Ensuring Leak Integrity in the Aerospace Industry

Nov. 4, 2005
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. A relatively new method of leak detection, using hydrogen as a tracer gas, is being used in a variety of aerospace applications.

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.

Pressure Decay

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.

Another consideration with pressure decay testing is the susceptibility to distortion by changes in the temperature of the air inside the test object. Temperature rises as air is compressed and the test processes must wait until the temperature stabilizes. Some pressure decay systems now employ software algorithms and thermometers that can compensate for temperature distortion to a limited degree, but it it is difficult to fully eliminate this problem.

Leak Testing With Helium Tracer Gas

Newer methods of leak detection utilize tracer gas testing methods to detect whether any tracer gas has penetrated through a leak.

The first such method, helium mass spectrometry, was first developed for the Manhattan Project during World War II to locate extremely small leaks in the gas diffusion process. Over the years, helium leak testing has been used for a number of aerospace applications, including testing the integrity of the inter-compartmental sealing technology of the International Space Station.

The helium mass spectrometer is a complex piece of equipment that requires a vacuum. One approach is to hook up the mass spectrometer to the compartment being tested and evacuate the air within the compartment. Once an acceptable vacuum is achieved, helium is sprayed on the compartment’s exterior, particularly at any suspect locations such as seals, valves, and welds. An increased level of helium detected by the mass spectrometer indicates that a leak exists near the location where the helium was sprayed.

The alternative approach is to use a “sniffer” tube to draw a stream of air (and potentially dust and other contaminants) to the mass spectrometer or ion-pump-based helium detector. The compartment being tested is filled with helium. The compartment’s exterior is scanned with the sniffer. The operator identifies the existence and location of leaks when the mass spectrometer detects an elevated level of helium in the air flowing through the sniffer tube.

The Hydrogen Method

Another tracer gas method, which utilizes hydrogen, has been used in a variety of manufacturing systems for the past two decades and is now gaining acceptance in the aerospace industry. The hydrogen tracer gas (a non-flammable and widely available mixture of 95 percent nitrogen and 5 percent hydrogen) is detected by a highly sensitive semiconductor sensor located at the tip of the hand-held device. The portability and flexibility of the probe make it particularly suitable for situations where operators must take the test apparatus to many different locations on the an aircraft where access to compartments and systems can be restricted.

Hydrogen gas also has certain properties that make it particularly effective for aerospace leak detection. It is the lightest element in the universe (15 times lighter than air), and its molecular velocity is much higher than that of air and helium. This means that hydrogen spreads easily throughout the test object (such as the odd geometries of fighter-jet fuel tanks tucked away in any available space), penetrates leaks more readily, and vents away significantly faster than any other tracer gas. Helium, in contrast, tends to stick to surfaces, thereby increasing background buildup in the areas surrounding test objects, causing false negatives or masking small leaks.

The hydrogen method offers the ability to pinpoint the exact location of leaks, with sensitivity comparable to the helium method. There is no need to create a vacuum or to employ a sniffer to remove gas samples and transport them over a distance of several yards for analysis by the helium detector.

While no single approach will satisfy the full range of leak integrity testing challenges, the advent of the hydrogen method provides a precise, cost-effective alternative to conventional leak testing methods. Differences such as size, shape, and location of the object being tested most often dictate the chosen leak testing methodology.

Maintaining a consistent level of quality through leak testing is a large part of the aircraft manufacturing and maintenance process. In order to ensure leak integrity, aerospace construction and maintenance businesses are demanding quality assurance methods which are customizable to their particular product. '

Quality assurance, productivity, and cost-effectiveness are growing ever more essential in the aerospace industry production and maintenance. Even the smallest leaks can endanger the life of an aircraft or spacecraft. It is crucial to choose the best-suited leak testing technology in order to potentially avert problems, and to ensure quality of the aerospace equipment.

About the Author

Dr. Claes Nylander