The Phrog – Boeing 107 Vertol/CH-46 Sea Knight

In 1956 the Vertol company, formerly Piasecki, began to design a possible replacement for the H-21 heavy transport helicopter, popularly known as “The flying banana”. The H-21 was a tandem rotor helicopter that had also been procured for the Swedish Navy and designated in Sweden the Helicopter Type 1. The manufacturer’s designation was the Vertol 107, later changed to Boeing-Vertol 107 after Boeing bought up the Vertol company. Although directly descended from the H-21, new solutions were found and improvements introduced, based on experience gained from the first generation of helicopters.

An important difference was that the Vertol 107 was equipped with gas turbine engines instead of piston engines to provide considerably better performance.

The Vertol prototype first flew on April 22nd 1958, and already in June the US Army signed a contract for 10 such machines, to serial production standard, designated YHC-1A. However this order was soon reduced to only three helicopters and that was the end of the story as far as the US Army was concerned. At the same time as the US Army lost interest, other protagonists were taking notice.

A “Phrog” of the US Marines in 2011. Photo: Andrew Schmidt

The US Marine Corps had been studying the need for a medium sized transport helicopter for both ordinary loads and troop transport. So it was in 1961 that the US Marine Corps selected the Boeing Vertol 107M as the basis for its own helicopter version, naming it the CH-46 Sea Knight. Among the Marine Corps troops the new helicopter was quickly called “The Phrog” (The Frog), a name that would stick with it all through its long career and also while being operated by the US Navy. It would became a vital adjunct to the US Marine Corps arsenal, and continued in service until 2015. The new version first flew on October 16, 1962, and in 1964 finally entered service with the Marine Corps as the CH-46A Sea Knight.

The US Air Force also initially ordered its own version of the helicopter, but just like the Army withdrew its original order and switched to a competitive helicopter. So in the end it was only the Marine Corps and to some extent the US Navy that procured the type. At that time no-one could have forecast that the CH-46A Sea Knight would become the US Marine Corps’ longest-serving helicopter.

A civilian version also appeared, and the first customer was New York Airways, which received its first machines in 1964. The next year there was a readjustment whereby Boeing sold the manufacturing rights to Kawasaki Heavy Industries in Japan. After this, the helicopter’s name changed to the Kawasaki Vertol 107 or Boeing-Kawasaki Vertol 107.

Problematic start, but long-lived affection

The CH-46A was thrust into the fray immediately. The war in Vietnam and the increasing engagement of the USA towards the middle of the 1960s led to the Marine Corps being among the first armed forces to do battle there. The first years would be plagued by a series of accidents resulting in many deaths. On top of this, some of the crashes were into the sea, so that the wreckage could not be examined. It was suspected that there were problems in the power transmission and failures of the rear pylons. It was only after a joint investigation by Boeing and the Marine Corps after a crash in August 1967 that it could be established that structural weakness in the rear pylon led to the rear rotor detaching. It was believed that this was the probable cause of several of the previous crashes, and the team could now provide recommendations as to how the fault could and should be rectified.

The resulting modifications to the CH-46A fleet were successful, shown by the value of careful studies into the reasons behind air crashes. And the Marines now needed a reliable machine more than ever. The CH-46A would be employed on a wide variety of different tasks. Cargo transport, carrying underslung loads, troop transport, evacuation of wounded personnel, airborne rescue and even delivering troops into battle zones. In order to be able to defend themselves and to provide fire support when landing troops, the helicopters could be equipped with a pair of 12.7 mm machine guns, one on each side. In addition, some were provided with additional armour plating to protect against enemy ground fire. This was very necessary, as more than 100 CH-46A helicopters were shot down during the Vietnam War.

A CH-46A Sea Knight is hit by Vietnamese anti-aircraft fire, on July 15th, 1966. Photo: Haas Faust

Despite these losses, the CH-46A acquired a remarkably good reputation, not least after the identification and correction of the reasons for the earlier accidents that had plagued the type. In general this helicopter was robust and reliable, could absorb punishment and its transport capability meant that it was very flexible. The cargo hold could relatively quickly be changed from carrying stretchers to holding about 20 fully equipped soldiers, and then transformed to fly out heavy loads.

Wherever the marines went, the “Phrog” could follow. During the invasion of Grenada an entire Marine Corps regiment was flown in and landed, and in 2003 the CH-46A was used alongside other helicopters to free the captured soldier Jessica Lynch. The expression “Phrog forever” was used to identify the faithful helicopter. A more amusing saying underlining how long it served and was admired was “Never trust a helicopter under 30!” More than 500 were built, most of them serving in the US Marine Corps. But Canada and Sweden would also purchase these helicopters and keep them in service for many years, although not for so many as the Marine Corps.

Time would however leave its mark. Production ceased as early as 1971, so in spite of the type being so well regarded and continuously upgraded with modifications, the loading was successively reduced and other parameters changed in order to reduce the risks to gradually deteriorating machinery. The death blow arrived in the form of the innovative MV-22 Osprey helicopter, a cross between a helicopter and an aircraft. As the Osprey came into service, so the CH-46A was phased out, the last flight of a CH-46A taking place in 2015, thus bringing a 50-year epoch to its grave.

An MV-22 Osprey in Iraq in 2008.

It seems a little strange that a type of helicopter that became so well-loved after surmounting its childhood problems was not kept in production after 1971. An interesting parallel can be drawn on how the CH-46A’s big brother, the CH-47 Chinook would develop. It has the same layout but can lift a heavier load, although production of this type also eventually ended. However it turned out that the intended replacement could not measure up to the Chinook so that production resumed when the desire to keep the Chinook became overwhelming. If the CH-46 had attained such wide use, perhaps it too would still be in production.

The manufacturing licence for the former Vertol 107 is currently held by the Canadian company Columbia helicopters, which is now the last major operator of the type (more on this in the section dealing with the Type 4 Swedish version) but there do not appear to be any current plans to reintroduce production. Considering its good reputation and the affection gained by the type among its users, it is a pity that it was never given a second chance like the Chinook. However the refusal of the Chinook to retire shows the advantages offered by the tandem rotor concept wherever a helicopter needs to demonstrate a little more lifting power.