AD 74-04-03

74-04-03 HILLER AVIATION: Amendment 39-1785. Applies to Hiller Models UH- 12D (H-23D) and UH-12E (3 and 4 place), (OH-23G, H-23F) helicopters certificated in all categories.

Compliance required within the next 50 hours' time in service after the effective date of this AD, unless already accomplished.

To detect main rotor drag struts P/N 52120 with unacceptably low heat-treat condition and prevent premature fatigue failure of this strut accomplish the following:

(a) Remove and prepare for inspection the two main rotor drag struts, P/N 52120, in accordance with the instructions contained in Hiller Aviation Service Bulletin No. 51-2, dated January 21, 1974, or later FAA-approved revisions, or an equivalent method approved by the Chief, Aircraft Engineering Division, FAA Western Region.

(b) Check the Rockwell hardness of struts P/N 52120 using the "C" scale in accordance with the instructions contained in Hiller Aviation Service Bulletin No. 51-2, dated January 21,1974, or later FAA-approved revisions, or an equivalent method approved by the Chief, Aircraft Engineering Division, FAA Western Region.

(1) Struts with Rockwell "C" from 31.5 to 36 may be returned to service for the remainder of the 2500 hour service life. These struts should be identified by etching an "H" before the serial number.

(2) Struts with Rockwell "C" less than 23 must be replaced within the next 50 hours' time in service or a total of 2500 hours' time in service whichever comes first.

(3) Struts with Rockwell "C" from 23 to 31 must be replaced within the next 50 hours' time in service or a total of 1200 hours' time in service whichever comes later, but must not exceed in any case, a total of 2500 hours' time in service.

(c) Repaint the drag struts and reinstall or replace them on the helicopter observing the proper rotor blade alignment (Refer to UH-12E Service Manual).

(d) In lieu of performing the above inspections, the struts, P/N 52120 maybe removed and replaced with struts P/N 52120-5. Struts P/N 52120-5 also have a service life limit of 2500 hours' time in service.

(e) Aircraft may be flown to a base where the maintenance required by this AD may be performed per FARs 21.197 and 21.199.

This amendment becomes effective February 18, 1974.