The new generation of milliohm measuring instruments for military and civil aviation: the isolated milliohm multimeter METRAHIT IM XTRA

Aircraft discharge brush test

Aircraft discharge brush and lap testing in aircraft overhaul

In an age of ultra-modern flight control and navigation technology for civil and military aviation, ensuring flight safety remains a top priority. Regardless of any internal and external influences, electrical systems must operate reliably and without errors. Air disasters can quickly lead to disaster. There is no compromise on safety even in severe weather conditions such as extreme cold, wind and snow, hail and thunderstorms. This presents a significant challenge for the onboard electrical and electronic systems (avionics), although the aircraft is equipped with a fully conductive enclosure that functions like a Faraday cage – meaning that in principle adequate protection against lightning strikes is provided hit.

Day in and day out, aircraft are subject to all conceivable environmental impacts such as electromagnetic fields (radar, radio/television, cosmic radiation), bird strikes, storms, hail, humidity, extreme pressure and temperature differences, and lightning.

Lightning strikes – damage and failure of equipment and systems

Environmental Impacts – Shock, Vibration, Radiation, Bird Attacks

Static buildup – interferes with communications and navigation systems

Temperature Fluctuations – Condensation Caused by Extreme Differences

Corrosion – reduced conductivity, corrosion damage

Aircraft discharge brush test

Danger from lightning

When two surfaces rub against each other, electrons are transferred from one surface to the other. In flight, the plane is constantly rubbing against the atmosphere. This friction causes static charges to build up within the fuselage. The excess electrons tend to accumulate on the thin trailing edges of the aircraft, such as aileron tips, rudders, elevators, flaps, etc. In addition, the fuel tank is also a high-risk area for static electricity. During the various flight processes of the aircraft, the fuel in the fuel tank keeps shaking and rubs violently with the fuel tank wall, which will also generate a large amount of static electricity. Because hydrocarbon fuel is an electrical insulating medium, the conductivity coefficient is small. Static electricity gathers quickly and disappears slowly. If the electrical charge is not uniform across parts of the aircraft, there is an increased likelihood of sparks, which could result in a fire.

Static charges can also cause interference with aircraft radios. To prevent this, the aircraft is equipped with so-called discharge brushes. This provides a means for the excess electrons to flow back into the atmosphere rather than stay inside the fuselage. This avoids static build-up. The ends of the discharge brushes look like brushes with very little resistance, making it easier for electrons to escape. In addition, accessories that easily accumulate static electricity, such as the fuel tank, need to be connected to the metal casing to transfer the static electricity to the discharge brush and finally release it.

Aircraft Overhaul Lap Test

test task

Whether an aircraft is used for military, public or private purposes, the outer shell, consisting of riveted, lightweight sheet metal parts, must be regularly tested for sufficient electrical conductivity to ensure that even in the worst case scenario, i.e. being struck by lightning, it It can also continue to act as a Faraday cage. If the impedance of the enclosure is not sufficiently reduced, partial interruptions may occur in the event of high lightning currents, resulting in failure of the Faraday effect. The lap test is the measurement of the resistance of each connection and the measurement of the connection resistance of part of the casing and the discharge brush.

Therefore, the test requires that the instrument must detect resistances of only a few milliohms and be able to record the test value. Since the discharge brush consists of a series of discharge paths, it is also necessary to test the correct function of the spark arrester (after the low impedance measurement) with the help of an insulation tester (METRAHIT IM XTRA or METRAHIT ISO AERO).

Insulation Milliohm Multimeter METRAHIT IM XTRA

The KELVIN measurement method is suitable for lap and discharge brush testing, also called 4-wire measurement method (voltage correction measurement method). All contact resistances and test cable resistances that would otherwise add error to the final measurement are compensated by this measurement method.

previous solution

Ohmmeters powered by large batteries, typically used for lap testing in the past, are also much more expensive due to their bulky and relatively heavy weight and mechanical complexity. They provide 1 or 10A of test current (pulsed current), while our instruments only provide 0.2 or 1A (pulsed) of DC test current, which is sufficient for most applications.

Forward-looking solutions from GOSSEN METRAWATT

Insulated milliohm multimeters METRAHIT series multimeters provide users with extremely light, compact and rugged, battery powered, high precision measurement and test instruments. These features are an important advantage for field use. In addition to this, the relatively low purchase price offers a compelling economic advantage.

Failure detection is important when servicing avionics:

The insulated milliohm multimeter METRAHIT IM XTRA from GOSSEN METRAWATT is an all-in-one handheld multimeter for non-hazardous areas. In addition to milliohm measurements, it includes insulation resistance measurements and all the features of a high-quality true-rms multimeter with temperature measurement. , due to its multiple measurement capabilities, it is considered an ideal solution for avionics service applications. The standard IZYTRONIQ evaluation and management software makes the documentation of measurement results even easier.

Field Verification

For many years, leading aircraft manufacturers have been using the METRAHIT 27EX to detect contact resistance in explosive components (fuel tanks). METRAHIT 27 has long been used extensively by professional service companies, aircraft manufacturers, airfields operated by airlines, such as: Lockheed Martin (F16 and F18 fighter service kits), Lufthansa airfields, a service company in Norway, etc. .

METRAHIT IM XTRA is a replacement for METRAHIT 27. It includes all the features of the time-tested METRAHIT 27, plus useful state-of-the-art enhancements.

Reviewing Editor: Tang Zihong

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