About Aircraft IR
The Most Effective Way to Detect Water Ingress in Airplanes
The use of honeycomb sandwich structures is widespread on aircrafts. They provide stiff lightweight structures which are ideal for control surfaces and other exterior structures, such as radome, rudders, stabilizers, ailerons, etc.
Water ingress in honeycomb composite structures can degrade epoxy adhesives, erode metal, and add unnecessary weight to the structure. The water in honeycomb cells is a potential destroyer to cells as it expands after freezing at altitude or in freezing climate areas of the world. So the Infrared Non-destructive testing technique for detecting water in honeycomb structures is very important and the most effective, efficient and cost effective in aircraft maintenance. We use the Top Of The Line, Highest Resolution FLIR T-620 Infrared Camera Manufactured (307,200 pixels – 640×480 resolution).
The traditional techniques used by aircraft maintenance technicians to inspect honeycomb composite material are ultrasonic and X-ray techniques. However, the point-by-point ultrasonic inspection is low-productive, while the X-ray technology is hardly used in situ because it requires two-side installation and strict radiation safety precautions. Infrared Thermography (IR Inspection) has been very successful in detecting water ingress in composite materials in honeycomb structures. IR Inspection of aircraft composites is a high-speed, large area, portable, non-contact, cost effective and a safety inspection technique. IR inspection can be used in one-side testing, so it can make up the deficiencies of the traditional methods.
When an airplane takes off and goes up to high altitude, where the pressure is lower, air is forced out of some honeycomb cells that are not bonded perfectly. The air at high altitude is also cooler so some water condenses out and remains in the honeycomb cells.
As the airplane lands, warmer and moister air reenters the cell. This process repeats itself over and over again every time the airplane takes off and lands, the cell and/or cells fill with water. Each time the airplane goes up to high altitude, the water in the cells freezes, and expands, and the bond is further weakened. In this manner disbanding can occur in neighboring cells and the problem will become pervasive.
Water can also enter the honeycomb structure when the bonding is damaged by hail or impact from any number of other circumstances while in the air or on the ground.
Water ingress in an airplane part can create a dangerous situation. Although the honeycomb is very strong and lightweight, it loses these characteristics when it is damaged, in this case ice. Furthermore, the ice will also separate the bonding from the honeycomb. Through vibration the total bonding structure can be weakened or even partly destroyed. This means that the airplane will lose it’s stability. The weight of the ice also influences the aircraft balance. Besides these potential dangerous consequences the fuel consumption is rising as well. The total weight of the aircraft is increasing, so it will need more fuel to fly. It is therefore of the utmost importance that water ingress is detected at an early stage so that necessary action can be taken.
Water ingress can lead to skin to core bonding degradation, effecting structural integrity. More significantly, water ingress may contribute to structural failure of composite honeycomb sandwich components. That is why it’s important to discover water ingress at an early stage, so the removal of the water from the honeycomb can be done on time avoiding bonding degradation. Our high-end thermal imaging cameras can identify anomalies invisible to the human eye, as we cannot see within the infrared light spectrum.
One of most severe problems that maintenance engineers encounter during hot-bonded repair is inadequate water removal, which can lead to high void content in the bondline, degradation of adhesive bonds and even blow-off of the skins due to sudden buildup of vapor pressure in the honeycomb cells. Various techniques have been developed and used to achieve efficient water removal, including the application of heat, vacuum, ultrasound, and centrifugal forces and electrical pulses. Among these test scenarios combined heating and vacuum drying was found to be the most effective method at removing water from aircraft composite surfaces.
Once our comprehensive infrared inspection is complete we deliver the IR report digitally documenting the details of all findings with thermal and digital images identifying the exact locations on the airplane that need repair or replacement.
Source: Investigation of an accelerated moisture removal approach of a composite aircraft control surface (Chun Li, Rick, Ueno, Vivier, Lefebvre, National Research Council Canada, The University of Ottawa, Department of National Defense Canada).
Credits: Mr. Carsten Holm, Vice president technical at Star Air, Mr. Ralf Grispen, Thermografisch Adviesbureau BV (www.thermografie.nl) Pictures: Thermografisch Adviesbureau BV