Controls and instrumentation - Vibration

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Vibration
44. A turbo-jet  engine  has  an  extremely  low vibration level and a change of vibration, due to an impending or partial failure, may pass without being noticed.  Many  engines  are  therefore  fitted  with vibration  indicators  that  continually  monitor  the vibration level of the engine. The indicator is usually a milli ammeter that receives signals through an amplifier from engine mounted transmitters (fig. 12- 11).

Fig. 12-11 Vibration transmitter and

indicator.

45. A vibration transmitter is mounted on the engine casing and electrically connected to the amplifier and indicator. The vibration sensing element is usually an electro-magnetic transducer that converts the rate of vibration into electrical signals and these cause the indicator pointer to move proportional to the vibration level.  A warning lamp on the instrument panel is incorporated in the system to warn the pilot if an unacceptable  level  of  vibration  is  approached, enabling the engine to be shut down and so reduce the risk of damage.

46. The vibration level recorded on the gauge is the sum total of vibration felt at the pick-up.  A more accurate  method  differentiates  between  the frequency ranges of each rotating assembly and so enables the source of vibration to be isolated. This is particularly important on multi-spool engines.
47. A crystal-type vibration transmitter, giving a more  reliable  indication  of  vibration,  has  been developed for use on multi-spool engines. A system of filters in the electrical circuit to the gauge makes it possible to compare the vibration obtained against a known frequency range and so locate the vibration source. A multiple-selector switch enables the pilot to select a specific area to obtain a reading of the level of vibration.