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Boeing 307 Stratoliner

The Boeing Model 307 Stratoliner is an American stressed-skin four-engine low-wing tailwheel monoplane airliner derived from the B-17 Flying Fortress bomber, which entered commercial service in July 1940. It was the first airliner in revenue service with a pressurized cabin, which, along with supercharged engines, allowed it to cruise above the weather. As such it represented a major advance over contemporaries, with a cruising speed of 220 mph (350 km/h) at 20,000 ft (6,100 m) compared to the Douglas DC-3's 160 mph (260 km/h), at 8,000 ft (2,400 m) then in service. When it entered commercial service it had a crew of five to six, including two pilots, a flight engineer, two flight attendants, and an optional navigator and had a capacity for 33 passengers, which later modifications increased, first to 38, and eventually to 60.

Development
In 1935, Pan American Airways, United Airlines, American Airlines, Eastern Air Lines, and Transcontinental & Western Air (T&WA) had each signed a contract with Douglas to develop the forty-passenger DC-4 (later known as the DC-4E). Each company contributed $100,000 to development costs and agreed to not operate other aircraft with a maximum weight of for revenue service. A ceiling of at least was required both to avoid summertime "chop" over the Rocky Mountains and to allow the aircraft to fly around the thunderstorms that can sometimes block mountain passes, which meant a pressurized cabin would be the most comfortable for passengers on long flights. Affected aircraft included the prototype, the first Pan Am machine, and some early production B-17s, and this resulted in T&WA having to have their engineers inspect every tube that was to be incorporated into their aircraft. All ten aboard were killed, Parachutes were available but the force of the spin prevented their use. The resulting redesign was also incorporated into the redesign of the rear fuselage of the B-17E bomber. The wings were reinforced and Handley Page slots were added to the outer wing leading edges to improve low-speed aileron control while the inboard flaps were also extended. On March 13, 1940, Approved Type Certificate (ATC) number 719 was assigned to the Pan Am Boeing 307s, allowing commercial deliveries to commence. T&WA was also concerned about the engine carburetor intake heaters being deliberately restricted by Boeing to prevent cooking the engines, which could potentially leave crews unable to clear ice. Their point was made when icing problems during a test flight on May 17, 1940, with NC19905, while carrying dignitaries in overcast conditions over mountains resulted in three of the four engines failing, while the fourth was losing power, despite every measure being taken to clear the ice. This resulted in the aircraft making a belly landing in a field with a partially lowered undercarriage, T&WA then modified the carburetor heating themselves and the aircraft was repaired and returned to service. However, its first flight, on December 31, 1938, was later than that of the Renard R-35, which also had a pressurized cabin for passengers and flew on April 1, 1938, but that craft crashed and its development was abandoned. As for employing a flight engineer, it was preceded in the US on a commercial aircraft by Maddux Air Lines Ford Trimotors, whose "Mate" had the same responsibilities as a flight engineer. ==Design==
Design
As built, the Stratoliner used the all-metal stressed-skin cantilever wings from the B-17C mounted low on the fuselage to a constant-chord center section faired to the fuselage, with four Wright GR-1820 Cyclone air-cooled radial engines. TWA examples used GR-1820-G105A engines fitted with two-stage superchargers for high-altitude performance, while the Pan Am examples used the GR-1820-G102 with a single-stage supercharger. To reduce noise, engine exhaust collector rings and mufflers were installed. All of the fuel was carried in the wings, with, in each wing, a tank between the inboard nacelle and the fuselage, a main tank, and a tank between the inner and outer nacelles, making a total capacity of in six tanks. After being modified, the SA-307B-1s used the wings and elevators from the B-17G, with split flaps, and Cyclones. The rudder and elevators also had hydraulic boost to lighten control forces. A parking brake was provided, along with an emergency air brake system run off a bottle of compressed air, while the tailwheel was fully retractable. The circular-section fuselage was of all-metal construction, skinned with 24ST Alclad The structure was designed with strength reserves so as to handle as much as , but a pressure relief valve prevented the pressure difference from exceeding . The structure consisted of continuous longitudinal stiffeners spaced every nine degrees around the fuselage with radial hoop stiffeners mounted every along the fuselage, reinforcing the similarity to a dirigible. The skin seams were sealed with tape impregnated with sealing compound trapped between lapped joints which were secured with two rows of rivets spaced apart, while doors and hatches were sealed with soft rubber gaskets and control cables entered the pressurized cabin through specially developed glands designed to allow free movement of the cables with a negligible amount of air leakage. The industrial designer Raymond Loewy designed the passenger cabin, with furnishings provided by Marshall Field's. Extensive use was made of the latest in soundproofing, and the Dynafocal engine shock mounts were designed to reduce vibrations felt by passengers. The flight engineer was also an aircraft maintenance engineer (AME) and, aside from operating the radio, which required training in Morse code, he was also responsible for all technical issues and would carry out repairs and maintenance on the 307. C-75 conversion Following the entry of the United States into World War II in December 1941, long-range transports were needed to ferry government and military officials around the globe and many aircraft, including T&WA's Boeing 307s, were pressed into service. Beginning in February 1942, these were flown to Albuquerque, New Mexico for conversion, which included the removal of the plush, but heavy, civilian fittings, including the massive mirrors in the women's "charm room", and the pressurization system, to save weight. A desk was added behind the pilot for a radio operator, who had a 50-watt Bendix TA-12 high-frequency Morse transmitter and a BC-348 tunable receiver. A trailing wire antenna was used with a lead weight on the endwhich the radio operator needed to remember to reel in, by hand, when landing. Inexperience sometimes led to the antenna either being torn off or lashing against the fuselage. An astrodome was fitted and the Perspex top windows, which produced excessive parallax, were replaced with optically flat glass to allow the navigator to compare the positions of stars to the horizon to determine latitude when crossing large bodies of water. The landing gear was strengthened, and the maximum takeoff weight was increased from , leaving the Stratoliner underpowered, and the climb suffered accordingly. Passenger facilities were reduced to four bunks which when folded away allowed seating for twelve, along with four seats along the opposite side of the aircraft. The names had previously been used in TWA publicity but not painted on the aircraft. while more powerful versions of the same Wright Cyclone engines increased power from , The tailwheel switched to using B-17G wheels and 26" smooth-tread tires, which required that the wheel well be enlarged and structure supporting the tailwheel reinforced. with the cabin divided into a 10-seat front section and a 28-seat rear section, with no sleepers. ==Variants==
Variants
;300 :Original unpressurized proposal, with seating for 16–24 passengers, which began as a four-engined Boeing 247. ;PAA-307 or S-307 Strato-Clipper :Designation for three aircraft built for Pan Am under 719. Visible external differences included engine cowlings without cowl flaps. Crew of six. Strato-Clipper was Pan Am's name for the type. Four Wright GR-1820-G105A Cyclone engines were fitted, with two-speed superchargers. Wright GR-1820-G205A Cyclone engines were fitted,) but development was cancelled in favour of the 377 Stratocruiser, based on the B-29. ;322 :Development of 307 with similar fuselage but with a mid-mounted wing and a nosewheel, as a bomber. Eventually evolved into the B-29. ==Operational history==
Operational history
Ten 307s were built. NC19906 was temporarily marked as NX1940 and NC1940 for publicity purposes. Prototype The first Boeing 307 Stratoliner, serial 1994, registration NX19901, made its first flight from Boeing Field, near Seattle, on December 31, 1938, prior to its intended delivery to Pan Am following testing and certification. As related above, it crashed on a test flight on March 18, 1939, killing all ten occupants, and forced several design changes, of which the fin and rudder are the most immediately obvious. Koninklijke Luchtvaart Maatschappij N.V. (KLM) KLM was considering four-engine airliners for the European routes and a longer-ranged four-engine aircraft for their routes to the Netherlands East Indies (now Indonesia). Aircraft considered included the Douglas DC-4(E), the Boeing 307, the Junkers Ju 90, the Focke-Wulf Fw 200 Condor, and the Bloch 160, of which only the DC-4 and 307 came close to meeting requirements. with options for 18 aircraft and with first delivery in 14 months. By January 14, 1937, the price had increased for 18 aircraft to $265,000 each, not including pressurization equipment. Following T&WA defaulting on payments, the first three T&WA aircraft were offered to KLM in late 1939, but a quick decision was needed by Saturday, March 18, 1939. KLM was unable to make that decision before the offer expired and requested an extension. Australian National Airways (ANA) Another company that Boeing was in discussions with was Australian National Airways, who they quoted $310,000 per Stratoliner, or $340,000 for a pressurized version, on July 27, 1938, registered as NX19904, for $315,000 had it fitted with much more powerful Wright R-2600 engines for its transformation into "The Flying Penthouse", which included a bedroom, two bathrooms, a galley, and a bar as well as a living room. renovating it so he could sell it, which, like the T&WA and Pan Am aircraft, then included an interior designed by Raymond Loewy. Oil tycoon Glenn McCarthy bought it to coincide with the opening of his new Shamrock Hotel, renamed it Shamrock, and had it repainted; however, McCarthy defaulted on payments and it was returned to Hughes. After various repossessions, deaths, and failed sales, it became the Cosmic Muffin, in which form it survives. Transcontinental & Western Air (T&WA or TWA) As one of the companies sponsoring the development of the Douglas DC-4, Transcontinental & Western Air lost interest due to delays and poor performance, but the agreement they signed with Douglas limited the maximum weight of any replacement design to . After discussing their needs with Boeing, T&WA signed a contract with Boeing to buy six 307s with an option for 13 more for $1,590,000 on January 29, 1937, with deliveries to be made in mid-1938. and, by March 1939, Hughes had a controlling interest in Trans-World Airlines (or TWA, as it was rebranded once he had taken over), with roughly 46% of the shares. By August 1939, TWA and Boeing had resumed negotiations, and TWA would get five 307s, and Hughes would get one. or $350,000 per aircraftthree times the cost for Douglas DC-3s. In early 1940, Hughes bought up all remaining outstanding TWA shares not otherwise reserved for employees. The date chosen for TWA's first service flight was July 8, 1940, to coincide with the anniversary of TAT's 48-hour coast-to-coast service, in which trains were used for night legs. A few weeks later, on September 26, a TWA Stratoliner bound for New York, at , with a jetstream providing a strong tailwind, reached a ground speed of . Flight 45 added stops at Philadelphia and Pittsburgh on December 1, 1940. TWA ICD Wartime operations On December 14, 1941, representatives from various airlines as well as the Air Transport Association of America (ATA) met with Colonel Robert Olds of the Air Corps Ferrying Command (later renamed Air Transport Command) over the use of their airliners in wartime. Pan Am had already signed a contract on the 13th, in which it would keep its 307s but sell the 314s to the government. ICD Supervisor pilots and captains were paid $1100/month, first officers $800/month, Navigators $600/month, Flight Engineers (FEs) $500/month, and Flight Radio Operators (FROs) $400/month. The ICD was initially set up at the crowded Washington Bolling Field but soon transferred all of their operations to nearby Washington National Airport. The first North Atlantic crossing was in March 1942, when a flight from Washington to Prestwick carried senior military and government officials including the Army Chief of Staff General George C. Marshall, General Dwight D. Eisenhower (to command Operation Torch for the invasion of North Africa), Deputy Chief of Staff of Army Ground Forces General Mark W. Clark, Operation Torch Air Force liaison officer Colonel Hoyt Vandenberg, Chief of the Navy's Bureau of Aeronautics Rear Admiral John Henry Towers, who oversaw Navy aircraft procurement and training, and presidential advisors W. Averell Harriman and Harry Hopkins, who were crossing to negotiate the Lend-Lease program. Many of the surviving members of the raid followed soon after in another Stratoliner. The first aircraft to land at these fields, which were north of the Arctic Circle, was an ICD C-75, on April 20, 1942. On July 10, 1942, Ascension Island's airfield opened in the South Atlantic, Two main routes were flown, between Washington, D.C., and Cairo across the South Atlantic, and between New York and Prestwick, Scotland, across the North Atlantic. They often flew non-stop the between Gander, Newfoundland and Prestwick, Scotland in the north, and the between Natal, Brazil, and Accra, Ghana, in the south. After July 1942 a refueling stop at Ascension Island was added in the South Atlantic. In the north, stops in Iceland or Greenland were often necessary when flying west against unusually strong prevailing winds. As Douglas C-54 Skymasters took over the Gander–Prestwick route, the C-75s operated between Marrakesh and Prestwick over the Atlantic. Cherokee, carrying returning American troops from Reykjavik to Gander at , was shot at by a US Navy ship that left over 200 holes in the aircraft's tail and nearly severed the elevator controls. At the same time Curtiss C-46 Commandos and Douglas C-47s and C-53s provided feeder links to the C-75s, C-54s and C-87s, as their range was inadequate for the Atlantic. to have the five aircraft rebuilt by Boeing, and the first of them resumed passenger service on April 1, 1945. The CAA recertified these as SA-307B-1 civilian airliners with their original registration numbers. TWA post-war service and disposal On April 1, 1945, the first post-war civil commercial flight was made by Zuni, as an SA-307B-1, from La Guardia to San Francisco via Pittsburgh, Saint Louis, Kansas City, Albuquerque, and Burbank, New TWA flight routes were added, with routes 370 & 371, between La Guardia and Spokane through Chicago, Albuquerque, for fuel, and Burbank, and routes 48 & 49, between La Guardia and Kansas City through St Louis. The first coach service, from New York La Guardia to Chicago via Pittsburgh, was made on June 1, 1949, with a full load of 38 passengers. In November 1950, TWA introduced the similarly sized but cheaper-to-operate twin-engine Martin 2-0-2A on domestic routes and transferred some DC-4s from international routes to domestic service, until TWA finally phased them out between April and July 1951. At that time the aircraft had an average of 25,205 hours flying time each, with Cherokee having the most at 26,324 hours, Pan American Airways (Pan Am) In 1937 Pan American Airways placed their first order for two Stratoliners, which they soon increased to six. Deliveries to Pan Am started in March 1940, and they had received their first three before war intervened and civil aircraft production halted. with service between Miami, Brownsville, Texas, and Los Angeles. The former Clipper Rainbow NC19902 was to have been sold to the short-lived Mercury Airways of South Africa and was even given the South African registration ZS-BWU. However, the sale was never completed, and, similarly, it was supposed to have been sold to Aerovias Ecuatorianas, with registration HC-SJC-003, but that sale also appears to have fallen through. In 1951 it was sold to Aigle Azur as F-BHHR. The former Clipper Flying Cloud NC19903 was purchased by the Haitian Air Force () in 1954 and assigned the number 2003, but plans to use it for a passenger service by the Compagnie Haïtienne de Transports Aériens (CoHaTA) were cancelled and it was fitted out as a presidential transport. When François "Papa Doc" Duvalier came into power in 1957 he chose not to use it and instead had the aircraft sold the same year, with the money from the sale going toward five North American T-6G Texan training aircraft. This aircraft returned to the U.S. and, after briefly being registered as N9307R and N19903, is now restored and at the Smithsonian Museum as NC19903. Aigle Azur and Union Aéromaritime de Transport TWA sold all five TWA SA-307B-1s to the French operator Aigle Azur (French for Blue Eagle) in April 1951, for $525,000, along with their remaining supply of spares. Aigle Azur received them between May 14, 1951, and December 19, 1951. The Stratoliners were no longer competitive against the larger and faster Douglas DC-6 and Lockheed Constellation then entering service, Union Aéromaritime de Transport merged with Transports Aériens Intercontinentaux to become Union de Transports Aériens (UTA) on October 1, 1963, but by then had already sold all of its Stratoliners to CITCA. Airnautic Airnautic (or Air Nautic) received three ex-Aigle Azur SA-307B-1s in 1955 and 1956 including F-BELU, F-BELY, and F-BELZ, which were operated in southern Europe around the Mediterranean providing charter flights, especially around Corsica. On December 29, 1962, F-BELZ collided with a mountain while on a charter flight with 22 basketball players and fans while flying from Bastia to Ajaccio, on the island of Corsica. and in 1966 Airnautic ceased to exist, having been absorbed into Air France. Compagnie Internationale de Transports Civil Aériens (CITCA) Compagnie Internationale de Transports Civil Aériens (CITCA) bought five Stratoliners, F-BELU, F-BELV, F-BELX, F-BELY, and F-BELZ, which were leased out to other operators. In 1965, Cambodia Air Commercial leased F-BELY as XW-PGR, and F-BELU as XW-TFP, before both went to Royal Air Lao the same year, which then leased all five of the CITCA Stratoliners, which were also assigned Laotian XW registrations, and F-BELV became XW-TAA and F-BELX XW-TFR. The Air Laos Transport Aériens and Royal Air Lao Stratoliners were flown between Vientiane and Hong Kong with a large detour around North Vietnam. The International Control Commission (ICC; , or CIC) and its successor, International Commission of Control and Supervision, leased three aircraft in 1964 under their old French registrations of F-BELV, F-BELU, and F-BELX, which then had seating for as many as 60 and were used to provide what were often hazardous diplomatic flights around South-East Asia until 1974, when it suspended operations with the impending defeat of US forces in Vietnam. They were flown under diplomatic immunity along specially delineated wide corridors between Saigon in South Vietnam, Vientiane in Laos, Phnom Penh in Cambodia, and Hanoi in North Vietnam. On this flight there were five CIC delegates from India, three from Canada, and one from Poland, as well as four French crew members, The pilot of XW-TFP was forced to ditch in the Mekong river on March 13, 1975, near the Laos–Thailand border while on a flight from Hong Kong to Vientiane. Both the pilot and co-pilot escaped the wreck but were captured by the communist Pathet Lao and held until May. The wreckage was still there in 1986. ==Operators==
Operators
Civilian operators ; • Howard Hughes bought one aircraft. • Inter-American Inc. (IA) bought ex-Pan Am Clipper Flying Cloud from the Haitian Government. • Pan American Airways (PAA: Pan Am) received three aircraft. operated ex-Pan Am Clipper Comet. ; • Aerovias Ecuatorianas CA/AREA Ecuador, operated an ex-Pan Am aircraft as HC 004. ; • Aigle Azur (en:Blue Eagle) operated five ex-TWA aircraft and one Pan-Am aircraft, all bought in 1951. • Air Nautic (or Airnautic) operated three aircraft. • Commission Internationale de Contrôle (International Control Commission in English) (CIC/ICC) and its successor International Commission of Control and Supervision operated at least three aircraft leased from UAT and CITCA. • Compagnie Internationale de Transports Civil Aériens (CITCA) leased out ex-UAT aircraft to other operators. • Union Aéromaritime de Transport (UAT) operated the Aigle Azur aircraft after buying them out. ; • (Fr: Air Laos/Air Laos Transport Aériens) leased one ex-Pan Am/ex-Aigle Azur aircraft, before being renamed Royal Air Lao. • operated four ex-Aigle Azur aircraft. ; • (Fr: Royal Air Cambodge) operated one aircraft with French registration. • Cambodia Air Commerciale operated several aircraft, under Laos registrations. Military operators ; • Haitian Air CorpsCompagnie Haïtienne de Transports Aériens (CoHaTA) (a Military transport organization) operated one ex-Pan Am 307. ; • United States Army Air Forces operated five ex-TWA SA-307Bs as C-75s. Three Pan Am 307s operated under USAAF direction, but ownership remained with Pan Am. ==Accidents and incidents==
Accidents and incidents
The Boeing 307 was involved in eight hull-loss incidents with 67 fatalities. Four of the ten incidents involved fatalities, with one craft likely being shot down while in a war zone. ==Surviving aircraft==
Surviving aircraft
The sole intact Boeing 307 Stratoliner, NC19903, is preserved in flying condition at the Smithsonian's Steven F. Udvar-Hazy Center. After having been restored to flying condition, it was being delivered to the Smithsonian on what was to be its last flight when it ran out of fuel and ditched in Elliott Bay in Seattle, Washington, on March 28, 2002. Despite the incident, it was raised and again restored, and it completed its flight to the Smithsonian, where it was placed on display. The forward fuselage of Howard Hughes' 307 (NX19904) also survives, although it was stripped of flying surfaces and the rear fuselage and converted into a houseboat. The aircraft was awaiting restoration at Fort Lauderdale International Airport in August 1964 but was severely damaged when Hurricane Cleo tore it loose from its tiedowns and it was blown into a stand of trees. The aircraft was later salvaged and converted into a houseboat, and the interior remains notable for the additions made when owned by Hughes. It was part of the Florida Air Museum collection. As per a Kermit Weeks Facebook post, in February 2024, it was stored with Weeks' collection at the Fantasy of Flight Museum in Florida. ==Specifications (Boeing SA-307B, #726)==
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