USA   F-111 Aardvark OZ

Cold Proof Load Test

Below is an article concerning the testing of RAAF F-111's at the Cold Proof Test Facilitiy at McClellan AFB.  The same technique was used on USAF aircraft at McClellan AFB, and on USAFE aircraft at Bristol in the U.K.

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Images supplied by Steve Kell

F-111C at Cold Proof Load Test
An F-111 under cold proof load test.

"You send 'em we bend 'em" is the motto of the crew at the F-111 Cold Proof Load Test Facility located at McClellan AFB in California.
Control Room
Control room at the CPLT facility.

The Cold Proof Load Test Facility (CPLT), the only operational facility of its type in the world, subjects RAAF F-111's to the most extreme loads the aircraft would encounter in its lifetime, resulting in a main wing bend that would send a shiver down any aviator's spine.

The RAAF is currently using the USAF Sacramento Air Logistics Command (SM-ALC) owned and operated facility for about four aircraft tests per year. Ensuring that the program runs smoothly and that the RAAF gets value for money is the responsibility of F-111 Liaison Office Resident Project Engineer and the Assistant Resident Project Engineer.

The CPLT procedure was developed following in-flight structural failures in early F-111 production aircraft, including several in-flight losses during the Vietnam War. CPLT aims to bring on metal failure that could occur in flight.

D6AC steel used in the manufacture of critical F-111 structural components has a very low fracture toughness value. This means it is very brittle and susceptible to failures from very small fatigue cracks or manufacturing flaws.
To simulate critical positive and negative load manoeuvres on the wings, four conditions are tested at two different angles of wing sweep. The maximum positive load condition of 7.33 times the force of gravity results in the wings being deflected over two metres at the tips. The structural load tests, conducted at approximately minus 40 degrees Celsius, verify the integrity of the high strength D6AC steelalloy used in the wing carry-through box, wing pivot fittings and certain fuselage bulkheads. These are critical areas in the aircraft where a failure could result in catastrophic damage to the aircraft in flight. Obviously, the high stress conditions of low-level tactical flying are more likely to produce this type of failure thereby reducing the chances of crew survival. The reason for testing at such low temperatures is that the critical crack length is significantly smaller for the steel alloy at the sub-freezing temperatures. The test simulates worse conditions than those encountered during flight and is therefore more likely to accelerate a failure that was 'waiting to happen'.

Before the test, fatigue cracks which are accessible and able to be identified by non-destructive inspection (NDI) are repaired, but if cracks are too small or inaccessible for NDI, and are of a critical crack length, they are detected as a failure of the part under test loads. The CPLT is a potentially life-saving inspection because if a part fails under CPLT loads, it could well have failed in flight. The majority of RAAF F-111 aircraft have undergone CPLT three times and during these tests the RAAF has had three significant structural failures. Two wing failures and a horizontal stabiliser pivot shaft failure would have resulted in loss of the aircraft had the failures occurred in flight.

The USAF has also had its share of failures. A recent wing carry through box failure on an EF- 111A (66-038) would have resulted in an aircraft loss had it occurred in flight. Throughout the CPLT program, numerous failures have been detected and rectified to keep the F-111 flying.

Original story written by Greg Hoffman.
(C) Strike Publications

Republished under permission of Strike Publications (

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