The enormous pressure fluctuations in flight can cause water to penetrate into the outer skin of aircraft. This freezes with increasing altitude and thaws during landing. Because this continually repeated process causes damage to the material, thermal imaging cameras are used to examine aircraft for water deposits after landing.

The composite materials of modern aircraft must be both extremely robust and light. In addition, airworthiness and performance depend on the materials. Modern aircraft therefore use a honeycomb structure in their construction. This is similar to the honeycombs in a bee hive and is both light and tough. As long as this structure is intact, there is a balanced strength-to-weight ratio - on condition that the cells are permanently adhered to the outer skin, as in the case of the wings. Often materials such as carbon fibres are used to produce the composite materials.
In spite of optimised adhesive processes, the honeycomb and the material of the outer skin are not perfectly bonded, so that due to large pressure fluctuations air is pressed into the cells and then sucked out again. Because aircraft are subjected to enormous pressure fluctuations when rising and falling, there is a relatively high risk that water may penetrate by this route. When an aircraft reaches high altitudes in which the air pressure is lower, air escapes from cells which are not completely sealed. In addition, at high altitudes the air is colder, so that condensation can form, which remains in the cells. When the aircraft lands, warmer and more humid air flows into the cells. If this process is repeated, the cell becomes full of water. Every time that the aircraft reaches high altitudes the water freezes in the cells and expands - which damages the bond. In this way, the bonding of adjacent cells may also fail. The bonding can also be damaged by hailstones, so that water can penetrate. Although the honeycomb structure is flexible an tough, it looses these properties if it is damaged. As well as this, ice attacks the adhesive bonds of the honeycomb structure. The entire bonding structure is weakened or even partially destroyed by vibrations. Ultimately, this weakens the strength of the aircraft. As well as this, ice also has an effect on the balance of the aircraft. In addition, the increase in total weight causes greater fuel consumption. Because of these facts, water deposits must be detected at an early stage, so that appropriate countermeasures can be taken.

Inspection under time pressure

One method of detecting water in the wings is the use of thermal imaging cameras. These are used by the Dutch company Thermografisch & Adviesbureau Uden, which has specialised in the performance of aircraft inspections. "Most of our work consists of electrical and mechanical inspections" says Ralf Grispen, Commercial Manager of Thermografisch & Adviesbureau Uden. Thermal imaging cameras are used for the regular, preventative maintenance inspections in the area of the engines and other parts of the aircraft, as well as to detect water deposits in the wings and fuselage. "At present we have gained experience from the inspection of 75 aircraft for several MROs (Maintenance, Repair and Overhaul - aviation maintenance companies)", continues Ralf Grispen.
"Thermal imaging technology enables us to discover water deposits", explains Paul Kennedy, Composite/Painting and Supervisor/Inspector for Air Atlanta Aero Engineering. "At high altitudes with temperatures of -40°C and below, the water freezes in the cells. This is still frozen on landing, so that the thermal imaging camera can clearly identify the cold spots. With thermal imaging technology we are able to examine large areas within a very short period, before the ice has melted. According to the temperature on the ground, you can say that we usually only have an hour to inspect an aircraft."

Convinced by speed

A Flir P660 thermal imaging camera is used to search for frozen water. This provides sharp thermal images with a resolution of 640 x 480 pixels, in which details can also be detected. As well as this, temperature differences of 0.03°C can be made visible. It has a large format, folding 5.6" LCD monitor screen as well as a viewfinder, with which inspections can be easily carried out in summer and when the sun shines on the display. "After the inspection, we generate a report with the aid of the Reporter software. With an easy-to-use computer program we can give the repair personnel a detailed and thoroughly documented report which contains all of our results. As the Flir P660 provides the facility for assigning the thermal image to a daylight image as a reference, the areas of the aircraft which require repair can be precisely located. If the camera discovers any area with significant water deposits, the outer skin is opened up and the water-removal programme starts. "Water which has penetrated into an aircraft can also be detected with other methods. However, these have considerable disadvantages in comparison with thermal imaging technology. For example, liquid crystal sensor films can be fixed under vacuum to the areas which are to be examined. This is a slow process, which also only functions for small areas. Inspection by X-ray is also time-consuming and requires a large amount of equipment and personnel.