Chapter 8 - Airborne Vibration Monitoring (AVM)

Question 1

Describe a vibration monitoring system using a velocity transducer?

Answer 1

A permanent magnet is situated inside a coil, and is held by two springs on either end

The magnet moves relative to the transducer base, and induces an oscillating voltage into the coil which is proportional to the vibration velocity

This signal is amplified and then rectified before being passed to drive the indicator

Question 2

What is the output of the transducer measured in?

Answer 2

Millivolts per inch per second

Question 3

What are the disadvantages of transducers as compared to other types of vibration monitoring systems?

Answer 3

• Prone to wear and damage
• Frequent re-calibration is required
• Relatively large
• Sensitivity changes can occur

Question 4

How does a piezoelectric transducer provide a measurement of vibration?

Answer 4

As the piezoelectric crystal is squeezed, it will provide an AC voltage that will be proportional in amplitude to the engine vibration

Question 5

Why might there be more than one vibration transducer on an engine?

Answer 5

Sensing vibrations at different locations

One to measure the fan and one to measure the turbine

Question 6

What is the sample period for each transducer signal in the signal conditioning unit?

Answer 6

0.5 seconds

Question 7

What is the output signal to the indicators of vibration in each engine?

Answer 7

Average of both the front and rear transducers

Question 8

What are the advantages of piezo type accelerometers?

Answer 8

• Very high/low temperature operating range
• Range of sub-miniature designs
• Wide dynamic range
• No moving parts
• Piezoelectric transducers have a self-generating output, no power supplies needed

Question 9

What are the additional advantages of piezoresistive accelerometers?

Answer 9

• Low impedance output - eases inter stage matching
• Measures steady state acceleration
• Frequency response from zero hertz

Question 10

What is the purpose of the filter selection switch?

Answer 10

Enabling by selection to check which shaft is contributing to the excessive vibrations

Question 11

What do the selections LOW, MED and HIGH refer to on the filter selection switch?

Answer 11

Low - LP
Med - IP
High - HP

Question 12

What is the purpose of the FAN or TURB switch?

Answer 12

Enables a check to be made on whether the vibration is coming from the front or rear of the engine

Question 13

When is the output of vibration transducers inhibited?

Answer 13

• Within two minutes of engine start-up
• 10 seconds after take-off
• Immediately when the aircraft lands