Thieves Of Aircraft Engine Power and How to Cope With Them.

It appears that some General Aviation pilots are not really aware of the number of detrimental influences on their aircraft engines which can be identified as thieves of engine power, and how they can create an unsafe flight condition. As an example, in most instances the moderate engine power loss that occurs from attempting a takeoff at sea level where an absentminded pilot has left the carburetor heat in full hot position from the previous landing, may cause a scare, but not necessarily an accident. But move the situation to a 5000 feet or higher (density altitude) small airport, again forget there is full carburetor heat, add a rich carburetor condition, and the sum total of these combined power thieves add up to a takeoff or go-around accident. Similarly, a review of accidents over the years tends to prove in the majority of cases that it has rarely been one factor responsible for a crash, but rather one small item, added to another small factor, added to a third; all of these small items finally add up to a total beyond the ability of the pilot to cope. This is how accidents tend to happen. So let’s identify several of these power thieves in an effort to make flight as safe as we can.


In cool or cold weather, pilots should take extra care prior to attempting to takeoff with a cold engine and cold oil, and thereby prevent a temporary power loss during a critical part of the takeoff. Cold or heavy oil can and quite often does affect normal operation of the hydraulic lifters. Remember that aviation lubricants are heavier when cold than the commonly used automotive engine oils and require a little more time in warm-up to obtain normal flow in order to function properly throughout the air cooled aircraft engine.

To prevent possible power loss, a proper warm-up should be conducted. The engine is usually warm enough for pre-flight ground check in above freezing temperatures after 2 to 3 minutes running at 1000 to 1200 RPM. Below freezing temperatures, the warm-up period should be longer. With turbocharged powerplants, cold oil and cold engines require a longer warm-up period to assure proper controller operation and to prevent manifold pressure overboost.

After the above recommended warm-up period in cool or cold weather, including magneto and runup check, if the oil pressure is consistently over maximum red line, have a knowledgeable mechanic adjust oil pressure so that it does not exceed red line at takeoff or climb powers, and yet it is within the recommended green arc area at cruise. Cold weather tends to require a longer warm-up period.

Another cause of the power loss under these temperature and flight conditions has been the use of a heavier weight viscosity of oil than recommended for the ambient temperature flight condition. A heavier weight oil than recommended in cool or cold weather will tend to prevent the normal operation of the hydraulic lifters and thereby cause a loss of power.

Thus, to prevent power loss on takeoff with direct drive engines, select the proper weight oil for your engine for cold weather operation Make a careful run-up prior to takeoff with cold oil and a cold engine and observe engine instruments. Extend your warm-up period in cold weather until oil pressure is within recommended limits, or consult a mechanic concerning a compromise adjustment. If in doubt about power output, a brief smooth full throttle check is recommended.


In the opening paragraph, carburetor heat was used as an example of a cause of power loss, but many pilots aren’t sure they understand the reason for it. Flight tests conducted many years ago with a precision torque meter installed made it possible to measure fairly accurately a loss of as much as 15% of engine power when full alternate air or carburetor heat have been applied. As a specific explanation, there is a small power loss when we use heat because the pilot has switched from the direct, colder ram air to an indirect carburetor heat muff, or a similar indirect source of warm air with an alternate warm air source from inside the cowling. This accounts for an average 3% power drop because of the loss of ram air. The major portion of the engine power loss is caused by the carburetor heat or alternate air heat. Aircraft engines are checked for their horsepower output at a corrected standard temperature of 59ø F. Engineering has provided a simple rule of thumb for the effect of heat on power, i. e. for every 10o F of heat above the standard 59o F, there is a 1% power loss. Since the average heat source on an engine provides at least 100o F of heat above standard, this heat condition causes an average power loss of 10%. Our measurable total power loss at sea level, standard conditions is already up to 13%.

When warm air is used by the pilot, the mixture becomes richer and the engine may roughen with another slight power loss as a result. In addition, the higher the altitude with its less dense air, the greater the enriching effect because the fuel metering device will become richer at altitude and the engine less efficient. Thus, there will be another small, difficult to measure, power loss to be added to the 13% loss already accumulated.

With full carburetor heat applied, most float-type carburetors react very sluggishly or inefficiently on a straight-arm throttle technique during a touch-and go landing or an aborted landing. In some cases, the float-type carburetor may refuse to accept the throttle when it is applied in this manner. A gradual, steady application of the throttle is always the best approach.

We should also remind the pilot that when he uses carburetor heat or alternate air heat at cruise power that he should adjust his mixture lean, otherwise he will have a rich mixture. If the heat causes an undesirable power loss at cruise, and the pilot has throttle available, he may bring the manifold pressure up at least to the power reading he had before application of heat; and if additional power is needed and available, he may add a maximum of two inches of MP, or 100 RPM (fixed pitch prop) above the previous power, and then adjust the mixture. It is possible to compensate for the horsepower loss due to heat by means of the latter technique if throttle or RPM are available.


To fly safely at a high altitude airport (5,000 ft. density altitude and above) on a warm weather day, we must consider the aerodynamic loss of efficiency on the airplane and propeller under these conditions, and the power loss effect on the engine. A good "rule of thumb" for the pilot to remember is - for each thousand feet above sea level, the takeoff run increases approximately 25 percent. In the case of normally aspirated engines (not turbocharged or supercharged), at an altitude of 10,000 feet, about one-half of available engine horsepower is lost.

We can create a practical flight problem for the pilot who is faced with a high elevation field takeoff. At Denver, Colorado where the field elevation indicated on the airplane altimeter is 5000 ft., the pilot should consult the density altitude chart for takeoff. He must know that the published performance criteria of an aircraft is generally based on standard atmospheric conditions (temperature 59o F, pressure 29.92 inches of mercury at sea level). In checking the density chart and applying the ambient temperature of a summer day of 80o F, the careful pilot will note that the density altitude is actually 7500 feet, and the takeoff distance at this density altitude will be 2.3 times the sea level takeoff roll shown in his Pilot’s Operating Handbook.

If the same pilot flew to Laramie, Wyoming for the next landing and subsequent takeoff, he might meet these typical flight conditions:

The field elevation is 7,276 feet, and with an ambient temperature of 60o F, his actual density altitude will be 9,300 feet, with a takeoff roll 2.9 times the sea level takeoff. Furthermore, the pilot must remember - the higher the ambient temperature indicates, the higher the density altitude becomes. At this elevation, the pilot of normally aspirated aircraft engines should consider takeoffs in the cool temperatures of early morning or evening hours, rather than during the hot hours of the day.

Summing up the specific flight condition just discussed, the pilot must remember—when the temperature becomes higher than standard (59o F), the density of the air is reduced and aerodynamically affects overall airplane performance. The horsepower output of the engine is decreased because its fuel air mixture intake is reduced. The propeller develops less thrust because the blades are less efficient. The wings develop less lift because the less dense atmosphere exerts less force on the wings as airfoils. As a result, the takeoff distance is increased and the climb performance reduced.

In order to cope with high elevation airport takeoffs with normally aspirated engines, whenever the density altitude is 5,000 feet or higher, the pilot must compensate on the ground before takeoff. With a direct drive engine and a fixed pitch propeller, run the engine up to takeoff RPM and lean the mixture until a maximum RPM is noted; leave mixture at that position and accomplish the takeoff. If the engine has a governor, run it up to takeoff RPM and then lean until the engine smooths out and gives the indication of maximum power. At 5,000 ft. density altitude or higher, the available horsepower has been reduced so that leaning as described will not damage a healthy engine. If an EGT system is available, lean to +100o F on the rich side of peak EGT on a direct drive normally aspirated Lycoming engine.

All turbocharged or supercharged engines must use full rich for takeoff at any elevation airport. This includes either manually operated turbos or the automatic type.


There are several possibilities whereby the ignition system can be the cause of power loss in the engine. There is a power loss of approximately 3% with a single dead magneto or running on one mag. In fixed wing aircraft, if the pilot lost a magneto in flight it might not be a serious situation to complete the flight safely provided other power robbers didn’t begin to add to the problem. But in the case of the rotary wing aircraft it could be serious during takeoff, hover or landing because there are the regular inroads on power - such as operation of the tail rotor, the cooling fan, the alternator, the transmission, and also power loss from any excessive rotor blade trim tab position beyond the manufacturers recommendation. Therefore, magneto maintenance really is a critical item on rotor wing aircraft.

Other power loss influences in the ignition system include worn or fouled spark plugs that tend to provide a weak spark. Likewise, deteriorated magneto points will have some power loss influence. We have also learned the difficult way that old, worn or cracked (insulation) ignition harnesses can cause a loss of power, particularly at altitude. If this is suspected, it can be checked by means of a harness tester.

We know that magneto timing, either early or late, has a detrimental influence on power. Sound maintenance can eliminate these problems. But coming back to spark plugs, the correct plug is most important for efficient engine operation, and Textron Lycoming Service Instruction 1042 is the official reference source. Maintenance must also be careful that long reach plugs are used only in those cylinders designated by an area of yellow paint in the fin area between the spark plug and rocker box. Cylinders designed for short reach plugs may be either gray, blue or unpainted in this area. If the wrong length plug is used in the cylinder, it will cause a loss of power and perhaps preignition or detonation.

Champion Spark Plug Company published a bulletin warning that one dirty cigarette or contaminated plug barrel can rob an aircraft engine of two horsepower during takeoff. When dirt and moisture are allowed to accumulate on the harness terminal (cigarette) or spark plug barrel insulator, connector well flashover can occur resulting in plug misfire. The high voltage current will take the easy path to ground rather than spark between the firing-end electrode gap. Cigarettes, harness terminals, seals, and spark plug barrels should be kept clean and dry. When cigarettes are clean, do not touch them as the moisture on fingers is enough to contaminate them again. Replacing these parts and pieces at reasonable periods is inexpensive insurance against the power thieves. When the latter are at work, sharp performance and economy are lost.


If the intake pipes are loose at either end, leakage that tends to lean the mixture will take place and cause a power loss. It could be critical in the takeoff or climb power ranges. In most engines the leakage can be detected by observing fuel dye evidence at the leakage area. Any time this condition is discovered, it must be remedied before the aircraft is flown again.

In those engines using a carburetor, we have observed power loss effects from worn air boxes where the carburetor heat flapper valve in the air box remains partly open. When the outside air temperature is above 59o F, this malfunction can create a sneaky power loss, particularly at higher than cruise power.


Another power loss condition is that of blow-by - or oil blowing by the piston rings and getting into the combustion chamber in more than desirable amounts. It occurs with broken or worn piston rings, scored cylinder walls, and bell-mouthed exhaust valve guides. Oil in the combustion chamber tends to foul spark plugs and reduce their efficiency. It also lowers the octane rating of the fuel and tends to cause a loss of power, particularly at takeoff or climb. If the engine is not close to its normal overhaul life, then a top overhaul would be in order if more than one cylinder showed this condition.

Power loss from valve leakage may not be noticeable to the pilot while in flight. If an exhaust valve becomes burned and deteriorated at the edge of the head, it may cause an engine miss in flight. But leaking intake valves are difficult to detect during flight. The latter either get irregularly seated and cause a compression loss, or they can also cause a loss if they get tuliped from preignition. A good differential compression check will pick up most of these discrepancies except for some occasions of broken rings. However, any oil in the combustion chamber from broken rings would, in addition, call for a visual inspection with a borescope or a gooseneck light.


We can’t list all the many power robbing factors here, but we have tried to list the important ones, along with recommendations on how to cope with them. Again we want to remind all concerned of the dangerous difference between an engine problem where both spark plugs fail to fire in a cylinder, which is immediately obvious as compared with the small power loss problem that is not as obvious. The power thieves take power away in small quantities per cylinder until several of them happen to occur at the same time, reaching serious proportions and a definite unsafe flight condition. Be aware - don’t become a victim of power thieves!