Noncontact Triage for Aircraft Damage Inspection

One click measures, coordinates, and models

Published: Wednesday, January 19, 2011 - 04:00

Because damage inflicted on aircraft can affect structural integrity and radar signature, specific aircraft types are inspected to triage damage and define repair actions. To radically improve current manual damage-identification practices, metrology specialists from Maryland-based SURVICE Engineering are integrating Metris iGPS, a noncontact, large-scale metrology solution, as the core of a turnkey damage-inspection solution. Metris iGPS quickly and accurately acquires locations and characteristics of aircraft damage. The spatial coordinates of visual damage are instantly tracked and marked on a digital 3-D aircraft model, simultaneously stepping up process accuracy and efficiency.

Consistent damage interpretation

Aircraft incur structural and other damage from a variety of sources. Currently, aircraft damage is detected through visual inspection or with the aid of handheld, nondestructive inspection devices. Regardless of the inspection method, capturing exact damage location and spatial orientation is critical for determining the effects on aircraft capability and accurately cataloguing damage for future reference. Typically, hand measurements are taken relative to structural features such as fasteners, doors, and panels, and are then manually noted and archived on paper. Using photographs and tape or caliper measurements makes this practice inaccurate, labor intensive, and incompatible with digital data-processing requirements.

To improve the speed and quality of aircraft damage-inspection procedures, SURVICE Engineering researched potential technology-enabling solutions and identified Metris iGPS as the metrology solution suitable for inspecting aircraft surface damage. “The principal asset of Metris iGPS is the capability to create a local, GPS-enabled environment using two or more iGPS transmitters,” explains Mark Butkiewicz, manager of SURVICE Metrology.

A proof-tested solution

Aircraft inspection personnel need a robust solution that is easy to operate, practical to carry around, and based on a large inspection volume that can cover aircraft measuring 20 m by 20 m (65.6 ft by 65.6 ft). Successfully proof-tested in a major aircraft program, the technical capability of commercial Metris iGPS hardware coupled with software developed by SURVICE Engineering confirms valid measurement and provides a controlled digital data flow, regardless of indoor or outdoor use.

Through the use of custom software, data are immediately translated, overlaid onto the aircraft CAD model, and made available to analysis tools for real-time processing. “The ability to use the Metris Software Development Kit (SDK) to dynamically interface with the iGPS hardware and develop custom solutions for our clients is one of the key elements that has led to the success of this program,” says Butkiewicz.

Figure 1: iGPS receivers incorporated into probe stay within reach of transmitters surrounding the aircraft.

How it works

To begin the process of locating damage, an inspection officer sets up the iGPS system near the aircraft. After calibrating the system by using fixed reference points on the aircraft, the software loads the geometric aircraft model and orients this model into the iGPS field coordinate system. Based on triangulation of transmitter signals, the iGPS system determines the location of the probe tip operated by the officer. Any location the officer measures on the aircraft is automatically superimposed to the same location onto the geometric model. Automatic mapping establishes accurate damage cataloguing and helps determine threat trajectory and radar cross-section changes. In this way, details of external damage and the direction of the threat provide useful information regarding potential internal damage.

“The system is truly portable and entirely battery operated,” says Butkiewicz. “It includes two environmentally sealed Metris iGPS transmitters mounted on tripods. The inspection officer operates the lightweight touch probe unit (including receivers) and wears a dedicated belt equipped with iGPS peripherals to wirelessly stream geometric data to a laptop computer. One click on the probe button is sufficient to take a measurement, acquire the 3-D coordinates, and visualize the location on the digital aircraft model. The system robustly operates inside aircraft hangars as well as on flight lines, carrier hangar decks, and forward-deployed areas. We are intimately familiar with Metris iGPS, since we successfully used it at diverse, remote locations, with weather conditions ranging from hot desert to subfreezing temperatures.”

More transmitters can be added to increase the size and accessibility of the GPS-enabled area, and multiple operators can run probe measurements simultaneously to compress inspection time. Using a set of extensions, the probe can be lengthened up to approximately 1.8 m (5.9 ft). Because the sensors are in the aft part of the probe bar, the tip can be placed deep inside the aircraft structure without compromising the signal. As long as the probe sensors are within visual range of at least two transmitters, Metris iGPS is able to perform a valid measurement. In a two-transmitter setup, 1 mm (40 mil) precision is guaranteed at 50 m (164 ft); in a configuration with more transmitters, accuracy increases to 0.4 mm (16 mil). At these levels, the system meets or exceeds any accuracy requirements that might be imposed.

Future trials

Measurements showed that the iGPS technology easily meets location, length, shape, and orientation requirements of damage, including scratches, gouges, bubbles, holes, and cracks. For accuracy requirements related to depth, height, and width characteristics of local aircraft damage, however, another form of measurement is required. During the second stage of this funded research and development (R&D) program, Metris K-Scan’s ability to execute laser scans of individual damage items will be evaluated. Although the position and orientation of the K-Scan device is dynamically tracked by a portable Metris Optical CMM unit, the handheld K-Scan acquires a detailed scan of the local damage area. The acquired cloud of hundred thousands of points features individual point measurement accuracy down 25 micron (1 mil). From the 3-D surface created on the basis of this point cloud, the exact depth, height, and width characteristics of a damage feature can be quickly retrieved.

The second phase of the funded R&D program focuses on optimizing the aircraft damage inspection solution. “In general, the project team will further streamline system set up, calibration, and alignment through simplified hardware, better user prompting, and more intelligent software,” Butkiewicz says. “The potential will be investigated to use the Metris iGPS system to improve time-critical damage inspection routines. The iGPS system can be set up and used within this time slot and will help determine suitability for flight. In general, this capability will save time on identification and planning of repairs by quickly providing the needed information in an electronic format suitable for expediting the entire process.”

About The Author

Nikon Metrology Inc.

Nikon Metrology Inc. offers the most complete and innovative metrology product portfolio, including state-of-the-art vision measuring instruments complemented with optical and mechanical 3-D metrology solutions. These reliable and innovative metrology solutions respond to the advanced inspection requirements of manufacturers active in aerospace, electronics, automotive, medical, consumer, and other industries. Systems are available in both fixed and portable configurations, equipped with single or multiple sensors.