While working at Agusta as a RMS consultant I brought the following to the attention of the A-129 Program Manager and other members of the engineering department. I informed them that the rotorblades had inadequate protection from a lightning strike (i.e. embedded aluminum mesh). I also indicated that the bonding of the blades to the rotorhead did not comply with normal design standards. They indicated that the blades, which were of carbon composite structure, would conduct the charge after an attachment. I further indicated that the carbon composition was capable of conducting any static charge off to the rotorhead but if the blade(s) suffered a lightning attachment the conductive capabilities of the carbon composite material would be overwhelmed and the carbon structure would then act as an insulator. As an insulator the temperatures internal of the blade would be so intense as to blow the blade apart. I do not know if they changed the blade design to incorporate the aluminum mesh or, do I know if they improved the bonding.
Normal dissipation of static build up on the rotor through the transmission will cause spark erosion on transmission gears and bearings. Assuming that the bonding is adequate on the rotor system to conduct the amperage of a lightning strike and the full charge passes through the transmission the damage could be much more than just spark erosion.
To: captk
“Asking around other pilots and engineers hasn’t produced any satisfactory answers. I have heard that lightning can cause static pitting of bearings if all the components are not bonded properly”.
There are several things going on inside of a helicopter transmission and they are dependent upon the condition of the oil. It would be assumed that the gears and bearings have solid metal to metal contact when in operation thus providing a bonding path. This is not true. Assuming a properly working lube system there is a very thin oil film at the point of tooth contact on the gears and ball to race contact on the bearings. Oil is an insulator so when the gear teeth are at their maximum point of contact there is no conductivity. As the teeth separate or the balls move in relation to the race the static charge will penetrate or puncture the oil film and the spark will cause pitting. This is called the puncture voltage. The static charge must be high enough to effect the puncture of the oil film. As hours accumulate on the oil wear metals start to build up in the oil. That is why SOAP tests are performed. The wear metals change the conductivity of the oil so the puncture voltage is reduced and the sparking intensifies. When a helicopter suffers a lightning attachment and depending upon how high the amperage is as the charge passes through the transmission the transmission gears and bearings can be severely pitted all the way to being welded together.
[ 08 November 2001: Message edited by: Lu Zuckerman ]