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Comparative Vacuum Monitoring System

Comparative Vacuum Monitoring (CVM) System - For crack detection in aeroplane fuselage

Ken Davey (IAWA)

Air safety demands integrity of structural components. Ken Davey's Comparative Vacuum Monitoring (CVM) System provides a real time system for crack detection and monitoring in skins, struts and other structures. It has a place in the world stage for structural strength monitoring.


(Photo:  RAAF FA18  Courtesy Australian Defence Dept )




In the 1960s Ken was a pilot for McRobertson Miller Airlines (MMA). On the 31st December 1968 a MMA Vickers Viscount crashed on approach to the Port Hedland airport. Ken had flown to Port Hedland as a passenger on that plane the day before and knew the crew that perished. The crash had been caused by an undetected crack in the lower boom of the main spar in the starboard wing.

He says: "Over the following years I thought of ways of preventing such a terrible accident and developed an idea for a structural monitoring system and was granted International Patents such as US 5,770,794."


 The McRobertson Miller Airlines Vickers Viscount VH RMQ which crashed in 1968 near Port Hedland with the loss of 26 people.

In concert with Perth financier, Mr Paul Kristensen of Capital Technologies, Ken set up a company in 1995 and developed the device to the stage of completing lab tests at the Aeronautical and Maritime Research Laboratories. In order to raise further capital, the now ASX - listed company, Structural Monitoring Systems (SMS) was formed.

How It Works

Ken explained that his invention took 21 years to develop to the stage of being accepted by major airlines and aircraft manufacturers. The structural monitoring device that Ken developed uses two cavities; one containing sub-atmospheric pressure and the other containing normal ambient pressure. The two cavities may be defined within the manufactured structure but more commonly, the cavities are formed by means of an attached polymer sensor pad which has interspersed channels in an adhesively coated surface. Using this method the channels are created when the sensor is bonded to a test surface.

The CVM sensor above contains a grid of channel galleries that form cavities when the sensor is cemented to the structure to be monitored.

His system, known as comparative vacuum monitoring (CVM), monitors the leakage of air from the normal cavity to the adjacent evacuated cavity when tiny cracks form in the structure. This allows the monitoring of the health of the surface by detecting cracks that form under dynamic loads, such as when the plane is in the air. The device detects cracks before they become a problem and can follow crack development over time. These cracks may start to develop some years before the part fails.




(Diagrams Courtesy Structural Monitoring Systems Pty Ltd, Perth)

The invention has attracted world-wide interest in the aviation industry including the European Airbus consortium, Boeing Aircraft Corp, US Federal Aviation Administration for aircraft certification. Northwest Airlines, Military forces including the RAAF, the Australian Army, the RAF, US Air Force, US Navy (who have had a CVM system installed on a helicopter since 2002).

The system has been evaluated and validated in over two years of flight tests by a team made up of engineers from a US airworthiness inspection authority, Boeing, US and Canadian airlines, a US university and Structural Monitoring Systems. As a result, Boeing has added this technique to its Common Methods NDT manual. The system is also being used in a number of structural health monitoring applications such as monitoring bridge structures.

Testing a landing gear oleo strut from an Airbus A340 in Gloucester UK.  Ken detected a sub-millimeter crack in the ridge as shown by the arrow head. Turning aircraft on tarmacs is a big problem with bogie wheels. The upper section of the landing gear suffers high torque loads as a result (photo Ken Davey).

A critical bulkhead of the F/A 18 Hornet with the apparatus attached and in a test rig at the Aeronautical and Maritime Research Laboratories in Melbourne (photo Ken Davey).

About The Inventor

Ken joined the Inventors Association about 30 years ago. The only IAA records that have survived from those days show that he invented a self-adjusting hook for a door, a folding shopping trolley, a digitally variable mechanical gear transmission and a constant flow line pressure regulator.

In recognition of the significant contribution that Ken's CVM system is making to aviation safety, the Inventors Association of Western Australia Inc. awarded Ken with Life Membership in 2008.

Ken now lives in retirement at Walpole WA which makes it difficult for him to participate in the association's activities.   However, he continues to work on further developments of the system. He is now also working on a wind power device.

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