Shutting down a Rotax 912ULS

[XP503]

When switching the mags off - the RPM should be as slow as possible or at least full idle, correct?

 

I have noticed pilots revving the engine then switching both mags off, is there a reason for this that I am unaware of?

 

Cheers

 

 

[Admin]

I don't know if it is right or not so I am with you to get a proper answer but I have found that if you time it right by switching the mags off just as you rev it, the engine stops a hell of a lot smoother then just getting a "clunk"

 

 

[facthunter]

Shutting Down 912.

 

I have always believed that having this engine as slow as possible is easier on the reduction gear/clutch. The throttle should certainly be right on the idle stop as having it open admits more air to be compressed and consequently more load on the parts. Why don't you ask the pilots why they do what they do? Some years ago when I was involved with them the larger radials were shut down with a few revs on and the throttle was opened to expell the corrosive combustion products. When there is 18 cylinders the engine turns over quite smoothly so there is no shaking or harshness. The engine components act as a massive flywheel. Nev

 

 

[Yenn]

You will not clear corrosive products unless you have a fuel cutoff, which I don't think applies to any Rotax, Jabiru or ex car engine. No matter what happens without a means of stopping fuel flow there will be fuel or exhaust gas in the system.

 

 

[facthunter]

Cutting Fuel.

 

Yes Ian. The engines were stopped by cutting the fuel. The Citabria (when I had it) was also stopped that way. Cutting the ignition can cause running on with "hot" spots igniting the fuel mixture in an uncontrolled way. This is VERY damaging potentially to an engine and another reason why I would ensure that the throttle is on the idle stop.The Tiger Moth was run out of fuel as it has a carburetter which some times has no mixture control fitted. Nev

 

 

[Spin]

And then the fight started.....

 

This one seems to depend on who you were taught by or who made the biggest impression early on in your flying life. Some instructors swear by a "soft stop", ie run up to 2400, then cut the mags whilst closing the throttle, others are of the minimum rev persuasion. I tend to the latter, although will sometimes go the other way if I know a particular instructor or aircraft owner prefers it.

 

 

[Guest Maj Millard]

I believe the book says to shut them down at the lowest RPMs...however I personally shut mine down with a few RPMs going mainly to assure there is a clean burn going, and to help with a clean start next time which I usually get. I'm probabily shutting the switches off at around 2300 to 2400 rpm.......................................................................Maj..

 

 

[Ultralights]

If you read the 912 owners manual, it states throttle at idle after cooling off period if needed then both ignition switches off. The "clunking" is normal and will not harm the engine as the clunking originates in the gearbox/ clutch assyembly, not the engine, and is a result of momentum of the prop/hub assembly.

 

 

[Guest JeffC]

Rotax SL-912-016 (SL-914-014) - July 29, 2009 - contains "Essential Information Regarding Engine Behaviour ..." and includes at 3.7 Engine Shut down tips:

 

1. to reduce loading on engine and gearbox (eg adjustable props) and throttle to idle,

 

2. after cooling down run, close throttle fully so engine is at minimum speed, switch ignition off on one circuit for 2-3 seconds then switch off the second circuit.

 

3. refers to 914.

 

I'm happy with the procedure in para 2. It seems to minimise the stopping shudder.

 

I was also taught by my father (and also the Army) to never stop an engine after revving to ensure the oil was not flushed from the cylinder walls. Makes sense to me.

 

Anyway, for what it's worth, that's Mr Rotax's view on the question.

 

 

[bones]

Rotax SL-912-016 (SL-914-014) - July 29, 2009 - contains "Essential Information Regarding Engine Behaviour ..." and includes at 3.7 Engine Shut down tips:1. to reduce loading on engine and gearbox (eg adjustable props) and throttle to idle,

2. after cooling down run, close throttle fully so engine is at minimum speed, switch ignition off on one circuit for 2-3 seconds then switch off the second circuit.

 

3. refers to 914.

 

I'm happy with the procedure in para 2. It seems to minimise the stopping shudder.

 

I was also taught by my father (and also the Army) to never stop an engine after revving to ensure the oil was not flushed from the cylinder walls. Makes sense to me.

 

Anyway, for what it's worth, that's Mr Rotax's view on the question.

Well the switching off the mags, one at a time is bad for a 912 from what i heard?

 

This was blamed for my sprag shatting itself just a wee while ago, must admit with the new sprag in and the bigger starter, that motor sure gets a rude awaking.

 

The 914 really doesnt clunk like the 912 because of teh lower compression.

 

 

[Ding]

Shutting down.

 

I'm with Jeffc.

 

By the book for me. Seems to give as smooth a stop as you can get.

 

Mr Rotax must know something.

 

Ding.

 

 

[XP503]

Yea I normally do it as JeffC has written.

 

Full idle, check mags

 

one off, other one off.

 

It stops much softer than those who rev it up as they shut it down.

 

 

[Guest milton56]

I run my 912uls to about 2000rpm and then as you pull the throttle switch off the mags, this seems to reduce the clunk effect.:thumb_up:

 

 

[dazza 38]

I run my 912uls to about 2000rpm and then as you pull the throttle switch off the mags, this seems to reduce the clunk effect.:thumb_up:

I do the same as Milton.

 

 

[Guest Howard Hughes]

I don't have a 912...

 

 

[Guest Walter Buschor]

I always pull the throttle back and switch both circuits off.

 

On another note: when I start a cold motor I only turn on one ignition circuit as this seems to generate less " kick back ". As soon as the motor runs I turn on the second circuit. I know there is a softstart option for the Rotax but mine doesn't have it but the starting on one seems to help a lot.

 

fly safe

 

Walter

 

 

[Tomo]

I haven't said anything, as I don't often run a 912.

 

But lets say you've been idling around in a Lyc or Cont or whatever, it's best to rev it up a bit before switching off, reason is to burn off the plugs, so they don't fowl up. Another reason you turn those engines off with the mixture, instead of the mag's.

 

I'm no expert on this, but this would me my take on this particular path.

 

 

[Spin]

Yeah it's a bugger when the chooks get to your plugs:big_grin:

 

Tomo the debate here is more about reducing the clunking or backlash on the gearbox of the 912's when you shut down, ie one second the motor is driving the prop the next the prop wants to keep going due to inertia but the motor has stopped dead on compression. I know that it can seem that there is less of that objectionable clunk if you shut the throttle from say 2000 rpm and simultaneously kill the mags, however logically I would expect the minimum force to be transmitted if the engine were turning at minimum speed when it happened.

 

The other subject you brought up, that of burning off the plugs, I can't see that a quick blip of the throttle is going to do that for you - I was taught to up the revs slightly then lean the mixture, but only if necessary.