In the summer of 2002 a China Airlines 747 en route from Taipei to Hong Kong disintegrated midair and crashed into the Taiwan Strait killing all 225 people on board. The deadliest crash in Taiwan’s history, the accident was later attributed to metal fatigue stress, cracking in the fuselage.
When the Taiwanese crash investigation team examined the fuselage they discovered the origin of the cracking was a faulty repair to the plane conducted after a tail-strike incident two decades earlier. According to the report, the maintenance crew had neglected to follow guidance from Boeing’s structural repair manual, which required the removal of the damaged skin and the installation of a doubler plate that extended beyond the damaged area. A measurement error in the dimensions of the doubler plate led to the propagation of fatigue cracks that over time compromised the aircraft’s structural integrity and caused the crash.
The incident is an example of the potentially fatal consequences that come from lapses in metrology. In aerospace, accurately measuring dimensions during production and maintenance is a critical aspect of safety.
“It’s about quality control and passenger safety,” says Jörg Deller, general manager of stationary metrology devices and machine tool measurement at metrology device manufacturer Hexagon Manufacturing Intelligence. “We want to know if parts are doing what they are supposed to be doing. For example, if an engine plate is shaped in the right way with the correct angle so that it can perform as it should. Or if two plates are set alongside one another are they measured accurately enough that they don’t create vibrations.”
ORIGIN STORY
In addition, metrology is important for traceability, the process by which aircraft makers determine the exact production and maintenance history of an aircraft part.
Some parts are critical for flight safety, such as engine parts and turbine blades and must always be inspected for dimensional integrity. In reality, says Deller, “everything that is necessary for the safety of airplane operations should be submitted sooner or later” for metrology testing, including structural parts.
The most common instrument for conducting high accuracy parts measurement is the Coordinate Measuring Machine (CMM), a device that typically uses a physical probe to precisely measure the dimensions of an object by sensing discrete points on its surface. High-end CMMs can reach accuracy levels of less than a micron, equivalent to one-thousandth of a millimeter.
Florida-based FARO Technologies’ range of CMM devices are widely used in aerospace for a range of applications that include “jig and fixture calibration, deflection testing, assembly tolerance stack-up, and as-found condition documentation,” says Les Baker, senior field application engineer at FARO, an AMETEK business. “These tools help to ensure dimensional integrity across every stage of production and maintenance.”
FARO’s CMM solutions are used “across every type of aerospace component and material, from composites and turbine blades to fuselage structures, nacelles, cowlings, shrouds, landing gear, aerodynamic surfaces, stabilizers, and jigs”. They are also commonly applied to engine components, simulators, satellites, rockets, helicopters, and drones and can handle a range of metals and alloys.
PORTABLE AND STATIC
According to Deller, there are two categories of CMMs: stationary and portable. Portable CMMs are lightweight and handheld and can be subdivided into two types: articulated arms and laser trackers. The articulated arm CMM usually has a seven-axis arm that either has a touch probe or laser scanners or both, and you “manually scan the part,” says Deller, adding that the device has limited range because you can only measure what you can reach with the arm.
Laser trackers use a laser beam to measure the 3D position of a target and unlike articulated arms, they are unrestricted in reach. For this reason, they can be used to measure very large aircraft structures like wings, fuselage sections, jigs, and engines.
Where the tolerances of aircraft parts are tighter, stationary CMMs are favored over portable devices for their high-accuracy and repeatability. There are two main types of stationary devices: bridge CMMs and gantry machines.
Deller describes the bridge CMM as the “work horse” of the metrology world. It is composed of a three-axis linear robot with a two-axis wrist at the end fitted with a tactile sensor for measuring the part and mounted on a granite table. A gantry machine, meanwhile, is “similar to a bridge CMM but way bigger”, he says. Gantry machines can reach three to 4m (10 to 13ft) in width, 3m (10ft) in height, and up to 20m (65ft) in length.
LOSING TOUCH
Traditionally, metrology was always carried out using tactile measurement, which means that the CMM probe physically touches the part it is measuring. In recent years, however, advance in laser technology has made it possible to conduct non-contact measurements. “For tactile measurement you move to the part and measure around it. It is the most accurate thing you can do, but it’s not the fastest,” says Deller.
The other shortcoming of tactile measurement is that in the case of aircraft materials such as titanium or carbon fiber, there is a danger that the contact of the probe on the material can leave behind scratches, which in the case of high-accuracy measurements could impact accuracy levels.
Portable laser trackers offer a non-contact alternative to tactile probes and are more flexible, says Deller. “A traditional horizontal arm CMM is limited by the size of what fits in it,” he says. “But the laser tracker can measure up to 50m away.”
In any case, most modern CMMs provide multi-sensor integrations that include both laser scanning and tactile measuring technology. “Historically, tactile CMMs were favored for precise geometric features, while laser scanning was used for complex freeform surfaces,” says Baker. However, advances in hardware and software “have largely eliminated these boundaries,” he adds.
Hexagon’s MAESTRO device, for example, is an all-digital, networked CMM system that can switch between tactile probing and non-contact laser-based inspection. Similarly, FARO’s FaroArm CMM offers integrated tactile probing and a laser scanning solution in a single device. “The current generation allows dual mounting points for scanners or probes, with three different scanner options offering tailored resolutions and fields of view,” says Baker. “This flexibility enables engineers to probe high-accuracy geometric datums while scanning complex freeform surfaces, capturing both types of data efficiently in one seamless workflow.”
ENVIRONMENTAL CHALLENGES
Since aerospace metrology requires measuring to such high degrees of accuracy, tiny environmental changes can jeopardize the results. The environmental factors that are most critical to take account of are temperature and dirt in the atmosphere.
Dirt that settles on either the machine or the measured part can lead to inaccuracies. Temperature differentials, meanwhile, can expand or contract the dimensions of the part. Granite is used for the tables of stationary CMM devices in part because it is unaffected by temperature variations.
When the test environment is controlled, bellows can be fitted to remove dirt from the air and the temperature can be precisely controlled. But for measurements conducted on the shop floor the CMMs integrate “temperature compensation technologies and environmental monitoring directly into the hardware,” says Baker, allowing quality assurance engineers to “achieve lab-level precision without sacrificing production flexibility.”
Since their products are often so specialized, when OEMs purchase one of Hexagon’s CMM devices the company frequently offers training in how to use them. “If it’s a new user, we do training about how to use the machine and the software,” says Deller. If they are complete novices, Hexagon will even provide training on what metrology is and how it is used in aerospace.
The company also offers paid programming services, and turnkey solutions that include installation and programming. Hexagon also conducts annual services of its equipment on site to check that it still fits the specifications and to guard against wear and tear.
INTUITIVE SOFTWARE
Advances in software have made it easier for those without specialized knowledge to CMMs. “It has transformed inspection from a specialized discipline into a more accessible, automated, and data-driven process, enabling engineers to focus on actionable insights rather than programming challenges,” says Baker.
FARO points to its CAM2 Software, which is purpose-built to work with the company’s FaroArm and Laser Tracker systems. “CAM2 streamlines the entire inspection process through an intuitive interface that reduces operator training time and programming complexity,” says Baker. “Its model-based definition (MBD) capabilities allow users to program directly from CAD data, while real-time feedback validates measurements instantly against design intent.
“These capabilities mean that complete inspection routines can now be created offline, simulated before execution, and then run confidently on the shop floor.”
MATERIAL DIFFERENCES IN METROLOGY
The kind of material that an aerospace part is made from can significantly impact how you measure it. Aluminum, for example, can pose challenges for laser tracking devices since its shiny surface can reflect a laser beam around a part, leading to noisy data.
Composites’ internal fiber structures or residues at drilled holes can contaminate a CMM’s probe stylus. To counter this problem, engineers will often prepare composites surface with a spray or matte coating.
Aluminum and composites also have low stiffness thresholds meaning that when they are clamped for measurement, the clamp force or the probe contact can slightly deform the part, leading to measurement error. Test engineers can get around this problem either by using very gentle fixturing, or by switching to a noncontact CMM device.
These issues can exacerbate with larger parts, with parts made from lighter weight alloys and composites in danger of flexing under their own weight as they get bigger. Moreover, certain exotic materials, such as the Inconel, which is used in turbine blades or the carbon-carbon composite used in spacecraft heat shields, behave less predictably and for this reason can be harder to measure.
TOOLING UP FOR METROLOGY
CMMs are the high accuracy backbone of aerospace metrology, handling the most critical measurement checks. But CMM devices are expensive and sometimes time-consuming to deploy.
There are many other metrology requirements within the industry that do not need the same degree of accuracy and can be serviced by more basic measuring tools. At the more rudimentary end of the scale are shopfloor gauges such as digital calipers, micrometers, and bore/ height/thread and pin gauges.
Deployed directly on the manufacturing floor, these gauges are far more ruggedized than CMMs and what they lack in accuracy, they make up for in ease-of-use and durability, allowing operators to check critical dimensions quickly at the machine, rather than having to send parts to an inspection laboratory.
There are also certain metrology tasks for which CMMs are not practical, such as the measurement of very large parts, or parts that are complex in shape, such as certain curved surfaces. In this case, photogrammetry systems are commonly used. Photogrammetry is an image-based measurement technique that uses photographs to calculate precise 3D coordinates of points on an object.
Like some of the portable CMM devices, photogrammetry systems are handheld and can be easily used in the field during production. The technology’s advantage over laser trackers is that it can capture many points at once. During the assembly of large-scale structures like wings or aircraft fuselages, photogrammetry kits can be used to locate reference points, check rib and spar positions, verify jig alignment and to validate aerodynamic surfaces.
