The European Transonic Windtunnel (ETW) is a test bed for new aircraft. It is normal to use scale models to test the aerodynamics of new designs, for example the Airbus A380. Wind tunnels operating at normal temperatures cannot fully simulate real flight. The key feature of the ETW is that it operates in nitrogen gas at about -160°C. These conditions allow a close match to a key aerodynamic parameter called the Reynolds Number which occurs in full size aircraft in real flight.
The most important properties of the material specified for the construction of the ETW were impact toughness and dimensional stability at the sub-zero operating temperature.The first requirement is easily achieved by any austenitic stainless steel. The second requirement is less straightforward due to the variation in the risk of martensite transformation between different austenitic grades. This leads to a volume expansion.Several grades of the 304L (1.4307) type were tested for resistance to martensite formation. The final choice was a 0.07% nitrogen enhanced variant. Nitrogen is a very powerful austenite stabiliser.
The ETW was built at Cologne in Germany and came on stream in 1994. Since that time it has tested many new aircraft, leading to improvements in safety and fuel efficiency.