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912 ULS caution

flyeyes

Well Known Member
We don't have an RV-12, but have had an AirCam in the family for a dozen years with a pair of 912 ULS engines.

We've learned most of the Rotax quirks over the years, but occasionally they still surprise us.

Broken "P" leads are somewhat common, especially since the engines (and associated wiring) are exposed to the slipstream on the AirCam. Since the 912 doesn't have a mixture control, pulling the engine to idle and hitting the primer will stop it instantly (flooding it).

The electronic ignitions are designed to not fire below 300 RPM, which means that the engines are impossible to hand start.

Except when they aren't.

One of the other pilots on the AirCam was turning the right engine through by hand as part of the normal preflight and the engine started.

We didn't think that was possible.

Fortunately, there were no injuries, but he had to run around to the cockpit and turn the master and boost pumps on to hit the primer.

The right engine otherwise runs flawlessly, I don't know if this represent a subtle failure in the ignition, or if it is possible anytime.

I do know that from now on I will treat the Rotax prop like I would a Lyc/Continental and touch it only with extreme caution.
 
Wow

James, thanks for reporting this. I'll bet that ROTAX would really to get a full report on this. There must be something that we all can learn from this. Turning the prop is a part of a pre flight as you know. Please keep us informed if you can figure out how/why this happened. I'm glad that no one or no airplanes were damaged!
 
Turning the prop is a part of a pre flight as you know. Please keep us informed if you can figure out how/why this happened. I'm glad that no one or no airplanes were damaged!
Yes, this is something to be aware of, but there's still quite a bit of difference between this and something like a Lycoming. I have no evidence other than my own mistake, but I would wager that so small percentage of accidental starts on a Lycoming don't come from a full-on swing like shown in the video. The one and only time I accidentally started my Lycoming was on a single, slow pull through of a blade. The difference between a Lycoming doing that and a Rotax doing that is that the Lycoming has an impulse coupler.

Impulse coupling:

Because the magneto has low voltage output at low speed, starting an engine is more difficult. Therefore some magnetos have an impulse coupling, a springlike mechanical linkage between the engine and magneto drive shaft which "winds up" and "lets go" at the proper moment for spinning the magneto shaft. The impulse coupling uses a spring, a hub cam with flyweights, and a shell. The hub of the magneto rotates while the drive shaft is held stationary, and the spring tension builds up. When the magneto is supposed to fire, the flyweights are released by the action of the body contacting the trigger ramp. This allows the spring to unwind giving the rotating magnet a rapid rotation and letting the magneto spin at such a speed to produce a spark.
 
912 ULS hand prop start

If there is ANY chance of this happening somebody is going to be killed! This MUST be understood and resolved ASAP. If the 912 ULS was a certified engine there would be an AD prohibiting further flight until the problem was corrected.

EBB:-(
 
If there is ANY chance of this happening somebody is going to be killed! This MUST be understood and resolved ASAP. If the 912 ULS was a certified engine there would be an AD prohibiting further flight until the problem was corrected.

EBB:-(
A few things to keep in mind
The prescribed procedure is to slowly turn the prop.
Tens of thousands of these engines are in use. Though this may be possible, it is not a serious problem IMO
If you are concerned it is very simple to mitigate.
At every shut down, do a quick ign check to verify rpm drop when switching off A or B. This assures that the ign disable function is working properly for both ign. Pretty much making it impossible for the engine to fire as long as you remember to turn the switches off ( which on a Rotax usually isn't an issue because that is how we shut the engine down).
 
A few things to keep in mind
The prescribed procedure is to slowly turn the prop.
Tens of thousands of these engines are in use. Though this may be possible, it is not a serious problem IMO
If you are concerned it is very simple to mitigate.
At every shut down, do a quick ign check to verify rpm drop when switching off A or B. This assures that the ign disable function is working properly for both ign. Pretty much making it impossible for the engine to fire as long as you remember to turn the switches off ( which on a Rotax usually isn't an issue because that is how we shut the engine down).


Just to be clear, the prop was being turned slowly.

I've always been extremely careful turning props on lycoming or continental engines unless the plugs were out of the engine. I -never- touch a prop unless I first check the mags are off. I don't recall having a "hot mag" in 34 years and a lot of hours except on the Rotax.

I'd always considered the Rotaxes more benign, but my point is that they aren't. The requirement for turning the engines through on the preflight (which I've done hundreds of times over the years) potential represents a real hazard. The CAA advisory Doug linked on the front page, if followed literally, would mean it's impossible to preflight a 912 without a minimum crew of two "qualified personnel:" one in the cockpit and one turning the prop.
 
Always remember that second hand info usually isn't quite correct and even some stories from the actual person can be skewed trying not to look guilty or embarrassed.
There are over 50K Rotax engines out there with more than 40 million run hours. I have seen the video of the guy hand proving the Rotax which was an 80 hp and he was trying to rotate at some speed. We also don't know if he had it as factory or modified it to impress the public. Many fake and or enhanced videos out.

The reason that this is so unlikely is we do not have mags like a Cont. Or Lycoming. With a mag even a slow rotation fires the mag as it clicks over. On the Rotax we have a CDI ignition. It must have a certain rpm to generate enough electricity to supply any fire to the plugs. This is why many with just a low battery and the engine turning slightly slower won't get a start.
The prop rpm needs approx. 95 rpm and that translates to 230 rpm for the engine to generate enough electrical juice to give the plugs enough energy to create enough of a spark to start. The human ear couldn't pick up the difference from an 85 rpm turn and a 95 rpm rotation and this is why many sit and crank an engine and it doesn't start. A good healthy starter and battery usually gets you around 300 rpm.
The chances of a start should be nil. First the engine isn't going to start unless you have all the ignition system turned on. That isn't the engine's fault. The second item is the very slow rotation of the prop to push oil back into the tank and check for any hydro-lock. This may be around 3-4 rotations in a minute.
 
I really don't get this 'beware the devil engine' stance!

As Loki said there are now 50,000 plus Rotax 91x engines out there and have amassed 40 million flight hours in 25 years and there are now 5 million more hours being added every year.

All you need on most Lycomings or TCM magneto-sparked engines is a broken P-lead and that ignition system is live. And in 24 years of maintaining light aircraft and their engines (Lycoming, TCM and the Rotax) I have seen a number of broken P-leads and ring terminals on Lycs and TCM engines.

Switched on pilots will notice the lack of mag drop when checking the mags prior to shut down. Others may not and as the heritage engines are shut down by taking the mixture to idle cut off then the fault will go unnoticed until someone pays attention.

It's pretty easy to start a Lycoming or TCM motor with a good impulse mag even when there's hardly any power in the battery and the starter is only just turning the motor through compression.

On the Rotax we use the ignition system to switch off the engine as Scott said. Pretty obvious if there's a live ignition system problem. Also as Scott said, the best way to gurgle the Rotax for the oil level check is turn the engine over s-l-o-w-l-y, pausing on compression. No need to rotate the prop anywhere near 123rpm to gurgle.

Please, less of the hysterics about the Rotax. It isn't the same as 'other' aircraft engines and requires to be treated differently when it comes to operation and maintenance.

It is exactly the same though in that the Rotax also drives a propeller and they can hurt just the same if handled incorrectly.


PS flyeyes - Doug's front page link to the UK AAIB accident report was supplied by me in post #5 of this very thread...

PPS EBB - Rotax are very good at providing Service Bulletins to cover potential or actual faults or problems. If the regulating authorities (FAA, CAA, EASA etc) think there's a problem then, fear not, an Airworthiness Directive will appear. The 912ULS is as identical to the certified 912S as it's possible to be. Same parts with added paperwork and a longer test period. Make no mistake, the RV-12 and the Rotax 912 make a superb flying machine. If you really despise most aspects of it then go back to being BB instead of EBB. Best suggestion I have is to get yourself on a Rotax maintenance course and understand the modern aero engine.... ;)
 
Loki and Aerofurb:

I'm not saying the Rotax is a "devil" engine and certainly not implying its unsuitability for aircraft use.

I'm well aware of how the engines and their ignitions work, we've operated a pair of them for over a dozen years.

Like a large number of people (if not the majority) on this forum I've also had a of experience with more "traditional" engines from Lycoming, Continental, Jacobs, Franklin, and Pratt and Whitney. I've flown airplanes without electrical systems, with impulse coupled mags, shower of sparks, and three different aftermarket electronic ignitions. I've intentionally hand propped engines as large as a Continental O-470 although I don't think it's safe to hand prop larger engines or three-bladed props.

Loki, I haven't put out any "fake or enhanced videos" and I'm certainly not stretching any facts to be sensational. I'm not even sure what I'd gain by that.

My point here is that, contrary to common belief, the Rotax can start unexpectedly if one of the ignitions is not grounded properly. I was surprised to learn that it can happen even when the prop is turned slowly. I recognize that it's not supposed to happen, but it did, so it's clearly possible.

My friend who was preflighting the airplane is an experienced CFI/commercial pilot who has years of experience on a wide variety of aircraft and engines.

I could be wrong, but I think that a lot of people think that turning the Rotax through by hand is basically risk free, although many of the same people would never treat a magneto fired engine the same way.

That belief, coupled with the -requirement- to turn the Rotax through on every preflight, could get someone killed or injured. I see people preflighting 912s at fly-ins all the time. How many of those people have the airplanes chocked or tied down, or have a second pilot at the controls? Most people locally with cubs/champs/Taylorcraft, etc are religious about those precautions. Not so much with the Rotax.

My point is to make people think, and hopefully prevent an injury.
 
Without any emotion ;) I ask the following. When the accidental start occurred, was the engine hot or cold?
 
Without any emotion ;) I ask the following. When the accidental start occurred, was the engine hot or cold?

Cold, but had run within the last day.

No prime, master off. Switches off, but one bad ground.

Hopefully no excess emotion ;p

Seriously, I have nothing against the Rotax. My family just likes aircraft, of pretty much every type. We currently have in the hangar two Rotax 912s, a lycoming, two continentals , one Pratt&Whitney, one Rolls Royce/Allison (not the cool kind) and a couple more belonging to friends' projects.

The 912s in the AirCam are set up a little differently from what I understand the RV12 to be. There are no chokes, just electric fuel pumps and primer solenoids. they are somewhat prone to overprime/flooding, so the normal start is boost pumps on, throttle cracked, hit the starter and while the engine is turning bump the momentary toggle for the primer.
 
Hydro-lock

slow rotation of the prop to push oil back into the tank and check for any hydro-lock

Loki. What is hydro-lock? And how often are you all performing this procedure? Certainly to visualize your oil quantity when interested but the engine will start just fine without...unless "hydro-lock" is something ominous.
 
What, exactly, is the purpose of "burping" a Rotax 912ULS engine? What is the risk of not doing that?

I have never heard or read the answers to those questions plainly stated.

Further it seems that the following questions need to be answered:

If the only reason to burp the engine is to check the level of oil available to the engine then it would makes sense not to burp it at the start of each flight day because if it was OK yesterday and there is no puddle of oil on the ground then it may be reasonable not to burp today? (please allow me to take that risk without a lecture on why I shouldn't)

If the reason to burp the engine is to ensure there will be no "hydro shock" then why is it not necessary to burp then engine after flying it recently today?

I would really love it if somebody with the proper credentials would take ten minutes and give us the answers to these questions. I have read the Rotax engine documents and instructions and there is nothing there that I remember that deals with these questions. There are all kinds of Utubes available on how to burp but none that go into detail about why.



EBB
 
The main reason to burp the Rotax is so that air leaking past the piston rings on the compression stroke will pressurize the crankcase and force oil out the bottom of the engine and up into the oil tank where the level can be checked accurately. You could check the oil after flying when excess oil will already be in the reservoir tank, with no burping necessary. Hydro-lock is a situation where oil seeps into the combustion chamber over time. If the engine is then started, the piston will hit the oil and not be able to compress it. Compare this situation to an irresistible force meeting an immovable object. Something expensive will break. I think that the engine would have to sit for a few months without running for enough oil to leak into the combustion chamber.
I think that if you fly regularly and check the oil within 24 hours after flying, you could get away with never burping the engine.
Joe Gores
 
Joe:
Thank you! That makes sense. I am amazed that such information has not been plainly presented heretofore. I have seen some of it....perhaps from you...but never so concisely and usefully stated.

Even though the risk of harm from slow rotation of the prop may be very, very slight I think that any pilot should and would avoid pulling a prop through without very good reason; if touching a prop at all can be avoided it should be avoided and I will start that now!

Thanks again,

EBB
 
As a few have stated here the prop rotation before start was just to help facilitate a little more accurate oil level check. Oil accumulates in the bottom of the case. It varys quite a bit between each engine. One time it may be 2-4 ounces in which case only a few rotations will push the oil back to the oil tank. Crankcase pressure as the engine rotates is the only thing that moves the oil back to the oil holding tank. It is usually around 4.5 to 5 psi. I have seen some engines keep as much as a quart back in a few isolated cases. In this case the owner would have to turn the prop many times to get all that oil back to the tank. You do not need to rotate the prop fast to get air pressure to move the oil back to the tank. Even if you hear the infamous gurgle, at times the oil level can still lie to you by a few ounces which may be the difference of the oil level half way up on the stick or at the top. Too many think that if the oil level was at the top one week and then half way down the next it means you are using oil. Most of the time it is probably lieing to you so it is better to start the engine an let it run a couple minutes then re-check it. If you are truly using this much oil you will see signs of an engine leak and usually have oil on the bottom of the plane, oil in the exhaust and or oily spark plugs. The old oil level from days gone past used to be 30% lower in the tank. Because of foaming issues with some oils Rotax raised the oil level in the tank so if your oil did foam it wouldn't suck in the air. The old dip sticks had a round grab handle and our new ones are square.

Okay now hydro-lock. This was more prevalent many years ago with the old oil filters. It was also more prevalent on the Kitfox and other aircraft that mounted the oil tank too high compared to the engine. Rotax specifies a correct mounting height in the installation manual. This is just another case and there are way too many of aircraft MFG's not following Rotax recommendations and doing what they want without the knowledge of the consequences.
The older filters had different anti drain back membranes, lower by-pass pressures and no internal check valve like our current oil filters. This is also why we should never use a car filter. When the engines with the high mounted oil tanks were combined with the older oil filters sometimes the oil would syphon back from the high oil tank and leak back with enough pressure to have oil accumulate in a cylinder. So if the owner failed to rotate the prop and check for the hydro-lock and just started the engine the oil in that cylinder would not compress and the starting forces would bend engine components.

It isn't as prevalent now days with better filters, newer engines and more people following the Rotax installation requirements.

I hope this helps.

P.s.
This info has been out for many years and is easily obtained in a Rotax class.
 
EBB

With all due respect, if you actually consult the Rotax 912ULS Operator's Manual (which, along with all the other current Rotax 912ULS documentation, is available 'on line'):

http://www.flyrotax.com/portaldata/5/dokus/d05649.pdf

and head to Section 3-3 'Pre-Flight Checks' (Page 3-6) under the heading of 'Oil' it takes you step by step through the gurgling/burping process - why you do it and how to do it.

There is normally a hard copy of this manual and a CD of various documents included free with every Rotax 912ULS.

flyeyes - I was not saying you were against the mighty Rotax. I am totally in agreement with the sentiments of your post and the care that should be taken when around piston engined propeller-driven aircraft - regardless of the engine manufacturer.
 
For Aaerofurb:
I read that long ago. Page 3.6 only explains how to burp the engine and check the oil. It does not tell the engine owner why it is important to check the oil by turning the prop until the burp; page 3.6 says nothing about why the burp is better than any other way to check the oil. Joe's response to my questions does that and it also offers a way to check the oil without burping and, thus, avoiding handling the prop at all.

Maybe the problem with the owner's manual is due to translations from German to English but I doubt it. It appears to me that nobody has thought through the reasons an owner would not want to burp.

I appreciate very much your effort here to help explain this. I think it's important for owners to have access to better information than is available in the owner's manual or on the net in all the places where I have looked.

Thanks,
EBB......BTW, I'm glad I'm an Ex Bonanza Bucko after 30 years and 3000 hours in my beautiful old V356A...I loved her but that is in the past and my RV12 is good enough for me now.
 
I co-owned a '66 V35 (not A or B) for several years. What a beautiful machine! Inspired confidence in those unfamiliar with GA that would walk up to it. Fast and roomy for 4 people! Wonderful handling characteristics and a delight to land. BUT THIRSTY! AND EXPENSIVE PARTS AND UPKEEP! My mission has changed and the RV12 suits it wonderfully and is a superb airplane in its own right.
 
From Bonanza to RV12

Bill:
I couldn't agree more...about both things you mentioned: the Bonanza being a superb airplane and the RV12 being "good enough" now.

I owned N7121N, a 1969 V35A, for 30 years and flew it 3000 + hours. We went from California to Canada to Mexico and back for years. We built a house on a remote beach in Baja California that was 1000 miles from our Sacramento home then and we pretty much made that trip every Friday evening and back every Sunday evening for a long time. The Bonanza made that easy. It is a fast (I cruised at 165Kts at 23/2300 and about 14 GPH), quiet and smooth airplane. Mine had an 840 pound payload with full fuel. We had a 900 mile range with reserve. It was a superb IFR platform and we were best friends for all those years.

I was a for free CFI in flying clubs all over California, North and South, as my job took me to those places. I flew just about every single and light twin airplane that was made between 1950 and 2005. The Bonanza was by far the best of the lot. We are an aviation family...the RV12 is our 8th airplane in the last 50 years and I think the RV12 will be the last one....it's good enough to be that.

Thanks,
Happy Flying.
EBB
 
EBB

As per the link I provided, the Operator's Manual describes the process for gurgling the 912 and why you do it - ie to scavenge the oil back to the engine's oil tank to enable the level to be accurately read.

It also goes through the reasons (ie because of the way that the system operates) in Chapter 7-3 (page 7-) of the Ops Manual and again the Maintenance Manual (Heavy) Chapter 12-10-00 Section 4-1 (Page 9).

It does state the reason for carrying out the gurgling process (to return all of the oil from the crankcase back to the tank) and it does state how it is done (by blow-by gasses pressurising the crankcase). How much more is there to know?

Okay, so you personally desire even more information about the process than is provided but I reckon that Rotax cover the systems pretty well in their freely provided manuals and documentation.

Do car (automobile) manufacturers provide such detailed information? Not many operators manuals go through all the engines' processes in intimate depth: 'need to know'?

It is Rotax's engine so I'm happy that they know the best processes and procedures for the safe operation of their engine.
 
For Aerofurb:
Thanks for all of that. I understand that the purpose of the burp is to return all the oil to the tank so the quantity can be checked and etc. That is easy.

But I think it's prudent to not touch a prop under any circumstances so I still need to know the following:

Is it a requirement to check the oil level by burping the engine? Said another way, can the oil level be checked by other means....such as at the end of a flight when all the oil has been pumped up into the tank?

Thanks again..I appreciate your interest in all of us understanding this stuff.

EBB:)
 
In cases where only a single pilot ever flys an airplane powered by a 4 cyl Rotax they can just check the oil immediately after shutdown and have a valid reading (there is tons of past discussions archived here in the forum about this subject), but what about an airplane rented from an FBO? Or one shared by a number of different pilots in a partnership or club? Would you trust that the last person that flew it checked the oil for you? Probably not.
That is the reason Rotax had to develop the procedure.

As already mentioned in this forum, if pilots would follow the recommended shutdown procedure .... pull the throttle to the idle stop, at the moment it hits minimum RPM switch off one ignition system and then about one second later switch off the second one, the engine will shut down with the minimum amount of shake, and if you pay attention, you will notice the RPM change as each switch is moved to off. This confirms that both ignitions are being Disabled. As long as no one messes around with any of the wiring in the airplane before the next flight, it is nearly impossible for the engine to accidentally start while turning the prop as long as the switches remain in the off position.

I don't know how to emphasis this any better....

Traditional aircraft engines are not shut down by turning off the ignitions, so there is no way to know if the engine is safe unless you do an ignition check just before shut down.

A Rotax is shut down by turning off the ignitions. If the engine on an RV-12 stops when both switches are moved to the off position, then the ignitions are safely disabled. If you have to shut off the fuel to get the engine to stop, THEN you should be very careful about moving the propeller.
If any maint. or repairs have been done in the engine compartment or on the electrical system since the last flight, it would be prudent to use care until you can once again confirm that the engine can be shut down with the switches.
 
(snip)I don't know how to emphasis this any better....

Traditional aircraft engines are not shut down by turning off the ignitions, so there is no way to know if the engine is safe unless you do an ignition check just before shut down.

A Rotax is shut down by turning off the ignitions. If the engine on an RV-12 stops when both switches are moved to the off position, then the ignitions are safely disabled. If you have to shut off the fuel to get the engine to stop, THEN you should be very careful about moving the propeller.
If any maint. or repairs have been done in the engine compartment or on the electrical system since the last flight, it would be prudent to use care until you can once again confirm that the engine can be shut down with the switches.


Almost, but not quite. The most frustrating issue we've dealt with over the years on the Rotax is wires breaking inside the PVC insulation. This has caused intermittent failure of the "P" lead to go to ground, and on one of the engines an intermittent failure of one ignition to fire, leading to hard starting and aborted flights. It's not uncommon to fail to get a "mag drop" on runup, an engine that won't shut down with the switches, but after the flight it shuts down normally.

We actually ferried the airplane to Lockwood for the intermittent problem, they swapped ignitions, bench tested everything (no anomalies), flew it a bunch, then pronounced it healthy after a very extended annual/test flying/troubleshooting. The airplane didn't make it home on the return ferry flight--it wouldn't start after a fuel stop.

I have a handful of pieces of wire taped to the wall of my shop that look OK, but will test open with a meter until you start bending them back and forth.

I realize that these issues are probably more common on the AirCam, because of the exposure of the wires to the slipstream, but the point here is that it's not completely safe, even with the switches off
 
Suppose that others have been flying the RV-12 or maybe you flew it last but forgot to check the oil after shutdown. Now check the oil without burping. If the oil level is above the add oil line, then go fly. You will not know the exact oil level, but you will know that that the engine will run without being damaged from lack of oil.
Joe Gores
 
it is nearly impossible for the engine to accidentally start while turning the prop as long as the switches remain in the off position.

Almost, but not quite.

Which is why in the portion of my post I said "nearly", but I agree.

I imagine all of the broken wires you have collected produced some type of indication that not all was right. Pilots need to pay attention to what the airplane is telling them... I have never seen a problem fix it self yet.
 
Suppose that others have been flying the RV-12 or maybe you flew it last but forgot to check the oil after shutdown. Now check the oil without burping. If the oil level is above the add oil line, then go fly. You will not know the exact oil level, but you will know that that the engine will run without being damaged from lack of oil.
Joe Gores

Been doing this for 7 years without problems.
 
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