Rotax 912 Engine: 2,500 Hours in 29 Years (achieved in 2024)

Rotax 912 engine

The Rotax 912 engine in our plane, a Zenith 601HDS, has reached an important milestone. Over the past 29 years, it has run 2,500 hrs without major repairs.

Zenith 601HDL, N314LB

It's appropriate to reflect about this performance and muse how this is possible. After all, when we bought the engine, in 1994, the suggested TBO was 1,200 hrs. 

Since then TBO has gone up, first to 1,600 hrs, and more recently to 2,000 hrs. Of course, those increases apply to later engines, not to our early model.

Let's see how the engine performs after all this time.

Performance after 2,500 hrs

Compression is 76-80 lbs/sq.in. on all cylinders. Oil consumption is less than 1/2 qt per 25 hours. 

During each oil change, we cut open the filter and look for metal. There is almost none, just one or two tiny specks. Essentially, the engine shows no sign of wear.

The engine performs well up to the legal limit of 14,000 ft MSL for flight without oxygen.

When flying without a passenger at 14,000 ft, the plane can still climb at 250 ft/min. Thus, we can cross the Rockies at almost any point.

How is this possible?

Demands on the engine

We always let the engine oil reach 120 deg F at 2,400-2,600 rpm before takeoff, as Rotax recommends. 

In flight, we cruise at low power settings. During the early years, that meant 4,800-4,900 rpm in cruise. 

In recent years, we have reduced this to 4,200-4,500 rpm, with 4,300-4,400 rpm preferred. Our current goal is to be in the air a long time, not to get somewhere fast! We revert to higher rpm only when strong headwinds would slow us down too much.

Cruise speed at the low power setting is 80-85 kts depending on payload, and at the higher setting 95 kts.  

Fuel consumption at the higher rpm is close to 4 gal/hr. For the reduced setting it used to be 3.5 gal/hr, but dropped after we replaced both carburetors. A recent cross-country trip required just 3 gal/hr. More on this later. 

At the reduced setting, the engine outputs about 50% of max horsepower. This estimate uses the reasonable formula that each gallon/hr produces 13 hp and also agrees with a chart in the original Rotax 912 Operator's Manual.

Engine operation

The water temperature typically hovers around 210-220 deg F. Oil temperature is 190-210 deg F. During a hot-weather take off and climb-out, the oil may reach 220 deg F. 

An oil fan controls the temperature during take offs in 100 deg F air and while cruising in humid 90 deg F air.

The plane originally supplied air under the cowl to the carburetors. That air can get quite hot in the summer. 

Before we redesigned the cowl, the air temperature in summer was more than 140 deg F during the early phase of flight. It remained beyond 100 deg F during cruise.

As a remedy, we designed a separate air supply that brings outside air to the carburetors. Here are the details.

Air Intake and Alternate Air

An air intake next to the reduction gear box funnels cold air via two hoses to the carburetors. 

We tested the system for heavy rain by pouring water from a fully turned-on garden hose into the inlet. There was no noticeable effect on engine performance.

Nevertheless, the cold-air supply can be shut off, in which case the carburetors use the hot air within the cowl.  

Intake for carburetors ...

This system does not force outside air into the carburetors, but blows it onto the air filters. If the outside air supply is turned off, the carburetors take in the surrounding air.

Hoses from air intake to carburetor filters

Caps distribute the cold intake air over the air filters.

The choke lever of the carburetor has been safety wired in the off position. Instead of the choke, a primer is used.

Throttle control relies on rods instead of cables. The rods never change length, so once the carburetors are balanced, they remain so.

The outstanding performance of the engine is most likely due to the modified air intake. Keeping oil and water temperatures moderate is also important.  

We now discuss how we have maintained the engine.

CAUTION: This is NOT advice what you should do. We simply describe what we have done. It is up to you to draw any conclusions.

Lubrication

Way back when, Rotax recommended that a good-quality automotive oil such as Pennzoil or Mobil be used. 

Over the years, Rotax has changed its motor oil recommendations multiple times. Currently, Rotax prescribes a special-formula oil produced by Shell. Despite these changes, we have continued to follow the original rule.

In the early years, we used Pennzoil 10W-40 mineral oil. 

More recently, it became clear that a mixture of mineral oil and synthetic oil was a good idea for aircraft engines. That is, the mineral oil part keeps the lead sludge in suspension, while the synthetic portion improves lubrication. 

So for a number of years, we have used two quarts of mineral oil and one quart of synthetic oil for each oil change, all produced by Pennzoil.

Pennzoil mineral and synthetic oil

By the way, we have found it convenient to check the oil level when we have reached the final destination of the day, and not just before the flight on the next day. That way we can add oil if needed and don't have to to do this in the morning just before the next flight. Of course, we still check the oil level as  part of every preflight inspection.

Oil filter

In 1995 we established that the Fram filter PH3614 had the required backstop valve and fit the engine.

The Fram filter was cheaper than the Rotax product and did not suffer problems such as leaks plaguing the Rotax-recommended filter.

Recently, Fram has begun offering a superior filter called Ultra Synthetic. We have switched to that filter, model XG3614.

Fram Ultra Synthetic oil filter in the box ...

... and installed

Fuel

During the early years, a number of airports in the US, in particular in the Midwest, had mogas. Since the Rotax 912 can run on regular unleaded autogas, we refueled at such airports whenever possible.

When flying locally, we always use premium unleaded autogas.

For years now autogas has contained 10% ethanol, a harmful additive. But Rotax says that this is okay. We have not seen any negative effects on the fuel system or the operation of the engine. 

During the last 15 years, hardly any airport has offered mogas. We have had to use avgas on virtually all cross-country trips.

Avgas contains far more lead than autogas did before it was removed entirely from autogas. We add Decalin to the tanks when refueling with avgas. It converts some of the lead during combustion into harmless white stuff.

Decalin additive for avgas

Apparently Decalin has worked well. During a recent repair of the engine mount, we removed the oil tank. We discovered that after more than 1,900 hrs of operation, the bottom of the tank had only a light coat of lead sludge.

Also, when we removed the reduction gearbox for reshimming one year ago, the housing was clean on the inside. There was no accumulation of lead sludge.

Coolant

When we began flying the airplane 29 years ago, a few drops of coolant appeared at the bottom of the water pump housing during each preflight. 

We hoped that this would go away, and indeed it did after a while. Since then, we have not have had any leak involving the water pump. 

In the early days, Rotax recommended use of undiluted antifreeze. But we quickly learned that the engine was insufficiently cooled. So we switched to a 50/50 mixture using distilled water.

Later, Rotax recommended long-life coolant. We switched to that superior coolant, again diluted by 50% distilled water.

Prestone long-life coolant

Cooling System Pressure Cap

The pressure cap failed after 28 years. The seal looked fine, but the hidden vacuum valve quit working, and any flight with coolant temperature above the boiling point resulted in loss of coolant. 

Upon landing, there was no sign that coolant had been lost, except when the engine had cooled down. At that time, the level in the overflow bottle was much reduced. Sometimes the bottle had become empty.

The replacement details and another mysterious loss of coolant are covered in the post Vanishing Coolant of Rotax 912 Engine.

Hoses

Manufacturers of light sport aircraft typically require a Rotax-designed replacement process of hoses and other parts every five years.

We have not done so. Instead we use the US Air Force concept of replacement upon condition. It has served us well. 

Recently, we had to pull off the engine to repair the motor mount. At that time, we had to cut off the water hoses since they seemed almost welded to the fittings. 

Checking the inside of the water hoses, they were in excellent condition. And that after 25 years! Of course, we had to replace all those cut hoses.

The oil hoses were also in good condition. We replaced them anyway since they had already been removed.

Ignition

Rotax recommends replacement of the spark plugs every 100 hrs. We replace them during every annual inspection regardless of time flown.

We use the NGK spark plug DPR7EA-9. Every motorcycle shop has it in stock. It is an appropriate replacement for the Rotax-recommended spark plug.

NGK spark plugs DPR7EA-9

Fuel Pump

We had a number of failures of the mechanical fuel pump. Eventually, we considered that pump to be a safety hazard. We then removed the mechanical fuel pump . . .

Gearbox without mechanical fuel pump

. . . and instead installed a dual electric pump system that has served us well.

Dual electric fuel pump system

Repairs

In 2019, we replaced the alternator since the wiring became frayed. Apparently this was a problem with the early engines. It is surprising that the wiring lasted as long as it did. 

Here are the details about the replacement of the alternator.

In 2022, the carburetors started leaking since the float needles no longer closed properly. Lockwood Aviation attempted to rework the seats of the needles. 

Upfront they said that they might not able to do so. That happened indeed, and we had to install two new carburetors. 

The replacement turned out to be a blessing: 

The engine idles much more smoothly. When we reduce power in flight, the engine no longer shakes.

 

Fuel consumption at our low-power setting has dropped from 3.5 to 3 gal/hr as described earlier.

In 2022, the exhaust pipe of the #3 cylinder broke off close to the cylinder head. A very dangerous situation, described in detail in the post Rotax 912 Engine Failure. 

The story includes extraordinary help by ATC (Air Traffic Control) and recommendations for handling such an event.

Summary

With our simple maintenance choices we made the engine last for so long. And it still does not show any significant wear! An amazing performance.

Coda: The Original Manual

When we bought the engine in 1994, it came with a small Operator's Manual.

Rotax 912 Operator's Manual, edition 1993

On 65 pages, size roughly 5 x 8 in., it provided:

• a description of the engine design
• technical data
• performance graphs
• handling instructions
• checks on engine and installation
• maintenance
• summary of service and repairs
• a table of lubricants
• torque specifications for bolts and nuts
• trouble-shooting instructions
• service partners
• warranty conditions
• warranty card

After installation of the engine we bought the 100 page list of spare parts and the 78 page repair manual. Thus, we had all information for:

• operation of the engine
• purchase of parts
• all engine repairs, including overhaul

Compare this with the size of today's manuals:

• installation - 203 pages
• engine operation for the Rotax 912 series - 91 pages
• parts catalog - 202 pages
• line maintenance - 174 pages
• heavy maintenance manual for the 912 and 914 series Rotax engines - 468 pages

These numbers indicate that we no longer live in a simple world.

Have any questions or feedback about the long-term performance of the Rotax engine? Please share your thoughts in the comments.