Flying quick-change artist

For more than twenty years, ATTAS (Advanced Technologies Testing Aircraft System) has been the large flying test bed of the German Aerospace Center (DLR). ATTAS was primarily designed as a 'flying simulator', to simulate the flying behaviour of other - real (existing) or virtual - aircraft.

The application portfolio of ATTAS was very wide-ranging. With its measurement and test equipment, ATTAS was used for numerous test duties, such as testing future air traffic control procedures and low-noise approaches, for example. Research into wake vortices was also carried out with ATTAS; these are air turbulences that occur as a result of the lift produced on the wings.

ATTAS is based on the 44-seater, twin-engine VFW 614 short-haul jet developed in the 1970s by Bremen-based VFW Fokker (now Airbus).

One of the core modifications was the integration of an electrohydraulic fly-by-wire flight control system (FBW), in addition to the usual mechanical controls. The changeover from mechanical to electronic control could be made as required during the test flight. This enabled scientists and engineers to investigate anything and everything that is possible with a computer in terms of flight control.

ATTAS was converted to a research aircraft at the end of the 1980s and since then has been used for aviation research for an average of 130 flying hours per year. 

Modifications

The following modifications differentiate ATTAS from the standard VFW 614:

• Electrohydraulic, digital fly-by-wire flight control system. It intervenes in the standard mechanical-hydraulic steering via electrohydraulically-operated linkages, which are connected to the control units of the safety pilot sitting on the right. The safety pilot can disconnect the fly-by-wire linkages at any time and take over control with the mechanical system. The fly-by-wire flight control system covers aileron, elevator, rudder, trimmable stabiliser, engines, landing flaps and the direct lift control flaps in the rear section of the landing flaps. The fast-moving flaps enable a direct change to the wing lift and so provide an additional degree of freedom in the longitudinal motion of the aircraft, which, amongst other things, is necessary for the in-flight simulation used in assessing the flight characteristics.
• Computer system for controlling the electronic fly-by-wire flight control system, with freely programmable, powerful experimental computer. The installed simulation software can simulate the flight behaviour of other (even virtual) aircraft. Because of the safety concept with the mechanical back-up control system, the software used does not need to be certified. The experimental system can be expanded with additional computer modules.
• Test pilot cockpit with freely programmable displays, fitted with both a sidestick and a control column.
• Measurement system for recording and displaying the data from the flow and acceleration sensors, avionics and air data, data from the FBW flight control system, and additional data from the computer system.
• High-frequency data link (downlink and uplink) for online data transfer between aircraft and ground station.
• Electrical and hydraulic power supplies for the experimental systems, independent of the systems of the basic aircraft.

Missions - research focus

Research into the life and decay of wake vortices

The aim of the experiments was to reduce the possible separation distance between aircraft coming in to land or taking off behind one another, by means of a more accurate computation of the evolution and decay of wake vortices. For this, the wake vortex produced by ATTAS was measured by LIDAR (Light Detection and Ranging) systems either stationed on the ground or installed in an aircraft. At the same time, there was also the attempt to accelerate the breakdown of the wake vortex by periodic aileron movements.

Simulation of flying through wake vortices

Using a simulation programme, and with the help of the fly-by-wire flight control system, flying into wake vortices was simulated without risk; and then, based on the pilot's subsequent assessment, criteria were developed for possible, acceptable vorticities. Strategies for automatic countersteering were tested too.

Testing future air traffic control procedures

Thanks to satellite navigation, aircraft today are able to navigate very precisely, independently of ground-based support systems. Over and above this, to enable optimal planning and control of air traffic, individual airspace users need to be managed with the timings as accurate as possible in that airspace. The solution lies in trajectories - so-called 4D trajectories - which describe the route of an aircraft precisely in both the time and space domains, and enable each individual leg of the flight to be strategically planned. This way, conflicts in the airspace can be detected and avoided more effectively. Finally, this capability will lead to higher airspace and airport capacities.

The DLR Institute of Flight Guidance (DLR-Institut für Flugführung) has already developed and tested an Advanced Flight Management System (AFMS) back in the 1990s, a system that makes precisely this 4D planning and management possible. Since then the system has been continuously improved. Additions include communication elements that link the traffic planning modules on the ground to the flight route planning modules in the aircraft by data link.

The AFMS has demonstrated its highly promising capabilities in numerous test flights with ATTAS, the DLR research aircraft.

Testing new low-noise approach procedures

With the Experimental Flight Management System, the approach profile is worked out accurately - depending on the wind and other factors - whilst the aircraft is still at cruising altitude, and then flown along fully automatically and with accurate timing. This enables, for example, low-noise steep approaches with the engines idling, and approaches to the ILS glide path from above that would not be possible with a conventional flight management system.