Lightning Strikes can and do occur to helicopters in certain atmospheric conditions where there would otherwise be no similar electrical discharge without the presence of the helicopter.
The helicopter-triggered lightning strike phenomenon has been found to occur during operations over the North Sea between November and early April when there is usually little or no natural lightning activity in progress at the time. Over the past 20 years it appears that there have been 1-2 of these strikes each winter. It has also been considered significant that no strikes have been recorded to North Sea helicopters in summer months when the level of natural lightning activity is significantly greater than during the winter months.
Although this phenomenon has occasionally been reported quite widely, the only other known area of relatively frequent occurrence is the Sea of Japan, also in winter. This area is geographically similar to the North Sea since both areas experience convective instability attributable to advection as dry polar air passes over a warmer sea surface. However, in the case of the Sea of Japan, the difference between the sea and air temperature is often 6 ºC or more and vigorous convection can result. North Sea temperature differences tend to be much less, leading to much weaker convective activity. Whilst visual avoidance of convective cloud is often possible in summer, relatively small winter Cb cloud over the North Sea in winter can be embedded in Sc layer cloud and consequently difficult to see.
The part of the helicopter struck is almost always the main or tail rotor blades. Although the immediate effect on safety has so far appeared to be limited, the amount of damage caused has often been high with affected rotor blades found after flight to be beyond repair and rectification expensive. Operationally, since a lightning strike to most helicopters requires a landing at the nearest suitable location as the helicopter is almost certain to have sustained some damage, the disruption caused is also likely to be expensive.
It has been found that, like fixed-wing aircraft, helicopters acquire a strong negative charge when they fly through air because of static charging. This is normally discharged once back in contact with the Earth’s surface, but if an aircraft comes close to a positively-charged region in a thunderstorm cell then there is potential for a lightning strike to be triggered to and through the helicopter. It is hypothesised that triggered lightning strikes may occur when a helicopter flies:
- into a positively charged base of a Cb cloud
- under the positively charged anvil of a Cb cloud
- from a positively charged region of cloud to a negatively charged one.
It has been found that most, but not absolutely all, triggered strikes are positively charged and it is noted that positively charged regions of a Cb cloud are located close to the freezing level (the 0ºC isotherm) where the rate of change from the frozen to liquid moisture state, which leads to the separation of electrical charges, is at its greatest.
Forecasting the Encounter Risk
Early attempts to forecast the triggered-strike risk have resulted in forecasts which had high false alert levels but work at the UK Meteorological Office has led to the validation of an algorithm based on outside air temperature and precipitation rate. Evaluation of this method against past instances of recorded helicopter strikes has returned an 80% success rate. The altitude band between 2000 feet and 3000 feet was of primary interest for North Sea helicopter operations and here, risk was characterised by precipitation rates above 4 mm/hour when the static air temperature was in the range -2°C to -6°C and the freezing level was between 1000 feet and 3000 feet. The same algorithm was also found to correctly forecast a similar proportion of natural lightning strikes recorded in the operating area during winter 2010–2011. It has been found that the areas of risk identified are usually small and so it should be relatively easy to plan helicopter operations so as to avoid high risk. The information from the study has also effectively defined the circumstances in which triggered lightning strikes occur. More detail on the field trials of this model during the 2011/12 and 2012/13 winters is reported in a presentation given to the UK CAA by the UK Meteorological Office in May 2013.