Recip Technology: Rotax Engines

Rotax engines are a popular choice for certified light and homebuilt aircraft, auto gyros, and military UAVs. Though you may not have encountered one yet, chances are you will in the future.

So what sets the Rotax apart, and what do you need to know to troubleshoot it? To answer these questions, AMT spoke with Rotax expert Dean Vogel of the Aero Technical Institute (ATI; Affiliated with Lockwood Aviation in Sebring, FL, ATI offers Rotax maintenance training courses.

AMT: First, tell us about the Rotax engine. They are quite different from a Lycoming or P&W, aren’t they?

Vogel: Yes. Rotax engines have much more in common with motorcycle engines than they do with traditional aircraft engines.

AMT: What makes them different?

Vogel: Well, Rotax engines use liquid cooling to keep their size and weight down. They have five-piece crankshafts, rather than the traditional one-piece forging used in most aviation engines. And their oil tank is located outside the engine proper; allowing for a lubrication system that is effective with a lower oil quantity and significantly less weight. As well, Rotax engines use motorcycle oil — aviation engine oils are totally unsuitable — and automobile fuel.

AMT: Now that we understand how different Rotax engines are, what are the three most common troubleshooting issues associated with them?

Vogel: The three most common troubleshooting items, in probable Pareto order, are carburetion, kickbacks, and ignition. Please understand, if these engines are properly set up and maintained, problems are next to non-existent. But if they do have problems, these are the most commonly seen.

AMT: What symptoms do carburetion problems present, and how do you diagnose them?

Vogel: With carburetors, the symptoms will be engine roughness, and it may or may not be accompanied by a decrease in power. Once in a while the problem will show up during the mag check, and it will be difficult to determine if the problem is ignition or carburetion.

One quick troubleshooting tip is to put the engine where it is not happy, rpm-wise. If that means on one mag, put it on the mag with which it’s the most unhappy. Then pull on the choke. If there is no change, it’s likely that the problem is not carburetion. If the problem disappears, or gets dramatically worse, the problem is the carburetor or something in the air or fuel delivery systems.

AMT: What about kickbacks?

Vogel: They show up as difficulty in starting the engine. There may be a loud banging during start with an associated, apparent pause of the propeller. When the problem is advanced, the propeller may stop altogether and the starter may spin free — and this often can be heard from the cockpit.

The problem with the diagnosis is that several possible starting problems will look the same, and — if left uncorrected — will eventually result in sprag clutch failure. So if you have kickback you may have need to replace the sprag clutch, but if you don’t correct the other issue(s), you’ll be going right back to damaging the clutch again.

One of the easier ways to tell if the sprag clutch has been damaged to the point it needs replacement — regardless of the presence of other issues — is the amperage going to the starter. Place a clamp-on type ammeter around the cable that powers the starter. You will need one that has data hold. Press the data hold button and hit the starter. If you see a startup amperage of less than 75 amps, the sprag is too far gone and needs replacement. You will obviously want to make sure your power supply is adequate before doing this test. Given the expense of changing the sprag, you wouldn’t want a false reading to come from a low battery.

The usual other culprits are a weak battery, too long a cable from the battery to the starter with insufficient gauge, carburetor problems, gearbox maintenance, a propeller with too high a moment of inertia, a resonance in the engine mount or airframe, or the need for a Soft Start module on the ignition.

AMT: As for ignition?

Vogel: Ignition problems will show up either in the mag check or at shutdown. If the engine won’t start, the problem is almost certainly not the ignition. The ignition system on this engine is dual, independent, redundant, and solid-state. The only moving part is the flywheel. The probability that both systems would fail at the same time in the same mode is extremely remote.

The major components of the ignition system — with the exception of the modules themselves — have resistance values that are published in the Heavy Maintenance manual. This provides one way to check for the health of those components. Beyond that, troubleshooting of this system is a little too complex to attempt a description in a magazine article, but suffice it to say that with complete parallel systems, it’s fairly convenient to troubleshoot by switching connections in different places to work the gremlin into a corner.

A word of caution: Do not treat this ignition like the magneto on a Briggs & Stratton. Putting a screwdriver in a spark plug cap and holding it next to the block to check for spark is dangerous — not to you, to the ignition module. Many people have started off thinking they had an ignition problem when none existed — until their troubleshooting technique resulted in one, a $1,100 one.

AMT: Please walk me through the process for resolving these problems.

Vogel: Carburetion problems are almost always a cleanliness issue. Often people will be struggling with issues and call us for help. After finding out how old the carburetors are, or how long it’s been since they’ve had any scheduled maintenance, we suggest that the carburetors be rebuilt and taken back to their original configuration so we can eliminate the obvious before continuing to troubleshoot. As you might expect, after the rebuild the carburetors are reinstalled and synchronized — and the problems are gone.

If the problem persists, there are other adjustments available, but you’ll want to consult with someone who possesses a lot of experience with these engines before toying with them.

Kickbacks: Replacement of the sprag is a heavy maintenance repair item. That means it should only be done by a facility that has the specialized tools and training to accomplish the task. In the vast majority of installations this means the engine must come out of the airplane. Now you know why we urge people to take care of starting problems before they get to the point of damaging the sprag.

One of the key ways to avoid doing this damage is to get a Soft Start module installed in one of the ignition circuits. Experience from the field indicates that even if the airplane has one of these other problems, the Soft Start may make up for it.

When it comes to ignition, the most frequent problem — when one shows up — is the wiring. Once it’s been located it is easily repaired — unless the break occurs very close to where the wire disappears into the component. Once the wire is repaired, take another look at the way the installation is done. If the connectors are not secured, they could be flopping around due to engine vibration and/or wind buffeting, and be the source of the wire failure. Make sure you secure the connectors — securing wire bundles next to the connectors will only make matters worse.

If troubleshooting indicates one of the actual components is the problem, replacement is the corrective step. All the components, as we said earlier, are solid state. There is no such thing as opening them up and repairing them.