View Full Version : B737-400/-NG Engine Vibration Indication

20th Mar 2002, 10:59
Okay - I'd like to know what what you know about the unit of measure that is displayed on the VIB indicator for the 737-400/-NG. I know that the range for the indicator on the -400 is a value between 0 and 5 and the -700 displays the data in a digital format. I also know the data originates at two sensors (accelerometers) - one on the forward end of the engine and one at the fan frame. The signal goes to the AVM signal conditioner in the E&E bay and is then displayed appropriately in the flight deck as a 'number'. I know that the 'number' represents how much vibration the engine is encountering, some being acceptable, too much being cause for concern and eventually in-flight shut down. What I want to know is - what is the unit of measure displayed on the VIB indicator? This is unclear from the data I have to review. <img border="0" title="" alt="[Roll Eyes]" src="rolleyes.gif" /> Thanks...

20th Mar 2002, 16:14
I think that Boeing just meant to display a scale of severity with 0 being "glider", and 5 being "I've spilled my coffee". I've never heard of a scientific quantity being referenced for engine vib but there must be one i suppose. Maybe youshould try the flight test forum? <img border="0" title="" alt="[Roll Eyes]" src="rolleyes.gif" /> ...so this reply was really useful then

20th Mar 2002, 19:23
Not sure about 737 but for 747 The AVM system generates signals proportional to engine motion in a radial direction. The indicator shows the engine vibration velocity in inches per second. <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

20th Mar 2002, 19:39
Dear forum readers. .. .(the following text is extracted from Boeing Airliner magazine, OCT-DEC 1987 issue). .. . Two types of vibration pickups have been used on Boeing airplanes, the electomechanical velocity pickup and the piezoelectric accelerometer. Signals from velocity pickups are converted into vibration displacement, measured in 1/1000 of an inch, peak to peak (mils double amplitude). These units represent the physical displacement of the vibration sensor. Signals from accelerometers are converted into velocity, measured in inches per second (ips). These units represent the peak amplitude of the velovity of the vibration sensor.. . Boeing airplanes typically use a 0-5 scale for cockpit display of vibration. On earlier applications, the scale provides an indication of absolute vibration units(ips or mils da) as output from the airborne vibration monitoring (AVM) signal conditioner. This approach is not optimum in that a display of displacement units will provide relative insensitivity to low speed rotor vibration. AVM signal conditioners used on 737-300 and latter, 747, 757 and 767 provide output data in scalar units. These scalar units are uniquely tailored for each engine type to match the operating speeds and vibration characteristics of the engine rotors. The scaling used on the 737/CFM56 installation provides display sensitivity for the high speed rotor proportional to the vibration velocity. The low speed rotor vibration is displayed in units proportional to displacement, except near idle conditions where velocity scaling is used.. . . .Regards

Al Weaver
21st Mar 2002, 08:18
Delarocha is totally correct which should be almost meaningless to anybody but an engineer who designs and developed the engine and its accessories.. .. .The user of the engine simply needs to understand relative experience which equates to soothing hums from the engine or your instrumennt dials and eyeballs jiggering so badly that you are convinced somethings going to break unless you shut it down promptly. Everything else in between is subjective and relates to time at the condition.