Boeing 377 Stratocruiser

The Boeing 377 Stratocruiser was a large long-range airliner developed from the C-97 Stratofreighter military transport, itself a derivative of the B-29 Superfortress. The Stratocruiser's first flight was on July 8, 1947. Its design was advanced for its day; its innovative features included two passenger decks and a pressurized cabin, a relatively new feature on transport aircraft. It could carry up to 100 passengers on the main deck plus 14 in the lower deck lounge; typical seating was for 63 or 84 passengers or 28 berthed and five seated passengers.
The Stratocruiser was larger than the Douglas DC-6 and Lockheed Constellation and cost more to buy and operate. Its reliability was poor, chiefly due to problems with the four 28-cylinder Pratt & Whitney R-4360 Wasp Major radial engines and structural and control problems with their propellers. Only 55 Model 377s were built for airlines, along with the single prototype. The 377 was also converted into the Aero Spacelines Pregnant Guppy by John M. Conroy for NASA’s Gemini space program. 
Design and development.
The Boeing 377 Stratocruiser was a civil derivative of the Boeing Model 367, the Boeing C-97 Stratofreighter, which first flew in late 1944. William Allen, who had become president of the Boeing Company in September 1945, sought to introduce a new civilian aircraft to replace reduced military production after World War II. Boeing saw in their large-bodied, fast, and long-ranged military transport potential for a passenger aircraft suited for premium service on long transoceanic routes, expanding on the precedent set by their Boeing 314 Clipper with Pan American World Airways. Despite a recession in late 1945, Allen ordered 50 Stratocruisers, spending capital on the project without an order from an airline customer. His gamble that customers would be interested in Boeing's unique and expensive new airplane turned out to be correct for a brief period.
On November 29, 1945 Pan American World Airways (Pan Am) became the launch customer with the largest commercial aircraft order in history, a $24,500,000 order for 20 Stratocruisers. Earlier in 1945 a Boeing C-97 had flown from Seattle to Washington, D.C. nonstop in six hours and four minutes; with this knowledge, and with Pan Am President Juan Trippe's high regard for Boeing after their success with the Boeing 314 Clipper, Pan Am was confident in ordering the expensive plane.
The 377 shared the distinctive design of the C-97, with a "double-bubble" fuselage cross-section, resembling an inverted figure-8, with 6,600 ft³ (187 m³) of interior space shared between two passenger decks. Outside diameter of the upper lobe was 132 inches, compared to 125 inches for the DC-6 and other Douglas types (and 148 inches for today's 737). The lower deck served as a lounge, seating 14. The 377 had innovations such as higher cabin pressure and air conditioning; the superchargers on the four Pratt & Whitney R-4360 engines increased power at altitude and allowed constant cabin pressure. The wing was the Boeing "117" airfoil, regarded as the "fastest wing of its time". In all, 4,000,000 man-hours went into the engineering of the 377. It was also one of but a few double deck airliners, another being its French contemporary, the Breguet Deux-Ponts, as well as Boeing's own 747 and the Airbus A380. A total of 56 were built, one prototype (later reconditioned) and 55 production aircraft.
First flight of the 377 was on July 8, 1947, two years after the first commercial order. The flight test fleet of three 377s underwent of flying to test its limits before certification.
Adoption of the Stratocruiser got a boost from the US government, with a controversial incentive package offered to Northwest Orient Airlines for its purchase. Its components were unusually generous mail contracts offered to Northwest for opening new routes to Hawaii and points in the western Pacific region that they were invited to apply for, and a Reconstruction Finance Corporation loan earmarked for the purchase of a fleet of Stratocruisers. Pan-Am saw Northwest's mail contract deal and appealed for new terms in their own international mail contracts, which were granted much to the consternation of Trans World Airlines, who were able to provide the same Atlantic mail services as Pan-Am with lower operating costs. The Northwest deal led to allegations of graft and political favoritism towards Boeing. The other carriers who adopted the Stratocruiser were British Overseas Airways Corporation, American Overseas Airlines (merged with Pan Am in 1950) and United Airlines. The last 377 was delivered to BOAC in May 1950. On this delivery flight, Boeing engineer Wellwood Beall accompanied the final 377 to England, and returned with news of the de Havilland Comet, the first jet airliner, and its appeal. The tenure of the Stratocruiser with United ended in 1954, when United had the opportunity to sell them to BOAC after finding them unprofitable without the extra mail subsidies enjoyed by Pan Am and Northwest.
Operational history.
As the launch customer, Pan Am was the first to begin scheduled flights, from San Francisco to Honolulu in April 1949. At the end of 1949 Pan Am, BOAC and American Overseas Airlines (AOA) were flying 377s transatlantic, while Northwest Orient Airlines was flying in the United States; in January 1950 United began flights from San Francisco to Honolulu. Stratocruisers were pressed into emergency military service after the onset of the Korean War. In late 1950 Northwest Stratocruisers were serving New York City, Chicago, Detroit, Minneapolis/St. Paul, Milwaukee, Spokane, Seattle, and Honolulu. By late 1952 Northwest Stratocruisers replaced DC-4s to Tokyo via Anchorage, Alaska; Northwest replaced the Stratocruiser on the Honolulu run in 1953 and to Tokyo in 1955. For a short time Pan Am 377s flew to Beirut, Lebanon, but after 1954 no 377 was scheduled east of Europe or west of Singapore. In 1954, United Stratocruisers flew Los Angeles to Honolulu and between Seattle and San Francisco; United's B377 flights to Honolulu were all first class. In 1955 BOAC 377s had 50 First Class seats (fare $400 one way New York to London) or 81 Tourist seats (fare $290). In 1956 Pan Am 377s flew from Los Angeles and San Francisco to Sydney with stops at Honolulu, Canton Island and Suva (via Nadi Airport in Fiji). By 1958 Pan Am was operating the Stratocruiser between Seattle, Washington and Fairbanks, Juneau and Ketchikan, Alaska and between Seattle and Whitehorse, Yukon. Within six years of first delivery, the Stratocruiser had carried 3,199,219 passengers; it had completed 3,597 transcontinental flights, and 27,678 transatlantic crossings, and went between the United States and South America 822 times. In these first six years, the Stratocruiser fleet had flown 169,859,579 miles (273,362,494 km).
The 377 was one of the most advanced and capable of the propeller-driven transports, and among the most luxurious, but it was troubled by reliability issues and maintenance costs. Problems included catastrophic failures of propellers, failures of propeller pitch control leading to overspeed incidents, problems related to the poor thermal design of the engine, and aerodynamic problems arising from design constraints imposed by the engine's thermal problems. Its service record was marred by a high incidence of in-flight emergencies and hull-loss accidents related to those issues. The propellers were the subject of Airworthiness Directives in 1955, 1957, and 1958.
In 1953 "United's Ray Ireland ...described the Stratocruiser as unbeatable in luxury attraction but is uneconomical. Ireland said PAA's Stratocruiser competition to Hawaii induced United to buy the plane originally." In 1950 United's seven 377s averaged $2.46 "direct operating cost" per plane-mile, and "Indirect costs are generally considered to be equal or greater than the direct costs." Most operators were using Stratocruisers on long-range routes where higher prices could be charged, off-setting the higher operating costs. The exception to this was Northwest Airlines, who managed to keep the aircraft competitive on shorter U.S. domestic routes where the aircraft's higher payload capacity benefited from lower fuel weights. United however could not integrate their six-plane fleet of 377s. By 1954 the lack of spares and the inability to cross-train their Douglas crews with the type relegated their Stratocruisers primarily to their Hawaii route, where they faced stiff competition from Pan American and Northwest. By the end of that year the six United 377s were all sold to BOAC in a deal orchestrated by Douglas Aircraft. BOAC, which was short of aircraft after the grounding of the Comet 1, paid between US$895,000 and US$995,000 per unit and spares for what were essentially five-year-old aircraft. An equivalent brand new Douglas DC-7 cost US$775,000 in 1954.
Boeing set never-exceed speed at IAS but in testing, the 377 reached IAS (about TAS) in a 15–20 degree dive at ; another report said it reached 498 mph true air speed while diving from 21000 feet altitude to 12000 feet in 50 seconds using "full rated power". Typical airline cruise was less than ; in August 1953, Pan Am and United 377s (and United DC-6s) were scheduled between Honolulu and San Francisco () in 9 h 45 min each way.
The longest (by distance) 377 nonstop flights were made by Pan Am from Tokyo to Honolulu during four winter seasons beginning in 1952–1953. In January 1953, two nonstops a week were scheduled with a flight time of 11 hr 1 min due to tailwinds; the following August all flights took 19 hours, with a stop at Wake Island Airfield. In winter 1953-54 one Tokyo-Honolulu flight took 9 hr 35 min for 3853 great-circle miles.
By 1960 Stratocruisers were being superseded by jets: the de Havilland Comet, Boeing 707, and Douglas DC-8. The last flight of the 377 with United was in 1954, the last with BOAC was in 1959, and the last with Northwest was in September 1960. In November 1960 only a weekly Pan Am Honolulu to Singapore flight remained, and the 377 was retired by Pan Am in 1961. High operating costs (notably the fuel consumption and maintenance of the Wasp Major engines) led to rapid abandonment of the 377 with the onset of the jet era. Its contemporaries such as the Douglas DC-6 and the Lockheed Constellation continued longer, on secondary routes or rebuilt as freighters. A few 377s were sold to smaller airlines, used as freighters, or converted by Aero Spacelines into heavily modified enlarged freighters called Guppies. During 1959 and 1960, Transocean Airlines assembled a fleet of fourteen at bargain prices. In 1960 TOA went bankrupt and only four were in operable condition. The hulks were stored at Oakland International Airport through the 1960s and cannibalized for parts, contributing some to the Aero Spacelines Guppies. Five remaining 377s were modified by Bedek Aviation to resemble former U.S. Air Force Model 367 Stratofreighters and pressed into service with the Israeli Defense Force. Two were shot down during the course of their service and the three remaining 377Ms were retired in 1978 and later scrapped. None of the 56 377s built were preserved for display; the IAI Museum in Israel has a C-97 (4X-FPM) on display painted to resemble their most famous 377M, 'Masada'.
Variants.
Aero Spacelines Guppy.
In addition to the Israeli Anaks a company called Aero Spacelines was converting old 377s to aircraft called Guppys in the 1960s. There were three types: the Pregnant Guppy, Super Guppy, and Mini Guppy. They had an extension to the top of the fuselage to enable them to carry large aircraft parts between manufacturing sites.
The first was the Pregnant Guppy, followed by the Super Guppy, and finally the Mini Guppy. The Super Guppy and the Mini Guppy had turboprop engines.
Accidents and incidents.
This aircraft type suffered 13 hull-loss accidents between 1951 and 1970 with a total of 139 fatalities. The worst single accident occurred on April 29, 1952. The aircraft type also experienced a significantly high rate of in-flight emergencies related to engine and propeller failure, resulting in Airworthiness Directives. Faults included structural failures of neoprene-cored propellers, failures of propeller pitch control resulting in overspeed, and failures related to engine cooling. Six propeller failures between 1950 and 1955 resulted in separation or near-separation of engines from mounts, with two resulting in hull-loss accidents. Directives were issued in 1950, 1955, and 1958 regarding enhanced maintenance and fault detection, in-flight vibration monitoring, and propeller replacement. A Directive concerning the pitch control system was issued after the October 16, 1956 hull-loss accident. A June 1957 overspeed incident occurred on "Romance of the Skies", after the compliance date of the Directive and less than six months before its fatal accident of November 8, 1957. No hull-loss accidents after the loss of the "Romance" have been attributed to an overspeed incident.