Aircraft Weld & Bond Inspection
GENEVA, OH, December 1, 2022
Lightning poses the most challenging and dangerous threat during an aircraft’s flight. It has been estimated that every aircraft in the U.S. commercial fleet is struck by lightning on average a little over 1 time per 3,000 hours. That translates to a little over once per plane per year. Surges caused by lightning can reach 200,000 amperes (200 kA) of direct current (DC). As long as the ground path remains intact, lightning typically enters the front of the plane or wing tips and exits towards the rear. It is therefore critical that no bonding sites exhibit resistance above 2.5 mΩ. Should a weld seam or bonding site record resistance higher than 2.5 mΩ, it would be above the industry standard. This could potentially compromise the ground path and therefore the safety of the aircraft.
Other potential impacts that can compromise the ground path include electromagnetic interference, decreased antenna performance, and current path hazards. This post covers more detail about mechanical bonds for Classes A, C, R, L and S (respectively, antennae, current path, electromagnetic interference, lightning, and static electricity).
Recent advances in milli-ohmmeter technology have resulted in convenient, hand-held portable instrumentation ideal for conducting these inspections. Well designed solutions provide an easy-to-read screen that displays the reading in mΩs while visually reporting the result as PASS or FAIL. Now a bluetooth enabled milli-ohmmeter assures high integrity inspections of critical aircraft assemblies. This advanced solution captures the crucial readings and seamlessly connects them to any software system from spreadsheets to Quality Management systems.
As the primary exterior of most aircraft, aluminum conducts electricity. As part of the environmental exposure to oxygen, the metal is also subject to oxidation. Aluminum oxide as opposed to pure metal is an insulator. To maintain the ground path of the aircraft, the structural integrity of rivets and weld seams must be maintained.
The more that aluminum oxide develops at any one location, the resistance increases as well. If the resistance of a mechanical bond increases beyond the required threshold of 2.5 mΩ, structural integrity is lost. This may compromise the plane’s ground path.
To test weld seams and bonds for aluminum structures, use a high-quality milli-ohmmeter. Because of their different electrical properties, composite structures require ultrasonic frequencies to detect voids and delamination.
Composites in aircraft production use an integral mesh in one layer to maintain the ground plane. The mesh also introduces resistance. To remain within standard, the composite structure also needs to be tested with a high quality milli-ohmmeter.
Lightning: The Most Challenging and Dangerous Impact
All calculations of lightning’s electrical impact are based on a maximum resistance of 2.5mΩ within the ground path. The energy in a typical lightning strike equals 200 kA (200,000 Amps) x 2.5 mΩ equals 500 V (volts). An intact ground path conducts the lightning from the impact area, usually near the front of the aircraft or the wings to exit towards the rear.
The last confirmed commercial plane crash in the U.S. that was directly attributed to lightning occurred in 1967. The lightning caused a catastrophic fuel tank explosion. If the ground path is compromised and fails, potential energy greater than 500V could result in the following dire impacts:
- Sensors may not record properly.
- Data may be corrupted and/or not transmitted accurately.
- Communications may be garbled.
- Instruments may display one or more false positives.
- Controls may be mis-activated.
- And for military aircraft, ordnance may be activated
Preventing Ground Path Issues
To test resistance at aircraft bond sites requires a Digital Low Resistance milli-ohmmeter (DLRO). This method measures resistance elements like seam welds and rivets to identify any location with resistance exceeding the 2.5 mΩ standard. If any location records more than the industry standard 2.5 mΩ, it compromises the ground path of the aircraft and must be repaired. Left uncorrected, a lightning strike may result in some or all of the extreme consequences noted above.
Electromagnetic Interference (EMI)
Besides lightning, other sources of EMI include radar; ground, air and satellite links; mobile phones and solar flares. The frequency range that can trigger interference ranges from 30 MHz to 30 GHz. As a result, aircraft designs include electromagnetic shielding.
However, electronic shielding degrades over time. Since electrical bonds are most subject to EMI, check those points first. In the chart below, note that the resistance of electrical bonds changes depending on the EMI frequency. To test the resistance of these bonds again requires a milli-ohmmeter.
Static Electricity: Danger of Explosion
Many sources contribute to increased static electricity. These include precipitation, air or dust friction, fluid flow electrification, charged clouds. Even the human body can carry a charge of up to 0.25 mJ. It takes a surprisingly small amount of energy to ignite fuel vapor, as little as the static electricity from human contact. Using an accurate milli-ohmmeter on bond sites and weld seams will identify any site that exceeds the threshold limit of 2.5 mΩ. If none exceed this limit, the risk of a static electricity discharge is greatly reduced. Note that threshold limits vary depending on application.
The above scenario can be described using the formula:
U = ½ cv² where U = energy stored, c = capacitance, and v = voltage, so…
½ x 0.1µF x 71V² = 0.25mJ = BOOM!
The ground plane produces counterpoise for the aircraft’s antenna. If the ground plane is poor, it steals transmitter power and increases the voltage standing wave ratio (VSWR), reducing the antenna’s range. A high-quality, accurate milli-ohmmeter is a shortcut to conduct a quick resistance test to determine that the bond is below 2.5 mΩ. It’s worth noting that the probe design used with the instrument matters; an unwieldy probe design makes it difficult for technicians to access the antenna mounting hardware.
Return conductors act as power sources. Electrical surges develop when large power loads are activated. This creates a potential fire and arc hazard. Large power loads on an aircraft occur when any of the following systems are activated:
- Fuel pumps
- Air conditioning
- Wing de-icer
- Landing Gear
- Engine startup
The following chart compares the fault current versus resistance.
If the maximum current shifts into fault current, one or more arcs can ignite fuel, onboard equipment won’t function, and breakers don’t trip on FAULT. Test the current path with a high-quality, high accuracy milli-ohmmeter.
The TEGAM 730A milli-ohmmeter has been designed and produced by the low resistance experts at TEGAM. The company is the largest manufacturer of bond meters for airframe testing in both military and commercial applications. They are also the only bond meters made in the U.S.A.
The 730A delivers a high accuracy measurement – ± (0.2% Reading + 0.02% Range). With designed-in Bluetooth, the 730A captures crucial readings and via the seamless data connection, exports results to ANY software system from spreadsheets to Quality Management solutions. This functionality eliminates the integration headache. In addition, a free download is automatically included with advanced new tools supporting iOS, Android, and Windows.
Drawing on our experience, we designed the smallest size bond meter available today. The 730A is portable with a hand-held ergonomic design, for easy one-handed operation. The design integrates our unique low-power techniques to eliminate power cords and battery chargers. The 730A runs on 3 AA batteries for up to 100 hours. The easy-to-read screen includes a battery health indicator to provide ample warning for replacement. The design also Includes an auto power-off to extend battery life.
The 730A has a range from 10 milli-ohm to 100 ohms, a resolution of 1 micro-ohm with a PASS/FAIL indication. Each instrument includes a 3-year warranty on Parts and Workmanship plus a 1-year calibration guarantee.
The handheld 730A provides a major testing advantage for producing reliable measurements in tight spaces found in aircraft.
Aircraft flight safety depends on the integrity of the ground path, crucial to prevent failure modes due to hazards. Ground path failures can be caused by lightning, EMI, static electricity, antenna bonds, and current path. Since commercial aircraft are constructed of aluminum, riveting and weld seams are crucial to achieve and maintain structural integrity. This in turn results in a high-integrity ground plane with resistance below the 2.5 mΩ limit.
The highly accurate handheld TEGAM 730A milli-ohmmeter provides the solution. It captures these critical readings and eliminates all manual data recording and entry into spreadsheets or Quality Management systems. The ergonomically designed bluetooth enabled instrument fits enables seamless data capture using only one hand. As the only milli-ohmmeter designed and manufactured in the U.S.A., it delivers the excellence, accuracy, and reliability necessary to assure aircraft flight safety.
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