Summary
The FAA proposes to supersede an existing airworthiness directive (AD) that applies to certain Cessna Model 500, 550, S550, 560, 560XL, and 750 airplanes. The existing AD currently requires installing identification sleeves on the wires for the positive and negative terminal studs of the engine and/or auxiliary power unit (APU) fire extinguishing bottles, as applicable, and re-connecting the wires to the correct terminal studs. This proposed AD would retain the requirements of the existing AD; add airplanes to the applicability; and, for certain airplanes only, require a review of wiring changes made using the original issue of one service bulletin and corrective actions if necessary. This proposed AD results from a determination that additional airplanes are subject to the unsafe condition described in the existing AD. We are proposing this AD to ensure that the fire extinguishing bottles are activated in the event of an engine or APU fire, and that flammable fluids are not supplied during a fire, which could result in an unextinguished fire in the nacelle or APU.