Accelerometer Mounting

The type, location and orientation of accelerometers required for specific rotor balance or vibration analysis checks are identified in the OEM Maintenance Manuals. They have been chosen and tested to ensure that they:

  • Are positioned close to the point of interest
  • Provide repeatable and reliable data
  • Do not compromise aircraft safety
  • Are easily accessible and convenient to install

Thus they have the potential to provide the information you require, however their integrity can only be guaranteed if the installation procedure is correctly followed and a few basic rules are observed.

Interface brackets. Many installations require the use of some form of bracket, or block between the aircraft structure and the accelerometer. Those supplied from official sources have been designed, manufactured and tested to ensure that:

  • Their construction and rigidity ensures their natural resonant frequency is outside the frequencies of interest.
  • The bracket face to which the accelerometer is attached is true and square, ensuring good contact with the accelerometer base.
  • The bracket face in contact with the structure is adequate for mechanical transfer of vibration without amplification or attenuation caused by trapped debris, paint flakes etc.

Tip - Do not substitute the authorised item with unproven, locally manufactured brackets.

Surface preparation. The transmission path of mechanical vibration from its source to the accelerometer must be accomplished without modification. This can occur if the accelerometer is loose or not in good surface to surface contact between bracket and accelerometer base.

Tip – Always torque load the accelerometer to the recommended figure.

Tip – Ensure the mating surfaces are free from debris and if necessary use a proprietary coupling agent.

Electrical Connection. The transmission of signal from the accelerometer back to the processor must be reliable under all flight conditions. Poor or intermittent connections will result in bad data.

Tip – Ensure that the bayonet connector on the accelerometer cable positively locks into place when connected to the accelerometer.

Tip – Ensure that the ‘O’ ring seal fitted in the head of the H-901 accelerometer is in place.

Tip – Never secure the accelerometer cable back to the body of the accelerometer, this provides a secondary vibration input path and can breach the isolation.

Tip – Use a cable tester to ensure that continuity is maintained when the cable is subject to movement and vibration.

Bonding and Earth loops. The Rotortuner system, components and cables have been designed to provide a high level of integrity and protection from earth / ground loops and other electro-magnetic interference. If this is compromised it is possible to induce unwanted noise into the system

Tip – Do not wrap locking wire around an accelerometer such that it bridges the case isolation.

Tip – Ensure that the 28V DC power input is grounded