Aviation Maintenance: Technology Focus: DSSP Parts Analysis in 3D

A 3D modeling technique, digital shape sampling and processing (DSSP), improves analysis of aircraft parts and assemblies. This technique may ultimately help airplanes return to flight much sooner.

Maintenance, repair and overhaul (MRO) has long been part of the aerospace vernacular. But only recently have service providers adopted automated tools to streamline the up-front process of capturing and digitally reconstructing physical parts that require repairs or redesign.

The central issues surrounding MRO — speed of repair, accuracy, reliability and quality — are relevant worldwide. In the U.S., it is a central issue with adherence to FAA parts manufacturer approval (PMA); similar certification processes are expected throughout Europe. In all cases, the combination of market demand and certification make MRO a prime application for digital shape sampling and processing (DSSP).

DSSP describes the process of digitally capturing physical objects and automatically creating accurate 3D models for downstream design, engineering analysis, inspection and custom manufacturing.

DSSP is ideal for MRO projects for four basic reasons:

• It captures and reconstructs parts as they exist, enabling engineers to analyze damage and its underlying causes.

• It enables engineers to accurately recreate parts for which no computer-aided design (CAD) data or documentation is available.

• It enables engineers to analyze situations where every part is different, due to factors such as manufacturing variability and tolerances, wear patterns and stress.

• It improves asset management by speeding part reconstruction and inspection of as-built vs. as-designed parts, helping airplanes return to flight much sooner.

Unlike CAD, DSSP does not require an existing part or assembly to be created again from a blank screen. Design, engineering and quality inspection processes spring from what already exists in the physical world.

MRO Applications

The following lists a few examples of MRO applications, with an eye on how DSSP could be applied to speed the process and ensure quality:

Commercializing New PMA Parts: A company wants to commercialize a new PMA part, but the OEM does not release drawings or CAD models. The company needs to be able to capture the part, digitally recreate it, and send data to a CAD program. From there, the company can create a model from which the newly fabricated parts can be manufactured with the same form, fit and performance of the OEM parts.

Structural Check: A UPS cargo plane goes in for a C-check. As part of the maintenance, the airframe needs to be checked for cracks and possible repairs. This requires comparing the actual airframe against the nominal CAD geometry to identify areas that are out of tolerance. Engineers need a way to perform a complete 3D analysis of all the surfaces, quickly compare them against the CAD model, and identify out-of-tolerance areas for repairs or part replacement. UPS must minimize downtime and get the airplane back in service as soon as possible.

Conversion from Pax to Cargo: A company has received a contract to convert a Boeing 737 into a cargo plane. The company does not have access to the original Boeing drawings, and since the 737 was first built in 1967, there are no CAD models associated with it. The entire existing environment — including electronic cables, hydraulic systems and other internal aspects — need to be captured so that engineers can design the new cargo areas. Engineers have four months to remove all passenger equipment, install the main cargo door and integrate the new cargo system.

Fixing a Crack in a Fighter: A fighter airplane undergoes routine maintenance and inspectors discover a crack in the structure. Repair requires a stainless-steel doubler fitting that is riveted into place, bridging the gap caused by the crack. Obviously, there’s no CAD model of the crack. The traditional repair process requires that an impression of the crack be made with dental putty, measuring the mold manually with calipers, and creating a CAD model that will be sent to a CNC to make the doubler fitting. Typically, it takes five or six prototypes to meet the fit tolerance required, grounding the plane up to six weeks. By contrast, if an engineer could capture the crack with a laser scanner, it would only take a few days to create a CAD model of the bracket to repair the crack, machine the bracket, scan the manufactured part and compare it to the CAD model to ensure that it meets tolerances, and install the new bracket.

Redesigning a Lavatory: An airline decides to remodel the lavatory of a Boeing 767 to increase passenger satisfaction and update accessories. Since the 767 first took flight in 1981, there are no CAD models available. New components need to fit into the current space while accommodating fixed infrastructure, such as water and power systems. The designer must start from existing conditions, taking out old parts and installing new ones within the same envelope of space. And, of course, this has to be done in a few weeks, as the airplanes need to return to flight as quickly as possible to reduce revenue loss.

Why Use DSSP?

There are two central similarities in these diverse cases: first, there is a need to capture existing conditions rather than create new models. Secondly, time-to-market, or getting back into service, is crucial for asset management.

Both of these requirements are perfectly suited for DSSP. Companies throughout the world are already successfully using DSSP for MRO applications. The Royal Australian Air Force has implemented DSSP to reduce repair time on its fighter planes from several weeks to two or three days. Eastern Technical Services uses hybrid modeling — a combination of DSSP and CAD — to not only improve quality, but also increase the number of PMA projects it can handle 10-fold.

To get a better picture of how DSSP can streamline the MRO process, let’s first review how CAD models are used in downstream applications. Then, we’ll take a look at the traditional process for creating those models for MRO parts and compare it to DSSP.

Importance of CAD Models

Accurate CAD models of parts and assemblies are required for a variety of downstream processes, including: generating 2D drawings for documentation and dimensioning; providing the model for CAE studies such as stress, thermal or fluid dynamics analysis; creating a numerical control path that drives a CNC machine for manufacturing a new part; producing physical prototypes to test parts and make sure they meet specifications; and inspecting the as-designed part against the as-manufactured part.

Traditional Method

The traditional process for creating a digital representation of an actual object begins with a manual measurement tool such as a caliper or a CMM machine to obtain dimensions. In both cases, users need to have the expertise to know which dimensions will be important for reconstructing the digital shape. With the manual process, the dimensions are in 2D. With CMM, dimensions are captured in 3D, but only a small number of pre-defined points are collected. Collecting a large number of points requires a significant time investment.

The output from the data-capture phase is a list of measured key characteristics that can be used to model a 3D CAD object. This process requires a great deal of time spent on sketching, extruding and rotating profiles, and blending all the surfaces together. Total time for this process is usually measured in weeks.

Creating a New Model

With DSSP, the starting point is the existing part. Or, it could be several examples of the same part, since DSSP provides the luxury of being able to easily obtain the average or optimal shape within a series of parts. Scanners used in DSSP are capable of capturing millions of points in minutes to accurately represent the physical part.

From those millions of points representing different examples of the part, DSSP software such as Geomagic Studio can create a new model that is an average of all the samples. Automated tools recognize primitives and profiles from the scan data, repair holes and noise, and wrap the point cloud to form a polygon model. The polygon model is then converted into a surface model that can be exported into CAD. If needed, users can further manipulate the model in their existing CAD software to create a parametric model.

The process typically takes a couple days. Since it is based on the detailed capture and automated reconstruction of the actual part, DSSP increases accuracy, removes ambiguity and ensures consistency between the CAD model and the manufactured part.

Bridging the Gap

By bridging the gap between the physical and digital worlds, DSSP offers multiple benefits for MRO businesses:

• Time savings: Automated data capture and reconstruction enables companies to shrink time-to-market or return-to-service cycles by 90 percent or more.

• Improved profitability: MRO service providers can handle many more PMA projects and dramatically increase capacity with little or no increase in staffing or resources.

• Better quality: The ability to collect and process greater amounts of data for as-manufactured parts helps ensure higher-quality CAD models, allows for CAE based on real-world data, and enables inspection of CAD models vs. as-built parts.