Bristol Freighter

The Bristol Type 170 was designed as a stop-gap project to provide work for the Bristol Aeroplane Company while the Bristol Brabazon was under development. Subsequently, the British Air Ministry expressed interest in the project, believing that it would provide a rugged transport aircraft capable of using unimproved airstrips; accordingly, a pair of prototypes were ordered on the condition that the design was modified so that it would be compatible with the air-transporting of a British Army 3-ton truck. For this requirement, the Air Ministry formulated and issued Specification 22/44 (which was later revised as C.9/45) around the envisioned design.[2]

As proposed, the aircraft was an all-metal, twin-engine high-wing monoplane based on the pre-war Bristol Bombay, having wings of the same section and taper but with a swept leading edge and straight trailing edge with two spars in place of the seven used in the Bombay.[3] The square-section fuselage was clear of internal obstructions; in the original design this was to be loaded via a trapdoor in the nose, but the Air Ministry requirements necessitated a change to clamshell doors in the nose. Considerable effort was made to maximise the usable internal volume of the aircraft in order to readily accommodate the carriage of bulky cargoes.[4] The flight deck was elevated above the load space on the nose as to not obstruct access; the crew would enter the flight deck via a fixed vertical ladder on the side of the cargo bay.[5] Power was to have been provided by a pair of a development of the Bristol Perseus using nine Bristol Centaurus cylinders, each engine rated at 1,150 hp (860 kW).[6]

Early on, it had been envisioned that the Freighter would play a logistical role in the South-East Asian theatre of the war, being operated by British Far East Command.[1] However, as it was becoming clear that the conflict would be over before the aircraft could enter service, an increasing emphasis was placed on suiting the aircraft to meet the demands of civilian operators. A cost analysis demonstrated that for civil use, it would be more economic to increase the all-up weight of the design from 30,000 to 35,000 lbs and to adopt the more powerful Bristol Hercules engine to power the Freighter. It was also considered that there would be a market for a passenger aircraft suitable for use from basic airstrips, and so a version without the nose doors and capable of carrying up to 36 passengers, to be known as the Wayfarer, was proposed.[4]

On 2 December 1945, the first prototype, registered G-AGPV, was first flown at Filton by Cyril Uwins.[1] Cyril found the aircraft to be generally satisfactory, but requested that the tailplane be lowered and increased in span as to enable the aircraft be trimmed to fly "hands off" over a wide range of centre of gravity positions.[7]

Bristol Freighter 31M in Norcanair markings at the Western Canada Aviation Museum in Winnipeg, Manitoba, 2007

The Bristol Type 170 Freighter was a twin-engine, high mounted-wing monoplane that was developed specifically for the economic carriage of freight by air.[8] It was a visually distinctive aircraft, possessing a 'boxy' fuselage, rounded nose, and a high-set flight deck.[1] In order to maximise the economical performance of the Freighter, compromises were implemented on other aspects of performance, resulting in a relatively low cruising speed; this was not viewed as being of importance to the role of a freighter and thus not a major diminishing factor.[9] According to aviation publication Flight, the economics of the Freighter were judged to be a major factor of its market appeal, as well as the wider economic situation of the UK at this time.[10][11]

Operationally, the Freighter was intended to be employed upon high-frequency short distance routes as opposed to long-haul routes.[4] Being flown at the low speeds and short ranges for which the aircraft was intended, the fuel economy improvements that would be provided by a retractable undercarriage was outweighed by the increase in structural weight; therefore, it was decided that a fixed undercarriage would be used, which also had the benefits of reduced production and maintenance costs.[3][12] The combination of a high-mounted wing and fixed undercarriage was considered to be atypical for the era, and resulted in greater drag than a low-mounted counterpart would have.[10] The main gear legs, which featured Dowty-built shock absorbers, were supported by an arrangement of strengthened vertical struts, positioned beneath the aircraft's engines and horizontally from the lower edge of the fuselage.[12]

The lower nose of the Freighter was covered by a pair of large clamshell doors, for easy access to the main hold; as a direct consequence of this arrangement, the unpressurised fuselage was somewhat breezy during flight.[13] The doors, which are hinged outwards, led into a main hold that had an internal volume of 2,020 cu ft; it was capable of being loaded with heavy payloads, up to a maximum of 350 cu ft per ton.[10][14] To better facilitate loading, a built-in hoist is installed on the fixed upper surface of the nose, which reduced the need for airport-based infrastructure. Fixed restraining points to secure payloads are present throughout the internal space.[10] On the passenger-carrying Wayfarer variant, the nose-mounted doors were substituted by a fixed shell and the area immediately behind would be used either as a cargo hold or galley.[4]

Bristol Freighter operated by Aer Lingus being loaded through the clamshell nose doors in 1952

The flight deck of the Freighter was positioned in an elevated position, directly above the clamshell doors.[15] In addition to providing a good all-round view for the flight crew, this placement kept the flight deck clear of the loading activity below. Flight stated of the cockpit: "The control and instrument layout is quite the best we have seen in any Bristol aircraft".[5] The major controls are typically comfortable and smooth, while each pilot is provided with a standard blind-flying panel. Instrumentation fittings differed dependent upon customer specification, but a Sperry Corporation-built autopilot would typically be installed.[15] The flight deck was typically operated by a two-man crew, a first pilot and second pilot or radio operator; in addition, space was provided at the rear for a third crew member.[15]

The Bristol Hercules 734 radial engines that powered the type, along with all of its major subsystems such as the cowling, oil tank, cooler and control cables, could also be easily detached for servicing; an entire engine replacement could be performed within 90 minutes.[3][16] The fuel system is uncomplicated, being contained within a pair of 300-gallon fuel tanks positioned in the interspar bays of the wings; neither fuel tanks or hydraulic pipelines were present in the outer wings.[12] The only elements needing hydraulic systems were the clamshell doors and flaps.[6] The electrically controlled carburation system had three different intakes, each suited to starting the engine under different conditions – these being hot, sub-zero, and temperate climates.[15]

The airframe itself is deliberately simple in its construction, while the use of maintenance-intensive components was kept to an absolute minimum and, where necessary, these were made as accessible and readily serviceable as possible.[10][6] Manufacturing of the Freighter was eased by many components being standardised and identical where feasible; this ideology was also applied to elements of the internal structure, such as the longerons, frames, and skin.[14] The floor of the main hold is covered by wooden panels, designed to be easily replaceable in the event of damage; these were supported by a floor structure of deep transverse beams with a pair of longitudinal strips, strengthened by a support beam.[14] Another ease-of-maintenance decision was to use cables for the control system where practical, while all flight control surfaces had fabric coverings, which was both light and easy to replace.[12]

Silver City Airways Freighter 32 loading a car for Cherbourg at Southampton in September 1954

On 30 April 1946, the second prototype, which was also the first 34-seat Wayfarer, registered G-AGVB, made its first flight. It quickly commenced proving flights in the colours of Channel Islands Airways, where it carried in excess of 10,000 passengers over six months.[1] The third aircraft, registered G-AGVC, was the first Freighter I and had fully operational nose doors. After a number of demonstration flights around the world, the Bristol 170 entered full production. One of the first sales was to the Argentine Air Force, which ordered 15 aircraft.[17]

The managing director of Silver City Airways was Wing Commander Griffith James Powell, who realised that he could adapt the Bristol Freighter to fly passengers with their cars from Britain to Continental Europe and Jersey. As an "air ferry", it would allow people going on holiday to avoid the lengthy waits and travel times involved in traditional sea ferries. On 14 July 1948, the airline made the first flight with a car, from Lympne Airport in Kent to Le Touquet on the northern coast of France.[18] Silver City Airways would become one of its most prolific operators: during 1954, each Freighter in the company's fleet averaged 2,970 landings and take offs — in excess of eight sectors per day for every day of the year.[1]

A Silver City Airways Bristol Freighter, viewed from under the wing of an Avro York at Berlin-Tempelhof, 1954.

In 1953, production of the freighter was moved to Whitney Straight's Western Airways factory at Weston-super-Mare airport. A lengthened version, the Freighter 32, which featured movable wooden partitions in the cargo compartment, was introduced; it could be configured to carry either three 14 ft (3.3m) cars and 20 passengers or two larger vehicles and 12 passengers, the passenger seats being in the rear section of the fuselage.[19] Silver City Airways dubbed this variant the Superfreighter and subsequently built an airport named "Ferryfield" at Lydd in Kent, beginning air ferry services in 1955. In the same year, Channel Air Bridge started operations from Southend, with four Bristol Freighters flying to Calais.

The last two Freighters of the 214 built were delivered in 1958, one to New Zealand in February and the last aircraft to Dan-Air in March 1958. The New Zealand aircraft was delivered to Straits Air Freight Express (SAFE), which eventually operated one of the largest fleets of Freighters. One of the lengthened aircraft, registered G-AMWA, had 60 seats fitted and was known as a Super Wayfarer.

Other civil uses

In New Zealand SAFE Air moved rail freight from Wellington (the North Island) across the Cook Strait to Blenheim (the South Island) and back, using Bristol Freighters, starting in 1951. The airline later reconfigured its aircraft to accept palletised cargo loaded on patented "cargons". This was a first anywhere in the aviation world.

A Bristol 170 Series 31 Merchant Venturer of SAFE Air at Auckland International Airport in 1973

Cargons were loaded near the rail yards and their load was calculated and arranged to remain within the aircraft's load and centre of gravity limits. They were then trucked to the airport and placed on the Freighter using a mechanical loading device. The loader accepted cargons from horizontal-tray road vehicles and then raised them to the level of the aircraft's cargo deck on electrically powered screw-jacks. The pallets could then be rolled into the nose of the aircraft. Other adaptations allowed the carrying of horses and other high-value large animals.

Freighters were the major link between the Chatham Islands and mainland New Zealand until Armstrong Whitworth Argosy aircraft replaced them. SAFE Air developed a soundproofed "container" for the half of the aircraft given over to passengers on these flights. Bristol 170s were still in commercial use with SAFE until the late 1970s.

Military uses

Royal Canadian Air Force Bristol 170 31M of the RCAFs European-based 1st Air Division in 1966

In military service, Bristol Freighters were operated by the air forces of Argentina, Australia, Burma, Canada, Iraq, Pakistan and New Zealand.[1] Bristol Freighters were operated briefly by the Pakistan Air Force. After withdrawal, some of the Pakistan aircraft were bought by SAFE Air and used in New Zealand. The Royal Canadian Air Force used five Freighters to carry spares and supplies between the United Kingdom and their bases in France and West Germany.

The Royal New Zealand Air Force ordered 12 Mk 31M Freighters in the late 1940s. RNZAF Freighters ranged as far as supplying the New Zealand Army in Malaya, the British High Commissions (and other support staff) in the Maldives, Ceylon, India and Nepal, performing Far East Air Force tasks in Malaya (often when other aircraft types were unserviceable owing to maintenance problems) and Hong Kong. They ran a highly reliable military shuttle service for allies in Thailand during the Vietnam War and served several other roles, being adapted for—amongst other things—aerial top dressing experiments, although to avoid competition with private enterprise, the New Zealand government did not use them in that role.

Sixty-eight of the 214 Freighters built were destroyed or damaged beyond economical repair in accidents. At least 45 of these were fatal, resulting in the deaths of at least 385 passengers and crew.[24]

Flight deck of a Freighter

Vehicle being loaded onto a Freighter via the open nose doors

Data from Jane's all the World's Aircraft 1953–54[29]

General characteristics

• Crew: 3
• Capacity: 22,300 cu ft (630 m³) cargo hold, 16,184 lb (7,341 kg) payload inc fuel
• Length: 68 ft 4 in (20.83 m)
• Wingspan: 108 ft 0 in (32.92 m)
• Height: 21 ft 8 in (6.60 m) [30]
  
• Wing area: 1,487 sq ft (138.1 m²)
• Airfoil: RAF 28 (modified)[31]
• Empty weight: 26,910 lb (12,206 kg)
• Gross weight: 44,000 lb (19,958 kg) all marks
• Fuel capacity: 1,170 imp gal (1,410 US gal; 5,300 l) total[32]
• Powerplant: 2 × Bristol Hercules 734 14-cylinder air-cooled radial piston engines, 2,000 hp (1,500 kW) each
• Propellers: 4-bladed de Havilland constant-speed fully-feathering metal propellers, 14 ft (4.3 m) diameter

Performance

• Maximum speed: 225 mph (362 km/h, 196 kn) at 3,000 ft (910 m)
• Cruise speed: 164 mph (264 km/h, 143 kn) recommended, at 5,000 ft (1,500 m)
• Range: 820 mi (1,320 km, 710 nmi) with 12,000 lb (5,400 kg) payload[33]
• Service ceiling: 23,000 ft (7,000 m) at max. continuous power and 38,000 lb (17,000 kg)[34]
• Rate of climb: 250 ft/min (1.3 m/s) On one engine at fully loaded weight
• Time to altitude: 10,000 ft (3,000 m) in 10 minutes at max. continuous power
• Power/mass: 11 lb/hp (6.7 kg/kW)
  
• Take-off distance to 50 ft (15 m): 2,500 ft (760 m) in a 5 mph (4.3 kn; 8.0 km/h) wind
• Landing distance from 50 ft (15 m): 2,300 ft (700 m) in a 5 mph (4.3 kn; 8.0 km/h) wind

External video
Video of the First Taxi Run of Bristol Freighter in 20 Years
Extended footage of a Bristol Freight in a film
Documentary of Freighter operations in New Zealand

Aircraft of comparable role, configuration, and era

• Armstrong Whitworth AW.660 Argosy
• Aviation Traders Carvair
• Blackburn Beverley

Wikimedia Commons has media related to Bristol Freighter.

• "Cargo Aircraft: Part II. Dealing with Some New Facts and Figures of the Bristol Freighter", Flight, 1945
• RNZAF Museum Freighter page
• Bristol Type 170 – Freighter, Wayfarer and Superfreighter versions
• Showing the Flag – The Bristol Freighter's Tour of North and South America
• "Bristol Freighter ... turns car freighter!" a 1948 advertisement for the Freighter in Flight
• "New Bristol 170" a 1948 Flight article
• LIFE photo of Bristol freighter locust sprayer in Iraq, 1959