The Hush-house was a strange construction in Bromma Airport, Sweden, which was used between 1957 and 1962 for reducing the noise from aeroplane engines under test.
Before the Hush-house
In 1957, SAS (Scandinavian Airline Systems) decided to buy Caravelles to add jet planes to their commercial fleet. This would naturally require new techniques and new technicians. A new airport at Arlanda, near Stockholm, was going to be the largest airport in Sweden, but would not be completed for a couple of years. Until then it was necessary to test the Caravelles' engines at night in Bromma, as that was where the testing and repair facilities were. Unfortunately, the Ministry of Health said that the engine noise could not be measured higher than 30dB in the nearest living areas.
Dawn of the Hush-house
The German airline Lufthansa had created a solution: a hangar with sound-proof walls to contain the entire aircraft. But that was too expensive and there was no room for a construction like that at Bromma, so another way needed to be found to silence the engine noise. SAS sent out several technical specification sheets around the world trying to find a plausible solution. In 1957, project engineers and group-leaders for the jet-engine testing and repair facility, Gunnar Wikner and Harry Liljeblad, came up with an idea: if you isolated only the back of the plane in a hangar, you could test the engines without any noise. A three-dimensional model was made and Sud-Aviation in Toulouse, France, and Rolls Royce in Derby, UK were asked if they could approve such a solution. The answer was not much to stand on: 'yes, if it doesn't effect the plane or the engines'. In March, 1958 the first test run took place. Everything worked - nobody in Bromma knew that they were testing the engines.
Design and Specifications
The Hush-house technical description was a 'ground run-up silencer for Caravelles'. Ground run-up tests are used for troubleshooting problems with installed engines, as well as making final adjustments and tuning the engines after overhauls. The Caravelles were pushed into the house, tail-first, using a tractor. As SAS at this moment only had Caravelles in its new jet-plane fleet, the hush-house was designed only for planes with engines in front of the tail-plane and behind the wings, mounted directly onto the fuselage. The tail and the engine nacelles were completely enclosed by the house, where the engines were then activated to their maximum output.
The house looked like a tall hangar made out of concrete, large enough to enclose the tail of the aeroplanes, with four large chimneys sticking out vertically from the corners and two large square tubes for exhausts with in-built silencers at the back. A concrete, industrial-type roof covered with metal on the outside completed the first impression of an industrial building. The chimneys contained the vertical sound-silencers, and the two horizontal tubes at the back connected to the horizontal exhaust silencers that were placed behind the engines to gather all exhausts, with a distance of about 15cm to avoid the silencers getting deformed by the pressure.
The house construction was steel-concrete, the walls were 12cm thick concrete slabs, and all the internal walls were covered with 10cm of mineral wool held in place by perforated steel sheets. These were installed to prevent any soundwaves bouncing off the walls and damaging the aircraft structure during testing. As with the walls, the ceiling was made of lightweight concrete and explosion panels that were released automatically when air pressure approached dangerous levels. The entrance consisted of two large steel doors, each of them having a large half-hole that together formed a hole enclosing the rear part of the aircraft. The edges were packed with about 20 - 30cm of air-empty foam-rubber to prevent any air penetrating. Inside the rest of the doors there was a similar thickness of glass wool.
Success and Failure
Many aviation companies, as well as individuals such as the Shah of Iran, were interested in the Hush-house and came to look at it. One day, however, Sud-Aviation detected fissures in the tail of the SAS-Caravelles. Sud-Aviation blamed the Hush-house and set up a delegation with a device to prove this. Tests indicated that the vibration environment inside the Hush-house was better than that experienced under flight conditions. But it was all called off a couple of weeks later when they found fissures of the same kind in Caravelles in France that never had been to Sweden. When Arlanda airport was complete in 1962 the Hush-house was dismantled and the engine testing facility was moved to Arlanda.