Rotax engines (2) carburettor floats

Rotax carby floatsAt last Rotax has issued a final service bulletin covering replacement carburettor floats. For those not fully up to speed on this, Rotax has had a problem with ‘permeable’ carburettor floats on most recent 912 series engines. Basically any faulty floats absorb fuel and sink to the bottom of the float chamber, causing rough running and potentially overflowing fuel. Floats on affected engines had to be checked every 25 hours and if needed, replaced.

The latest service bulletins (click on the numbers to see & download): SB-912-067 and SB-912-067UL list the affected engine numbers and necessary actions.

In addition, Rotax has posted information on their blog about how to get your replacement floats.

In Australia, contact Bert Flood Imports – phone 03 9735 5655 – near Melbourne, give/send them your engine number, and they will send you replacement floats and an invoice for them. Returning the old floats quickly together with the invoice will result in a refund/credit or cancellation of the invoice.

Hopefully this will see a conclusion to the concerns over faulty floats.

Rotax engines (1) scheduled service intervals

Rotax serviceOn what basis do you record your times for scheduled services on your Rotax engine? Engine running hours – start to stop? Starting to taxi to stopping – wheels turning to wheels stopping? Flight time via an air switch – wheels off to wheels on the runway?

At bigger and/or busier airfields there could easily be a big cumulative difference between engine start/stop and wheels off to wheels on the runway – as much a 25% or more by some LAME accounts. So what is the correct procedure for recording times to determine scheduled maintenance on your Rotax engine? The answer, it seems, has been about as clear as year-old engine oil!

The initial response from the Rotax service department to my query was that maintenance must be carried out based on engine running time as recorded by ‘an electronic engine hours timer’ – ie start-up to shut-down. This is different to some of the statements in their maintenance manuals and certainly not the way most flight schools and clubs record time for engine servicing; they usually use take-off to landing times for overhaul.

Please note:
What follows has now been superseded by the following: all Rotax engine scheduled maintenance times are currently based on engine running times – ie from start up to shut down. Therefore, for maintenance purposes, record engine running times accordingly. Also check to ensure that the airframe manufacturer aligns their mandatory maintenance schedules with the engine maintenance requirements. Most of them do – including Aeroprakt.

Please also see my updated blog post on scheduled maintenance times:

When questioned a little more, Rotax finally clarified that their engine scheduled maintenance times comply with standard FAA (and CASA) practice – that is, take-off to landing time, ideally measured with an air-switch hours meter. I now have this in writing (by email) from them, so if you or your engineer need written confirmation, let me know.

So, repeating, for the sake of avoiding all doubt – record your engine scheduled maintenance times from take-off to landing. Maybe even fit an air-switch-operated hours counter if you need to. In the long run, this can save you quite a lot when it comes to servicing, likely well more than the cost of fitting an air-switch.

PS – Don’t forget: if you are late with a service – eg 55 hours instead of 50, the next service is still due at 100 hours, not 105. Read the Rotax manuals!

Oil stains on the bottom of your aircraft?

One or two Rotax-engined aircraft owners have commented to me about ‘overflow’ from the oil breather tube which comes out of the Rotax oil reservoir/tank – often leaving a big brown stain all over the bottom of the aircraft. This can be very noticeable and leads to all sorts of concerns about what’s causing the tank apparently to become pressurised and release oil overboard. In fact, the amount of oil is very small, but the slipstream spreads it everywhere. Often, once the paint is stained, it’s almost impossible to clean off.

I’ve even heard from engineers, who are convinced there is something seriously wrong with the engine. While I’m not ruling this out in all cases, sometimes the problem is much simpler. There are basically three items to check before investigating the engine:

First, make sure the oil level is correct and not above the top mark on the dip stick after ‘bleeding’ (often called ‘burping’) the engine. Second, make sure the breather tube exits correctly through the cowling under the aircraft – contrary to one engineer’s assertion, the breather tube should NOT terminate INSIDE the cowling – see the Rotax engine installation manual.

Oil breather restricted

Oil breather tube restricted

Oil breather unrestricted

Oil breather tube unrestricted

Finally, have a look at these two photos. One shows an oil breather tube with a kink in it. The other photo is of a correctly installed tube. As you can see, the kinked tube is brown – mainly due to oil staining – and so is the bottom of the aircraft. The correctly installed tube, with no obstructions, is clean as is the bottom of the aircraft. It is essential that the oil reservoir is free to breathe!

So, before you start worrying about major engine problems, check the oil overflow tube. If It’s brown and discoloured, make sure it isn’t kinked or tied too tightly with cable ties. If necessary, replace it – or get a qualified engineer to do so. Bunnings Aerospace has an excellent selection of quality plastic tube which will do the job very nicely. Make sure the replacement tube is correctly cable-tied and routed away from the exhaust. And ensure there are no kinks or pinches in it.

Chances are, the overflow/breather problem will disappear.