Airworthiness Directives; Boeing Model 727-100 and -200; 737-100, -200, -200C, -300, -400 and -500; and 747 Series Airplanes

This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 727-100 and -200; 737-100, -200, - 200C, -300, -400 and -500; and 747 series airplanes. This amendment requires, among other things, preparation of the electrical bonding faying surfaces for the tubing penetrations of the hydraulic heat exchanger on the forward and aft surfaces of the rear spars of the fuel tanks of the left and right wings, a one-time measurement of the electrical bonding resistances, and follow-on actions. This action is necessary to ensure adequate electrical bonding between the penetration fittings of the hydraulic heat exchanger and the rear spars of the fuel tanks. Inadequate electrical bonding, in the event of a lightning strike, could cause electrical arcing and ignition of fuel vapor in the wing fuel tank, which could result in a fuel tank explosion. This action is intended to address the identified unsafe condition.