Aviation Oil Consumption - Are you flying low on oil?

Engines don't "find their level" and stay there, it don't work that way!

OIL LEVELS: As long as I have been flying aircraft I have heard from nearly every person I have come in contact, "I don't fly her full on oil because she just blows out that first quart"; or, "all aircraft find their comfort level and they just seem to stay there". THIS IS NOT TRUE! The fact of the matter is, those who say this don't have a clue as to the oil burn rate of the aircraft they are flying. If you want to lose a quick $500 bring your aircraft to me; we'll fly it for 10-hours and I will prove it to you; but I won't fly it more than a quart low on oil. If I'm wrong, I'll pay you the going rate on your aircraft plus an additional $500.

First, one must understand that oil is not just a lubricant. Oil has a secondary purpose which you should be learning in your flight training; oil provides internal cooling. That is, oil absorbs heat generated inside the engine and transfers this heat in the oil cooler (heat exchanger) or in the oil pan. Less oil means less heat absorption/transfer capabilities and consequently a hotter internal temperature. As the internal temperature rises, oil consumption increases. Why? As the temperature of the oil rises, it becomes thinner; thinner oil means smaller drops; smaller drops leak faster than larger drops. This means that the thinner oil will leak past the rings and valve guides into the combustion chambers at a higher rate as well.

Leak Rate: Each quart of oil contains a given number of drops. If a leak develops, the leak will occur at a "rate" or drops per second or minute. If you make a habit of running your engine low on oil you will decrease the time your engine will live if a leak develops.

I was flying a Bell 47 with a Lycoming 435 several years ago when it developed an oil leak at the engine filter container. I had flown for two hours when I landed at an airport for fuel. Oil was everywhere. I landed with roughly 6-quarts remaining in a 12-quart system. Had I the habit of filling the oil only partially as often occurs, this engine would almost surely have failed as the critical level of oil would have been reached. When I say critical level, I mean that level where there is some oil in the engine, but the majority may be in lines, the heat exchanger etc. With minimal oil in the crankcase to transfer heat and to lubricate, the engine will overheat internally and bearing, metal fatigue, and other internal damage will certainly occur.

Most importantly, I will not drive my car low on oil and I can coast it to the side of the road. Why would I fly an aircraft low on oil where if an engine problem develops, I have a serious emergency?

Engine manufactures do not put the "Full level" on the indicator for their health. They do it because they know what the engine oil requirements are. They also do not indicate the "minimum low level" so that foolish pilots can run the aircraft at this level for extended durations. The minimum low level is where heat transfer and burn rate will no longer be as needed for the engine.

I don't care what your instructor is telling you, keep your aircraft full of oil (add 1-quart as soon as it indicates 1-quart low). If your instructor or the school you are attending don't want to keep it full, then you need to refuse to fly. They need your money so the oil will be added to the aircraft. You just need to be the squeaky wheel.

Does oil "wear out"? No, oil does not wear out. However, oil contains additives that do wear out, burn, or dissipate over time. In fact, as much as 50% or more of what is contained in a bottle of oil is additives. These additives are important, without them your engine would build harmful sludge and the oil would not lubricate as well, nor transfer heat as well. The loss of these additives also causes oil to burn at a faster rate as it ages. Additives in oil do many things, and are very important.

Oil Change Intervals

Oil change intervals are a topic of much debate. It has been proven that oil changes of certain intervals lengthen the life of any given engine. Aircraft engine manufacturers commonly recommend oil change intervals of 50-hours with oil filters, and 25-hours without (Engines without filters have fine screen filters). This is not to say that you have to replace the filter with every oil change, you don't. The price of oil and filters is inflated beyond reason, and that is the reason that many people extend their oil change intervals beyond the recommendation. Take for example one individual who reported that he commonly paid $300 for his 50-hour oil change; that is $6 per hour, or $12,000 over the TBO duration of 2000-hours. First of all, $300 for an oil change is robbery, but hey isn't all aviation maintenance robbery? You know the old maintenance saying, "Never feel sorry for a man who owns an aircraft".

Any individual can learn to change the oil on his own aircraft (except for part 141, 135 or 121 aircraft). You don't have to use that over priced "pilot store or FBO" oil; you can buy the same oil by the case from the same place they get it for much much less. If you only change your oil once or twice a year, then those over priced oil changes probably won't seem so high, but if you're like me, and do two or more oil changes a month, then you can save a lot of money doing those oil changes yourself. For example, if I used FBO oil at $4 per quart (which I don't), and $15 for an over-priced filter would cost me $39 for an oil change if I change my filter everytime (which I don't). I change my filter every other oil change because that is all that is needed, and I buy oil by the barrel for $2.50 per quart so my average oil change costs me just $22.50 and an hour of my time. This comes to $900 over the 2000 -hour TBO. I saved $300 in filters over the TBO which may seem petty, but I am not going to simply hand $300 to someone who is ripping me off for something more frequently than makes sense.

If you think of oil as engine insurance; it is the cheapest insurance you will ever buy. It is the insurance that will keep your engine working at its best for the longest period of time. However, this doesn't mean that it is wise to simply throw thousands of dollars at unnecessary oil and filter changes but you can if you want to. It makes more sense to document your oil additions to your engine; that's right, write down the hours everytime you add a quart of oil. This is studying your engine. You will find that there is a point where the engine starts to burn oil at a faster rate which will depend greatly upon a wide variety of conditions. When oil begins to burn at this higher rate, it is a sign that the additives in the oil are used up or have reached their useful life. This is the interval at which you might consider changing your oil, and it may not always be the same.

A prudent pilot will know a lot about their aircraft or that which they are flying. To gain this knowledge, the pilot must document everything including the hours at which oil and fuel is added religously. By documenting in this way, a pilot will know the first sign that something is abnormal and get it addressed as soon as possible. This could be an increase in fuel or oil consumption either of which will be a sign that there could be something about to go wrong. Of course in most cases, you shouldn't take one instance of consumption increase as hard evidence unless you see a leak, but rather back this up with vigilance, and more documentation.

What is TBO?

TBO (To Be Overhauled) is the common term used in aviation for the "useful life" of any given item. Manufacturers through extensive study determine the useful life of any given engine or component. Not everything has a specified "TBO". The only time that an engine TBO must be adhered to is when the aircraft is being used in a controlled environment such as under Part 141, 135, and 121. Under part 61/91 operations, an engine TBO can be/is typically extended by the normal 100-hour and annual inspections. The thing about these inspections is that they can't really tell the internal condition of the engine such as normal wear over time. They can tell you if you have a bad cylinder, or if you have chunks of metal in your oil change, you would know something isn't normal. One way to enhance the effectiveness of these inspections is to get your oil analyzed periodically, for examply every x number of oil changes or hours. I don't really think it is money well spent to do it every oil change as some will say, but the choice is yours.

There comes a point where if you throw away to much money on oil and filter changes and analysis, it would have just been cheaper to plan to overhaul or replace your engine at TBO which isn't necessary at all if you took good care of it. I know of one Lycoming 435 that was flown to 3500-hours, 1500-hours beyond TBO at which time it was overhauled. If the cost of overhaul was $20,000, where the normal cost of operation per hour was $10 per hour, this engine actually only costed $5.71 per hour, which is significant savings.

There are also what is known as hard times, where a component must be retired at a specific period of time such as calendar months or flight hours. Sometimes these times are determined by the manufacturer and sometimes by AD (Airworthiness Directive). In any case, hard times can not be overflown except by ferry permit regardless of the type of operations.

Engine damage from Leaning, and Carburetor heat

More engine damage occurs from over-leaning than any other single issue including oil. The excessive use of carburetor heat is also a significant factor which causes engine damage. Most pilots just don't understand what takes place inside the engine when the mixture is leaned or when carburetor heat is applied.


Leaning is something that must be done at higher altitudes, and also something which most pilots really don't understand the effect on the engine if not done properly. First, it is important to understand that fuel serves three important roles. It is the source of fuel for combustion, it provides top engine lubrication, and finally, it provides engine cooling. There are three common methods for leaning, two of which are acceptable, and a third which causes significant damamge over time. The two acceptable methods are by cylinder temperature which is provided by sensors and displayed by instrument indication, and secondly by lean-drop. If the engine cylinder temperature is used, it should be backed up by a lean-drop test. The final common method which should not be used is engine max power which is similar to lean-drop but missing one important step.

  • Lean Drop - The lean-drop is the most reliable method and should be verified periodically in flight. From stabilized flight, slowly lean the engine to obtain the maximum power/rpm, then richen the mixture for a 100-rpm drop.
  • Instrument indication - This is self explanatory, but unless an owner has installed the sensors and indicator, most engines do not have this luxury as it is very expensive. When I have flown or owned aircraft with this capability, I always back it up with a lean drop test. When using this method, the indication of the hottest cylinder should be used. There can be significant difference in temperature from one cylinder to the other.
  • Max Power - The maximum power method of leaning is the method most pilots use, but they forgot the other half, that is richening it back up for a 100 rpm drop. Remember that fuel is also cooling and lubrication for the top of the engine.

Some time ago while I was working for a large part 135 operator, I was talking to the mechanics one day, and the topic of discussion was top engine damage from over-leaning.They said that the most of their time was spent repairing damage (cylinder replacement) from over-leaning. The damage caused by over-leaning is the same as that caused by excessive carburetor heat; that damage is warped and burned exhaust valves that no longer fit the valve seat.

Carburetor Heat

The cylinder head is the hottest part of the running engine excluding the exhaust stack. The carburetor heat is taken off the exhaust pipe stove (collection box) where the temperature of the pipe is several hundred degrees. This heat is then routed into the inlet end of the carburetor where it enters the engine still at very high temperatures which increases combustion temperatures accordingly.

Without carburetor heat, aircraft engines would starve for fuel and stall due to frozen carburetors very frequently. With regard to helicopters, we have temperature gauges to indicate the ideal temperature where icing won't occur, and we should be applying carburetor heat to maintain the temperature in that range at all times. There is an exception however, that is the approach to landing segment where the carburetor heat is fully applied regardless of temperature until the hover where it is finally taken off. It is during this approach where in warm climates especially that top engine damage can and does occur from excessively high engine temperatures mostly in the cylinder head.

The unfortunate part with Carburetor heat damage to the cylinder heads is that there is no method to fix the issue. The reason for the full application of carburetor heat is to prevent icing especially when engines are operating at low power settings. It does indeed save lives and aircraft despite the fact that it damages mostly exhaust valves where the temperatures are the highest in the head. There is no fix, and we will keep replacing cylinders prematurely as a result. END. Jump to Top