If 2009 was a year of take-offs, 2010 is without doubt the year of flight tests. In Spain the flight test campaign for certification of A330 MRTT is in progress, and the one of the A400M military transport has just started.
Also in progress in the rest of the world, are the campaigns of the Canadian jet Bombardier CRJ100, the Russian Sukhoi Superjet100, the Chinese COMAC ARJ21, the Americans Boeing 787 and 747-8 or the Japanese military transport Kawasaki XC-2, to mention but a few. These tests involve very high costs and occasionally also significant risk. The crew for this task of extraordinary responsibility is part of an elite trained in hardly five schools all around the world. But, why are these tests carried out, what do they involve? The objective is to gather data during flight of the aircraft to evaluate its behaviour and validate its design. This allows, in the first place, finding and troubleshooting any design problems, and secondly, documenting the capabilities of the aircraft in a way that can be used later for certification in front of the authorities or for customer acceptance.
The Prototype Aircraft and the Flight Test Team In order to achieve these goals, the prototype aircraft is fitted with plenty of flight test instrumentation (FTI): thousands of sensors of diverse types connected to electronic racks, monitors and recording systems so as to process, analyze and record data on board, since current technology does not allow transmitting by telemetry all the volume of information registered during flight. The organization of the flight test team may vary, but typically responds to that shown in the figure. In the cockpit, the test pilot and the test flight engineer (TFE) execute the manoeuvres. The Co-pilot is in charge of the communications and navigation to keep the aircraft inside the area authorised for the test. The leader of the team is the flight test engineer or FTE (not to be confused with the TFE).
The FTE conducts the flight from the FTI station, from which he monitors the test, and manages the recording and transmission of data to the ground station. In the telemetry room, a team of specialists receives the data in real time and supervises the status of the aircraft systems, under the coordination of the test conductor, the only one allowed to communicate with the crew during the flight. Once on the ground, the FTE is responsible for the test plan, and coordinates the analysis of results by the specialists. The TFE on the other hand is in charge of coordinating the maintenance and troubleshooting activities on the aircraft by the ground support team, and must guarantee traceability of all interventions in the aircraft technical logbook.
Flight tests can be classified in four basic types according to their goal: envelope expansion, handling qualities, performance flights, and system and equipment testing. In each case there will be a technical instruction to specify the objective and success criteria for the test, the initial conditions (e.g. configuration of flaps or engines), the procedure of execution and data gathering, and the safety instructions in case something goes wrong.
The flight envelope defines the manoeuvring capabilities of an aircraft. The V-n diagram in the figure is a typical representation, showing the acceptable range of load factor as a function of speed. The load factor measures the stress at which both the structure and the crew are subject during a manoeuvre, and is normally measured in multiples of the acceleration of gravity (“1 g”, “2 gs”, “1 g negative”, etc).
The limits of the flight envelope are given on one side by the conditions for stall, an aerodynamic effect that appears at low speeds, and on the other side by the maximum levels of load factor and speed, which are defined by design so as not to compromise the structural integrity of the aircraft.
Envelope expansion tests consist in taking the aircraft to the limit of its capabilities, and therefore involve a significant risk. It is necessary to watch the onset of instability phenomena such as flutter, a vibration of aero-elastic nature that can potentially cause catastrophic failure. Anemometric calibration flights are also included in this category. Their goal is calibrating the on-board instruments that measure e.g. speed, altitude or angle of attack of the aircraft.
When we speak about the handling qualities of an aircraft we refer to the study of its characteristics of stability and control. The goal is to reach equilibrium between the stability against perturbations and the ease of handling, and maintain this equilibrium when modifying weight, position of the centre of mass, speed or altitude. The prototype aircraft employs ballast with water deposits and a transfer system to modify the position of the centre of mass in flight and check the stability and control in static and dynamic regimes. Stall characteristics are also studied, as well as the recovery of control of the aircraft. The goal is to demonstrate that the stalling is progressive and symmetrical, to prevent the aircraft from entering abnormal positions that could lead to a dangerous situation such as entering a spin or a deep stall. In these tests it is sometimes necessary to use recovery systems that use parachutes or boosters to stabilize the aircraft and regain control. Another test that is both spectacular and risky is the identification of VMU (minimum unstick speed). Here the goal is to take off at the minimum speed possible, with the maximum angle of rotation, actually dragging the tail of the aircraft along the runway.
The goal of Performance flights is to validate parameters such as range, autonomy, or take-off and landing distances, which define the operation of the aircraft in the different flight phases: takeoff, climb, cruise, descent, and landing. These parameters are very important for the client. It is necessary to prove that the values foreseen by design can be reached throughout all the range of conditions of use of the aircraft, so the campaign typically includes also tests in extreme cold, hot or altitude conditions.
These flights are designed to test the performance of particular aircraft systems (e.g. electrical, hydraulic, power plant, etc) or equipment (e.g. communications, radar, navigation, etc). Specific campaigns are also included for military aircraft according to the different types of mission, e.g. low level loads delivery, on-air refuelling, landing on unprepared fields, etc. Finally, there are also so-called mixed flights, which goal is to test specific equipment or secondary systems designed precisely to improve the handling qualities or performances of the aircraft. One such example could be the testing of a system that modifies the range of movement for the rudder, in order to facilitate handling of the aircraft when an engine fails.
A number of certification flights are performed at the end of the flight test campaign, normally with the certification authority on board. The goal is to demonstrate that the aircraft, or a particular system, comply with the certification requirements.
The A400M flight test program is particularly complex, because it comprises all the civil certification requirements of the European Aviation Safety Agency, in addition to a whole range of tests to comply with the military requirements defined by the air forces of the client nations. The campaign will make use of five prototypes, three of them based in Toulouse (MSN001, MSN003, and MSN006), and the other two (MSN002 and MSN004) based in a new Flight Test Centre created for this purpose in Seville. In the work-sharing arrangements, Seville has capitalised on its experience and know-how in the development of military transport. Thanks to the A400M, Seville recovers this high value-added activity, in which the city has an extensive tradition, even though in recent times it had progressively shifted to the Getafe factory in Madrid.
Manuel Heredia Ortiz, Industial Manager A400M Flight Test Centre Seville, Airbus Military
Rubén Carvajal vázquez, Automation Group Coordinator for Airbus Military Industrial Means and Tooling Engineering Department.
The authors of this article are the managers of http://aergenium.es, the first web devoted since 2008 to the aerospace industry in Andalusia, Spain. Special thanks to Jose Antonio Ojeda Rubio, Chief Test Flight Engineer, Flight Operations, Airbus Military
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