Description

On 22 February 2008, a Eurocopter AS332 L2 Super Puma being operated on a day passenger flight from the Bruce Offshore Oil Platform to Aberdeen was transiting a squall line in Instrument Meteorological Conditions (IMC) at the cruise altitude of 2000 feet when it was struck by lightning. There was no loss of systems or indication of any other adverse effect and the flight was completed as planned with no subsequent consequences for the operation of the helicopter or its 17 occupants. Significant main rotor blade damage was found during post flight inspection.

Investigation

An Investigation was carried out by the UK AAIB. It was established that the aircraft had encountered a line of rain showers across the intended track and had selected a crossing point which had appeared to offer the shortest transit and avoided weather radar red returns. Soon after beginning the transit, both pilots saw “a bright flash at the rotor tip in the one o’clock position, accompanied by a ‘bang’ or ‘pop’ sound”.

Although there were no obvious consequences, it was initially decided to begin a diversion towards the nearest available platform which did not involve returning through the line of showers, in accordance with the Quick Reference Handbook (QRH) requirement to land as soon as possible after a lightning strike but it was then established that adverse and unsuitable weather conditions prevailed at this and all other platforms in the vicinity and that diversion to either Sumburgh or Kirkwall would involve further transit through adverse weather. The crew therefore elected to continue to Aberdeen as originally planned.

A post flight inspection found main rotor blade damage including arcing damage to the leading edge anti-erosion strips, broken bonding leads and damaged trim tabs with high energy tracking visible on two main rotor pitch link ball joints and one main rotor servo upper ball joint. The main rotor head and other components were removed and returned to the manufacturer for detailed investigation where it was found that one rotor blade was damaged beyond repair limits and the other three were repairable.

It was found that although air-ground lightning strikes (normally regarded as the most damaging type) had been recorded in the general area around the time of the strike to the helicopter, none of these had been anywhere near the position of the helicopter. It was concluded that the strike had probably been either an inter-cloud or intra-cloud discharge which are known to frequently be triggered by the presence of an aircraft. The fact that “a very large number of lightning strikes have been encountered by AS332-series helicopters operating over the northern North Sea, since the type’s entry into service” was noted as was the fact that these “appear to have occurred predominantly during the winter months”. The response to this in the form of numerous design changes and modifications to the type was also noted as was the apparently positive effect on type vulnerability.

The Final Report of the Investigation was published in September 2008.

Further Reading

• Helicopter-triggered Lightning Strikes
• Health and Usage Monitoring System (HUMS)
• Offshore Helicopter Safety Review 2014 UK CAA
• Engine Malfunction Trigered by a Lightning Strikes, UK CAA AIC 29/2004 (Pink 64), April 29 2004