(ref: FLIGHT SAFETY AUSTRALIA, SEPTEMBER-OCTOBER 2002 pp36-38)
If you are going easy breaking in your new, remanufactured or overhauled piston
engine, you could be doing more harm than good.
EVER MET AN “engine babyer”? You know the type, power partial on take-off and low power in the cruise.
“Engine babyers” usually believe that by using low power settings they are conserving their engine.
Usually this type of thinking is the result of experience with stock automobile engines. In reality, the operation of an aircraft engine is very different. You will not do an aircraft engine any disservice by using full power for take-off and 65 to 75 percent for cruise
Many pilots are still reluctant to fly their engines at high power settings simply because they think they might hurt the engine. If you are one of those pilots, perhaps it would help if you knew what your engine had to do to prove itself before it was awarded a type certificate.
Let’s say you are flying an aircraft powered by a Teledyne Continental Motors model 0-470-R. The first production engine off the assembly is usually the test engine for US Federal Aviation Administration type certification.
Here is what a type test involves. The type test engine is mounted in a test cell and given a standard production acceptance test. After completion of this series of runs, the engine is ready to begin its type test (also called an endurance run). The engine is then operated at full power (maximum rated manifold pressure and maximum rated RPM) for a period of 50 hours. During this period, the engine’s cylinder head temperature (CHT) and oil temperature (OT) are held in the normal range (first 2/3 of the green arc on your aircraft gauges).
After the first 50 hours, the engine is checked and made ready for the second 50-hour test. During the next 50 hours, the engine is once again at full power – only this 50 hours is at redline CHT and OT.
Imagine that: 50 hours at “full bore” at red-line temperatures.
The third and last 50-hour phase includes alternating sequences of 65-75 percent power – all at redline CHT and OT. While the minimum number of hours required by the FAA for testing is 150 hours, engine manufacturers often exceed this figure by as much as two or three times, simply to satisfy themselves the engine is trouble free.
After all type testing is completed, the engine is removed from the test stand, completely disassembled and every part carefully examined. Each moving part is checked with measuring instruments to determine if any appreciable wear has taken place. Each part must be able to pass the blueprint specifications for new parts.
To replicate the type testing regime in real-world conditions, you would have to fly your engine at full power for six minutes every hour for 1,000 hours.
Now let’s talk about all the other production engines that will follow the type test engine. Each new production engine is given a standard production acceptance test. The last part of this test is the oil consumption run which is conducted at full throttle. The purpose of this test is initial seating of the piston rings to the cylinder walls. The run is conducted at full power because that is where greatest brake mean effective pressure (BMEP) occurs and a high BMEP is necessary for good piston ring break-in.
The test house at the factory determines initial piston ring seating by the amount of oil consumed by the engine during this run. Only a few hours are involved in the acceptance test and the new engine is by no means completely “broken in.” The final “break-in” rests with the pilot who will be flying the engine during the first 100 hours of its life.
Correct “break in”: The cylinder walls of a new engine are not mirror smooth as one might imagine. A special hone is used to put a diamond like pattern of scratches over the entire area of the cylinder wall. The crosshatch treatment of the cylinder walls plays an important role in proper break-in of piston rings to cylinder walls.
Proper break-in of piston ring to cylinder wall requires that the ring rupture or break through this oil film and make contact with the cylinder wall. During such “metal-to-metal” contact, the little peaks on the ring face and cylinder wall become white hot and rub off. This condition will continue to occur until the ring face and cylinder wall have established a smooth compatible surface between each other.
At this point, break-in is said to be relatively complete and very little metal-to-metal contact will occur hereafter. In fact,as the break-in process progresses, the degree of metal-to-metal contact will regress.
To get some minute metal-to-metal contact during the break-in period, rupture of the oil film is necessary. Two factors under the pilot’s control can retard this necessary rupture: low power and improper lubricating oils during the break-in period.
Engine lubricating oils can be divided into two basic categories, compounded (detergent and ashless dispersant) and non-compounded. The compounded oils are superior lubricants with greater film strength than non-compounded oils. Consequently, only non-compounded oils should be used during the break-in period.
Some owners insist on using additives or super lubricants along with the regular engine oil during the break-in period. They believe that this will aid the engine during its breaking in. With all due respect to such good intentions, this practice is wrong and actually causes harm.
During an oil film rupture, only the ridges on the piston rings and cylinder walls contact each other. The little “valleys” between the ridges retain a film of oil and prevent a total dry condition between piston ring and cylinder wall.
BMEP or combustion pressure forces the ring against the cylinder wall. This is the “key” to the break-in process.
Glazes, valleys and scratches: You can see then that low power (low BMEP) won’t provide the same results and the break-in process will require a longer period of time. However, time in this instance will have a detrimental effect on your engine because any prolonged, low power break-in procedure usually leads to “glazed” cylinder walls.
During each power stroke, the cylinder walls are subjected to very high temperatures, often 4,000°F or higher. This period is very brief but nevertheless long enough to cause oxidation of minute quantities of some of the lubricating oil on the cylinder walls.
Some of this oxidation will settle into the “valleys” of the honed cylinder wall “scratches.” Eventually this situation will fill the “valleys” of the cylinder walls creating a smooth, flat surface.
This is also a normal situation; however the ring break-in process practically ceases when these valleys become filled or “glazed” over. If this “glazed” over process occurs before break-in is complete, in modern day language, “you have had it.”
Excessive oil consumption resulting from incomplete ring seating will present itself and the only certain remedy is re-honing the cylinder walls. This is both expensive and unnecessary.
Well now you know the whole story, let’s examine the few simple steps necessary for proper break-in of any new, re-manufactured, major or top overhauled engine.
Pick a good quality, non-compounded aircraft engine lubricating oil and stay with it throughout the break-in period. Duration of the break-in period is usually defined as the first 50 hours or until oil consumption stabilises.
Do drain and replace engine oil as often as recommended by your owner’s manual. If operating conditions are unusually dusty or dirty, more frequent draining may be necessary.
Remember, no one ever wore out an engine by changing oil too often. Oil changes are more critical during the break-in period than at any other time in the engine’s life.
Power: Do use full rated power and RPM for every take-off and maintain these settings until at least 400ft of altitude above the departing runway is attained. At this point, reduce power to 75 per cent and continue the climb to your cruising
Do maintain 65-75 per cent power for all cruise operation during the break-in period. Avoid high altitude operation with non-supercharged engines during the break-in period. Altitudes in excess of 8,000ft density will not permit sufficient cruise power development with non-supercharged engines.
Interrupt cruise power every 30 minutes or so with a smooth advance to full available manifold pressure and RPM for 30 seconds then return to original cruise settings (non-supercharged engines only).
This procedure helps to hasten a good break-in. The procedures suggested in this paragraph apply primarily to the break-in period and are not necessary thereafter.
Avoid long power-off let downs especially during break-in period. Carry enough power during let down to keep cylinder head temperatures at least in the bottom of the green.
Keep ground running time to absolute minimums especially during warm weather. During the break-in period, it will be better to delay departure than to sit at the end of the runway for 15 minutes or more running in high ambient temperatures.
Be generous with mixture controls and cooling air during break-in. All take-offs should be with a full rich mixture except from altitudes in excess of 5,000ft and then take care to lean only enough to restore power lost from overly rich mixtures. Make your climbs just a little flatter in hot weather to assure adequate cooling air.
Follow these simple recommendations during break-in and your engine will reward you with a healthy service life. And above all “Don’t baby your engine during its break-in period.”
Reprinted with permission from the US AOPA Pilot magazine.
(This article has been archived by the National Library of Australia. It can be found at: