Vacuum Monitoring May Predict Impending Structural Failures

A China Airlines Boeing 747 suffered catastrophic structural failure in May 2002, crashing and killing 225 people. The cause was a 180-centimeter (5.9-foot) crack on the under fuselage which had been hidden from view externally by a tail strike doubler in place for 25 years. Internally it was hidden by an accumulation of dust and debris. That repair would have undergone inspection before reaching 22,000 flights, but the accident occurred just a few cycles shy of that mark as the aircraft had just completed its 21,398th flight.

An Australian company, Structural Monitoring Systems (SMS) (www.smsystems.com), has devised a patented technology to detect unseen defects such as the China Airlines crack. Comparative Vacuum Monitoring (CVM) technology, invented in 1994 by a retired airline pilot named Ken Davey, is designed to provide a structural health monitoring system for aircraft. CVM is a measure of the differential pressure between the “vacuum” side, next to the surface being checked, and the non-applied side, which has openings vented to the atmosphere. If no flaw is present, the vacuum will remain at a stable level. If a flaw develops, air will flow through the passage created from the atmosphere to the vacuum side. Imagine a suction cup being placed on a glass surface. It would stick reliably to the glass, but if there were a crack in the glass where the cup is placed, it would not maintain suction. Sensors may either take the form of self-adhesive polymer pads or may actually form part of the component.

A basic system consists of three main components:

• An inert sensor that can be adhered to (or embedded within) the structure during manufacture.
• A regulated vacuum source to apply and modulate a low vacuum.
• A measuring device.

Flat self-adhesive polymer sensors are most commonly used but sensors can be designed and manufactured to conform to two- and three-dimensional surfaces. The sensors can be made in a range of materials to suit even extremely hostile environments. These sensors can be embedded within the mass of a structure or encased within bonded joints and lap joints to monitor for internal failure. This design can enable significant cost savings when compared to retrospective installations.

Today’s aircraft are susceptible to cracking, not just older aircraft. The Australian airline Ansett went out of business following two lengthy groundings by the Australian Civil Aviation Safety Authority. The airline’s maintenance group had missed the target dates on two important service bulletins, one involving a potential crack in the tail of its 767s and another addressing engine pylon cracks. Once an audit picked up these oversights, the carrier learned that it did indeed have fleet-wide cracks in both areas. Perhaps even the China Airlines accident could have been prevented with a system like CVM as an integral sensor can be sandwiched between the doubler plate and the surface with the cracking problem.

SMS is looking to have an offline monitoring system hooked up to the monitoring system of a widebody aircraft some time next year, and to have an online monitorable system interfaced with the aircraft’s health monitoring system by the following year. This story was adapted from AM sister publication Air Safety Week . For more information about ASW visit www.aviationtoday.com.