Introduction
On 16 December 2006, a Cessna 404 Titan, registration G-OOSI, was en route from San Pedro Airport, Cape Verde Islands, to Dakar Airport, Senegal. The aircraft, equipped with two Continental GTSIO-520-M piston engines, was on a private flight with one pilot and one passenger. No injuries or damage were reported.
History of the Flight
The aircraft had been conducting map survey operations since late November 2006 without significant unserviceability. The pilot, aged 32 with 504 total hours and 35 on type, held a Commercial Pilot’s Licence with Instrument Rating. The flight departed at 1855 UTC. During the climb, the pilot used his oxygen mask intermittently, more frequently as altitude increased. When not using it, he placed it on his lap. At 10,000 ft, the passenger moved to the rear of the cabin, as instructed by the pilot, to optimize center of gravity. However, at the rear, the oxygen and intercom leads were too short to connect simultaneously.
At a planned cruise altitude of FL210, the pilot engaged the autopilot. The passenger later noted that the pilot’s voice seemed “a little slurred” during radio transmissions. When asked, the pilot confirmed he was using oxygen. Shortly after, he was seen adjusting the engine controls, having removed his mask to do so in response to a perceived engine issue. Soon, the passenger heard a change in engine noise, felt vibration and a sensation of being pushed into his seat. Attempts to contact the pilot via intercom failed. With the aircraft descending at high speed in a spiral, the passenger twice called for a ‘mayday’ transmission. The pilot responded to the second call and declared an emergency. At about 5,000 ft, the passenger opened the emergency escape hatch in preparation for a possible sea ditching. The pilot then regained control, stabilized the aircraft, and requested a diversion to Amilcar Cabral Airport, Cape Verde Islands, where a safe landing occurred at 2005 UTC.
Post-Landing Actions
After landing, the pilot checked the aircraft and considered it fully serviceable. He believed the aircraft had remained within normal operating parameters and that no negative-g maneuvers had occurred. Reflecting on the incident, he concluded that he had begun suffering from hypoxia during the climb and that the perceived engine problem likely resulted from not correctly adjusting the engine controls at altitude. The pilot contacted the company engineer in the UK and conducted uneventful engine ground runs the next day before flying the aircraft to Dakar in daylight. At Dakar, the engineer assessed the aircraft as fully serviceable; the oxygen contents gauge read 600 psi.
Oxygen System
The Cessna 404 normally has an oxygen system activated by a knob, but G-OOSI had been modified for survey tasks, with the oxygen cylinder installed at the rear of the fuselage. The control knob was at the rear, requiring the pilot to activate it before takeoff or instruct passengers to do so in flight. Three pilot-style oxygen masks were available, each with a rubber restraining strap and a microphone. However, the pilot knew that at least one mask had a “crackly” microphone and avoided using it for transmissions. The passenger’s mask had a broken strap held together with adhesive tape. The Pilot’s Operating Handbook listed an altitude operating limitation of 30,000 ft with oxygen equipment.
Regulations
All aircraft must operate below 10,000 ft unless equipped with a pressurised cabin or the pilot uses an individual oxygen source supplied by a personal mask. The source also recommends using oxygen at lower altitudes when flying at night.
Pilot’s Assessment
The pilot stated that although he was aware of the insidious nature of hypoxia and attempted to recognize symptoms during the flight, he underestimated the risks of becoming hypoxic by not wearing the oxygen mask continuously.
Company Actions
The company concluded that the main contributing factor was the pilot not using his oxygen mask continuously above 10,000 ft, and another contributing factor was that at least one oxygen mask may have had a defective microphone, requiring removal for communication. The company plans to implement more stringent hypoxia training and changes to procedures, including annual flight checks with oxygen system briefings, reporting unserviceable equipment when operating abroad, requiring camera operators to attend CRM courses, and prohibiting night flights above 10,000 ft in unpressurised aircraft. In view of these actions, the AAIB did not issue safety recommendations.