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85000 "FLIGHTS" WITHOUT TAKE-OFF

Basically every new aircraft must undergo two especially critical tests. First, the measurement of its ultimate strength, which is done by loading the wings until they breakdown, and, second, a test to prove the lifespan of the aircraft. At the facilities of the Industrieanlagen-Betriebsgesellschaft (IABG) in Ottobrunn, such a fracture test has just been successfully completed. Over a period of four years, a fuselage/wings combination of the Airbus A330/340 was evaluated. Since IABG had already conducted the tests for the A300 in 1972, the placing of the order for the A330/340 was almost a routine.

The preparations for the extensive test began well ahead of the actual testing. An IABG team drove to the wing manufacturer British Aerospace in Chester. The team needed four weeks to glue approximately 1000 load application points to the wing to mark the areas where loads would be applied on from hydraulic cylinders during the test.

At the same time, the scaffold, which was to hold the aircraft during the test, was set up. Approximately 500 tons of steel were needed to accomplish this task. Then the fuselage and wing parts, which had been flown to Munich with a Super Guppy, were mounted into the scaffold by a group of Aérospatiale employees. Just the mounting of the test fuselage took three months.

A total of 92 push-pull cylinders were installed to simulate the forces of the air. A hydraulic system supplied the 1200 kilowatts to move the cylinders and an additional compressor generated the 300 kilowatts which were required to pressurize the fuselage for simulating the changes in altitude. The internal loads on the fuselage ñ in the real aircraft coming from fuselage fittings and passengers ñ were simulated by hydraulic cylinders as well.

Following the integration of the hydraulic lines and the strain gages, the test was started in 1992. The hydraulic cylinders picked up their work, moving the wings up and down, shaking the installed landing gear and engine imitations, while the pressurized fuselage was pumped up to simulate the inner pressure at altitude, expanding the fuselage diameter up to 12 mm.

There was a method to the madness. The computer was accomplishing a program of 21 different missions. The program generated so called load collectives. These are normalized loads representing the forces the aircraft would be exposed to during a real flight. The calculated load collectives were the basis to determine the forces which had to be applied on the test airframe by the hydraulic actuators. The aerodynamical data of the aircraft and the turbulence data of the air were part of the calculation. The computation of the load collective was accomplished by Dasa with the help of extensive computer programs.

Continuous walkaround inspections were carried out during the test. A team of eight inspectors constantly searched the structure for indications of fatigue. The test was interrupted when a defect was found and the respective part manufacturer was informed, who then decided whether the test could be continued or if the part had to be repaired or modified. Such modifications where also carried over to the series production of the aircraft.

By the end of the test this year, the test airframe had accomplished 85000 "flights", the first 40000 of which in the A340 configuration with four engines. The following 45000 flights were carried out in the two-engine A330 configuration. This was due to the fact that there is a significant difference in the load spectrum of the two aircraft. While the A340 operates on medium to long range routes, the A330 is used for service on short and medium range connections, resulting in a higher number of flights, as well as, take-offs and landings, which also have an significant affect on aircraft fatigue. As a result of the extensive testing both aircraft are now certified for a lifespan of 20 years.

The accumulated load during the tests equals 50 years of operation, such demonstrating a safety factor of 2,5. This is a very conservative number. The manufacturers in the USA, such as Boeing with its 777 airliner, work with the factor 2.

Manfred Kalweit, the IABG engineer responsible for the Airbus test, did not want to reveal details concerning the defects that were discovered during the test: "Strict confidentiality is an important principle of our company. Only our customers get the results."

Since the tested fuselage/wings combination cannot be used for any operational purpose again ñ it is sent back in pieces for some further testing to the manufacturer ñ the costs of such a test are high, in this case in the three-digit million range. However, the result justifies the effort. "We can testify the best grade for the Airbus airframe. There is absolutely no risk concerning structural fatigue with this aircraft", says Manfred Kalweit.

From page 80 of FLUG REVUE 10/96