Loop resistance test on aircraft: ensuring safety in lightning strikes

A loop resistance test (loop resistance test) is performed during the production or maintenance of an aircraft to check that it is safe in the event of a lightning strike.

Why are loop resistance tests necessary?

Aircraft are struck by lightning on average once a year*, according to data from the US Federal Aviation Authority (FAA). That’s surprisingly often, isn’t it? It is much more common than most people think.

The good news is that standard aircraft are designed to withstand lightning strikes. A low-resistance path is integrated into the design, allowing lightning current to flow from the point of impact to the tail of the aircraft and be safely dissipated there.

The principle behind this design is quite simple. But a single connection with high resistance can disrupt the path of the lightning current, with potentially catastrophic consequences.

What can go wrong with a high-resistance connection?

A correct electrical connection is crucial to ensure aircraft and passenger safety. As mentioned earlier, just one connection with high resistance is enough to render a lightning protection circuit inoperable.

Worse, a high-resistance connection becomes the focal point of as much as 200 kA of lightning current seeking an outlet, which can lead to a dangerous situation.

High-resistance connections can occur for a variety of reasons, including:

• Surface contamination
• Improperly prepared connection surfaces
• Missing parts
• Defective materials
• Loose crimp connections
• Loose ring terminals
• Wrong specification bonding straps

Aircraft are complex mechanical and electrical structures with thousands of connection points and earthing circuits that must be tested to ensure a low-resistance path. A loop resistance test is therefore essential, but can be time-consuming and complex.

What is tested during a loop resistance test?

The various elements that together provide a low-resistance path for the lightning current are called a connecting circuit. These elements consist of aircraft structural elements, equipment enclosures, cable shields, piping systems and connections.

The complexity of these elements makes the effectiveness of test methods and validation of test results all the more important.

What is the best method for testing connections and loop resistance?

Simple electrical connections between two separate elements are relatively easy to test. Using the Kelvin measurement principle, bond meters allow a current to flow between the two elements, measure the voltage drop across the connection and report the resistance.

Is the Kelvin measurement principle suitable for all circuits?

This method is not suitable for testing circuits with parallel paths. Yet bond meters are often – and wrongly – used in such situations.

Why this is so is made clear in the example below (Figure 1). Two parts of an aircraft structure are connected by a series of bonding straps; however, one of the connection points is poorly mounted and forms an open circuit.

When this assembly is tested with the bondmeter described earlier, the parallel resistance paths allow current to flow between the meter’s measuring probes. The voltage drop is measured, but this voltage drop takes place across the parallel bonding straps. As a result, the measured resistance results in a sum of the parallel resistance paths, which can lead to the test accidentally registering an approval when it should have actually failed.

In a real lightning strike, with a current of up to 200 kA flowing through this system, this current would be forced through the faulty connection – with likely major safety consequences.

Types of tools for loop resistance testing

The most effective method for loop resistance testing is with a specially designed tool, such as the ‘LRT’ or MK Test Systems’ tools.

These tools all use the same technique: a current is injected into the loop via clamps (or ‘couplers’). The current is then measured as it flows through the loop. The voltage required for the current to flow is monitored, and the impedance of the loop is calculated.

By applying phase correction, the resistive element is isolated and the resistance of each individual loop can be reported.

In the example shown above (Figure 2), the loop test system would report the loop as an open circuit, allowing engineers to detect and correct the problem.

Different requirements of OEMs in loop resistance testing

Original manufacturers (OEMs) have different requirements when it comes to loop resistance testing. Maintenance operators should always consult the Aircraft Maintenance Manual (AMM) to ensure that the right tool and method are used.

Want to know more?

IONIO and MK Test systems offers a wide range of bond and loop test tools suitable for various applications:

ExLRT

The ExLRT is our intrinsically safe loop resistance tester, specifically designed for use in MRO (Maintenance, Repair, Overhaul) environments. This tester is more than 80% lighter than the industry standard LRT and is mentioned in Boeing maintenance manuals as an approved tool.
Visit the ExLRT product page here.

BLRT

The BLRT is a fully automatic bond, loop and connection resistance tester with built-in operator instructions. This tool speeds up the testing process by quickly diagnosing faults and automatically uploads test results to production systems once all checks have been completed. Boeing recommends the BLRT for testing joints, loops and couplings during the production of aircraft and aircraft components.

Visit the BLRT product page here.

BLTU4

The BLTU4 is suitable for use on Airbus aircraft. This is an automatic, portable bond and loop resistance tester with operator guidance. The BLTU4 performs automatic tests on earth points of aircraft and other large structures and records test data for full traceability.
Visit the BLTU4 product page here.

Comparison of models

Want to know how our ExLRT and BLRT models compare or do you have questions and are looking for more information? Feel free to contact us!