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Boeing Seaknight


November 18, 2014

Posted on November 18, 2014
By Ray Panko | [email protected] | Pearl Harbor Aviation Museum

Key Points

  • The United States Marine Corps realized that beach assaults in the atomic age would be suicide. It needed to move large numbers of Marines far inland very rapidly, to sites without runways.
  • For mass troop delivery, the Marines needed something larger and faster than the Army’s Hueys. In the meantime, they used the piston-engined CH-34 Seahorse.
  • The Boeing/Vertol CH-46 Sea Knight arrived in 1965. In Vietnam, it could carry a squad of 12 or 13 Marines, compared to the Huey’s five or six. It also cruised at 155 mph — 25 mph faster than the Huey.
  • The initial CH-46A version was unreliable. The CH-46D model was better, but it suffered several initial crashes. The flaw was fixed, and the CH-46D became an excellent assault helicopter. After the war, many Phrogs, as the Marines called them, were upgraded to the CH-46E standard. Ours was one of these.
  • Sea Knights are now being almost entirely replaced by MV-22 Ospreys, which give the Marines an even better way to move large forces deep inland with stunning speed.

Phrogs Evolve

The Boeing/Vertol CH-46 Sea Knight, better known as the Phrog, became operational in 1964. However, it was preceded by almost two decades of thinking that refined the United States Marine Corps’ doctrine for troop assault.

Figure 1: Phlight of Phrogs

Source: U.S. Navy

Before the Phrog

Nuclear tests after World War II convinced the Marines that the days of massive beach assaults were over. The Corps would still have to deliver thousands of troops ashore in a few hours. However, it would have to deliver them far beyond the beach, and disperse them to locations devoid of airfields. Then, it would have to support them as long as they stayed. Paratroops and gliders had done this sort of “vertical envelopment” in World War II, but the losses had been great. The Marines would use a different approach. They would use helicopters — big, fast helicopters, not the little Hueys the Army used. Massive troop delivery required big rotorcraft. Survival required faster delivery.

Figure 2: Nuclear Test in the Pacific

Source: U.S. Air Force.

Initially, the Marines pinned their hopes on the Sikorsky H-37 Mojave. This heavyweight was powered by two Pratt & Whitney R-2800 radial engines — the same engines that had driven the P-47, F6F, and F4U in World War II. Seeing its two enormous engine nacelles, Marines immediately called it “the Deuce.” Unfortunately, the H-37 pushed piston-era technology too far. The Mojave had disappointing reliability, disappointing performance, and disappointing range. When the Vietnam War started, the Marines were already developing replacements. In the meantime, however, the H-37 was all they had. In Vietnam, they used the Deuce to lift heavy cargo, not for combat assault.

Figure 3: Sikorsky CH-37 Mojave

Source: Ray Panko ([email protected]). Evergreen Aerospace Museum.

For their battle taxi at the start of the war, the Marines used their old standby, the CH-34 Seahorse. This was an even older piston-engine helicopter. Each had an early World War II Wright R-1820 engine in its nose. Compared to Hueys, Seahorses were a little larger and a little slower. They were not a permanent solution for the combat assault needs, but they were available. Although its size and speed were not what the Marines wanted, these mature and reliable helicopters did solid work for the Marines. The Marines continued to use them for a good part of the war while waiting for their new troop assault helicopter to arrive. For more information on the CH-34, click here.

Figure 4: Sikorsky CH-34 Seahorse Engine

Source: Ray Panko ([email protected]). Udvar-Hazy Center, 2010.

Fortunately, the Marines’ next-generation helicopters were not too long in coming. The Marines settled on a large/medium mix for their future helicopter inventory. Their large cargo helicopter would be the massive yet fast CH-53 Sea Stallion. The big hauler reached Vietnam in early 1965, and soon took over the cargo mission. However, the CH-53 was not what the Marines wanted for troop assault.

Figure 5: CH-53 Sea Stallion

Source: U.S. Navy

As a side note, Pearl Harbor Aviation Museum has a CH-53 Sea Stallion. This particular aircraft flew its last missions in Afghanistan in 2011. For more information about the Museum’s Sea Stallion, click here.

For troop assault, the Marines chose the medium-size Boeing/Vertol CH-46 Sea Knight. It looked odd, sitting with its tail lower than its front. In addition, the two sponsons in the rear looked like legs coiled to spring. Overall, the CH-46 looked like a frog about to jump, and the Marines quickly dubbed it “the Phrog.” Its maniacal, wide-mouth, frog-like grin added to the image. Substantially larger than the Huey, with a cabin 24 feet long, 6 feet high, and 6 feet wide, the CH-46 could carry a full squad of 12 or 13 combat-loaded Marines. (It was designed to carry 18 to 25 troops, but in Vietnam, the added armor and .50-caliber side guns and gunners, plus the hot climate that lowered air density, sapped its lifting capacity.) It had a big rear ramp that could be raised and lowered quickly for rapid troop delivery and recovery. The ramp could even be left open in flight or removed. In addition, the Sea Knight was much faster than the Seahorse. While the Seahorse lumbered around below 100 mph, the CH-46 cruised at 155 mph. This was also about 25 mph faster than the Army’s UH-1 Hueys. At the same time, the Phrog was still small enough to operate in most landing areas.

Figure 6: CH-46 Troop Loading and Water Delivery

Source: U.S. Marine Corps photos 120114-N-PB383-096 and 11117-N-SK590-189

For cargo duty, the CH-46 was no slouch. The definitive CH-46E model could carry 5,000 lbs. of internal cargo and a little over 2,000 lbs. as a slung load. Some sources note that the sling was stressed to carry 10,000 lbs., but the helicopter had nowhere near that much lift.

Figure 7: a squad of marines on a CH-46

Source: U.S. Navy 140620-M-WH399-067

Figure 8: Navy UH-46 Sea Knight Doing VERT-REP (Vertical Replenishment) at Sea

Source: Department of the Navy, Naval Historical Center, 2000.

For protection during combat assaults, Army UH-1s had machine gunners in the rear seat on both sides. This provided .30-caliber protection to the aircraft’s flanks. For their beam protection, the Marines adopted .50-caliber machine guns to extend their protection range and to slice through dense jungle foliage on the sides of landing zones. One pintle-mounted machine gun was installed on each side, firing through the removed cover of an escape hatch. The Phrog was big enough to carry the weight of these heavy guns.

Figure 9: CH-46 with .50-Caliber Machine Gun

Source: U.S. Navy

Although the Marines thought the Huey was too small for combat assault, they decided that it made a good gunship platform and was ideal for light utility work. The Marines flew UH-1E models, which had most of its magnesium parts replaced with aluminum. This made them more corrosion-resistant than Army Hueys in ocean environments. They replaced these with AH-1 Cobras later in the war. For utility work, the Marines continue to fly Hueys and Cobras today. Of course, current models are far more advanced than their Vietnam forebears. All have two engines for greater survivability, as well as far better avionics and combat electronics.

Figure 10: United States Marine Corps UH-1E Huey Utility Helicopter

Source: U.S. Marine Corps

CH-46s in Vietnam

In late 1965, the first CH-46As reached Vietnam, where they soon proved to be maintenance nightmares. The biggest problems were their transmissions and blades. Weaknesses in these vital components forced the Marines to restrict the A model’s cruising speed from its normal 168 mph to only 143 mph. In addition, its two engines produced only 1,250 shp apiece. This gave indifferent performance in lifting cargo. Although far from perfect, the A model did decent work. Boeing/Vertol produced 160 of these aircraft for the Marines and another 14 as UH-46As for the Navy. The UH-46s were modified to improve their use in the VERT-REP (vertical replenishment) job of moving cargo from ship to ship at sea.

In late 1967, improved CH-46Ds began to arrive in Vietnam. By the end of 1968, all of the A models were gone or upgraded to the D specification. With stronger T58-GE-10 engines (1,400 shp each), improved transmissions, better-cambered rotor blades, and many other improvements, the CH-46D promised to answer all of the Marines’ needs. Instead, six of these helicopters quickly crashed with fatal results. Their pylons and tails simply broke off in flight. These breaks were all in the region of frame 410 on the fuselage, indicating a common problem. The fleet was grounded, and the Marines had to make do with their remaining CH-34D Seahorses until the problem was solved. Not until 1969, in fact, would the last Marine “Dogs” leave the war.

Investigation found that designers had given pilots a special “hover aft” control. It allowed the pilot to put the tail on the ground while keeping the front of the helicopter in hover. This permitted faster egress after unloading troops, and it kept the front wheels from getting caught in obstructions. Resourceful Marine pilots quickly realized that if they used hover aft while entering a hot landing zone, the nose would pitch up to act like a speed brake. This reduced approach time significantly. Pilots had done this with the CH-34, so it seemed like a logical tactic. Unfortunately, the Sea Knight fuselage design assumed that hover aft would be used only at speeds below 30 kt. When pilots used it at 70 kt or even faster, it created stresses that weakened the tail. Eventually, catastrophic failure occurred in normal flight. The tail and pylon broke off, and the aircraft fell to the ground in two large pieces.

All CH-46Ds in Vietnam were retrofitted in Taiwan. Under this Sigma 1 program, 168 Phrogs were upgraded. Their tails were strengthened, giving them the nickname “Iron Tails.” The hover aft control was locked so that it could not be used above 30 kt. Without hover aft, Marine pilots experimented with other ways of bleeding off landing speed. They found that a sideways flare on final approach worked effectively and safely.

After this brief loss of service, the CH-46Ds flew back into combat and demonstrated just how right the aircraft was for troop assault and other missions. The USMC accepted 266 of these aircraft as CH-46Ds. The Navy accepted ten UH-46Ds and converted one CH-46D into the UH-46D specification.

Before the war ended, the CH-46F model arrived with the same engines as the D but with better avionics. Boeing/Vertol delivered 174 of these F models to the Marines between 1968 and 1971. This was the final production model of the Sea Knight. Production was ended after the 524th H-46 in February 1971. (Some sources list the number as more than 600.)

Although the Phrog proved itself to be an effective and reliable combat assault platform, its higher speeds did not make it immune from antiaircraft fire. The Marines lost 106 of their Sea Knights to enemy fire. Before the war, skeptics had argued that helicopters would be highly vulnerable in combat assaults. Advocates said that helicopters would be effective tools in assaults. Both were right.

Figure 11: One of Five CH-46 Sea Knights Lost in Operation Hastings

After the War

Although Boeing/Vertol did not produce any new CH-46s after the war in Southeast Asia, the CH-46 continued to be upgraded in later years. In the late 1970s, about 275 Fs and earlier models were upgraded to a new specification, which had a glass cockpit, fiberglass rotor blades, a stronger drive train, a redone electrical system, improved hydraulics, and two T58-GE-16 or 16/A engines with 1,870 shp output. Changes in the seats and fuel systems made it more crashworthy, and a third .50-caliber machine gun was added at the top of the rear ramp. The avionics were improved considerably, making the CH-46E a true all-weather, day/night helicopter. The new specification was called CH-46E. Further upgrades were done in the 1980s and 1990s. After the U.S. invasion of Iraq in the spring of 2003, the Marines switched from heavy metal armor to lightweight armor to increase lift. This was followed by a program to replace E-model engines with easier-to-service T58-GE-16A engines.

Thanks to these upgrades, the Phrog continued to serve the Marines ably. The Marines said, “Never trust a helicopter under 30.” The Sea Knight was still in heavy use during the invasion of Iraq in 2003. In 2006, however, the Marines began to phase out its beloved Phrogs with V-22 Ospreys. Although the CH-46 had good capacity, its age brought increasing maintenance problems, and its speed no longer gave sufficient survivability in the modern battlefield. More speed was also needed to respond more quickly to changing conditions in the battlefield. The V-22 Osprey is a tilt rotor aircraft that can take off and land like a helicopter but fly like a conventional airplane en route. After an appallingly politicized development process, the Marines finally declared it an “all up” troop carrier in 2006 and began to phase out its Phrogs. The last CH-46 squadrons are scheduled to convert in 2015. While the retirement of the Phrog is sad, the Osprey finally gives the Marines the ability they envisioned after watching nuclear tests in the Pacific — a way to move large numbers of troops inland rapidly to locations without airstrips.


Our Phrog

Our Sea Knight has tail code 153965. We know little about its history. Its construction number at Boeing/Vertol was 2316, and it was a Boeing/Vertol model BV-107M. It appears to have been ordered in 1967. It served in Marine Medium Helicopter squadrons HMM-162 and HMM-362. It was flown to the museum on June 26, 2014. Any additional information would be gratefully appreciated.


  • Piasecki introduced the twin-rotor system for building large helicopters. However, Piasecki was ousted from the company, which was renamed Vertol. In 1960, Boeing purchased the company, and renamed the company Boeing Vertol.
  • In 1958, Vertol flew its prototype of the Model 107 helicopter. It had rotors in the front and rear. It was driven by two Lycoming T-53 engines that were used in UH-1 Hueys. This commercial machine became the basis for the CH-46 with the addition of more powerful engines, a tail ramp, and other military accessories.
  • The Sea Knight was originally designated the HRB-1, where H stood for helicopter, R stood for transport, and B stood for Boeing. Not many people remember this because right after the first flight of the HRB-1 Sea Knight in late 1962, the Tri-Service Aircraft Designation System went into effect, and the aircraft became the CH-46 Sea Knight.
  • The H-46 was selected by the Marines in 1960 and formally ordered in 1961. Its first flight came in August 1962. In 1964, it began to be delivered to the Marines. It reached Vietnam in late 1965.
  • The UH-46B was a design for the U.S. Air Force. The Air Force cancelled the order and instead bought the Sikorsky HH-3C, which became the first Jolly Green Giant combat search-and-rescue helicopter. NASA purchased a single CH-46 for autolanding tests. This aircraft was designated the CH-46C.
  • The Navy traded its UH-46s for SH-60 Sea Hawks in 2004. This was actually a slight downgrade in capability.
  • CH-46E squadrons are authorized to have 12 aircraft. Their authorized air-crew levels are 28 pilots, 19 crew chiefs, and 19 aerial gunners/observers. A standard CH-46 crew consists of two pilots, one crew chief, and one gunner/observer. A squadron should be able to sustain twenty 1.5-hour sorties each day, assuming 70 percent availability of the aircraft and 90 percent of the air crew. Squadrons should be able to achieve this sortie rate from expeditionary bases or carriers. Individual CH-46Es require about 26 maintenance hours for each flight hour.
  • From a distance, the CH-46 resembles its bigger cousin, the CH-47 Chinook. Both are classic Piasecki/Vertol/Boeing twin-rotor designs. The Phrog is smaller, but this is difficult to tell at a distance. The major give-away is the number of positions on the landing gear. The Chinook has four, while the Phrog has only three. In addition, the CH-47 has sponsons all along the base of the fuselage from front to back. The C-46 has sponsons only near the two rear landing gear positions.

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