Investigators are puzzled by an incident in which a Eurocopter AS332 Super Puma helicopter's main rotor head (MRH) spindle yoke failed (Flight International, 24-30 October), and the European Aviation Safety Agency has made clear that the event could have been disastrous.

Close industry sources say the failure probably originated as a fatigue crack, but it is not clear why it should have affected the part as it did. Also, such a developing fault would normally have been detected by the aircraft's health and usage monitoring system, but it was not, say the sources.

EASA has issued an emergency airworthiness directive describing "the failure of the lower material section on the leading edge of an MRH spindle yoke on an AS332L helicopter", explaining that "failure of the spindle yoke can lead to the loss of the blade". Blade loss can result in the separation of the entire rotor head, with disastrous consequences. Operator Bristow Helicopters says, however, that in the 13 October incident - which took place as the aircraft was taking off with 15 on board - the crew felt "abnormal" vibrations and immediately carried out a precautionary landing.

EASA says that "the crack growth that led to the failure is visible from outside without disassembly", and has ordered all operators to check their aircraft. Bristow says it checked all its 33 Super Pumas and no faults were found. The UK Air Accidents Investigation Branch is conducting an inquiry.

 Bristow 5 Dec 06
©Wieste De Graaf/
 EASA has ordered all Super Puma operators to check their aircraft

Source: Flight International