For cooling fans that are a part of a powerplant installation the
following apply:
(a) Category A. For cooling fans installed in Category A
rotorcraft, it must be shown that a fan blade failure will not prevent
continued safe flight either because of damage caused by the failed blade
or loss of cooling air.
(b) Category B. For cooling fans installed in category B
rotorcraft, there must be means to protect the rotorcraft and allow a safe
landing if a fan blade fails. It must be shown that --
(1) The fan blade would be contained in the case of a failure;
(2) Each fan is located so that a fan blade failure will not jeopardize
safety; or
(3) Each fan blade can withstand an ultimate load of 1.5 times the
centrifugal force expected in service, limited by either --
(i) The highest rotational speeds achievable under uncontrolled
conditions; or
(ii) An overspeed limiting device.
(c) Fatigue evaluation. Unless a fatigue evaluation under
§29.571 is conducted, it must be shown that cooling fan blades are not
operating at resonant conditions within the operating limits of the
rotorcraft. (Secs. 313(a), 601, and 603, 72 Stat. 752, 775, 49
U.S.C. 1354(a), 1421, and 1423; sec. 6(c), 49 U.S.C. 1655 (c))