NTSB · Safety Recommendation
Safety Recommendation A-67-013
Recommendation
BOARD RECOMMENDS THAT THE PRESENT FLIGHT MANUALS FOR ALL HILLER UH-12 HELICOPTERS SHOULD BE REVISED TO INCLUDE THE PROPER PROCEDURES FOR ENGAGING THE MERCURY DRIVE CLUTCH ALONG WITH A FUNCTIONAL DESCRIPTION OF ITS OPERATION. THE BOARD WOULD LIKE TO SEE, IN ADDITION TO THE ABOVE, A PLACARD MOUNTED NEAR THE ENGINE/ROTOR TACHOMETER WHICH WOULD SPECIFY MAXIMUM ACCEPTABLE ENGAGEMENT TIMES.
Safety Issue and Context
THE BOARD'S ACCIDENT INVESTIGATION RECORDS REVEAL SEVEN HILLER MODELUH-12 HELICOPTER ACCIDENTS SINCE DECEMBER 1963, ALL OCCURRING SHORTLY AFTER TAKEOFF AND ALL WERE ATTRIBUTED BY THE SURVIVING PILOTS AS HAVING BEEN CAUSED BY LOSS OF POWER TO THE MAIN ROTOR DRIVE SYSTEM. DETAILED EXAMINATION IN EACH CASE DISCLOSED THAT THE LOSS OF POWER TO THE MAIN ROTOR DRIVE SYSTEM WAS CAUSED BY SLIPPING OF THE MECHANICAL CLUTCH ASSEMBLY. THE SLIPPAGE, WHICH WAS REVEALED DURING FUNCTIONAL TESTS AT THE CONTRACTOR'S PLANT AND, ALSO, DURING LABORATORY DISASSEMBLY AND EXAMINATION, OCCURRED AS A RESULT OF GLAZED AND/OR WORN FRICTION SHOES.
Response History
April 17, 1967 · Recipient Response · FAA
WE HAVE BEEN WORKING WITH HILLER ON THE CLUTCH SLIPPAGE PROBLEM. THE SERVICE INFORMATION LETTER REFERRED TO ABOVE RESULTED FROM THIS COMBINED EFFORT. FURTHER, SERVICE BULLETINS HAVE BEEN ISSUED TO MODIFY THE CLUTCH BY THE ADDITION OF MERCURY. CURRENTLY, HILLER HAS A PROGRAM UNDERWAY TO TEST AN IMPROVED CLUTCH THAT HAS BEEN MODIFIEDBY (A) THE ADDITION OF A SHAFT FROM THE CLUTCH TO TRANSMISSION TO CORRECT FOR MISALIGNMENT, (B) THE ADDITION OF A ROLLER BEARING TO FURTHER CORRECT FOR MISALIGNMENT, (C) AN INCREASE IN MERCURY CONTENTTO DECREASE ENGAGEMENT TIME, AND (D) AN INCREASE IN THE WALL THICKNESS OF THE DRUM TO PREVENT HEAT WARPAGE.
WITH REGARD TO THE PROCEDURES IN THE SERVICE INFORMATION LETTER MENTIONED ABOVE, IT IS OUR BELIEF THAT THERE HAS NOT BEEN ENOUGH SERVICE EXPERIENCE USING THIS PROCEDURE TO MAKE A DETERMINATION THAT THIS WILL CORRECT THE PROBLEM. ADDITIONALLY, IN VIEW OF THE FACT THAT THERE HAVE BEEN NO ACCIDENTS ATTRIBUTED TO CLUTCH SLIPPAGE IN THE PAST 16 MONTHS, WE BELIEVE THAT THE ADDITION OF MERCURY MAY HAVE IMPROVED THE SITUATIONTO SOME EXTENT. THERE IS, HOWEVER, AN APPARENT NEED FOR FURTHER IMPROVEMENT AND IT IS BELIEVED THAT THE NEW IMPROVED CLUTCH MAY WELLPROVIDE THE ANSWER. A 100-HOUR TEST HAS BEEN PROGRAMMED FOR THIS CLUTCH AND SHOULD START WITHIN THE NEXT MONTH.
IN VIEW OF THE PENDING TEST PROGRAM, THE INCONCLUSIVENESS AS TO THE EFFECTIVENESS OF THE CURRENT STARTING PROCEDURES IN PREVENTING CLUTCH SLIPPAGE, AND THE SATISFACTORY RECORD OVER THE PAST YEAR, WE BELIEVE THAT FURTHER ACTION IS NOT NECESSARY AT THIS TIME. WE DO FEEL, HOWEVER, THAT SHOULD THE STARTING PROCEDURE GIVEN IN THE INFORMATION LETTER PROVE EFFECTIVE, A CHANGE IN THE FLIGHT MANUAL WILL BE NEEDED. ON THE OTHER HAND, INSTALLATION OF THE MODIFIED CLUTCH WITH THE OLD STARTING PROCEDURES MAY PROVE TO BE MORE DESIRABLE.