Airco was only one of a number of companies owned by Holt Thomas. The Integral Propeller Co built wooden propellers in premises in Colindale near the Airco factory. Engines were manufactured by Peter Hooker Ltd of Walthamstow (previously Gnome & le Rhone Engine Co Ltd ?). Flying boats were built by May Harden & May Ltd at Hythe, Hampshire, and Airships Ltd, formed in 1914, built balloons at their factory at Merton in Surrey. Some aircraft components were built by the coachbuilding company Vanden Plas which was bought by Holt Thomas. After the war an airline, Aircraft Transport & Travel Ltd (AT & T), was formed.
In a manner similar to Holt Thomas the de Havilland group of companies had interests in many other aspects of aviation.
The de Havilland engine division was formed at Stag Lane near Edgware and it remained there when the airfield was closed. Most de Havilland aircraft were powered by engines produced by the division. At the time of the acquisition of the Group by Hawker Siddeley the company had also expanded into helicopter engine production and the design and manufacture of rocket motors. The company was sold to Bristol Siddeley in November 1961.
The success of the Comet racer created an interest in variable pitch propellers and a license to build the American Hamilton propeller was acquired. Over 100,000 were built during the Second World War. de Havilland Propellers Ltd also built guided missiles. The main ones were the Firestreak and Red Top air to air missiles. The company had also been working on the Blue Streak long-range ballistic missile but the project was cancelled in 1960.
Alan Cobham worked for de Havilland before becoming famous for his long-distance flights. He was the Chief Pilot and Manager of the de Havilland Aeroplane Hire Service and undertook work for Aerofilms Ltd, a company which specialised in air to ground photography. Aircraft were also hired to transport photographic negatives from country events back to newspapers in London . Cobham developed his skills and interest in long-distance flying while working for the company. Air Taxis Ltd at Stag Lane and Imperial Airways at Croydon Airport acquired the pilots, aircraft and work from the company.
The de Havilland School of Flying opened at Stag Lane on 1 April 1923 . It was the first of four clubs opened to enable pilots in the Reserve of Air Force Officers to maintain their flying hours. Its pupils included de Havilland's older sons as well as Amy Johnson, John Cunningham and many other famous aviators.
In 1935 the de Havilland School of Flying, which had moved to Hatfield, became No.1 Elementary and Reserve Flying Training School (E & RFTS) and a second one, No.13 E & RFTS, opened at White Waltham in Berkshire . They were renamed Elementary Flying Training Schools (EFTS) in 1939. In 1940 both became RAF units but No.13 EFTS was disbanded in 1941. Panshanger, near Welwyn Garden City, was used as a relief landing ground but later became the main base for No.1 EFTS. It was renamed No.1 Reserve Flying School in 1947 and continued to be operated by the de Havilland company until the school's closure on 31 March 1953.
The de Havilland Aeronautical Technical School was founded to provide owners of Moth aircraft with the technical skills required to maintain their aircraft. It opened in 1928 and was staffed initially by designers, aerodynamicists and stressmen. The school prospered and the syllabus later included aircraft design. Its pupils built seven de Havilland aircraft and a Druine Turbi. They also designed and built four aircraft of their own. The school also built the replica Wright Flyer now in the Science Museum , London . Work began in 1946 and was completed two years later. A working replica engine was also made as a separate project and presented to the Science Museum .
The name of de Havilland is known throughout the world. In 1927 de Havilland Aircraft Pty Ltd was formed to assemble aircraft in Australia . It produced its own designs later which were not very successful. In 1928 de Havilland Aircraft of Canada Ltd was formed which also assembled aircraft from components shipped from the UK . In 1946 it began designing its own aircraft. The Chipmunk was exported world wide and was also built in Britain , as was the larger Beaver. Other subsidiaries were formed in India (1929), South Africa (1930) and New Zealand (1939). Some Hornet Moths were assembled in India and South Africa and de Havilland Aircraft of New Zealand Ltd assembled Tiger Moths for the Royal New Zealand Air Force.