Colin Flint, retired Head of Ground Test, recalls his time with the Mithraeum test frame…
    With the closure of Langley in the late ’50s there was a requirement either to move the Abbey test frame to Kingston or build a new one so that structural testing could continue. Given that aircraft were getting larger it was thought sensible to build a new one. About the largest aircraft that Kingston could envisage building was something the size of a B-58 Hustler and so that was the chosen size.

People employed in the Research and Development Department where tasked with the design of such a test frame which was to be called Mithraeum after the Roman temple recently discovered during building work in London (all previous test frames were called after religious buildings). Derek Thomas was the lead engineer on the task.  

The Mithraeum Test Frame

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In 1959 the frame was constructed in Scotland and brought down by road in parts over a three week period. Because of its height the frame was erected in a 10 foot deep pit dug towards the northern end of the new Research building at the Hawker Aircraft site in Kingston. This was cheaper than raising the roof of the 500 foot long building by 10 feet.

The basis of the frame consisted of a pair of keel members some 95 feet long and overhead warren girders 105 feet long; these were mounted on four massive columns. Eight loading bridges were mounted on rails attached to the warren girders and associated structure, each having facilities for manual loading by turnbuckles via linkages to the structure under test.

The pit was finished in waterproof cement so that it could be used for underwater pressure tests if required. A 23,000 gallon water tank and pumping facilities were included.
    The first major job was the P1127 static strength test done in 1960, the load being applied manually via turnbuckles. Some twenty tests were carried out in various configurations, each taking about six weeks including rigging for the case, testing and data analysis. Each test required four people to apply the load, two to apply fuel tank pressures ( including myself), six to read the 800 strain gauges and four to read the deflection gauges.

The maximum load required was divided into eight or ten increments, each of which was applied before the instrument readings were recorded. Each test took one day.

As an aside, the last strength test carried out in the Mithraeum was the Hawk Mk1 test series where the six weeks per case was reduced to half a day using automatic load measurement and mechanical servo-controlled hydraulic valves. Just four people ran the test and graphs of strain gauge and deflection measurements where available one hour after the test was completed.
    The next major test was a research programme on twenty Hunter Mk5 airframes for the RAE (Farnborough). The RAE was to test using constant load cycles from start to finish at various levels; Kingston was to use programmed loading ( a programme consisted of 311 cycles which included one cycle of 7g to -2.5g and 5 other levels of cycles down to 2.5g to 0.5g ). Two airframes could be installed in the test frame at the same time. On completion of each simulated 500 flying hours inspections were carried out overnight.

The Kingston tests used four ultra low friction hydraulic tension jacks (two per wing) pulling up via linkages attached to soft rubber pads glued to the wings. The RAE tests used compression jacks pushing up under the wings. Kingston completed five airframes at three different levels. One additional airframe was added to the programme, a T Mk 7 XL574, to clear the trainer for use in the Royal Navy. This test used the same wing linkages with a modified fuselage linkage, and a different programme was used. An aircraft life of 3,400 hours was required with a factor of 5, meaning 17,000 flying hours and 34 inspections. This was accomplished in 49 working days! The test then continued to 63,000 flying hours when the fuselage failed. Three wing failures occurred, the wing being replaced each time.
    At the same time as the Hunter tests, a Kestrel fatigue test was carried out in the same test frame. This test used 49 hydraulic tension jacks, again using linkages to distribute the load to rubber pads attached to the wing.

It was not uncommon to go home with three fatigue tests running, unattended, overnight. Finding enough Inspectors the following day to inspect the airframes was the most serious management problem.
    The next test carried out in the Mithraeum was the Harrier GRMk1 Strength test. Mechanical hydraulic servo valves developed by Kingston were used for this test series which utilised multiple tension jacks to ensure rapid case changes. Strain gauges were read using PDP11 computers with suitable data collection software thus shortening the overall timescale. The wing failed marginally below the required load and as a result the wing skin thickness was increased on all service wings.

These tests were followed by the Harrier GRMk1 fatigue test and then the TMk 2 fatigue test. Both used the by now well tested system used on the GRMk1 strength test but the mechanical hydraulic servo valves were motorised so that case changes could be accomplished without human intervention during testing. Both these tests reached 200 % of the require life.
    The final full scale tests carried out in the Mithraeum were the Hawk strength test, mentioned earlier, followed by the first Hawk fatigue test which was stopped at 60% of the required number of cycles when the wing failed, there being too much damage to make repair an option. This test was the first to apply simulated manoeuvre loads, for example rolling pull-outs and Cuban eights. Specific sortie patterns were also run.

Many smaller tests were carried out using the Mithraeum but my memory is not good enough to list these here. Upon the Kingston site closure the Mithraeum test frame, which had been used for so much vital work, was scrapped and later structural tests were carried out at BAe Brough    .
    My thanks go to Richard Cannon and Brian Indge who did so much original thinking in constantly improving test techniques, and to the late Derek Thomas who created the environment in which people could advance.