What happened
On 12 May 2001, a Piper PA-2/4-250 Comanche, registration G-ARIE, was conducting a private flight near Osea Island, Essex, for the purpose of re-validating a pilot's licence. The flight involved a pilot and a qualified flying instructor. During the flight, the aircraft entered a steep, descending spiral. Witnesses on the ground and a pilot in a nearby aircraft observed the plane spinning to the right with a significant nose-down pitch. The aircraft impacted the ground at a high angle, resulting in 2 fatalities and the total destruction of the aircraft.
The investigation
Investigators focused on the cause of the uncontrolled descent after post-mortem examinations revealed significantly raised carboxyhaemoglobin levels in both crew members, indicating inhalation of carbon monoxide (CO) during life. The investigation examined the engine's exhaust system and found a fatigue crack in one of the stub pipes of the right-hand exhaust manifold. This crack allowed exhaust gases to escape into the engine compartment, eventually leaking into the cockpit through the firewall bulkhead.
Further examination of the manifold revealed that it had been modified with reinforcing doubler plates welded to the pipes. The investigation established that this part was not an original manufacturer component but was obtained via a Parts Manufacturing Authority (PMA) from the USA. The addition of these doublers was deemed an unacceptable design change by the aircraft's type certificate holder, as it likely introduced unpredictable thermal stresses that accelerated the fatigue cracking process. Additionally, there was no record in the aircraft logbooks regarding the installation of this specific manifold.