exhaust manifold was manufactured under a PMA by a company other than the aircraft manufacturer and had been installed without a corresponding logbook entry. The Type Certificate holder considered the addition of the doubler an unacceptable design change.
The AAIB determined that the carbon monoxide exposure impaired both pilots, leading to loss of control and an unrecovered spin.