The static charge that can build up on an airframe in flight can be quite substantial. Helicopters doing sling loading some times have the hook up person earth the helicopter before hooking up the load, have heard of cases where the hook up individual has been given a very large jolt, charges of 250,000 volts are mentioned in CAP 426 (Helicopter external sling load operations).
The CAP says,
Helicopter pneumatic tyres are made from electrically conducting rubber to ensure that helicopters with this type of landing gear make a good earth connection
Wonder if you FW folk are the same? One reference says while the tires will discharge static electricity, because of the presence of carbon black within the rubber matrix, they are not relied on to earth the airframe; therefore, when buildup of static electricity is of concern, it is always important to use mechanical means to ground the aircraft.