EASA issued an Emergency Airworthiness Directive (AD) following reports from Gulfstream Aerospace that an improper, flammable sealant has been used on the exterior of the APU enclosure (firewall) on G-IV, GIV-X, GV,
and GV-SP aeroplanes. This condition, if not corrected, and under certain conditions such as an APU failure or an APU compartment fire, could lead to ignition of the exterior surfaces of the APU enclosure, possibly resulting in propagation of an uncontained fire to other critical areas of the aeroplane.
The AD concerns aircraft with specific serial numbers and calls for a one-time inspection for sealant applied to the exterior of the APU enclosure, and, depending on findings, a revision of the Airplane Flight Manual (AFM) to prohibit operation of the APU during certain ground and flight operations. (EASA)