Arsenal – CH-46F Sea Knight: “Phrog”

Click on image for expanded view. (Illustration by Gregory Proche)
Click on image for expanded view. (Illustration by Gregory Proche)

Click on image for expanded view. (Illustration by Gregory Proche)

On January 31, 1968, eight CH-46s from the Marine Medium Helicopter Squadron (HMM) 165 delivered part of the 4th Battalion, 2nd Regiment, of the Army of the Republic of Vietnam into the Citadel in Hue. The weather was marginal, ceiling at less than 300 feet and the landing zone was under enemy mortar fire. Nevertheless, during the 30-day Hue battle, HMM-165 pilots flew wherever there was a need, delivering troops, ammunition and food to ad hoc LZs around the city and evacuating the wounded.

The Marines’ primary lift at Hue, the CH-46 Sea Knight—known as the “Phrog” for its head-on appearance—began life as the YHC-1A, one of two proposed medium lift transports for the Army. The Army chose the larger, more powerful YHC-1B, which became the CH-47 Chinook. When the Marine Corps required a gas turbine–powered replacement for its UH-34 medium lift assault helicopter, Boeing Vertol proposed a smaller version of the YHC-1A, which was designated the HRB-1. Powered by two 1,250-shp T58-GE-8-8B gas turbine engines, the CH-46A, as it was redesignated, carried 17 troops or 4,000 pounds of cargo.

Entering production for the Navy and Marines in 1964, the CH-46A proved underpowered and its transmission unreliable under combat conditions in Vietnam. The shortcomings led to development of the CH-46D, with a more powerful engine, a more robust transmission and stronger, lighter rotor blades. It could descend and climb much faster than the CH-46A or UH-1. Despite the 46D’s improvements, six crashed in 1967 when the entire aft pylon came off. Investigators found the cause was whirl mode flutter—the rotor blade generated “twisting” air pressure, inducing metal fatigue. Engineers strengthened the fuselage, changed the rotor blade design and directed pilots not to do the “hover aft” maneuver.

The more powerful CH-46F entered service in 1968 with improved avionics and all-weather performance, a better navigation system and three M-2 .50-caliber machine guns. It quickly became the Marine Corps’ workhorse in Vietnam, being used in airmobile assault, logistics and combat support; even in medevac and combat search and rescue roles.

Production of the CH-46 ended in 1971 with more than 600 delivered. Ubiquitous in the skies of Afghanistan today, all current Phrogs are Vietnam-era models, refurbished and upgraded. The CH-46 reflects the designs of the early helicopter pioneers and stands as enduring testament to their brilliance and vision.

5 Responses

  1. John Joseph Hemstreet

    My father served with the 95th Division WW2

  2. Jon Ness

    I was with the 1st MarRegt, 3/1 in the late 70’s and some of us signed up for a parachute class in the evenings and when we made our first jumps we jumped from the rear ramp of the CH-46 from static lines. This was somewhere near the airstrip in what was called “Main-side. Must have been near the 5 marines ? We practiced our PLF’s by jumping off a “pic-nic table! Only in the Marines corps!

  3. Gene Kruger

    I was Crew Chief on ET-42 in HMM-262 in ’66-’67. The 46 was a tough old bird and did everything we ever asked her to do. It was a great aircraft and anyone who served as a pilot or crew, I’m sure would say the same thing. I recently found out ET-42 went down in a blaze of glory a year or so after I returned home. Although I miss the old gal, it’s somewhat comforting knowing she went out while on a mission, as opposed to becoming razor blades after her active duty time.

  4. Sam Beamon

    I was a crew chief and deployed to Vietnam with HMM-262 in Dec. 66. The CH-46 was a tough old bird and some where along the line became known as the Phrog and Battle Phrog. It certainly earned those nicknames. For 50 years, it has been the backbone of the Marine Corps and now being retired for a new style of aircraft – the V-22 Osprey. I was transferred to HMM-164 in Feb 67, where I did my combat flying. I wrote a book about my time in the Marines, Vietnam and member of a combat flight crew. The book is entitled, “Flying Death The Vietnam Experience”. The life of a combat helicopter crew chief.


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