Landing Gear Health Management
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Positron Systems' two new NDE technologies address many of the problems involved with the manufacture and maintenance of aircraft landing gear. This information provides a high level view of how each technology can be applied to landing gear. It describes some of the capabilities and benefits. For more detailed information about a particular landing gear issue you have request more.
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Landing gear is made with components that our tests respond very well to. The low-alloy high strength 300M steel is the most common material and achieves a 280,000 PSI tensile strength. Titanium alloys are also used. Steel and titanium are our best metals for test response. Modern landing gear uses more composite materials now, but could use more now that a reliable test is available.
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It is important to develop new NDE test applications for landing gear, because 20% of non-pilot related accidents involve landing gear problems. Any improvement in NDE techniques will have a big industry benefit. To the right is a list of the typical types of defect mode found with landing gear - although this list is not exhaustive. These defects can occur in manufacturing, maintenance or both. Matching an NDE solution to them depends on material, configuration and scope. |
- hydrogen embrittlement
- burns (electroplating, grinding)
- inclusions
- stress corrosion fracture
- low cycle fatigue
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You may already be familiar with how Positron’s technologies cover the spectrum of lifetime material management. The diagram below shows the primary Positron technique that is applied to each landing gear defect type, although both techniques can sometimes be used together.
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IPA
fatigue
residual stress
burns
hydrogen embrittlememt
coatings |
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PCA
inclusions
scanning solutions
corrosion fracture
in-situ measurement |
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A key point with the IPA technology is that each measurement is discrete - a spot measurement. We can develop a ‘scan’ by taking multiple spot measurements in sequence. This is very useful when looking at residual stress from shot peening for example. However it is best to have a localized area of interest for the use of IPA. PCA on the other hand is a highly tuned form of x-ray and can develop a picture having scanned an area. That area can be a few inches, or an entire landing gear strut - it’s only dependent upon the extent of the supporting jigs. If an NDE technician finds something in the scan that gives concern, or the software analysis determines an unusual situation, manual adjustments can be made to hone in at a focus area.
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- works best on small areas of interest
- material and configuration dependent response
- residual stress specialty
- component life management
- primarily used for metals
- manufacturing benefits
- maintenance operations benefits
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The IPA test we are deploying is the surface test. Depending on the design and type of the positron probe, depths of up to 4 mm can be achieved, even through coatings or plating material. It is well known that most fractures manifest on the surface of components, so IPA gives us flexibility. IPA is best applied to applications where change (degradation) over time needs to be measured. When an IPA test is developed, a component of given material is tested new, at near failure and many points in between. Failed components are also measured. IPA then builds a ‘lifetime degradation’ model that mirrors the actual wear on the component. This provides a baseline.
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IPA is a reference technology, in that any component of the same part number and specifications can be measured and compared to the lifetime degradation curve. The component may be at ‘design life’ but in fact have the characteristics of a component that has operated much less. The opposite is also true, IPA can find a component that may be 20% ‘used’ but be close to failure. Tests can be developed for specific phenomena - for example the presence of alpha-case or hydrogen embrittlement can be detected for a given area of interest. Sampling techniques can be developed that can give an overall picture of the presence or absence of these defects.
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PCA requires a defect to be present - unlike IPA where we can see a forming defect. That said, the resolution of the defects that can be found are significantly smaller than current NDE capability - and - the test works on advanced composite materials as well as metals. Imagine seeing defects down to 10 nanometers.
Imagine, after casting main landing gear for a large aircraft, being able to put it in a jig and perform a complete scan with the gear rotating at intervals. Hands off, the software detects anomalies and provides the scan and report. NDE Technicians then accept the report as good, or perform manual focus studies if something appears amiss.
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In routine operation, the ability to detect corrosion fractures is a major benefit. Corrosion is not caused by the bumps of landing, taxy and runway movement - the defect mechanism is corrosive metal or fluid, sustained tensile stress and internal nitrogen pressure. Most of that happens when the plane is at rest and a PCA scan can be used quickly during that rest period.
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Some of the best solutions may come from a combination of IPA and PCA. For example, casting steel causes residual stresses up to 40,000 PSI. The only way to determine what the residual stress is inside the part itself is to use IPA. Then when stresses cause the onset of fracture, PCA can find it during routine scans. The cross-reference table gives a quick view of the failure mechanism and the likely application of IPA and PCA. It is not an exhaustive list, but there are sufficient benefits to landing gear manufacture and maintenance represented here. Using the word “spot” is not to be construed as a limiting factor, except that there is a practical limit to the number of spots than can be combined for a scan using IPA. The time to perform long scans is too much. Residual stress, especially that induced by processes such as cold working or shot peening can now be accurately determine inside the actual component - and not just on a substitute - the Almen strip. Shot peening is used frequently in landing gear repair and maintenance - now IPA can tell us when the residual stress has relaxed and by how much.
To the right is a list of the major strengths these technologies have when applied to landing gear solutions. There are a variety of ways in which they can be used - singly or together - and a significant amount of cost savings when applications are put into production.
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- reduced material discarded
- reduced manufacture, reduced hard chemicals used
- reduced aircraft down time through in-situ evaluation
- reduced warranty costs for the manufacturer
- increased reliability and availability of aircraft
- increased margin of safety
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How quickly can you access these tests?
IPA requires a set of equipment and a table top in its simplest form. Positron develops an integrated workbench - called the PS6100. The workbench can be built to order in 90 days, given current lead times on the components. If a customer chooses to develop their own tests, the workbench can be supplied and their technicians trained. An application must be developed and that requires a design and qualification phase that can last 6 - 18 months depending on the design criteria to be met.
The PCA technology has a 9 month lead time to develop the equipment, however it has a shorter time to develop the application. This is due to the fact that you can see the results in image form.
For either technology, it is safe to say that a considerable time needs to be taken into account for design and delivery of the production test equipment. Compared to the cost benefits however, it is well worth taking that time.
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- IPA and PCA together find most structural problems in landing gear
- IPA is the only NDE method that can detect hydrogen embrittlement
- PCA can scan rapidly - low NDE hours
- available in customized format
- IPA and PCA detect at the nano- and micro- level sizes
- both tests used for manufacturing and operations
- only NDE method to track operational relaxation of residual stress after peening
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For more detailed information about a particular landing gear issue you have request more.
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