In Field Micro Vib Testing

The Euravia MRT can perform on-site services to minimise Aircraft On Ground (AOG) time: in addition to this the PT6 MRT perform the MORE STC Propellor balancing and engine vibration surveys (Micro Vib)

MORE Company Inc. provides eight FAA approved Supplemental Type Certificates (STCs) for certain models of PT6A engines to extend the engine TBO to 8,000 hours. The MORE STC uses an enhanced maintenance method to find and correct problems in their early stages. The STCs were designed to extend engine life, reliability, safety, and to reduce the cost of operation and ownership.

The MORE Company Inc. instructions for continued airworthiness use the Pratt and Whitney Canada Maintenance Manual as a foundation and then add several additional inspection methods, these are:

Periodic Borescope inspection of the Hot Section, periodic inspection of the compressor and exhaust duct areas, periodic power plant-adjustment test (to monitor engine performance), periodic spectrometric oil analysis, periodic oil filter debris analysis, periodic vibration analysis, and preventative maintenance.

This article focuses on the periodic vibration analysis requirements.

Performed on a routine basis, propellers are balanced using an electronic balancer. Euravia uses the DSS Micro MicroVib II Aircraft Analyser, this is the latest generation balancing equipment approved by MORE Company Inc. for use with the STC’s. Once completed, the full spectrum of vibration is recorded, specific engine speeds (rpm) and balances are reviewed and identified to the corresponding speeds of critical parts in the engine. The results are used to compare readings from one inspection to the next and any changes are noted or corrected. Increases in vibration can indicate a deterioration of rotating parts. Early identification will keep the engine running safer and keep maintenance costs down.

The Method

Equipment setup and installation consists of a Vibration Sensor (accelerometer) and a Photo Tachometer sensor (Tach). These are attached to the engine utilising special brackets to existing engine bolts. The Tach obtains its signal from a piece of reflective tape which is attached the Propeller Back plate, this will give us our Propeller speed.

These are attached by cables which are routed from the engine over cowlings, wings, into the cockpit and then attached to the MicroVib II Analyser. Once the equipment has been checked for correct function and security the tests can be performed.

The engine is started and run at a suitable power and speed setting, this is usually around a high cruise or a shallow climb power, and the same settings should be used for all subsequent engine runs.

The analyser will collect data from the accelerometer and tach to compute a position for temporary weights, small washers, to be added to the spinner to reduce to an acceptable level any propeller imbalance. This may require several ‘balancing’ runs to achieve this. It is critical that any residual imbalance of the propeller is removed, or reduced to an acceptable level, usually below 0.2 ips (Inches per Second) to ensure that any imbalance in the propeller does not interfere with the engine vibration surveys as this could alter or distort results.

Once an acceptable level of imbalance is achieved the analyser is programmed to receive data for the engine vibration survey.

The same engine power settings are used as previously and NO changes should be made during the surveys.

Once the surveys are complete, data is checked and the results can be displayed on 3 specially produced graphs and can be interpreted by specialists to indicate any abnormal vibration within the engine. Even if vibration levels are within limits, over time and frequent vibration surveys any deterioration can be detected and rectified before any serious damage occurs.

Any temporary weights that have been installed to complete the propeller balance can if required be permanently installed to the propeller spinner back plate.