AD 56-21-01

56-21-01 BELL: Applies to All Models 47G, 47G2, and 47H1 Helicopters Except As Noted Below and All Spare Hydraulic Boost Power Cylinders Part Numbers 1090, 1190 and All 1270 Cylinders Prior to and Including Serial Number 733. Model 47G, Serial Numbers 1530 and 1687 and Up; Model 47G2, Serial Numbers 1494, 1505, 1506, 1508, 1617 and Up; and Model 47H1, Serial Numbers 1534 and Up Will be Modified Prior to Delivery.

Compliance required as indicated.

Malfunctioning of the hydraulic boost cylinder in service has necessitated the replacement of the hydraulic boost power cylinder barrels and clamps in the fore and aft and lateral cyclic boost power cylinder units and the establishment of a service life of 100 hours on the cylinders which have not been modified. These hydraulic boost cylinder barrels must be replaced by the time 100 hours service is obtained, except barrels that have already accumulated 100 hours service prior to the date of this directive may be used until 125hours are obtained or until December 1, 1956, whichever occurs first.

(Detail procedures for installation of cylinder barrel P/N 1097 and clamps P/N 1029 are contained in Bell's Mandatory Service Bulletin No. 114 dated August 27, 1956.)