Ian Sheppard/LONDON
The US Federal Aviation Administration is testing a new inspection technique which could allow rapid on-the-spot diagnosis of the integrity of aircraft skin panels in routine maintenance.
Advanced dynamic thermography is a non-contact, non-destructive inspection method which involves heating composite panels by around 4°C, with an induction heater or flashlamp, and then viewing the area with an infrared camera as the heat dissipates.
Mike Valley, head of thermal and optical projects at the FAA's Nondestructive Inspection Center in Atlantic City, envisages that, once commercialised, the equipment for such inspections will cost around $100,000, but adds that payback time for a repair station could be "less than a year".
Valley says that it is "an order of magnitude" faster inspection method than ultrasonics, although it is not a replacement as thermography is "not viable for crack inspection". A thorough inspection of a McDonnell Douglas DC-9 underside was made "in less than an afternoon".
The technique is also proving valuable in tracking problems such as water ingress. By using built-in software to analyse the infrared images, the FAA has discovered that it is possible to identify the layers between which fluid is present and even to distinguish between water and hydraulic fluid. Aircraft materials can also be identified, so that steel rivets show up as "bright" images and aluminium as "dark".
The FAA team is running a series of military and commercial projects to qualify the technique, with a competition launched to find an equipment vendor for a US Air Force programme. A winner is to be selected early in May, followed by a year's work to apply the technique to "real aircraft" and develop an non-destructive manual.
Meanwhile, the FAA is working with Northwest Airlines after the airline raised concerns over current methods for inspecting disbonding under the nose of its Boeing 747s. The final proving tests are proving elusive, says Valley, as the two aircraft still known to have the disbonding are with international operators, one of them El Al.
Northwest is also co-operating in an effort to extend the method to quantitative corrosion inspection. The aim is to "find the extent of material loss as well as the fact that it is there", says Valley. Tests have already shown that damage assessment is accurate at damage levels above about 5% of the material. Proof of concept is now being undertaken with inspections of operational aircraft alongside existing techniques. Approval will be sought in 1999, says Valley.
Source: Flight International
