Investigating and predicting helicopter-triggered lightning strikes
October 2013 - Recent research from the Met Office has investigated instances of North Sea helicopters triggering lightning strikes.
The North Sea Oil and Gas fields are a challenging environment to work in. Day-to-day operations are subject to the weather conditions present. The industry is reliant on helicopter operations to transfer personnel quickly and safely to work on the oil rigs. Each day, there are as many as one hundred helicopter movements in the North Sea to the north-east of Aberdeen, plus a significant number of flights over Danish and Norwegian waters.
For a number of years, it has been known that these helicopter operations have been particularly susceptible to lightning strikes during the winter season. These are not usually natural lightning strikes, but lightning caused by helicopters charging up and 'triggering' the lightning. Triggered lightning strikes have been reported over the North Sea between early October and the end of March. Past triggered lightning strikes have forced a helicopter to ditch in the North Sea in January 1995 (AAIB, 1997) and have been a causal factor in a fatal accident that occurred in July 2002 (AAIB, 2005).
Although more recently there have been no events of this severity, helicopters do continue to suffer lightning strikes, with one to three strikes occurring in the North Sea each winter season. Figure 1 shows the locations of such lightning strikes from 1992 to 2013.
Due to improvements made in helicopter design, all helicopters are now expected to survive a lightning strike. However, as a lightning strike to a helicopter is inconvenient, expensive and has many safety-related issues, the most sensible option is to try and prevent the lightning strikes from occurring by keeping the aircraft away from at-risk areas.
Investigating why helicopter-triggered lightning occurs
The absence of little other lightning activity in the areas around where the helicopters were flying at the time they were struck by lightning suggests that it is the helicopter itself that is triggering the lightning strike. In addition, pilots who have been struck by lightning say that they have had little or no warning of lightning activity in the area. Therefore, it is necessary to both understand why the lightning strikes are occurring and also to predict the areas which are likely to have the highest level of risk, to allow the helicopters to maintain a safe distance from the danger areas.
Under contract to the UK Civil Aviation Authority (CAA), the Met Office has been able to investigate why these lightning strikes have occurred. Research led by Dr Jonathan Wilkinson and Dr Helen Wells has found out why helicopters trigger lightning strikes and has also suggested a method to predict when and where the strike conditions will occur. This work has been published as a research article (Wilkinson et al, 2013) in the journal Meteorological Applications.
The paper makes a direct link between helicopter-triggered lightning and cumulonimbus clouds that occur during cold-air outbreak cases associated with polar lows. Here strong winds advect cold air over relatively-warm waters. This leads to a field of deep convective clouds, often with little or no organisation. An example from February 2013 is shown in Figure 2. Here, the helicopter was struck while returning to Aberdeen airport from an offshore rig.
It is believed that the strikes occur as a result of the helicopters acquiring a negative charge during flight. If the helicopter flies close to, or within an area of cloud that is positively charged, then the lightning is able to discharge on to or through the helicopter. This could occur if the helicopter flies close to an area of positively-charged hail or graupel, which can exist near the base of the cumulonimbus clouds. Alternatively, the helicopter may fly beneath the anvil of a cumulonimbus cloud, which can discharge to the helicopter as a 'bolt from the blue'.
It is thought that the cumulonimbus clouds found in the North sea during winter are frequently able to carry an electrical charge. However, this charge is not usually strong enough to discharge to the Earth's surface in the form of a natural lightning strike. The authors have found no evidence to suggest that helicopter-triggered lightning strikes occur in deep stratocumulus clouds, which has been suggested in the past. Instead, it is likely that deeper cumulonimbus clouds have become embedded within stratocumulus, disguising their presence when viewed from a helicopter.
Predicting areas at risk from lightning strikes
The second half of the paper is devoted to an algorithm, which uses temperature, the height of the freezing level and total surface precipitation rate to pinpoint the location of deep cumulonimbus clouds in the North Sea. This algorithm has been tested on the data from the Unified Model , which has been run for ten strike cases occurring between 2003 and 2010. It was found that the algorithm predicted a triggered lightning strike risk for 8 out of 10 cases where a strike had occurred. In addition, the algorithm predicted a region of risk for 8 out of 9 occasions when a natural lightning strike had been observed in the North Sea during winter 2010-2011.
The results of the algorithm can be used to generate a forecast of the regions of risk, which can then be passed directly to the companies which operate the helicopters that serve the North Sea. The pilots are able to view the regions of risk when flight planning, enabling them to avoid the areas of highest risk when the meteorological conditions suggest that triggered lightning is a threat in the North Sea.
A version of the algorithm was trialled during winter 2012-2013, with feedback from the helicopter operators. The outcome of this trial was generally positive and as a result, the algorithm is intended to be used operationally in the near future. In the meantime, the Met Office will support the product while continuing research into lightning and cold-air outbreak storms, assessing the ability of the Unified Model to predict lightning risk. The Met Office is grateful for the continued guidance from both the UK Civil Aviation Authority (CAA) and the North Sea helicopter operators, who have continued to support this work.
AAIB Report No: 2/1997. Report on the accident to Aerospatiale AS332L Super Puma, G-TIGK, in North Sea south west of Brae Alpha Platform on 19 January 1995. Link to report
AAIB Report No: 1/2005. Report on the accident to Sikorsky S-76A+, G-BJVX near the Leman 49/26 Foxtrot platform in the North Sea on 16 July 2002. Link to report
Wilkinson, J. M., Wells, H., Field, P. R. and Agnew, P. (2013), Investigation and prediction of helicopter-triggered lightning over the North Sea. Met. Apps, 20: 94-106. Link to article