engine vibration

[prasanna]

a friend told me about an recent aircraft encountering engine vibration in flight . there had been a brief eicas warning as well. the one engine had relatively significant vibration and even the other had developed it. they could feel the vibration even to the flight deck. i have a few questions ! what does engine vibration signify ...can it mean fan asymetry due to a blade problem ? does one land asap ? also why did the other engine also vibrate ...is it common for the other engines to vibrate due to transmitted vibrations from the defective engine? and finally, has vibration measurement been available to pilots of the old generation aircraft ?. .. .prasanna

[411A]

Well, on the RB.211-524, it could be a fan locator bearing failure...have had two of these. Many of the guys off the DC8-707 thought that the eng vibs could/should be ignored...not a good idea at ALL.. .Picture this, B747SP on takeoff in JED, max weight headed for JFK.. .Number two engine vibs off the scale at 700agl, gear coming up, very noticable vibs in the airframe. Captain retards throttle toward idle, vibs decrease, with the gauge. Captain thinks....ah well maybe it was 'da gauge, pushes throttle back up and...boom, engine comes apart, and as part of the uncontained failure, bits go in the number one engine and the wing. Now down to three engines AND fuel streaming out of the wing, F/E notices number one fire handle illuminated, but no bell so....proceeds to pull number one fire handle without consulting ANYONE.. .Aeroplane now down to two engines, going downhill rapidly. Fuel dump and APU started immediately, flaps retracted (descent stopped at 200agl)and back 'round for landing on 34L.. .In short, 'ole captain had a few more grey hairs...and F/E out of a job.. .Better to NOT ignore high vibs.. . . . <small>[ 08 March 2002, 20:11: Message edited by: 411A ]</small>

[Herod]

Prasanna. I assume you want a reasoned answer, and not another war story. I'm no expert but: All jet engines will have a little bit of vibration, the indicators will very rarely be at zero. generally it is indicating an imbalance somewhere within the engine. If it is severe enough to cause an alert, chances are either an imbalance in the fan (damage of some sort), a bearing on its way out or possibly icing. I think most aircraft drills call for power to be reduced then, if the vibration reduces to acceptable limits, continue operating at the lower power, otherwise shut the engine down. The exception may be icing where the drill may call for power changes, in an attempt to dislodge the ice. I think the older generation of jets had vibration indicators, they have been around a long time. Not the piston era though, enough vibration there anyway!! Any engineers like to comment/shoot me down in flames?

[polzin]

Herod......... 411 's first comment was quite true. And I understand your emotional comments about 411. I have had them. But what he said about the subject was true . On the big fans , if there are high vibs it is a shut down, period. There is just a lot more mass turning around out there and the vibs just get more important. . .. .But i could tell you the time i ignored them if u really insisted?

[polzin]

Herod......... 411 's first comment was quite true. And I understand your emotional comments about 411. I have had them. But what he said about the subject was true . On the big fans , if there are high vibs it is a shut down, period. There is just a lot more mass turning around out there and the vibs just get more important. . .. .But i could tell you the time i ignored them if u really insisted?

[Al Weaver]

No intent to be hard over on any of this but the general idea is:. .. .Vibration meters are unreliable indicators by themselves of engine health and the pilot should consult other indicators before shutting down an engine (EGT, oil temp, oil pressure).. .. .Seat of the pants is actually more reliable, but do you really know that the vibration you feel is actually an engine. . .. .Engines do not fail catastrophically due to continuing vibration. Catastrosphic failures are decided in less time than it takes to glance at a gauge.. .. .Vibration can lead to plumbing on the engine failing and of course secondary fuel leaks and/or oil leaks.. .. .Vibration as displayed or felt, from an engine is that which is reacted through the mounts. There is lots of other vibrations going on in an engine which will never be displayed or felt, yet can cause plumbing or other failures.. .. .I'm not sure what failure was being mentioned by 411 above coming out of JED of that magniture unless it was the SV RR uncontained turbine event.. .. .The more serious vibration events are fairly rare (less than one per year) and have been associated with fan bearing failures. None of which actually pose a structural threat to either the engine or the aircraft (they are very uncomfortable to the crew though).

[ft]

FWIW, vibrations are quite a useful diagnostic tool - if you know what to look for. Many kinds of failures or conditions requiring maintenance cause vibration in different kinds of the spectrum. It has been used on recips as well, there's always vibration but certain frequencies of vibration can spell impending doom for machine parts you really don't want to live without.. .. .I beg to differ regarding the failures due to continuous vibration as well. Over time, you might end up with fatigue in some affected component.. .. .Generally, if someone has gone through the trouble of incorporating the equipment to sense and monitor a vibration frequency into the design, I'd say it is for a reason and I would certainly hope that you pay attention to it and consider the implications before choosing to react to or dismiss it.. .. .Another war story is the aircraft (jet fighter) where they had to replace lockpins by the dozen - they just snapped off, apperently for no reason! Turned out they had a high frequency vibration outside of the spectrum they could measure coming from the engine.. .. .Cheers,. . /ft

[Jet II]

Most of the large transport A/C nowdays have vibration meters fitted. As they can be unreliable - usually any vibration shown is down to the pick-up not the engine - some of the older aircraft (DC10's etc) had the vib meters disconnected. This wasn't such a problem as the crew could feel any bad engine vibes thru the throttles and fuel selectors. (A lot of vibration on the older jets was due to cowling stays rattling around the engine). .. .On the modern fly-by-wire (FADEC) engines with no mechanical linkage from the flight deck to the engine it can be more difficult to feel any vibration, but vibe meters have become more reliable and most aircraft now have them fitted. Usually (not accurate figures for all A/C) 1 - 2 units of vibration are acceptable and 4 means get engineering to have a look. <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />