CH-46B Sea Knight

In the CH-46, as in most aircraft, the original design underwent a series of improvements. These resulted from development of new manufacturing techniques, experience in actually operating the aircraft, modifications to improve the aircraft's ability to perform its intended mission, and redesign to accept equipment or engines not available when the machine was first built. Under the Department of Defense designation system, if these modifications were significant but did not alter the original purpose of the aircraft, these variations on the initial design were denoted by different letters following the basic model number. So it was for the CH-46. The first model which Boeing-Vertol built for the Marine Corps was designated the CH-46A, and the improved version now to be bought was to be the "D" model."

In theory, there could have been a CH-46B and C, but, as was not uncommon, changes could be proposed, and a designation assigned and then no aircraft of that designation, or possibly only a test aircraft, ever built. Some military purchasers might accept a modification while others did not. Hence, helicopters in the Marine Corps did not always have their letter designations in strict sequence, and the CH-46 jumped from an "A" version directly to the "D".

If the mission of the aircraft were changed in modification, the first letter of its designation, which indicated purpose, also changed. The Navy SH-34 series, for instance, was the same helicopter as the UH-34, except the former was modified for anti-submarine warfare and the latter as a troop carrier and cargo transport. Except for the special equipment for the different missions, the two aircraft were almost identical.

The H-49 Sea Knight was the original designation of CH-46B. The HX/H-2 was very briefly designated as XH-49A before finally becoming the XCH-46B. The designation XH-49A was briefly allocated in 1962 to a Boeing-Vertol Model 107 helicopter for the USAF. The designation was changed to XCH-46B before September 1962.

According to one report by the Vietnam Helicopter Pilots Association (VHPA) and Vietnam Helicopter Crew Members Association (VHCMA), CH-46B tail number 152552 crashed into a mountain during bad weather on a routine flight between Marble Mountain and Chu Lai for mail and passengers on 01/07/69. This helicopter was said to have been purchased by the US Army. Other sources report that U.S. Marine Corps helicopter tail number 152552 was a CH-46A. The Army Aviation Safety Center database reported that on 18 November 1968 a USMC CH-46B landed in close proximity to an Army OH-6A, and the rotor wash of the CH-46B damaged the revolving m/r blades of the OH-6A.