top of page

Curlin Air

The Curlin Air - Air Coupled Composite Bond Tester & Ultrasonic Flaw Detector

The Curlin Air - A High Performance, Non-Contact, Air Coupled Ultrasonic Flaw Detector & Composite Bond Tester manufactured by NDT Systems.

 The amazing penetrating power allows the inspection of many materials/structures not normally considered testable by conventual NDT methods.  No need for couplant avoids contamination of surfaces and allows for the testing of porous materials.

Contactless Air-Coupled Ultrasonic Flaw Detector

The portable Curlin Air utilises air-coupled ultrasonics and requires no surface preparation and no direct contact with the test object, meaning no couplant for porous surfaces.  It performs sensitive inspections to detect voids, impact damage, and disbonds in a wide variety of water-incompatible materials.

 Originally used primarily for aerospace composites, the Curlin Air is referenced specifically by major aerospace manufacturers in their published inspection procedures.  It has since evolved to evaluate a diverse range of materials including testing propellants, wood and foam panels.

 Air-coupled ultrasonic techniques are used when testing sensitive materials which are at risk of contamination during the testing processes or in situations when the cost of removing special coatings from the material to be tested is impractical. Wetting agents or powders typically used in conventional ultrasonic testing can lead to degradation of sensitive components.

Curlin Air Flaw Detector - Advanced NDT LTD

Curlin Air Key Features:


High Penetration air-coupled ultrasonics for attenuating materials

RF display mode with flaw gate and alarm modes

Non-contact, no surface preparation

Robust aluminium construction with rubber end caps

Remarkable penetrating power

Alarm modes

Programmable user set-ups

Custom Yokes for easy alignment of probes

NDT Systems - Curlin Air Ultrasonic Flaw Detector & Bond Tester.

As the use of adhesively bonded structures has increased across many industries, the need & importance for testing bond integrity has grown.  Multiple glue lines, foam sandwich constructions with honeycomb cores, bonded carbon fibre composites and even wood have all become important in manufacturing.  The integrity of these bonds is critical to the quality & safety of the final product. 

 Conventional ultrasonic methods can be limited for these applications and so a variety of alternative methods have been developed to handle this range of material combinations.

The CURLIN AIR, Airborne Ultrasonic Flaw Detector was initially developed to ultrasonically inspect an ever growing class of materials/products that are too attenuative to inspect with conventional (Megahertz Frequency) Ultrasonic Flaw Detectors.

 However, because of the Curlin Air's non-contact airborne beam feature, the Curlin Air, Airborne Ultrasonic Flaw Detector, also, offers applications advantages for certain materials/products that are routinely inspected with conventional equipment.  Refer to the Application Notes (AN) which are published on an ongoing basis for the Curlin Air Airborne Ultrasonic Flaw Detector.

Curlin Air Applications

General Applications:

Integrity of Composites and adhesively bonded structures

Inspections on highly attenuating multi-layered structures including carbon-carbon, wood, foam, glass fibre / carbon fibre composites and honeycomb

Delaminations, disbonds, skin to core flaws, crushed core, voids, Impact damage, Liquid Ingress

Foam core laminates

Graphite Composites

Far-side defects

Bonded honeycomb structures

Construction materials: Drywall, plywood, particleboard & cement

Structural urethane foam

New & Re-Tread Tyres

• Far Side Defects

Unique Applications:


The Curlin Air has become synonymous with specialised inspections including:-


• Carbon-Fibre sandwich with honeycomb core: Aerospace application for control surfaces, thrust reversers, cargo doors. Inspecting for skin-core disbonds or damaged core.

• Carbon-Carbon:

Space application used for heat shields. Inspecting for delaminations, disbonds in highly porous material.

• Metal sandwich with foam core:

A typical construction used for fire doors and walls. Inspecting for voids in the foam core.

• Solid Foam: Inspecting for cracks or voids internally.

• Drywall: Used extensively in the construction industry.


• Multiple Core Sandwich:

Marine composites application with multiple core sheets and glue lines between. Inspecting for skin to core and core to core disbonds

Curlin Air Applications.png
Curlin Air Applications.png
Curlin Air Specifications.png

High Penetration Ultrasonics

The Curlin Air is a non-contact, through transmission, air-coupled ultrasonic flaw detector.  With an operating frequency of 50kHz, which is very low compared to conventional ultrasonic testing, the system is able to detect anomalies in the sound path through even the most attenuating materials.

 The Curlin Air requires no surface preparation and can penetrate up to 6 inches (150mm) of foam, wood, honeycomb & other materials with a high air volume ratio.  A transmitting and receiving probe are positioned on opposite sides of the test object by means of a mechanical yoke or a pair of magnetic wheeled carriers.  The live display shows instantly if the sound path is interrupted by a defect in the material.  Flaw gates and alarms can be programmed and stored for each application.


• The Curlin Air - Air-Coupled Composite Bond Tester & Ultrasonic Flaw Detector weighs 6.3 lbs. (2.9 kg) including batteries. • Has an easy-to-read ultra-fast LCD display with A-trace.

• User-friendly menu structure with context-sensitive help screens for every menu item.

• Membrane push button controls.

• Direct access to range, delay & gain controls, audio / visual flaw alarms, as well as various menus through a new direct access keypad.

• Features 115db gain.

• Uses 5 'D'-size alkaline or rechargeable Ni-cad batteries for about 24 to 8 hours, respectively, of typical use (with optional • LCD back-light off).


Standard package includes instrument, Pelican style shipping case, manual, batteries and AC charger

Remarkable penetrating power

Through transmission, non-contact

A-trace display

Alarm: adjustable trigger level, audible and visual

Adjustable range, delay and gain

Options Include: Transducers, Yoke Adaptors, Magnetic Wheeled Carriers

Curlin Air - How it works & ultrasonic principles.


• No Liquid Couplant Needed

• No Surface Contact

• Portable

• Analogue Output for Use With Data Acquisition Systems


In order for the Curlin Air Ultrasonic Flaw Detector to perform, the following major technical issues needed to be addressed:

  Test Frequency: A frequency of 50KHz (having a wavelength in air of 1/4") was chosen because it was low enough so ultrasonic attenuation (scatter and absorption) is greatly reduced to levels which permit ultrasound to readily propagate through both air and the categories of materials targeted for inspection, yet high enough so satisfactorily small-diameter, well-collimated ultrasonic beams can be generated by acceptably small-sized probes (transducers).


The 1.9" OD Model ATI Probe has an effective beam diameter of about 1 3/8" at the probe face, with a beam-spread of only about 91/2° (half-angle at - 20dB).


Tone Bursts: Generate pulsed ultrasonic energy in the form of rapidly reoccurring tone bursts (cycle packets at the 50KHz test frequency) which possess the necessary duration (pulse length) and amplitude to deliver the desired "penetration power", yet eliminate/minimize standing wave interference.

Sensitivity: Provide the high levels of low-noise adjustable amplification needed (up to 100dB or 100,000X) to compensate for the large amount of ultrasonic energy lost by reflection at both material surfaces (airborne ultrasound experiences an exceptionally large acoustic impedance mismatch at air-solid interfaces - far greater than the liquid couplant-solid interfaces experienced with conventional flaw detectors).

Special Probes (Transducers): Provide exceptionally effective energy coupling to transfer satisfactory levels of ultrasound into and back out of the air.  An easy way to initially visualise what's happening ultrasonically and why flaws are detectable is to take a "trek" along with an ultrasonic tone burst (longitudinal wave mode) as it travels from the transmitter probe, through air to the material being inspected, through the material and eventually through air again to the receiver probe.

The Ultrasonic Trek

• Transmission Into Air:

 Initially, a portion of ultrasonic energy is lost (due to the transmitter probe/air impedance mismatch) as the tone burst is initially launched into the air.

• Airpath To Material: Once launched, the tone burst travels through the air and loses further energy due to attenuation (absorption) and beam-spread (diffraction). For example, about 3dB of tone burst amplitude is lost by travelling across an airpath of 6". (There is also a "complex" radiation field, called the Near Field, which extends about 1.9" in front of the Model ATI transmitter probe).

• Entry Into Material: A large amount (over 99%) of the incident tone burst energy is lost due to reflection at the material surface, with only a small amount of the ultrasound actually entering the material. This reflection loss is caused by the extremely large acoustic impedance mismatch at the air-material interface. The acoustic impedance mismatch can be thought of as a "valve or shutters" which determine how much ultrasound is permitted to cross the interface (a perfect impedance match allows all the ultrasound to pass-no reflection).

• Travel Through Material: As this weakened tone burst travels through the flawless solid material, it experiences further energy loss caused by attenuation (both absorption and scattering, in the case of a solid), as well as by additional beam-spreading.

• Exit From Material: The remaining burst now experiences another huge energy reflection loss (again over 99%) due to the same excessive material-air acoustic impedance mismatch as was experienced at the above-mentioned entry surface. At this point, the tone burst has lost more than 99.9% of its energy due to only the two surface reflections.

• Airpath To Receiver Probe: After exiting the material, the now extremely low energy tone burst experiences further attenuation and beam-spread loss as it travels along the airpath to the receiver probe.

• Reception From Air: Finally, when this low-level tone burst impinges on the receiver probe, an additional energy loss is experienced during its ultrasonic transfer to (ultrasonic activation of) the probe.

• Impact Of Material Flaw: Ironically, while impedance mismatches at the two material surfaces can be considered energy-stealing enemies, the large air-material mismatch caused by a flaw (delamination, split or cavity) becomes a great ally. Basically, the tone burst experiences another similar 99%-plus energy loss when it impinges on a flaw (assuming the flaw is large enough to intersect the complete beam). Even"pressed together" delaminations at this test frequency (50KHz) will still reflect 99%-plus of the signal energy (because there is a microscopically thin layer of air still present at the flaw site). Thus, the flaw "blocks" a huge percentage of the normal (flawless) tone burst energy - making the flaw readily detectable.ENERGY AND AMPLITUDEThe above-mentioned ultrasonic energy (energy intensity) losses are electronically detected in terms of overall signal amplitude losses. Signal amplitude is related to the square root of the energy intensity.CURLIN AIR, Airborne Ultrasonic Flaw Detector COMPENSATIONIn the "real world", many Inspectable products create actual energy intensity losses which require compensatory signal amplitude boosts by the receiver amplifier that are in the range of 1,000x to 10,000x (60 to 90dB). The gain required mainly depends upon the product's acoustic impedance, attenuation and thickness

bottom of page