View Full Version : Engine Vibration Measurement

9th Feb 2017, 12:44
What unit of measurement is used to monitor engine vibration, The aircraft I currently fly is GE turbofan, has a digital, in green 1.0..1.1...1.2 and so on until it reaches 3.9 then turns to an amber 4.0. My question 4.0 what? %? mm?

9th Feb 2017, 12:54
4.0 units according to my B737 FCOM. The vibration monitoring system measures vibration in both turbines, the high pressure compressor and the fan, and displays the highest out of these four values. You then need a table to convert this value to mils (thousands of an inch).

FE Hoppy
9th Feb 2017, 13:00
Inches per second.

9th Feb 2017, 13:08

includes a long and detailed explanation of how Boeing do it.

9th Feb 2017, 20:51
This is the key statement from the DaveReid link:
These scalar units are uniquely tailored for each engine type to match the operating speeds and vibration characteristics of the engine rotors. In short, there are engineering units associated with the vibe measurement, but that's not what gets displayed. The bottom line in what's displayed is:
1.0 or less is real good
4.0 or greater is real bad
On the newer installations (787, 747-8, and I presume the same is being done on the 737MAX and the 777X) the pointer pegs at 5.0 units, but the digital display goes to 9.9. The digital display goes "inverse video" when it reaches 5.0 to help get the crew's attention (instead of white numbers on a black background, the background turns white with black numbers).
With the notable exception of some Rolls installations, there is no Boeing or engine company requirement to shut down an engine with high vibes - rather the vibes are simply considered another piece of data for the crew to decide what action to take.

12th Feb 2017, 04:00
Very interesting topic. While I understand the cockpit display using a linear scale to indicate the relative severity of an abnormal engine vibration condition, I don't understand the basis for using units of inches/second which is velocity, rather than inches/second^2 which is acceleration.

Vibration forces are related to accelerations, and not velocity. Can someone explain why this approach is used?

12th Feb 2017, 04:59
I suspect that they use velocity for two reasons:

- As the integral of acceleration, i'd expect it to be a good deal less noisy than raw acceleration - which then makes it much easier for a human to interpret.

- What they're actually interested in is displacement (from the link above, displacement is what gets used whenever the engines aren't at idle). Large acceleration doesn't matter much (and may even be normal) as long as it's only occurring over a very short distance. In this case you'd expect the velocity to be small (over a short distance, even at high acceleration there's not space for a large velocity to build up). On the other hand, vibration over a large distance (even if the peak acceleration is small) suggests that something is loose. In this case the velocity could be substantial, since even at a small acceleration over a long distance will cause a high velocity.