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A340-600 FATIGUE TESTS AT DRESDEN

It is not exactly every day that an Airbus outsize cargo aircraft lands at Dresden-Klotzsche airport. It is hardly surprising therefore that the arrival of the Belugas last year, bearing their valuable cargo - three fuselage sections and the wings of the future longest passenger aircraft in the world - should have attracted so much media interest.

The modules were hauled into a hanger belonging to IMA Materialforschung und Anwendungstechnik GmbH by low bed truck and crane. Then specialists from Toulouse arrived and over a period of ten weeks they assembled manually what otherwise would be put together by ultra-modern automated machines, the complete testbed of an A340-600. Several truckloads full of tools and jigs were needed for the job.

Then began the work of setting up the test equipment, including hydraulics, control unit and measuring system, which had been developed by IABG mbH. IMA is responsible for installation of the sensors, the pneumatic load equipment and for carrying out the tests. The paramount objective is to assess the fatigue properties of the primary structure of the aircraft, for which purpose loads are introduced into all three levels of the fuselage, the wings, the undercarriage and the engine pylons, using the ingenious hydraulic system. 94 power cylinders have been installed for this purpose. These are supplied with 4,400 litres of hydraulic fluid per minute by two central pumping stations. Once the tests begin, they will pull, push and shake the structure, mimicking the conditions that a production aircraft would experience during daily flight operations.

35,000 flights will be simulated in this way, from taxiing to take-off, cruise, landing and finally taxiing back to park the aircraft. Short-, medium- and long-range flights will be simulated under a wide variety of weather and wind conditions, and concluding with both "hard" and "soft" landings.

The data used to define the flight profiles comes from a statistical database held by Airbus Industrie and is based on airlines' actual operational experience. When testing gets under way in Dresden, appropriate software will ensure that the different flights are presented in random order. The figure of 35,000 flights was set by the manufacturer, which guarantees that every aircraft sold is capable of at least 16,600 longhaul flights over its lifetime. In this way the test schedule will actually result in a safety factor of 2.5, as all the trials are in fact run with a 10% greater load than is normal in practice. All this means that testing can be completed in less elapsed time, while another benefit is that the engineers gain additional experience which may even subsequently enable the guaranteed lifetime of the aircraft type to be extended.

In 1996 the four-year basic fatigue test programme for the A340-300 was completed at IABG's Ottobrunn site. Immediately afterwards work was begun there on the development of a new test procedure and, in particular, new control technology was introduced. The result was a test technology that is at the forefront of technology today and enables testing to be completed in half the time required under previous methods, so that the Dresden trials can be completed in little more than two years.

For this purpose it is sufficient to test two fuselage sections and one central segment with the two wings. The loadings which occur at the nose and tail are also simulated using ingenious hydraulics and are introduced at the ends of the pipes. The open fuselage is sealed using heavy calottes, and a complicated compressor system simulates the internal pressure at cruise altitude. Before this one can vary the fuselage volume using a number of polystyrene blocks which are suspended on rails and can be moved into the passenger cabin and also into the freight area.

The loading values for nose and tail are derived once again from the database. The hydraulic cylinders involved here are adjusted with regard to their transverse forces, torque and twisting moment, and the load sequences applied to individual cylinders are specified by Airbus Industrie.

But even during normal flights load peaks can occur, for example, due to gusts of wind or wind shear. In extreme cases the wingtips can be displaced up to 2.7 m upwards or 1.5 m downwards as a result. However, such extremes occur on average only once in a thousand flights and, due to their infrequency, have only a small effect on the fatigue of the structure. Much more important for the tests are the many small load variations which occur in the course of an aircraft's life and slowly wear it down. For this reason the test spectrum is designed in such a way that under maximum loading the peaks are "cut off" to avoid distorting the test results.

To establish the fatigue of the structure, about 3,700 strain gauges, small rectilinear metal structures to which a tension is applied, are stuck on over the entire airframe including the wings at precisely pre-specified positions. If the airframe stretches and with it the rectilinear wire structure, then its resistance will change so that one can draw conclusions about the change. Many kilometres of cable connect the transducers to a measurement amplifying device under the fuselage, from where signals are passed on via photoconductors to the measurement computer. This initiates the measurements and stores the data. The control system builds up the load, and when the required level is reached, measurements are automatically taken. Additional static measurements are taken in the middle and at the end of every trial.

There is also a clever system of inspections which fall into four different categories. After each precisely defined inspection programme, 15 technicians and engineers go over the entire test aircraft with magnifying glasses, lamps and mirrors. This occurs both during the trials, from outside, and also during regular interruptions to the test which last different lengths of time and accordingly vary as to how much work is entailed. In this way the test team obtain information about possible weaknesses in the aircraft, and this is sent to the relevant production departments immediately.

The test results will be fed into the continuous further development of the aircraft which, it is planned, will be built and sold over a period of many years. In order therefore to gain still more safety-relevant data about the aircraft, as well as normal fatigue testing the aircraft's damage tolerance characteristics are also examined. How much damage can the aircraft tolerate without compromising safety? Do small cracks endanger flight safety? In a later stage of the test programme, the Dresden team will thus be applying an abrasive cutting machine to the structure and inflicting artificial cracks on it. The effects of missing screws, rivets or even door mountings will also be examined in painstaking detail.

And only when the tests are finally complete and fully documented will it finally be the end for the experimental aircraft: cut up into manageable pieces, the sections will be returned to their respective manufacturers which will then carry out further studies on them. By then the equivalent, costwise, of one-third of a new aircraft will have been sunk into the tests. Not too much in the interests of flight safety.

From page 84 of FLUG REVUE 4/2001