Airworthiness Directives; Boeing Model 747-200B, -200C, -200F, -300, -400, -400D, and -400F Series Airplanes; and Model 747SP Series Airplanes

This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 747-200B, -200C, -200F, -300, -400, -400D, and -400F series airplanes; and Model 747SP series airplanes, that requires repetitive functional tests of the auxiliary power unit (APU) and engine fire shutoff switches and repetitive replacements of the APU and engine fire shutoff switches. The AD also provides an optional terminating action for the repetitive functional tests and replacements. This action is necessary to prevent mineral build-up on the APU and engine fire shutoff switches, which could lead to failure of the switches to discharge fire suppressant in the affected area and could result in an uncontrolled fire that could spread to the strut, wing, or aft body of the airplane. This action is intended to address the identified unsafe condition.