F-104
6th Jun 2009, 11:51
Lightning strikes attaching to the structure of a metal aircraft (and leaving) cause heavy currents in the skin - no surprise. These curents create strong magnetic fields, which again induce high currents in underlaying structures of the aircraft. Any cable/cable bundle, exposed to this, will carry this induced current in it's outer metal shielding. (Underlying inner metal shieldings are there to take care of EMI). (Bundles typically has an outer overall metal braiding and the carried single cables each their own braidings).
This heavy shielding, and all connections of it, are be designed to carry these currents and propagate them.
Any transition in the cable runs (be it at connectors, at overlapping metal braiding intersections, terminations etc.) has an ohmic value. It is a design goal to keep these as low as possible. According to e.g. present military standards, not above 4 milli ohms, which can be quite hard to acheive.
Deteriorated transitions (due to grit, oxydation etc.) can have quite higher values and the connection can have gone all together. This is tested for on a regulsr basis.
If a lightning strike induces high currents in such a fault ridden cable (e.g FBW runs) it gives rise to high tension (voltage) at the high ohmic transitions, causing the current to spark e.g. to the flimsier signal carrying cunductors underneath, creating total burnouts. (If Kapton insulation is involved, this will detonate if the temperature gets high enough - easily attained in a heavy sparkover !!).
A lot of effords are put into composite structures to make safe passage of either direct lightning strike currents and/or of the induced currents in the structures. Fuel tank protection is obviously of great concern.
Lightning strike handling is not just a simple matter of viewing the aircraft as a simple Faraday Cage.
Cheers.
This heavy shielding, and all connections of it, are be designed to carry these currents and propagate them.
Any transition in the cable runs (be it at connectors, at overlapping metal braiding intersections, terminations etc.) has an ohmic value. It is a design goal to keep these as low as possible. According to e.g. present military standards, not above 4 milli ohms, which can be quite hard to acheive.
Deteriorated transitions (due to grit, oxydation etc.) can have quite higher values and the connection can have gone all together. This is tested for on a regulsr basis.
If a lightning strike induces high currents in such a fault ridden cable (e.g FBW runs) it gives rise to high tension (voltage) at the high ohmic transitions, causing the current to spark e.g. to the flimsier signal carrying cunductors underneath, creating total burnouts. (If Kapton insulation is involved, this will detonate if the temperature gets high enough - easily attained in a heavy sparkover !!).
A lot of effords are put into composite structures to make safe passage of either direct lightning strike currents and/or of the induced currents in the structures. Fuel tank protection is obviously of great concern.
Lightning strike handling is not just a simple matter of viewing the aircraft as a simple Faraday Cage.
Cheers.