The programme ran to a tight schedule from the contract signature in spring 1983, to the roll-out on 16th April 1986, and the first flight on 8th August 1986. Impressively, the first flight lasted 67-minutes and included supersonic flight at Mach 1.1 and at altitudes up to 30,000 ft. The aircraft flew nine further flights during its first week of operation.
When the Experiment Aircraft Programme concluded, ZF534 had completed 259 test sorties, totalling some 195 flying hours, during which it had exceeded Mach 2 and had flown at angles of attack of over 35 degrees.
The aircraft not only delivered on the original programme aims, it also provided data to validate load prediction modelling for the Typhoon / EFA programme. ZF534 is widely acknowledged as playing a crucial role in the development of the Eurofighter Typhoon, shaving a whole year off the development programme, at a saving of £850 million.
Flight control system and fly-by-wire controls
EAP was intended to demonstrate active, full-authority digital fly-by-wire controls in a significantly unstable canard delta configuration. The flight control system was derived from the one flown on the ACT (Active Control Technology) SEPECAT Jaguar (XX765). It emphasised carefree 'hands on throttle and stick' handling, with high angle of attack, manoeuvrability and a departure prevention system.
Designed for superior manoeuvrability
Unlike traditional aircraft designed to naturally return to steady flight, EAP was aerodynamically unstable. It relied on its onboard flight control systems and computers to process the pilot's inputs and commands. These were computed at a rate of thousands per second, constantly adjusting the aircraft's control surfaces to keep it upright and stable. That in turn allowed EAP to respond very quickly to the pilot's actions, resulting in superior manoeuvrability.
Manufacture and materials
EAP featured new manufacturing materials and methods, including carbon fibre composites, aluminium lithium alloys and super-plastically formed diffusion-bonded titanium. It also carried advanced digital electronic systems, among them a glass cockpit with a wide field-of-view head-up display (HUD). For cost reduction purposes, it was fitted with a modified Tornado fin. An additional emphasis was based on the ability to design and manufacture primary structures in advanced materials, using processes with a clear path to full production capability. Many companies in the supply chain, from the UK, Germany and Italy, contributed to the programme. The sole EAP aircraft was assembled within the development facility at No. 2 Hangar in Warton.
Engines
To reduce risk to the programme schedule, EAP was powered by two Turbo-Union RB199 Mk104D engines, the standard Panavia Tornado F.3 engine less its thrust reverser.
Weaponry
As an experimental and technology demonstration aircraft, EAP carried no weapons or military equipment. Dummy weapons were carried in a low-drag position.
The EFA (European Fighter Aircraft) was a collaborative international program established in the 1980s that eventually resulted in the Eurofighter Typhoon. The development of EAP helped with the risk reduction testing on this programme. This included the following activities:
In-flight loads measurement
An external array of pressure sensors were fitted to the left wing, the left foreplane and both sides of the fin, as a proof of concept for measuring loads in flight. Each sensor sat within a black rubber disc shaped to fair in the sensor and avoid false results. The system calculated the loads experienced on the airframe in flight and allowed Aerodynamics experts to compare them with predictions. This avoided the cost of instrumenting the test airframe with a multitude of strain gauges and then carrying out a loads calibration ground test.
Dummy airbrake trials
EAP was modified to carry a dummy airbrake mounted on the spine, as on Eurofighter. This dummy couldn't be adjusted in flight, and EAP's own airbrakes were locked closed. EAP was flown with the spine airbrake set at 15, 30 and 45 degrees, to assess the aircraft handling and measuring fin loads and fin vibration.
Towed decoy flying
EAP was flown with a towed decoy, an aerodynamic shape only, with no avionics fitted.
Synthetic Target Tracking
Further trials explored a synthetic target projected onto the HUD, allowing a pilot to practice air combat manoeuvres without the need for a target aircraft
(lose the bit about production aircraft loads measurement system – turns out to be false)
EAP experience read across to Eurofighter
The whole structure and concept of EAP served as risk reduction for Eurofighter. Several specific problems were understood and avoided on Eurofighter as a direct result of what was learned flying EAP.
Rate of change of angle of attack
During practice air combat flying, EAP crossed the target aircraft's trailing vortex obliquely. The flight control system's airflow sensors first saw a very large positive angle of attack, then moments later an equally large negative one. The hydraulic systems only just kept up with the demands that the flight control system made. As a result, Eurofighter's flight control system was given sensible limits on the rate of change of angle of attack that can be experienced in the real world.
Wing fuel depletion warnings
During EAP's early flying, repeated warnings of wing fuel depletion proved very distracting, as each one triggered the voice warning system. The wing tanks were empty apart from residual fuel, and aircraft movement kept wetting the sensor, which moments later were dry again. It sounded simple, but it took a while to resolve on EAP before the fix was then deployed to Eurofighter.
Confidence in wind tunnel data
The in-flight loads measurement work brought a further benefit. It allowed how closely the pressure measurements taken in the wind tunnel matched those seen in actual flight. When the measurements correlated, wind tunnel results could be used with greater confidence to define the aircraft design at a very early stage.
A proven track record
ZF534 earned its place in aviation history, and its influence is immeasurable. The programme proved the technologies, matured the processes and built the confidence that made Eurofighter Typhoon possible. That same approach, proving the hard things early and carrying the experience forward, continues to guide our work today.
See EAP ZF534 up-close
The airframe of the EAP arrived at the RAF Museum in Cosford, Shropshire in 2012 following its departure from Loughborough University. In February 2014, the airframe was officially donated to the museum to form the centrepiece of a new exhibition featuring the ground-breaking achievements of the aircraft.
Visit the museum’s test flight hangar to view the exhibition.
The EAP story
See more on our dedicated EAP (Experimental Aircraft Programme) page featuring an exclusive video on the history of EAP ZF534.