Vibration tester enhances jet engine safety

Snecma and Bruel and Kjaer have developed a Pulse-based, state-of-the-art portable system to perform ground vibration tests on Snecma M53 and Larzac gas-turbine engines.

Snecma is one of the world's leading aerospace corporations, specialising in propulsion, equipment and associated services.

Snecma customers include Airbus, Arianespace, Boeing, Dassault Aviation, Eurocopter and other leading aerospace companies throughout the world.

The Snecma group is active in the military aviation sector, providing engines, equipment and services for all types of aircraft.

More than 50 aircraft types deployed by air forces around the world use Snecma products.

Snecma Moteurs and Bruel and Kjaer have been co-operating for more than three years on a project to design a Pulse-based, state-of-the-art portable system to perform ground vibration tests on Snecma M53 and Larzac gas-turbine engines.

The Snecma group is highly active in the military aviation sector, providing engines, equipment and services for all types of aircraft combat, training, support, transport and special-mission.

Snecma's military aircraft engines are known for their reliability, carefree handling, fast throttle response and easy maintenance.

Snecma Moteurs, located at Evry, 40km south of Paris, designs, develops, manufactures the group's range of aerospace power plants.

With several thousand engines in the military aviation sector in service around the world, it offers tailored solutions from documentation and maintenance, crew training to complete custom-designed maintenance programs.

Owing to the close co-operation developed over the years between Snecma and Bruel and Kjaer, when a new engine test solution was needed, it was natural for Snecma's Logistic Integration Support Department and Bruel and Kjaer's French office, supported by the aerospace and special project teams at Bruel and Kjaer's headquarters in Denmark, to work together.

And so, three years ago, detailed discussions began to define the specifications for a new test system for vibration-analysis based ground engine testing.

The goal of the design team was to develop a customised system and user interface that was almost automatic in use, and would provide diagnostic data.

The Snecma Logistic Integration Support Department and Armee de l'Air (the French Air Force) together determined the parameters that should be visible on the user interface.

The test uses order analysis data in the range from 0Hz to 25kHz acquired during an engine run-up/run-down.

The simple user interface provides a simple pass or fail indication.

If unexpected vibration is monitored, the service technician is prompted to follow a series of further investigation checks.

The last resort is to remove an aircraft from operational service and to replace the engine, a time consuming and expensive procedure.

The number of input signals associated with the two engine types makes the use of the rugged and compact five-channel Pulse 3560B front-end ideal for data acquisition and analysis.

The Pulse hardware and power supply are each mounted on special shock resisting damping material.

The system is housed in three waterproof rugged carrying cases.

One contains the Pulse system, another houses the PC and a third transports the cables and accessories.

Each carrying case weighs about 8kg.

The connectors and fittings used are to fully approved military standards.

Each carrying case has been subjected to a full range of MIL tests specified by the Logistic Integration Support Department and based on published standards, including: shock, vibration, CE Mark, drop test, humidity, temperature, EMC compliance, immersion and a salt-spray test.

The data acquisition unit communicates with the PC either using a wireless LAN (range up to 40m) or direct cables.

The complete test system can either be powered from its internal batteries, from a 110/240V AC mains supply or from an external 12 to 32V DC supply, allowing it to be powered from the aircraft's own 28V supply, if required.

To set up the system on an aircraft takes ten minutes.

The transducers are not installed permanently on the engine but are mounted for each test using screw fixings on permanently installed special brackets in predetermined positions.

The number of tacho and vibration signals used depends on the type of engine being tested.

With the M53, one tacho and three vibration signals are monitored.

The Larzac engine test uses two tacho and two vibration signals.

The accelerometers are piezoelectric high temperature single axis types with balanced differential output.

They are calibrated once per year and the user interface displays the system calibration parameters.

Bruel and Kjaer is a major supplier of transducers to the Snecma group.

The actual test takes about five minutes.

A mechanic in the aircraft cockpit starts the engine and it is run up to a predefined speed and then run down.

Monitoring green or red indicators on the display, the test technician can immediately see if the engine is performing normally, and the aircraft is safe to fly, or if unexpected vibration is present.

The test data is recorded on the PC's hard disk for standard post-processing analysis in the laboratory, or detailed investigation by a specialist.

The test data also form an important part of the test and performance record of the individual engine.

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