By the summer of 1967 the Marine Corps had ten squadrons of CH-46 Sea Knights. Half were equipped with the early A model and the rest were being equipped with the newer D model. Three of the squadrons were in Vietnam and one was on board the assault ships of the Special Landing Force in the South China Sea. In addition, three of the Corps five remaining UH-34 squadrons were in Vietnam and another was with the Special Landing Force.
On 3 May, a CH-46D crashed at the Marines Santa Anna base in California, killing all four members of the crew. Apparently the main transmission mounting brackets had failed, allowing the front and rear overlapping rotors to intermesh. As a result, all CH-46s were grounded pending inspection and modification. To compound the problem, a CH-46A crashed in Vietnam when the tail pylon, containing the engines, main transmission and aft rotors broke off in flight.
The grounding order was lifted after ten days, but two more CH-46As crashed in June with the loss of six of the eight crewmen. On 30 June a CH-46D at Santa Anna crashed when a rotor blade separated from the aircraft. Miraculously all three of the crew survived. All D models were immediately grounded again while the blades were checked. Still another A model went down in Vietnam three days later, due to failure of the main transmission.
A CH-46 Reliability Review Conference was convened at the Vertol plant in Pennsylvania on 1 August. The Marines denied that the quality of their maintenance crews was to blame and suggested that the fault lay in the rotor blades, drive shaft bearings and excessive vibration of the aft pylon. The wrangling did nothing to prevent the deaths of the five crew of an A model from HMM-262, when it lost its tail pylon at 3,000 feet on 31 August. The next day, a tail pylon separated from a Sea Knight as it was landing, but the crew walked away from the crash.
In October, the first Sea Knights arrived at Okinawa from Vietnam for a repair and modification program. A total of 325 A and D models underwent extensive overhaul amounting to 1,000 man hours each, including the strengthening of structural members in the aft pylon and along the ramp closure area and modifications to the engine and transmission mountings. To alleviate the drastic shortage of helilift caused by these problems, ten additional CH-53s and 23 UH-34s were shipped to Vietnam and 31 UH-ls were borrowed from the Army.
The exact causes of the problems encountered by the CH-46 were never pinpointed with accuracy and complete assurance. There is no doubt that some blame was attributed to the extensive modifications made to the YHC-IA in order to sell it to the Marine Corps. The blade folding mechanism imposed new loads on the transmission and fuselage. The widening of the ramp door and the resulting smaller support on the sides of the fuselage for the 'shelf' on which the main components were attached, would have weakened the structure of the aircraft and more powerful engines would have added to the strain. Regardless of the exact cause of the problems, the modification program corrected the problem.
Vietnam-the Helicopter War
by: Philip D. Chinnery