How North Sea helicopters cause lightning to strike
Each year a small number of helicopters working in the North Sea oil and gas sector are struck by lightning.
It is an unpleasant experience for those on board but thankfully improvements in design and technology mean a strike is unlikely to knock the aircraft out of the sky.
Being hit though can cause substantial damage, immediately prompting an emergency and then taking the helicopter out of action for maintenance and repairs.
In light of this, the civil aviation industry asked scientists at the Met Office to examine lightning strikes in the hope of coming up with a forecasting system to help pilots avoid them. What they found was a surprise.
The conclusion was that in many cases it was the helicopter itself which caused the lightning.
Other factors are needed to create the right conditions for helicopters triggered lightning, which has been observed in very few areas of the world. Besides the North Sea, only a small section of sea near Japan reports instances.
For a start, positively charged cumulonimbus clouds need to be present and these are formed by cold air from the Arctic moving south during winter months.
Dr Helen Wells, head of the civil aviation research and development group at the Met Office, said: "In a typical winter season there are about 10 cold air outbreaks in the North Sea operating area, each lasting around three days.
"This means that there are typically 30 strike risk days per season."
Pilots whose helicopters have been hit by lightning have reported there was no sign of it present in the area before they were struck.
Forecasters now know the negatively-charged helicopter acts as a conductor for the lightning, aiding its passage to the earth.
Typically, it would enter one of the main rotor blades and leave at the tail but could knock out instrument panels, heat up components or leave burn marks. It may also magnetise navigational equipment.
Although studies have been ongoing for several years, the 2014/15 winter was the first in which a forecasting system - trialled the previous year - was fully introduced.
Pilots can see on computer modelling data where the high and moderate risk cloud is located. This allows them to delay flights or plot alternative routes around the weather system.
If a red area is over the destination platform or heliport, such as Aberdeen Airport, then the flight may have to be cancelled and this season has proved particularly disruptive.
Dr Wells adds: "Last winter was unusually mild with fewer than normal risk days, whereas this winter so far has been more active than usual."
On offshore installations this year there are posters in communal areas explaining the concept of triggered lightning to passengers who use the 61,000 return flights annually.
John Hopkinson, manager of flight operations for CHC Helicopter, said: "Inevitably, this means we will have to change our behaviours. It is a balance between minimising the risk and the disruption. We are trying to find a good balance."
The CAA has received notification of six lightning strikes on offshore helicopters since 2010.
In the latest, on an Airbus EC225 on 27 December 2014, the crew described being "aware of a flash and a sharp 'tap' which was audible above the background noise of the aircraft."
The helicopter's Terrain Awareness and Warning System subsequently failed but the aircraft was able to land safely.
The Met Office is still evaluating the effectiveness of its forecasting system and hopes to carry out more research to help with understanding triggered lightning.