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Common causes of Vibration

Safety Officer



Common Causes and Fixes


   I get many phone calls on vibration in owner’s aircraft.  Here we’ll talk about some of the common causes, where to look and what you can do to help mitigate these vibration issues.

Here is a common list, but is not all inclusive;

1.     Carbs not synced properly.

2.     Carb vent hose improperly placed or removed.

3.     Carbs not opening equally or fully.

4.     Prop blades not the same pitch or out of track.

5.     Prop out of balance.

6.     Aircraft wheels not balanced.

7.     Old rubber engine mounts.

8.     Mag drop difference too wide between ignition modules.

9.     Trigger coil air gaps too wide.

10.  Gearbox worn, damaged or in need of maintenance.


  So let’s address each of these.

  The carb sync (#1) should be fairly obvious to most now. The carbs should be synced at each annual / 100 hour inspection or anytime they have been removed for maintenance or you suspect a problem like vibration and you need to rule this in or out as the problem. Carb sync is vital to a good smooth long lasting running engine. You don’t want one side trying to run at 5100 rpm while the other may be trying to run at 5200 rpm. Sync those carbs. Once done it’s easy to keep them there.

  The carb vent hose (#2) that may be attached to the standard Rotax air box, a small clear plastic tube on the side of the carb under the carb bowl bale or some others have them routed to different places. These hoses should be as close to equal length as possible and be routed to the same area of pressure. If one hose has fallen off the side of the carb and the other is still attached it will cause the carbs to become unbalanced which will cause your vibration. Do not place these hose ends in the air stream outside the cowl. These only take a minute to confirm their attachment and placement.

  Check to see if the carbs open equally (#3) by moving the throttle from idle to wide open when the engine is off. You may see some signs of this during a carb sync, but most people don’t go above 3500 rpm for a carb sync so you need to double check this while the engine is off to see if they do in fact reach WOT at the same time or if one hangs up slightly.

  The prop blades all too often are not the same pitch (#4) from blade to blade. This is easy to double check and can be done with either a prop protractor and or a 12” digital level. Measure back from each tip 8”- 9” and put a mark on each blade. Make the blade out to your right level with the floor and then put the level on the back of the blade where you made the line from the tip. The blades should be no more than one tenth of a degree out from each other. That measurement seems small, but it is quite easy to accomplish. The Sensenich prop gauge pins are not accurate enough. Check them by hand with a prop gauge or level once you are close. To check tracking place a box underneath the bottom tip of a blade pointing straight down. Put a line on the box where that tip just barely touches the box. Then swing the other blade(s) around and see if they all cross at the exact same mark. If they don’t you’ll need to loosen the prop flange bolts and

re-torque them to get the blades to all track over your line on the box.

  Prop blades now days are much better in balance (#5) than they were decades ago, but all props should still be dynamically balanced. All wood blades in humid climates can change due to moisture absorption. With all the new composites that aren’t susceptible to this anymore I’m not a fan of all wood blades. Even the main bolts change torque with humidity changes. A dynamic balance will not only help vibration, but will help save your gearbox from wear or damage. The heavier the blades i.e. long Warp Drive props the more important this becomes.

  I have never found an aircraft wheel (#6) in balance. Most do not ever think about the smaller aircraft wheel being out of balance as a vibration cause, but over the years I have cured many a vibration just by balancing the wheels. I always balance all new wheels I install. I see some occasionally that would need up to 20 x ¼ oz. weights to bring them in balance. If you failed to balance your wheels you would never find this huge disparity. These come off and go back to the distributor. What I normally see is 2 – 8 x ¼ oz. weights per wheel. It usually takes me about 3-5 minutes to balance a wheel after it’s off the plane. Don’t disregard this when you are looking for a vibration cure.

  Old rubber engine mounts (#7) are a common problem. Rotax wants a 5 year rubber replacement which I’m a fan of. This includes the rubber engine mounts.  Rubber can get hard or soft from repeated heating and cooling cycles plus chemical exposure and just the ozone in the air. I replace these every time I do a rubber replacement on an aircraft. It usually isn’t hard or expensive.

  The mag drop vibration (#8) should be obvious when you do your mag drop check. Most see anywhere from 40 rpm – 100 rpm as a normal drop and usually both mags are within about 10 rpm – 30 rpm of each other. If you experience

300-1000 rpm drop then it’s time to troubleshoot your ignition system. There are documents out there that tell you how and where to look for ignition issues.

It could just be a bad plug, too wide a plug gap, a bad plug boot, a bad connection at the plug boot where the wire screws in. If it is a large drop like 800+ rpm it may be a bad ignition module. These are all items you need to rule in or out. Always start with the most common, easiest and cheapest first. Do not just throw money at everything hoping to hit the jackpot. Most ignition issues are simple common issues.

  The trigger coils (#9) in the flywheel compartment can at times have too wide an air gap between the pick-up and flywheel trigger point. These are checked by using a feeler gauge and checking the gap tolerances listed in the Heavy Maintenance manual and setting them to the proper gap. These can even be off from the factory so check them before installing a new engine when they are easy to get to. You not only are checking the gap, but the screw torque for tightness.

  Gearbox (#10) care is important. As you look for your vibration issue consider the gearbox. It has maintenance service times at either 600 or 1000 hours. Using an automotive oil over a motorcycle oil can cause premature wear and damage. At your 100 and annual inspections you should be doing a gearbox friction torque check. Normal measurements that I usually see in the field is between 425-490 in. lbs.  There is a low limit, but I personally don’t like to see anything in the 300 in. lb. numbers. It only takes a few minutes to perform. Checking the magnetic oil plug for debris at every oil change is another check for gearbox wear and damage. Prop strikes should have the gearbox removed and sent to a distributor for a special inspection. Gearbox’s when taken care of tend to last a long time, but there have been a few with excessive wear in early run hours. There have been some with the 912iS engine.


  These are the 10 common causes for unwanted vibration. Most are easy to fix and find. When trouble shooting start with the cheapest and easiest to rule in or out and progress to the harder least common when you do your checks. Whatever you do be methodical and don’t jump all around to exotic areas to check. Most Rotax issues are easy to find when you start at “A” and then work to B, then C and so on.


I hope this helps some reduce any frustration in locating an unwanted vibration.


Signed your friendly,

Safety Officer. :)


Recommended Comments

In addition to Bill, I also would like to know who authored the original post.  I think it is poor practice to give technical guidance without accountability.  

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That is the choice of a poster.  The credibility of a poster is determined by their historical posts, not their name.  I think Safety Officer's posts speak for themselves.

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I work in a safety sensitive business and anonymous guidance is not accepted. In my work, professionals are willing to put their name to their recommendations.  No matter the rationalizations here, it still doesn't fly.  Clearly, the administrators of the list think this is ok, so this is how things will stay.

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I don't get it.  Whats the difference between his name and yours?  Or anyone's?  I don't see a CV/Bio posted with your name or any other poster.  Would you take advice from a poster with the name "RotaxGuru" with no posting history, or poor history?  Or "JoeBlo" with 1,000 thoughtful, informative posts.  In any case, you could always PM the person to discuss your questions or comments.  What exact policy would make you happy?







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Welcome to the hospital.  Your surgery will be done by Jane Doe and supervised by John Doe.  Best of luck.

If you don't get it then you don't get it. I can't fix that for you.

I don't give guidance on how to maintain an airplane.  When Corey and Tom give advice, I actually know who I'm getting advice from.  And, when they post, they do include their credentials.  That is how the rest of the world works.  I get it, the CT Flier "Safety Officer" is special.  

What exact policy would make me happy?  That the "Safety Officer" - who appears to consider his guidance to be authoritative - identify himself.  As you already knew.

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There's value in a name. FredG wants to know what that name is, to discern the value of the advice provided.

If safety officer's credentials were posted and verified, that probably would go far enough to help considerably, but not being provided a name can still feel like something is being hidden.

While I know what is said here to be true and it doesn't bother me, I can understand where people come from when they want to know about the person providing it.

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I guess I don't get it, either.  If you physically took your plane to a mechanic and he/she refused to provide a name and credentials, then I would agree with the hospital analogy.  But this is an open internet forum -- nothing here is official, even if it comes from someone who does provide their name and certifications.


As a newbie I have found the advice here invaluable, from many sources.  When I did my first oil change, I printed out a post from Roger for reference.  Roger provides his credentials, but I used the information because it was well-reasoned, made sense, and I from past forum use have a positive opinion of Roger's expertise.  But could I use that information in any legal capacity; i.e. logbook entry or justification of method used?  Of course not.


Like most here I have also gotten plenty of good advice from "non-credentialed" posters.  FredG, a few months ago you gave me some advice about flap use, which I found helpful.  I used the advice despite the fact that you are anonymous and provide no credentials because it was logical and made sense and I had a good opinion of your previous posts.


I can think of several scenarios where a poster might wish to remain anonymous, for reasons unrelated to the usefulness of the information.  I would hate for that person to stop posting in response.

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This does not make sense to me. This is a discussion forum. Some of the info posted on this forum does not make sense to me. If it is relevant I still check other sources or make contact with the poster to better understand. 

Nothing here is officially Flight Design or Rotax approved. If that is what you are looking for you need to look elsewhere.

Having said that there is a wealth of excellent info from excellent people here. They have helped a lot of people who use good judgement.

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Sounds like you are making my point for me.

If, as you correctly state, nothing here is Flight Design or Rotax approved, and this is a discussion forum, and you always double check relevant information against other sources, then why do you have a problem with a poster who provides valuable information but chooses to remain anonymous?


Edit: ignore me, I'm an idiot.  I replied to DougG, when I thought the response was from FredG.  The similar "G"s confused my lame brain...

Edited by JLang
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I suggest some look at this differently. 

The user "Safety Officer" took the time to make a pretty extensive post full of information.  Instead of saying "thanks" or perhaps critiquing anything in the post, you crap all over it because you don't know the user's real name.  Then, the logic becomes that the post is bad data because you don't know the user's name.  To put it politely, that's absolutely foolish.  There have been plenty of times well known "names" give bad advise.  Name or no name make no difference as to the value of said advise.  Having a title or no title doesn't makes no difference.  If you feel otherwise, you're bound to be fooled. 

So, does anyone have anything to say about the actual post by S.O.?  Does anyone agree with the content he/she posted (for free)?  Or does anyone have a critique of the content?

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Names don't make an article or writer, content accuracy does.

If you know Rotax you'll know what's in the article is all good info. If you don't know Rotax then research. Rotax seems to like the article when I ask them about it and it should be on their forum blog shortly. If you read Rotax blogs many times it just says Rotax owner. Of course you can choose not to believe and that's okay to.

If you develop a vibration or already have one you don't have to read this for help. :eyebrow-1057:



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