14 CFR § 25.901 - Installation.
(a) For the purpose of this part, the airplane powerplant installation includes each component that—
(1) Is necessary for propulsion;
(2) Affects the control of the major propulsive units; or
(3) Affects the safety of the major propulsive units between normal inspections or overhauls.
(b) For each powerplant—
(1) The installation must comply with—
(i) The installation instructions provided under §§ 33.5 and 35.3 of this chapter; and
(ii) The applicable provisions of this subpart;
(2) The components of the installation must be constructed, arranged, and installed so as to ensure their continued safe operation between normal inspections or overhauls;
(3) The installation must be accessible for necessary inspections and maintenance; and
(4) The major components of the installation must be electrically bonded to the other parts of the airplane.
(c) For each powerplant and auxiliary power unit installation, it must be established that no single failure or malfunction or probable combination of failures will jeopardize the safe operation of the airplane except that the failure of structural elements need not be considered if the probability of such failure is extremely remote.
(d) Each auxiliary power unit installation must meet the applicable provisions of this subpart.