Dinger's Aviation Pages

What is an "interceptor"? What makes an aircraft an interceptor? How did the term arise?

Most aviation enthusiasts think of "interceptors" in terms of the supersonic jets of the 1950s and 60s. Aircraft such as the English Electric Lightning, Lockheed Starfighter, MiG 21 and Convair F-106 Delta Dart. Aircraft launched in response to incoming bombers detected on radar, climbing quickly under the direction of either radio or a data-link from a ground-based controller or AWACs, to make an "interception" to bring down their target, usually with air-to-air missiles. The term is less frequently used today, the term "fighter" or "air-superiority fighter" being more common.

If you go back in time you find the term applied to aircraft in the Second World War, aircraft such as the Spitfire and Hurricane, and before that you find it widely used in books and magazines in the 1930s. So when does the term come from? When was it first used? The answer is probably not what you would expect, and its first use is in many ways the exact opposite of what the term represents today. You might be surprised that the first "interceptor" to see service was a failure in the role when first introduced, yet is remembered as a very successful aircraft today. The twists and turns of the "interceptor" story had a direct effect on British success in the Battle of Britain.

Have I piqued your interest? Let's begin...

Fairly quickly in World War One, the diversification in the role of combat aircraft led to the adoption of names for the various types. For the class of aircraft designed to attack enemy aircraft in the air the French used the term "chasseur" and the Germans "jager", both meaning something akin to "hunter" or "chaser" in English, although also redolent of their use for classes of lightly equipped, fast soldiers. The British Army's Royal Flying Corps first used the term "scout" (the RFC still saw the prime use of single-seat aircraft was for reconnaissance) while it was the Royal Navy's Air Service that first used the term "fighter", the term that ended up being adopted when both the British services were united as the Royal Air Force in 1918. The US Navy also adopted the term "fighter", probably influenced by their brief but close involvement with the Royal Navy in 1918. Meanwhile the US Army Air Corp was most closely involved with the French (many of the US airmen learnt their trade flying with the French Lafayette Escadrille) and they adopted the term "pursuit plane" because of the similarity in meaning between "pursuit" to the French "chasse", as in to hunt, pursue or chase (French fighters were organised in "Groupe de Chasse"). The US Army Air Force used this term into World War Two, and it is the reason US Army fighter aircraft had the appellation "P" (eg P40, P47, P51).

For most pilots in the First World War, air combat was largely a matter of chance encounters over the front. Efforts to do reconnaissance, artillery spotting or bombing meeting standing patrols of enemy aircraft. However, the German attempts to bomb the UK, principally London, first with Zeppelin airships and then with Gotha bombers, prompted the need for some means of getting a fighter aircraft off the ground and as near to the airship or bomber in the quickest possible time.

Flying standing patrols was wasteful and unproductive, while waiting until the enemy aircraft were spotted by observers on the ground and then launching the fighter aircraft to try and find them in the vast sky seldom met with succes. If the enemy was spotted it was usually after they had already dropped their bombs and the fighter would end up in a stern-chase, rarely gaining a victory. Although sometimes launched together as flights or squadrons the British inevitably split up to search a bigger area, so any attack on a German bomber formation was inevitably delivered by single aircraft. The Germans usually lost more aircraft to accidents and the weather than to British fighters.

The British response was a streamlined reporting system, an outer gun barrage around the East of London and behind that fighters launched to patrol over the outskirts of London when an incoming raid was reported. There were also fighters based further out along the Thames Estuary and Kent, but these were usually launched to try to catch the enemy on their way back after having dropped their bombs. But one particular raid, on 12th August 1917, may have had a particular impact on future British thinking. A raid by Gotha bombers was detected and it was assumed that it would be another attack on London. The squadrons on the Thames estuary were put on readiness to try to get them on their way back. In fact, the Germans had been blown North of their intended track and slowed down by a strong headwind. They made landfall near Clacton and headed over the Blackwater estuary and River Crouch to regain their track. Number 61 Squadron, a newly formed unit equipped with Sopwith Pups, was awaiting the call to take off at their base at Rochford (now London Southend airport). Suddenly the German bombers appeared overhead. It seems that at the same time the commander of the German aircraft gave up the fight against the strong headwind and fired a flare to order the formation to turn towards Southend. As they flew over Rochford airfield they dropped some bombs on the airfield itself. 61 Squadron was able to take off and keep the Germans in view and deliver a concerted attack. The German formation went on to overfly Southend, dropping their bombs as they went, causing 32 deaths and numerous injuries. Although 61 Squadron chased them out to sea the Germans lost only one bomber to a fighter that day (and that was not one of those attacked by 61 Squadron, but one that had earlier fallen out of formation) four more crashed on landing, probably because of the high winds. However, the British saw it as a notable victory, convinced that they had driven off a major attack meant for London. The Germans gave up daylight attacks on the UK at the end of August, switching to night bombing but it seems that an increase in the volume and accuracy of the anti-aircraft artillery defences of the UK played the major part in this decision.

It is during this time, toward the end of World War One, that the term "intercept" or "interception" starts to be seen in the descriptions of a fighter aircraft catching its target. Up until then, it had its original meaning of waylaying a person or message (as in "to intercept a letter") and in mathematics where two lines or curves meet. It had an obvious use for describing the attempt of one aircraft to find another. Yet the use of the term "intercept" and "interception" in newspaper reports and official documents of the time does not imply the use of the term "interceptor" defined any specific type of aircraft.

When the Great War ended, the new RAF saw a drastic reduction in size and resources (the home-based RAF was reduced to only ten squadrons of both fighters and bombers). To ensure its survival it quickly secured a role in policing Britain's empire but also made sure the strategic aspect of airpower was not forgotten (after all that was the reason for the creation of the RAF in the first place). There was widespread fear that any future war would involve attacks by massive fleets of aircraft, armed with both high-explosive bombs and poisonous gas. It was widely assumed such an attack would take the form of a knock-out blow to the enemy's capital city, causing massive casualties and widespread panic in its wake.

Now the only great power within flying distance of London at that time was France. Despite having just fought alongside each other it was thought prudent to provide some defence for London against such a knock-out blow, just in case there was some radical realignment in French politics (communist revolution was the big fear at the time) or in case some colonial rivalry caused conflict.

So a plan for an air-defence system for London was needed. Prepared by a committee headed by Air Commodore J.M. Steel and Colonel W.H. Bartholomew the "Steel-Batholomew plan", released in 1923, envisaged the home-based RAF being increased to twenty-three squadrons with nine of them being fighters. The fighters would be disposed in an arc around the south and east of London and would defend a belt of sky known as the "Aircraft Fighting Zone". This zone was some 10-miles wide and divided up into eight blocks. Each squadron would be allocated one of these eight blocks to defend (one of the nine squadron was a "spare"), being launched when approaching enemy aircraft were detected by sound-locator equipment on the coast and observers inland. In front of the Aircraft Fighting Zone was a belt of AA guns and searchlights, and another area of AA guns and searchlights covered the heart of London itself. The harbours and navy bases at Dover, Portsmouth, Harwich and Chatham would have their own AA and searchlight defences but no defence by fighter aircraft.

A very simplified diagram of the Steel-Bartholomew plan. The red area is the "Aircraft Fighting Zone", divided into eight blocks, each to be allocated to a fighter squadron. The blue areas are defended by anti-aircraft (AA) artillery. An outer artillery belt around London meant to break up bomber formations before they entered the Air Fighting Zone, and central artillery zone over the heart of London itself. The harbours of Portsmouth, Dover, Harwich and the Medway (Chatham) had their own AA defence, but no fighter cover.

Here a few points need stressing. The fighter aircraft that were to be used to do combat in the "Air Fighting Zone" were expected to do so both by day and night. They had to be able to climb swiftly to height and operate from relatively small grass aerodromes. These aircraft would be in touch with their base by radio. Such aircraft became known within the RAF as "Zone Fighters".

Back in 1921 a proposal from Group Captain TCR Higgins on the subject of the use of any "spare" fighter squadron had suggested they be deployed at Detling in Kent "devoted to the role of interception by day".¹ Some collective memory of the events of 12th August 1917, kept alive in RAF messes and meetings, may well be at work here. It's only a short jump from an aircraft "devoted to interception by day" to an "interceptor".

Before the Steel-Batholomew plan could be implemented a change of government prompted another review and its replacement by a "Fifty-Two Squadron Scheme" which saw the Air Fighting Zone extended up into East Anglia and West to Gloucestershire with ten boxes of larger size. These were allocated fourteen squadrons of zone fighters. But, in front of this zone, on airfields near the coast, would be three more fighter squadrons, equipped with a new type of fighter aircraft expected to only fly during the day and have no radio to weigh it down. It was to be able to keep up with and overtake a formation of bombers passing overhead of its airfield. Such aircraft were deemed INTERCEPTORS.

Simplified map of the "52 Squadron plan". The Air Fighting Zone has been extended North and West, while two bases for specialist "interceptor" squadrons are provided near the coast.

Therefore, in its genesis, the interceptor was the antithesis of what it is now understood to be. Rather than a fighter aircraft launched under the guidance of ground-based or AWACs controllers, relying on radar to guide it to its target, the original interceptor was a fast, quick-climbing aircraft under no outside control at all, having no radio, relying entirely on its pilot to see its target flying overhead, take off, overtake and destroy it.

The role of zone fighter had been filled by The
Gloster Grebe and Gamecock and the AW Siskin in various marks, to be replaced by the Bristol Bulldog. These were designed for both day and night flying and carried short-wave radio. The inability to catch the Hawker Hart bomber on exercises prompted the adoption of a two-seat fighter version of the Hart, first a version with minimal modification called the "Hart Fighter", then one with more refinements called the "Demon". These also operated in the zone fighter role.

The first specification for an "interceptor" was F20/27, issued in September 1927, for an aircraft without radio, able to overtake in the shortest possible time an enemy aircraft overflying its airfield at 20,000 feet at 150 mph. It was expected that the interceptor's minimum time to 20,000 feet would be 12 minutes and that the speed at that height would be at least 200 mph. It was stressed that the view from the cockpit must be good (to keep the enemy aircraft in view during the pursuit). Preference for an air-cooled engine was expressed, as was a desire to explore monoplane designs (either parasol or low-wing). The British aircraft industry was prodigious in its response with 11 designs being tendered. ² Four of these designs were monoplanes (three low-wing, one parasol) the rest were biplanes. Testing was prolonged and in general disappointing, the designs either did not reach the required top speed of 200 mph or could not climb as fast as required. This latter requirement was particularly hard for the monoplanes to reach with their reduced wing area. The delay in testing enabled Hawkers to supersede their original radial-powered F20/27 design with a liquid-cooled design called the Hornet, this reached the desired performance and resulted in an order for a further refined design called the Hawker Fury. Now regarded as perhaps the most pre-eminent RAF fighter between the wars. In fact, it was originally only ordered in small numbers, only enough to equip the first three squadrons allocated to the interceptor role, to be based at the forward coastal airfields (Nos 1 and 43 Squadrons at Tangmere, No 25 at Hawkinge).

The Classic Fury - Epitomising the RAF of the 1930s, yet a complete failure in air exercises when tried in its original role of "interceptor".
Note the lack of any radio aerial.

The first Furies were delivered to Number 43 Squadron in May 1931, in time to take part in the big air exercises that summer, when simulated attacks against London were flown. Various books written about the Hawkler Fury, along with histories of Number 43 Squadron, assert that the Fury was a great success in these exercises.³ However, documents in Goverment archives paint a different picture; when operated in their designed role they could not keep sight of, let alone catch, the bomber aircraft (even though some of the raids were deliberately flown directly over their base). "The most unsatisfactory feature of the exercises was the failure of interceptor squadrons to engage the bombers." ran an official report.⁴ Thus the whole idea behind the "interceptor" class was shown to be faulty.

Thankfully there was enough room in the fuselage of the Fury to allow a radio to be fitted. The following year all three Fury Squadrons took place in the Air Exercises again, this time alerted to their targets position by radio, albeit restricted to use during the day only. In this role they were much more successful.

It should be noted that there had been another interceptor specification in 1927. F29/27 called for a performance identical to that of F20/27 but its armament was to be a big 8 foot long
37 mm COW cannon mounted to fire upward at a no-deflection angle. This bought forth two prototypes.The anachronistic looking biplane Vickers Type 161 COW gun fighter and the Westland COW gun monoplane (based on Westlands F20/27 design). Neither design received a production order.

The other interceptor - Vickers Type 161 Cow gun fighter. Able to fire clips of 37mm ammo the gun was mounted in a no-deflection mounting. It had about the same speed as its monoplane competitor from Westland, but its biplane layout and increased wing area allowed it to climb much more quickly.

The performance of the Fury with radio fitted prompted a later version of the Fury with a more powerful engine and a higher speed called the Fury Mk II. This was ordered in greater numbers to replace the earlier mark in front-line squadrons and expand the number of Fury squadrons to six. But even this new version was not adapted to fly at night and lacked instruments able to be read at night or navigation and landing lights. But from then on, when specifications for new fighters came out they combined the role of interceptor and zone fighter.

Fondly remembered, but its original role forgotten. Contemporary book plate illustration of Furies in flight by C R Moore ARCA.

One of the other contenders for the F20/27 specification, the Gloster SS1B, had meantime matured into a very capable aircraft, with a more powerful engine it could outpace and outclimb even the Fury Mk II. It entered service as the Gloster Gauntlet and it certainly flew as a night-fighter. Its follow-on design, the Gloster Gladiator, incorporated wing flaps, which cut down the landing speed to make it even more suitable for night-fighting. And so onto the Hurricane and Spitfire, both fitted with flaps and landing-lights, when they entered service they were expected to fight by both day and night, even though they were classed as "interceptors" and carried full radio equipment. Thus we see that the two classes of fighters the RAF thought it needed, the zone fighters and interceptors, had merged into one type of aircraft.

The term "interceptor" found its way into the British aeronautical press during the 1930s. Sometimes you see it correctly in context, but as the decade wore on it became applied to any type of fighter. Over the Atlantic, in the USA, the term was also in increasing use through the 1930s and again just became another term for "fighter".

The real divide between fighter types came with the introduction of specialised night fighters during the early years of World War Two. Usually with two engines and equipped with radar and night-flying aids they formed a distinct branch of fighter development during the 1940s and 1950s until day and night fighters were combined in the new class of day-and-night, all-weather, high performance jets that started to appear in the 1950s and dominated in the 1960s and 1970s where they were inevitably called "interceptors".

One ramification of the RAF's Fury interceptor experiment was the use of bases on the English South coast, Hawkinge and Tangmere. These were vital bases during the Battle of Britain, although some pundits have suggested Fighter Command would have been better off not using these coastal bases (and the others at Manston and Lympne). Suggesting instead they should have operated from bases further inland, allowing fighters to gain more height before meeting the incoming waves of German aircraft. As it was, RAF fighters on these coastal bases often flew inland, away from the incoming Germans, to gain height, before turning to intercept, guided by radar.

Another implication of the history of the evolution through "zone fighter" and "interceptor" was the number of pilots and aircraft allocated to each squadron. During World War One it had been usual for a RFC squadron of single-seat "scouts" to have only enough pilots to fly the aircraft available. So a compliment of as few as 12 pilots was possible. When the RAF adopted the "zone fighter" it was obvious that to keep the squadron flying both day and night double the number of pilots would be required, so the number of pilots on a squadron was increased to 20 -25 and usually 19 aircraft were held on strength⁵ (even though the squadron was only expected to put up a maximum of 12 aircraft at one time). When the interceptor squadrons were formed it seems that a compliment of 20-25 pilots was retained, even though the interceptors were only expected to fly in daylight. The Hurricane and Spitfire Squadrons in the Battle of Britain, combining as the did the roles of zone fighter and interceptor, likewise started off with 20 to 25 pilots per squadron and 19 aircraft, even though they were not called upon to operate at night at anything like the rates expected pre-war. This was absolutely crucial to British success in the Battle of Britain. German intelligence seems to have assumed that British fighter squadrons operated with the same number of men and machines as a German Staffel, thus seriously under-estimating the size and resilience of RAF Fighter Command.


¹ See Chapter 1, page 17 "The RAF and Aircraft Design 1923-1939 Air Staff Operational Requirements" by Colin Sintott published by Frank Cass. ISBN 0 7146 5158 3

² See F27/33 Spec in "The British Aircraft Specification File" by KJ Meekcoms and EB Morgan, published by Air Britain, ISBN 0 85130 220 3, for full details of the designs - to list them all here - AW Siskin IIIA without radio, Bristol Type 107 Bullpup, de Havilland DH 77, Fairey Firefly Mk II, Gloster SS1B, Hawker 20/27, Hawker Hornet, Saunder/Saro A10,
Vickers Type 151, Westland F20/27, Westland Wizard Mk II.

³ For example, Putnams "Hawker Aircraft since 1920" by FK Mason and "The Fighting Cocks - 43 (Fighter) Squadron" by Jimmy Beedle.

⁴ Public Records Office AIR/ 2/1069 40A. See Chapter 3, page 70 "The RAF and Aircraft Design 1923-1939 Air Staff Operational Requirements" by Colin Sinnott published by Frank Cass. ISBN 0 7146 5158 3

⁵ The strength of a Fighter Command squadron in the 1930s was made up of 14 aircraft in "first rate" condition and a further 5 aircraft in reserve. The reserve aircraft would usually be the oldest aircraft with the highest flying hours. See the Profile Publication "The Gloster Gauntlet" by Francis K Mason.


"The RAF and Aircraft Design 1923-1939": By Colin Sinnott, published by Frank Cass. ISBN 0 7146 5158 3. Is a brilliant source that deserves to be more widely read. It directly prompted the writing of this article.

"Air Defence of Great Britain": By John R. Bushby, published by Ian Allan, 1973. ISBN 0 7110 0476 5. Great for laying out the changes in RAF planning between the wars. While all the pieces of the jigsaw are there, the author has not made the final connections between the Fury, and interceptor specifications, Tangmere and Hawkinge.

"The British Aircraft Specification File": By KJ Meekcoms and EB Morgan, published by Air Britain, ISBN 0 85130 220 3.

"Battle of Britain 1917 - The First Heavy Bomber Raids on England": By Jonathan Sutherland and Diane Canwell, Pen and Sword. ISBN 1 84415 342 2