This is a digital surface model patch structure of an MD88 aircraft engine inlet scoop created using data from a 3-D scan of the physical part. Source: Delta Air Lines

With a fleet of more than 500 aircraft that need to be kept airworthy, Delta Air Lines' technical operations center is a key part of the airline's activities, providing a comprehensive maintenance service to Delta, as well as to other airlines. To ensure spare parts are available when needed, it uses ICEM Surf software from ICEM (Southampton, United Kingdom) in the reverse engineering of parts for in-house manufacture.

When Huff Daland Dusters was founded in 1924 as the world's first aerial crop dusting organization, maintenance and overhaul of the firm's aircraft was done only when necessary. That situation changed in 1928 when the company became Delta Air Service, and in June 1929, they inaugurated airline services with the first passenger flights from Dallas to Jackson, MS, via Shreveport and Monroe, LA. From that point on, aircraft maintenance and repair climbed to the top of the ladder of priorities.

Today, Delta Air Lines is the world's second-largest airline in terms of passengers carried and the leading U.S. carrier across the Atlantic, offering daily flights to 490 destinations in 85 countries through its various airline operations and partners.

Delta TechOps' Technical Operations Center (TOC), located at Hartsfield-Jackson Atlanta International Airport, is one of the largest and most modern aircraft restoration facilities in the world. The TOC, a one-stop shop, provides a comprehensive range of aircraft maintenance services designed to keep Delta Air Lines' fleet safe and reliable. It also makes its services available to other aircraft operators on a commercial basis.

Final digital surface model of an MD88 aircraft engine inlet scoop, created using data from a 3-D scan of the physical part. Source: Delta Air Lines

One-stop shop

The services provided by the TOC include airframe maintenance for both wide-body and narrow-body aircraft; engine maintenance, including full overhaul, repair and support capabilities for engine parts and components; component maintenance, including repair, overhaul and test facilities for electromechanical components and avionics; line and maintenance, for a full range of ground services; and technical support and technical operations training.

Without the wide range of on-site engineering expertise and support provided by Delta TechOps, Delta Air Lines would need to purchase more spare parts and components from third-party suppliers to meet flight schedules. And while many airlines contract all part overhauls to third-party stations, Delta limits its need for out-sourced composite overhauls by engineering its own in-depth, in-house procedures.

The Shop Support Engineering office has responsibility for ensuring that these procedures are undertaken in a cost-effective and timely manner. It supports all requests for repairs to Delta's and other commercial airlines' aircraft structural components, including slats, spoilers, flaps, engine cowls, composite skin panels, doors, elevators, rudders, tabs and ailerons.

It also is where the reverse engineering facilities within the ICEM Surf surface modeling, surface model validation and design visualization software suite are used. This software enables design-engineering teams to create and analyze complex 3-D digital surface models to the highest possible quality, in the shortest possible time. This model data is then used in the tooling development process to design the mold tools and dies required for manufacturing.

ICEM Surf also offers special functionality to enable the easy input and manipulation of data digitized from physical items, such as an existing aircraft component. Digital surface models can be developed quickly and automatically from ordered or unordered point cloud digitized data in a fraction of the time required by other methods. Special diagnostics tools in the software guide the user to the correct balance between surface smoothness and adherence to the digitized data. This enables the user to develop the best fit to the data, based on the user's own criteria, not the software's internal algorithms.

Delta's main focus in using this software suite is to reduce the need to scrap and replace components, as well as to reduce the need to outsource composite component overhauls and repairs. In practice, the software generates the data needed by Delta maintenance personnel to computer numeric controlled-machine special tooling in order to maintain complex surface geometries during elevated pressure and temperature cure cycles for composite components.

Data from a 3-D scan of a physical MD88 aircraft door pan are used to create a digital surface model for manufacturing purposes. Source: Delta Air Lines

Reverse engineering

In special cases, where a component's contour information is not available from the OEM, Delta's engineers use software to reverse engineer the contour of an existing spare part. In general, this is used only to design composite flat pattern ply templates, using a sophisticated ply design software suite in conjunction with this software.

In other cases, where .stl data files are available for OEM sheet metal parts, Delta Engineering uses this software to pursue parts manufacturing authority (PMA). They do this by taking a sample of OEM parts, digitally scanning them and then using this software to establish tolerances based on the dimensional range of the parts. These tolerances are then sent for material testing to determine the alloy and heat treatment in order to prove that Delta can meet all strength and dimensional requirements during the manufacturing of the part.

In addition, by using this software's advanced surface quality analysis and diagnosis capabilities, repetitive inspections of tool surfaces can be performed on a periodic schedule using portable coordinate measuring machines. The raw data obtained from these machines is imported into this software and then compared with nominal mold tool models. With a typical ideal maximum deviation of 0.025 centimeter, or 0.01 inch, this software enables Delta's engineers to monitor tool surface integrity over time.

Currently, the primary use for scan data with this software at Delta is for checking the final surfaces of a component against a master data set. This gives Delta's engineers the ability to adjust the surface models that they generate to the original scan data. For most reverse engineering projects at Delta, they are dealing with relatively simple geometries. For example, if they are dealing with an inlet scoop on an engine cowl, they know that the basic shape is going to be around third order. They can start a project by using the software to sketch the basic shapes of all loft lines on top of the imported scan data and create profiles or surface patches by four curves to give them a starting point.

Using the surface analysis and diagnosis facilities in this software, they can then start adjusting the surface model to the scan data. After all of the basic surface patches have been created, they can then go through the model and start filleting all the corners and use the edges of the scan data to trim the surface patches to the digital edge-of-part.

"Actually, we use only a small portion of ICEM Surf's capabilities to do what we need to do," says Todd Herrington, who is responsible for reverse engineered surface modeling at Delta. "However," he adds, "to do these same operations with other CAD (computer-aided design) software products would be much more cumbersome and time consuming. We feel that this software gives us the best opportunity to maintain complete control during our decision-making process."

The final digital surface model of an MD88 aircraft door pan was created using data from a 3-D scan of the physical part. Source: Delta Air Lines

Quality assurance

Understandably, it is essential that Delta's engineers achieve the original manufacturer's specifications and tolerances and meet any applicable U.S. Federal Aviation Authority guidelines.

This software's deviation diagnosis functions are important when dealing with parts manufactured in-house from the reverse engineering process. The software provides facilities for interrogating the created surfaces in order to identify any deviation from the original scan data. A colored contour plot shows the difference, in millimeters, between nominal and actual values. Out of position and partly scanned data also can rapidly be positioned using a register function. This capability reassures the engineer that his surfaces and tolerances are correctly specified.

Other surface diagnosis functions that help Delta's engineers reduce design time are the ability to create sections and to examine surface-to-surface intersections. Because most commercial aircraft components have only gentle blends, the ability to approximate a cross section quickly with a Bezier curve and to arrive quickly at the most likely surface patch structure to suit the specific requirements are important capabilities.

The bottom line

In case of severely damaged components, without the reverse engineering and surface modeling and analysis facilities, Delta Air Lines would have little choice but to purchase new assemblies or to send parts out to third-party repair providers. This would increase the overall cost of replacement parts and could affect Delta's ability to meet its flight schedules because of the loss of control over the manufacturing process.

However, use of this software has enabled the Shop Support Engineering office to provide Delta's Technical Operations Center with the surface model data needed to keep the majority of replacement part manufacturing in house, and as a result, to deliver the quality and cost-effective maintenance and repair service needed to keep the airline's fleet flying.

From the point of view of the Shop Support Engineering office, it also enables them to keep control of the design-to-manufacture process to ensure that in-house manufactured replacement parts pass all regulatory and product quality requirements.