Tokyo-Chōfu

It is highly probable that this accident occurred as the speed of the Aircraft decreased during takeoff and climb, which led the Aircraft to stall and crashed into a residential area near Chōfu Airport. It is highly probable that decreased speed was caused by the weight of the Aircraft exceeding the maximum takeoff weight, takeoff at low speed, and continued excessive nose-up attitude. As for the fact that the Captain made the flight with the weight of the Aircraft exceeding the maximum takeoff weight, it is not possible to determine whether or not the Captain was aware of the weight of the Aircraft exceeded the maximum takeoff weight prior to the flight of the accident because the Captain is dead. However, it is somewhat likely that the Captain had insufficient understanding of the risks of making flights under such situation and safety awareness of observing relevant laws and regulations. It is somewhat likely that taking off at low speed occurred because the Captain decided to take a procedure to take off at such a speed; or because the Captain reacted and took off due to the approach of the Aircraft to the runway threshold. It is somewhat likely that excessive nose-up attitude was continued in the state that nose-up tended to occur because the position of the C.G. of the Aircraft was close to the aft limit, or the Captain maintained the nose-up attitude as he prioritized climbing over speed. Adding to these factors, exceeding maximum takeoff weight, takeoff at low speed and continued excessive nose-up attitude, as the result of analysis using mathematical models, it is somewhat likely that the decreased speed was caused by the decreased engine power of the Aircraft; however, as there was no evidence of showing the engine malfunction, it was not possible to determine this.

It is estimated that in this accident, while the aircraft was on a company test flight prior to an airworthiness certification inspection, it entered spin and because it was unable to recover, it crashed, destroying the fuselage and killing the captain and the passenger. Because the left engine’s oil tank cap had not been normally locked, abnormal engine oil temperature and pressure occurred, and it is estimated that the aircraft’s airspeed decreased to near the stall speed. The captain increased power on the right engine to regain airspeed, which induced a yawing moment. It is considered possible that the aircraft then entered a spin because either it was uncontrollable due to being below the minimum control speed and safe one engine inoperative speed, or the captain had been incapacitated by hypoxia and was unable to cope with the loss of airspeed.
The following are considered possible reasons as to why the aircraft did not recover from the spin; however, the precise cause could not be clarified.
① Because the aircraft type is prohibited from spins, the captain could not have been practiced in spin recovery for the aircraft.
② The spin developed without being arrested in the early stages, until flight conditions exceeding the aircraft’s design limits so that the aircraft could not be recovered by normal control forces.
③ The aircraft was in a state of spinning without a reduction of engine power, which made recovery difficult.
④ The captain had been incapacitated.