Over the last decade, the aerospace industry has experienced significant improvements in technology processes; specifically, there has been a greater emphasis on improved quality and consistency to ensure greater productivity and reduce costs. In order to rise above their competitors and comply with increasingly stringent environmental regulations, original equipment manufacturers are seeking economic and ecologic solutions to close gaps between shorter innovation cycles and longer product lives.
Leak detection is an essential quality assurance process in the construction and maintenance of aircraft. Fuel systems, oxygen systems, bleed air systems, coolant systems, and fire extinguishing systems all need to be tested for tightness. Leaks can render critical systems inoperable and even cause explosions.
There’s no argument that leak testing is important, but the question remains which leak testing methods are the most practical and effective. For years, the simpler methods have been the most popular; methods prevalent in every industry include soap bubble testing and pressure decay. While each of these methods offers the advantage of minimal investment, they also pose significant drawbacks.
A relatively new method of leak detection, using hydrogen as a tracer gas, is being used in a variety of aerospace applications. The “hydrogen method” is particularly well suited to the aerospace industry because of its sensitivity in detecting extremely small leaks, portability, and flexibility which allow it to probe locations that are difficult to reach with other methods.
Soap Bubble Testing
The oldest method of leak detection, soap bubble testing, is simply the observation of a pressurized component that has been sprayed or brushed with a soapy solution. Soap bubble testing can detect very small leaks allowing the operator to pinpoint the exact location of a leak. However, the process is highly dependent on the skill and patience of the operator. This can be a factor if the operator’s perspective is limited. For example, small leaks may remain hidden on the reverse side of a component or in a recess. This is a common occurrence in the aerospace industry where components are usually packed into tight spaces.
Another consideration when using soap bubble testing is that sometimes larger leaks do not cause the formation of bubbles. Instead, the compressed air blows away the soap solution, and the inspector may fail to observe such a leak.
Another factor to keep in mind is that with small holes, the capillary force can be extremely strong. The result is that liquid that has been sucked into a micro leak by capillary action, cannot be forced out with compressed air, and therefore no bubbles will appear.
Another widely used leak detection method is pressure decay, where compressed air is simply injected into a test object, and a decrease in air pressure over time signifies a leak. While the pressure decay process is uncomplicated and inexpensive, it is an “integral” test, meaning that it measures the total leakage from an entire object but does not indicate the location of any leaks.
More significantly, the pressure decay method provides limited sensitivity, especially because leak testing procedures can only use limited amounts of pressure to protect the items being tested from damage. Medium to large objects require an unreasonably long cycle time to achieve an adequate level of sensitivity for most applications. For medium-size objects, sensitivity is limited to the detection of leaks emitting 0.5 – 1.0 cc/min, 10 times less sensitive than typical tightness specifications for components containing fuel and several orders of magnitude away from the requirements for components containing gas.