The present invention provides a tailcone and power assembly mountable to the body of an aircraft using a height adjustable dolly. The tailcone assembly comprises a longitudinal support member, a gas turbine engine mounted to the support member; a firewall; two curved rotatable casings hingeably connected to the support member; an inlet duct extending form an aperture in one of the rotatable casings to the engine inlet; an integral exhaust casing, and interface means for making necessary engine accessory connections to the aircraft. The tailcone is installed on the aircraft by mounting the tailcone in the adjustable dolly, rolling the dolly up to the aircraft, adjusting the dolly until the auxiliary power assembly is properly aligned for attachment to the aircraft, connecting the engine accessories to the aircraft, and bolting the assembly to the aircraft.