Hardwick, J., Lees, M., Eaton, D., Halsey, N. et al., "Lightning Strikes to Helicopters in the North Sea," SAE Technical Paper 1999-01-2399, 1999, doi:10.4271/1999-01-2399.
Concern over severe lightning strikes to helicopters in the North Sea resulted in a UK Civil Aviation Authority sponsored research programme. The programme investigated the polarity of lightning strikes in this region by remote sensing with a view to determining the likelihood of severe positive strikes. It also investigated correlations of individual incidents with meteorological conditions and .the lightning detected by remote survey on the day of the strike. The study shows that the strikes to helicopters occur only in winter, possibly triggered by the helicopter itself. The proportion of positive strikes, believed to be on average more energetic than negative strikes, is higher than the World-wide average, particularly in winter. The meteorological investigation found that temperature and updraft parameters allowed a predictive algorithm for helicopter strikes to be constructed.