De Havilland Mosquito

The de Havilland Mosquito was a British combat aircraft that excelled in versatility during the Second World War. It was known affectionately as the "Mossie" to its crews and was also nicknamed "The Wooden Wonder" or "The Timber Terror" as the bulk of the aircraft was made of laminated plywood. It saw service with the Royal Air Force (RAF) and many other air forces in the European theater, the Pacific and Mediterranean.

Originally conceived as an unarmed fast bomber, the Mosquito was adapted to many other roles during the air war, including: low to medium altitude daytime tactical bomber, high altitude night bomber, pathfinder, day or night fighter, fighter-bomber, intruder, maritime strike, and fast photo reconnaissance aircraft carrying out aerial reconnaissance. It was even used by the British Overseas Airways Corporation (BOAC) as a transport. It was also the basis for a single-seat heavy fighter, the de Havilland Hornet.

Upon the Air Ministry's decision to enter the Mosquito into production in 1941, it was the fastest operational aircraft in the world. Entering widespread service in 1942, the Mosquito supported RAF strategic night fighter defense forces in the United Kingdom from ''Luftwaffe '' raids, most notably defeating the German aerial offensive ''Operation Steinbock '' in 1944. Offensively, the Mosquito units also conducted night time fighter sweeps in indirect and direct protection of RAF Bomber Command's bomber fleets to reduce RAF bomber losses in 1944 and 1945. The Mosquito increased German night fighter losses to such an extent the Germans were said to have awarded two victories for shooting one down. As a bomber it took part in "special raids", such as pinpoint attacks on prisoner-of-war camp, Gestapo or German intelligence and security force bases as well as tactical strikes in support of the British Army in the Normandy Campaign. Some Mosquitos also saw action in RAF Coastal Command during the Battle of the Atlantic, attacking ''Kriegsmarine '' U-Boat and transport ship concentrations, particularly in the Bay of Biscay offensive in 1943 in which significant numbers of U-Boats were sunk or damaged.

Design and development

Earlier designs

Throughout the 1930s, de Havilland established a reputation in developing innovative high-speed aircraft such as the DH.88 Comet mailplane and DH.91 Albatross airliner that had already successfully employed the composite wood construction that the Mosquito would use. The construction of their aircraft compensated for the general low power of aero engines available at the time. The 22-passenger de Havilland DH.91 Albatross was capable of cruising at 210 mph (338 km/h) at 11,000 ft (3,350 m). The wooden construction and stressed skin covering not only saved weight and compensated for the low power engines, but simplified production and enabled a fast construction rate.

The potential of their construction techniques could be seen in the design of the DH.88, which heavily influenced the Mosquito. In October 1934, the purpose-built Comet Racer C-ACSS ''Grosvenor House'' won the London-Melbourne Centenary Air Race, flying 11,300 miles (18,180 km) in 70 hours 54 minutes and 18 seconds. Later, the aircraft flew from Gravesend to Sydney, Australia before flying to Blenheim, New Zealand in 10 days, 21 hours, 22 minutes, covering a distance of 26,450 miles (42,566 km).

Just under two years later, de Havilland took an interest in Air Ministry specification P.13, issued on 8 September 1936 which asked for a twin-engined "medium bomber" suitable for "worldwide use" which could carry a 1,000 pound (450 kg) bombload for while operating at and and a maximum 8,000 lb (3,630 kg) bombload. Major aviation firms entered heavy designs with new high-powered engines and multiple defensive turrets, but de Havilland felt that a smaller aircraft could do the same job. The firm had little experience of working with the Air Ministry, and their all-wood approach was considered to be out of keeping with official policy. P.13 would instead lead to the Avro Manchester and Handley Page Halifax.

Concept and design

In 1937, the concept of a fast unarmed bomber including a design comparable to P.13/36, was put before the Air Ministry by Volkert of Handley Page. Within the RAF it had support as an idea worth pursuing in case the need appeared as well as raising concerns that the (economic) benefits over conventional bombers were marginal given the limited operational role it could play. The Ministry was also considering the maximum use of non-strategic materials for aircraft production, leading to the development of the Albemarle medium bomber which was largely constructed from spruce and plywood on a steel tube frame.

Although de Havilland wanted to go further than the original P.13 requirement, at first he looked at bomber adaptions of existing designs, such as the Albatross airliner. Eventually however, de Havilland settled on a new design for a twin-engine wood constructed bomber. It would be aerodynamically clean and powered by the Rolls-Royce Merlin engine which even at that early point, offered huge promise. The original plan of the Mosquito was to be faster than enemy fighter aircraft, and therefore not need a defensive armament consisting of gun turrets, which in turn would slow it down and make interception more likely. This was in complete opposition to contemporary RAF design philosophy, which required well-armed heavy bombers. The design received the designation DH.98.

The de Havilland design was deemed radical by the Air Ministry and received several rejections owing to the RAF desire for heavy well-armed bombers. The last frontline operational aircraft the RAF had taken from de Havilland was the DH.9a. The RAF required an aircraft that could act as a heavy bomber and a long range aircraft, and had sufficient speed to reduce the time spent over defended airspace. The priority given to speed was emphasized - one basic design able to combine medium bomber capability with reconnaissance and "general purpose" classes. The aircraft was also to be capable of maritime strike, so specifications for torpedo carrying equipment were to be part of this new design. In addition, the Ministry suggested two forward and two rear firing machine guns for defense.

The Air Council then applied further requirements, such as remotely controlled guns, and a top speed of 275 mph (442 km/h) at 15,000 ft (4,570 m) on two-thirds engine power and a range of 3,000 miles (4,830 km) with a 4,000 lb (1,810 kg) bomb load.

In April 1938, de Havilland's initial design had started off as an adaptation of the Albatross, armed with three gun turrets and a six-man crew, and powered by two Rolls-Royce Merlin engines. It would carry 6,000 lb (2,720 kg) of bombs to Berlin and back at 11,000 ft (3,350 m). Geoffrey de Havilland was concerned that the Air Ministry would not allow a design constructed of wood carrying bombs 3,000 miles (4,830 km) in wartime. On 7 July, he sent a letter to Air Marshal Wilfred Freeman, an old friend of de Havilland since the First World War, and now the Air Council's member for Research and Development, asking for help in case of opposition. Freeman gave the plans the green light.

The Albatross design produced would need at least a three man crew and six to eight forward firing guns and a couple of handheld defensive weapons, with provision for a rear turret. All this produced a total weight of 19,000 lb (8,616 kg), a top speed of 300 mph (483 km/h) and cruise speed of 268 mph (431 km/h) at 22,500 ft (6,860 m). Geoffrey de Havilland considered this impossible for the time being, and expected the aero companies in Britain to struggle with the specifications of P.13/36. As it transpired, the Avro Manchester and Handley Page HP.56 were problematic designs because of the problems with the Rolls-Royce Vulture. Changing to four Merlins gave the successful Lancaster and Halifax.

Ronald Bishop, de Havilland designer, sketched out the Mosquito at Salisbury Hall. Bishop had worked on the Comet under de Havilland's chief designer and the Albatross under A. E. Hagg.

Later development, the Mosquito design takes shape

The restrictive design features placed upon the P.13 specification prompted de Havilland to approach the medium bomber project with a radical new design. They decided to eradicate the defensive armament altogether as it would save one-sixth the total weight of the aircraft. In consequence, production would be easier and faster with a delivery rate far in advance of any competing designs. Without armament, the crew could be reduced to two, a pilot and a navigator. The chief designer, Ronald Eric Bishop, made provision for the addition of 20 mm cannon installed in the nose. This forward firing heavy armament proved very successful in wartime.

The Air Ministry was not happy with de Havilland's proposals. They wanted a bomber to conform to their concept that heavily-armed formations of bombers would make it through to their targets. On 27 July 1938, de Havilland distanced themselves further by stating the P.13 specification could not be met by a two Merlin engine aircraft unless the Air Ministry only wanted half of the 4,000 lb (1,810 kg) bomb load, otherwise a larger and slower bomber would be needed.

During the Munich Crisis in September 1938, the de Havilland company presented their brand new design which was manned by two crew and powered by two Merlins without any defensive armament. Speed, de Havilland vowed, would be its defense. In the First World War, de Havilland was satisfied that the Airco DH.4 bomber was able to outpace the fighters of the day. The wood design would also shave a year off the time it would take to build a metal prototype.

In October 1938, the Ministry rejected de Havilland's proposal, skeptical about the idea of a wooden aircraft and the concept of the unarmed bomber. The Air Ministry were interested only in all-metal bombers that could defend themselves with their heavy defensive armament. To accommodate them, de Havilland built a series of mock-ups with a turret installed in the fuselage armed with two machine guns. But other than this, the de Havilland company refused to change the fundamentals of the design.

On 20 September 1939, de Havilland wrote to Freeman again:

We believe we can produce a twin-engined bomber which would have a performance so outstanding that little defensive equipment would be needed. This would employ the well tried out method of construction used in the Comet and Albatross and being of a wood or composite construction would not encroach on the labour and material used in the expanding RAF. It is especially suited to really high speeds because all surfaces are smooth, free from rivets, overlapped plates and undulations and it also lends itself very rapid and subsequent production.

In November 1939, de Havilland were progressing through various schemes using different engines and considering the effects of defensive armament on their designs. A four-gun turret defence was possible with Griffon engines while keeping speed up. The Ministry were still supportive for the aircraft for development purposes, but availability of engines and turrets would make production of any more than the prototypes impracticable. Rather than commit the design team to a single aircraft, 50 Merlin-engined DH.98s and two turreted prototypes were decided upon. At a meeting considering de Havilland's, Blackburn's and Bristol's fast bomber ideas in November, Wilfrid Freeman gained support for directing deHavilland to produce a fast aircraft with increasing power, firstly with Merlin engines, then RR Griffons, and after that Napier Sabre s.

Freeman was supportive and agreed to the lack of armament but doubted de Havilland's performance estimates which claimed a two crew wooden twin engine bomber could fly 1,500 miles (2,410 km) and at a speed faster than a Spitfire. He did believe, by using ducted air radiators and faired propeller roots, the aircraft could reduce drag to a large extent. Despite the agreement there was still strong opposition to a two crew bomber, the Air Ministry wanted at least a third crewman to reduce the workload of the others on long flights.

On 12 December 1939, the Vice-Chief of the Air Staff, Director General of Research and Development, Air Officer Commanding-in-Chief (AOC-in-C) of RAF Bomber Command met to thrash out the details of the design and its uses for future strategy. The AOC-in-C would not accept an unarmed bomber, but insisted it would be suited for reconnaissance missions with F24 cameras. On that day, the Air Council ordered the DH.98 prototype built under specification B.1/40.

After more talks between the company and Air Ministry and operational commands, on 29 December 1939, the project received official backing when the Air Ministry asked for a "basic" bomber requirement for a bomber capable of carrying a 1,000 lb (450 kg) bomb load and with a range of 1,500 miles (2,410 km). Finally, on 1 January 1940, a further meeting, chaired by Air Marshal Wilfrid Freeman took place with Geoffrey de Havilland, John Buchanan, Deputy of Aircraft Production and John Connolly, Buchanan's chief of staff. Claiming the Mosquito was the "fastest bomber in the world", de Havilland added "it must be useful". Freeman supported its production for RAF service and ordered a single prototype for an unarmed bomber variant to specification B.1/40/dh, powered by two Merlin engines. The aircraft was to have a speed of 397 mph (639 km/h) at 23,700 ft (7,220 m) and a cruising speed of 327 mph (527 km/h) at 26,600 ft (8,110 m) with a 1,480 mile (2,380 km) range at 24,900 ft (8,500 m) on full tanks. Maximum service ceiling was to be 32,100 ft (9,780 m).

Project Mosquito

Air Marshal Roderic Hill ordered a contract for 50 DH.98 aircraft, including one prototype, which was finally placed under Specification B.1/40 on 1 March 1940. Design and prototype construction was able to begin almost immediately, but work was cancelled again after the Battle of Dunkirk in order to focus on existing types.

Instead, the Supermarine Spitfire, Hawker Hurricane, Vickers Wellington, and Bristol Blenheim aircraft got priority. It was decided there was no production capacity for aircraft like the DH.98, which was not expected to be in service until early 1941. This was mostly as a result of the shortage of Merlin engines which were needed in fighter aircraft. The Mosquito was only reinstated as a priority in July 1940 after de Havilland promised Lord Beaverbrook 50 machines by December 1941; although de Havilland knew this was unlikely. Only 20 Mosquitos were built that year. During this time Beaverbrook, as Minister for Aircraft Production, asked Air Marshall Freeman to cease work on the project. Freeman did not issue such an order but it seemed the project would be shut down when the design team were denied the materials with which to build their prototype. Only when Beaverbrook was satisfied that work would not divert effort from the repairing of Merlin engines and production of Tiger Moth trainers, was it allowed to continue.

The Battle of Britain raged while the prototypes were being built, and 25% of the factory time was lost in the bomb shelters. Nevertheless, the original day bomber prototype, ''W4050 '', was rolled out on 19 November 1940, and first flew on 25 November, only 10 months after the go-ahead. The original estimates were that as the Mosquito prototype had twice the surface area and over twice the weight of the 1940 Spitfire Mk II, but also with twice its power, the Mosquito would end up being 20 mph (32 km/h) faster. Over the next few months, ''W4050'' surpassed this estimate, easily besting the Spitfire Mk II in testing at RAF Boscombe Down in February 1941 at a top speed of 392 mph (631 km/h) at 22,000 ft (6,700 m) altitude, compared to a top speed of 360 mph (579 km/h) at 19,500 ft (6,000 m) for the Spitfire. It also bested the Spitfire at low to medium altitude, some 6,000 ft (1,830m).

The Air Ministry was still not happy with the concept of the unarmed bomber. Nevertheless, in the aftermath of the Battle of Britain, interest in long-range heavy fighter and bomber destroyers began. The original 50 prototype order was changed to 20 bomber variants and 30 fighters; it was still uncertain whether the fighter version should have dual or single controls or should carry a turret so three prototypes were required. This caused some delays as half built wing components had to be strengthened for the expected higher combat load requirements. The nose sections also had to be altered, from clear perspex, to solid noses designed to hold heavy air-to-air combat weapons.

Prototypes and test flights

Construction of a prototype fighter version was carried out at the secret Salisbury Hall facility. The prototype was given the factory serial E-0234 and later allocated the military serial W4050.

During testing, it was found that the day bomber prototype had the power and internal capacity to carry not the of bombs originally specified but four times more. In order to better support the higher loads the aircraft was capable of, the wingspan was increased from 52 ft 6 in (16.0 m) to 54 ft 2 in (16.5 m). It was also fitted with a larger tailplane in comparison to later production types, an improved exhaust system, and lengthened nacelles that improved stability. These modifications became standard across the production versions. The skin on the tail surfaces was all wood and plywood covered, while the rudder and elevators were made of aluminium, and the elevators fabric covered. In 1943, the elevators were covered in metal to increase speed in dives. Two Merlin 21 two-speed single stage supercharged engines were installed. The propellers were de Havilland Hydromatic constant-speed three-blade airscrews.

On 3 November 1940, the aircraft was transported by road to Hatfield, where it was placed in a small blast-proof assembly building, where successful engine runs were made on 19 November. On 24 November 1940, taxiing trials were carried out by Geoffrey de Havilland Jr, who was the chief test pilot and responsible for maiden flights. The tests were successful and the prototype was subsequently readied for flight testing. On 25 November, the aircraft made its first flight, piloted by Geoffrey De Havilland Jr., who flew W4052 off a field beside the shed it was built in.The flight was made 11 months after the start of detailed design work, a remarkable achievement under the conditions of the time (without the assembly line method ).

John E. Walker, Chief Engine Installation designer, accompanied de Havilland. The takeoff was "straight forward and easy". The undercarriage was not retracted until a considerable height was obtained. The aircraft reached 220 mph (354 km/h), and the only problem was the failure of the undercarriage doors to fully close. They remained open by some 12 in (31 cm) as speed increased. This problem persisted for some time. Another teething problem was the inability of the tailwheel to caster properly, which caused the airframe in the rear fuselage to fracture during tests at Boscombe in 1941. This problem was solved in June–July 1941 by the fitting of a Dowty unit. The left wing of E-0234 had a tendency to drag to port slightly, so a rigging adjustment was carried out before further flights.

On 5 December 1940, the E-0234, now with the serial W4050, experienced tail buffeting at speeds of 240–255 mph (386–410 km/h). The pilot noticed this most in the control column, and handling became more difficult. After de Havilland design staff explored the problem, the conclusion was it was a stall generating, away from the engine nacelle s and on the underside of the wing. The disturbed airflow was striking the tailplane, causing the buffeting. Slots were fitted to smooth the air flow and deflect it from striking the tailplane with such force. Wool tufts were attached to the suspect areas as well during testing to investigate direction of airflow. The slots failed to achieve anything. The buffeting was eliminated by introducing stretched nacelles which extended back beyond the trailing edge of the wing, in February 1941. This meant the flap s had to be divided into inboard and outboard sections.

With teething problems largely solved, John Cunningham flew W4050 on 9 February 1941. He was greatly impressed by the lightness of the controls and generally pleasant handling characteristics". Cunningham concluded that when the type was fitted with radar, it would be a perfect replacement for the Beaufighter.

During its trials on 16 January 1941, W4050, in a bomber/reconnaissance configuration, the prototype outpaced a Spitfire at 6,000 ft (1,830 m). During February, the main priority was speed trials in level flight, during which it reached 390 mph (627 km/h) at 22,000 ft (6,700 m). Official trials began at Boscombe Down on 19 February which satisfied the de Havilland company. On 20 April 1941, it was demonstrated to Lord Beaverbrook, the Minister of Aircraft Production. The Mosquito made a series of flights, including one rolling climb on one engine. Also present were US General Henry H. Arnold and his aide Major Elwood Quesada:

I do recall the first time I saw the Mosquito as being impressed by its performance, which we were aware of. We were impressed by the appearance of the airplane that looks fast usually is fast, and the Mosquito was, by the standards of the time, an extremely well streamlined airplane, and it was highly regarded, highly respected.

The trials set up future production plans between Britain, Australia and Canada. The Americans did not pursue their interest. It was thought the Lockheed P-38 Lightning could handle the same duties just as easily. Arnold felt the design was being overlooked, and urged the strategic personalities in the United States Army Air Force to learn from the design if they chose not to adopt it. The USAAF then requested one airframe to evaluate, on 12 December 1941 after the Attack on Pearl Harbour, and the USAAF entered the war without a fast dual-purpose reconnaissance machine.

W4051 was to be the second prototype, but an airframe fracture in W4050 meant that W4051's airframe was used as a replacement. W4051 received a production standard fuselage and actually saw combat operations. W4050 flew on 10 June 1941, the third Mosquito after W4052, which flew on 15 May 1941. W4055 flew the first operational Mosquito flight on 17 September 1941.

During the flight testing, the Mosquito was fitted with a number of innovations. W4050 was fitted with a turret behind the cockpit for drag tests. It was not adopted and the idea was abandoned in July 1941. W4052 had the first version of the Youngman Frill airbrake fitted to the fighter prototype. The frill was opened by bellows and venturi effect to provide rapid deceleration during interceptions and was tested between January - August 1942, but was abandoned when it was discovered that lowering the undercarriage had the same effect.

W4052 was extensively used as the fighter variant prototype. It differed from its bomber brethren in a number of ways. It was powered by an upgraded variant of the Merlin 21 of 1,460 hp (1,088 kW), and also had a flat bullet-proof windshield. It was tested with powerful armament such as 40 mm cannon and other modifications, bomb racks, fuel drop tanks, barrage balloon cable cutters in the leading edge of the wings, Hamilton airscrews and braking propellers, as well as drooping aileron systems which enabled steep approaches, and a larger rudder tab. The prototype continued to serve as a test machine until it was scrapped on 28 January 1946.

W4050 continued to be used for long and varied testing programs. In June 1942, it was powered by two Merlin 61s, a two-stage supercharger powerplant. It was used to eliminate problems on W4052. Most of this related to the former's exhaust and armament systems. Flash eliminators had to be fitted to the NF.II to prevent the crew from being blinded by muzzle flash during the firing of the weapons. The cooling intake shrouds that were to cool the exhausts overheated after a while. Flame dampers prevented the exhaust glow on night operations, but they had an effect on performance. Multiple ejector and open-ended exhaust stubs configuration solved the problem and were used in the PR.VIII, B.IX and B.XVI variants. This increased speed performance in the B.IX alone by 10-13 mph (16–21 km/h).

By the time testing was over and the Air Ministry had authorised mass production plans to be drawn up on 21 June 1941, the aircraft was the world's fastest operational aircraft. The Air Ministry authorised 19 PR. models and 176 fighters. A further 50 were unspecified. In July 1941, the Air Ministry confirmed these would be unarmed fast bombers. The Mosquito would enjoy its fastest aircraft status for another two and a half years. On 18 and 19 July 1941, M4050 was fitted with Merlin 61s and reached 433 mph (697 km/h) at 28,500 ft (8,690 m). Multiple ejector exhausts contributed to the increase in speed. On 20 October 1941, W4050 achieved a speed of 437 mph (703 km/h) in level flight. Using two-stage Merlin 77s, it reached 439 mph (706 km/h) in December 1943.

By the end of January 1942, contracts had been awarded for 1,378 Mosquitos of all variants, including 20 T.III trainers and 334 FB.VI bombers. Another 400 were to be built by de Havilland Canada.

Basic design (based on the FB Mk VI)

The Mosquito was a mid-wing (or shoulder-wing) aircraft powered by two Merlin engines. In the most produced variant, the FB 6 (Fighter-bomber Mark 6), two Merlin Mk 23 or Mk 25 engines driving three-bladed hydromatic propellers powered the aircraft.

Construction

The airframe was built mostly from wood. The oval-section fuselage was built in two halves in jigs over a mahogany or basic shape. The join was along the vertical centre line. The spilt construction greatly aided the assembly of the internal equipment as it allowed the technicians easy access. The fuselage construction joints were made from balsa wood and plywood strips. The spruce multi-ply was connected by a V joint, which was balsa made, along with the interior frame. The spruce would be reinforced by plywood strips at the point where the two halves joined to form the V joint. Located on top of the joint the plywood formed the outer skin. During the joining of the two halves ("boxing up"), two laminated wooden clamps would be used in the after portion of the fuselage to act as support.

The fuselage was a frameless monocoque shell made of sheets of Ecuadorean balsa wood sandwiched between sheets of Canadian birch, but in areas needing extra strength—such as along cut-outs—stronger woods replaced the balsa filler. The plys were formed to shape by band clamp s over large concrete moulds, each holding one half of the fuselage, split vertically. While the casein -based glue in the plywood dried, carpenters cut a sawtooth joint into their edges while other workers installed the controls and cabling on the inside wall. When the glue was completely dried, the two halves were glued and screwed together. A covering of doped Madapolam (a fine plain woven cotton) fabric completed the unit. The fuselage could be made in a week by six technicians.

The fuselage was separated by seven bulkheads made up of two plywood skins parted by spruce blocks, which formed the basis on each half for the outer shell. The bulkhead was a repeat of the spruce design for the fuselage halves; a balsa sheet sandwich between two plywood sheets/skins. Among the glues used was Casein resin plus many other screws and flanges (made of various woods) which held the structure together. After securing the two halves, the fuselage was covered with fabric and was then doped with cellulose dope which was either sprayed or brushed onto the surface. A coat of silver dope was then applied before exterior camouflage was applied. The underside was cut to allow for wing assembly.

The all-wood wing was installed into the roots by means of four large attachment points. The wing was built as a one-piece structure and was not divided into separate construction sections. It was made up of two main spars, spruce and plywood compression ribs, stringers, and a plywood covering. The outer plywood skin was covered and doped like the fuselage. The engine radiators were fitted in the inner wing, just outboard of the fuselage on either side. These gave less drag. The radiators themselves were split into three sections: an oil cooler section outboard, the middle section forming the coolant radiator and the inboard section serving the cabin heater. The wing contained metal framed and skinned ailerons, but the flaps were made of wood and were hydraulically controlled. The nacelles were mostly wood, but the engine mounts were all metal as were the undercarriage parts (for strength). Engine mounts of welded steel tube were added, along with simple landing gear oleos filled with rubber blocks. Wood was used to carry only in-plane loads, with metal fittings used for all triaxially loaded components such as landing gear, engine mounts, control surface mounting brackets, and the wing-to-fuselage junction. The outer leading wing edge had to be brought 22 in. (55 cm) further forward to accommodate this design. The main tail unit was all wood built. The control surfaces, the rudder and elevator were aluminium framed and fabric covered. The total weight of metal castings and forgings used in the aircraft was only.

In November 1944, several crashes occurred in the Far East. At first, it was thought these were as a result of wing structure failures. The casein glue, it was said, cracked when exposed to extreme heat. This caused the upper surfaces to "lift" from the main spar. During the ensuing investigation it was concluded that there were construction defects found at two plants; Hatfield and aircraft built at Standard Motors. The gluing there left “much to be desired”. However, the main reason for the failures, the Air Ministry concluded on 1 January 1945, owed to the weather conditions in Asia, endorsing the view of Major Hereward de Havilland, leading the investigation. To solve the problem, a sheet of plywood was set along the span of the wing to seal the entire length of the skin joint along the main spar.

The casein glue was replaced by "Aerolite ", a synthetic urea-formaldehyde adhesive developed by Dr. Norman de Bruyne at Aero Research Limited (ARL), which was better able to resist deterioration in high humidity conditions. The de Havilland company also pioneered the use of radio frequency (RF) heating to accelerate curing of the adhesive.

Fuel, landing gear, electrical power and armament systems

The main landing gear were housed in the nacelles behind the engines. These were raised and lowered hydraulically. The main landing gear shock absorbers were of the rubber in compression type with twin pneumatic brakes for each wheel. The tailplane wheel was retractable.

The fuel systems allowed the light Mosquito frame to obtain a considerable range. There were nine fuel tanks. The two in the outer wing, the "outer tanks", each contained 58 gallons of fuel. The two inner wing fuel tanks, located between the wing root and engine nacelle contained 143 gallons each. The central tank mounted in the fuselage between bulkhead number two and three aft of the cockpit, consisted of two 25 gallon fuel tanks. The total fuel load of 452 gallons was initially deemed appropriate for the type. The range could be increased further; the Mk 6 could have larger fuselage tanks, increasing the capacity of the fuselage from 50 to 63 gallons. Two 100 gallon fuel cells were added under the wings to increase the total to 715 gallons. Both the inner wing, and fuselage tanks are listed as the "main tanks".

The central and inner wing tanks were pressurised to reduce fuel vaporisation at high altitude, a feature needed in the photographic reconnaissance variants. The pressure venting cock located behind the pilot's seat controlled the pressure valve; as the altitude increased the valve increased the volume applied by a pump. This system was extended to include field modifications of the fuel tank system.

The engine oil tanks were in the engine nacelles. Each nacelle contained a 15 gallon oil tank, including a 2.5 gallon air space. The oil tanks themselves had no separate coolant controlling systems. The coolant header tank was in the forward nacelle, behind the propeller. The remaining coolant systems were controlled by the coolant radiators shutters in the forward inner wing compartment, in between the nacelle and the fuselage and behind the main engine cooling radiators which were fitted in the leading edge. Electric-pneumatic operated radiator shutters directed and controlled airflow through the ducts and into the coolant valves to predetermined temperatures.

Electrical power came from a 24 volt DC generator on the starboard (No. 2) engine and an alternator on the port engine which supplied AC power for radios.. The radio shutters, superchargers, gun camera, bomb bay, bomb/rocket release and all the other crew controlled instruments were powered by a 24 volt battery. The radio communication devices included VHF and HF communications, GEE navigation, and IFF and G.P devices.

The electric generators also powered the fire extinguishers. Located on the starboard side of the cockpit, the switches would operate automatically in the event of a crash. In flight, a warning light button would flash to indicate a fire, should the pilot not already be aware of it. In later models, presumably to save liquids, the fire extinguisher was changed to semi-automatic triggers.

The design of the Mark VI allowed for a provisional long-range fuel tank to increase range for action over enemy territory, for the installation of bomb release equipment specific for depth charge explosives for strikes against enemy shipping, or for the simultaneous use of rocket projectiles and a 100 gallon drop tank under each wing for a 200 gallon supplement to the main fuel cells. The FB 6 had a wingspan of 54 ft 2 in (16.51 m), a length (over guns) of 41 ft 2 in. (12.55 m). It had a maximum speed of 378 mph (608 km/h) at 13,200 ft (4,023 m). Maximum take-off weight was 22,300 lb (10,124 kg) and the range of the aircraft was 1,120 miles (1,802 km) with a service ceiling of 26,000 ft (7,925 m).

Its armament was four Browning.303 machine guns and four 20 mm Hispano cannons for air and soft ground targets. Its bomb load consisted of 2,000 lb (907 kg) of bombs or eight RP-3 unguided rockets.