Airworthiness Directives; Gulfstream Model G-IV, GIV-X, and GV-SP Series Airplanes and Model GV Airplanes

We are adopting a new airworthiness directive (AD) for certain Gulfstream Model G-IV, GIV-X, GV-SP series airplanes and Model GV airplanes. This AD requires, for certain airplanes, a one-time inspection for sealant applied to the exterior of the auxiliary power unit (APU) enclosure (firewall), and, for airplanes with the subject sealant and certain other airplanes, a revision of the airplane flight manual to prohibit operation of the APU during certain ground and flight operations. This AD results from notification from the airplane manufacturer that an improper, flammable sealant was used on the interior and exterior of the APU enclosure (firewall). We are issuing this AD to prevent this flammable sealant from igniting the exterior surfaces of the APU enclosure (firewall) under certain anomalous conditions such as an APU failure/APU compartment fire, which could result in propagation of an uncontained fire to other critical areas of the airplane.