Apu powerplant installation:

The APU engine mounts consist of a number of supports with vibration isolators fitted to the end of each support. The tubular supports are bolted to the plenum chamber and when correctly attached, hold the APU against the air inlet duct in the plenum. The vibration isolators dampen out any vibration effects that the APU would have on the aircraft structure whilst it is running. Attached to the vibration isolator is a cone bolt that passes through a similar hole on the APU mounting bracket. When in position, the bolt is secured by a nut and washer arrangement and torque loaded to the set figure laid down in the Aircraft Maintenance Manual. (Figure 18.36).

Most APU's are located in a fire proof box made of titanium. Some aircraft have the APU shrouded in a close fitting Titanium case.