With a few extra simple checks before, during, and after each flight, you can gain a broader picture of your engine's health, and increase your confidence in your aircraft.
February 5, 1996
|About the Author ...
John Schwaner is AVweb's powerplant expert. John is a world-class
authority on piston aircraft engines, and a specialist in the
engineering analysis of engine failures. John runs Sacramento
Sky Ranch, Inc., a leading distributor of aircraft and engine
parts, and probably the foremost aircraft hose shop and magneto
overhaul facility in the U.S. John and his wife live in Sacramento,
John has also written two superb technical books:
Sky Ranch Engineering Manual
The Magneto Ignition System.
Both can be previewed in and ordered from the
AVweb Online Bookstore.
Inspect the aircraft's belly. On most aircraft, any fluid
leaks from the engine compartment ends up on the belly. Fresh
oil is a sign of an oil leak. Dark soot is a sign of rich engine
mixture or increased combustion gas leakage past the piston rings.
Fuel dye is a sign of a fuel leak. One quick look at the belly
and you know whether there are any leaks in the engine compartment.
Take your finger tip and touch the inside edge of the exhaust
pipe. If your engine's mixture and oil consumption are normal,
then your finger should be clean, or possibly have a slight tan
ash deposit. If your finger tip has dry black soot on it, then
your engine at a rich fuel/air mixture. If your finger has oily
black soot, then your engine's burning too much oil.
Smell inside the engine compartment for any fuel smells. Small
fuel leaks evaporate fuel as they leak and may not be enough to
drip. Leaks may occur at primer fittings, hose connections, or
the hose itself. One sniff in the engine compartment and you've
checked all of the fuel connections for leaks.
Check the color of the oil on the dipstick. If it looks like
black lacquer then the piston rings are leaking combustion gas
into the oil.
Listen for any out of the ordinary noises as the starter turns
your engine. You should hear the starter, the clanking of the
impulse couplings, and no wheezing of air out the engine breather
On your Continental 6-cylinder engine, does the propeller
turn with the starter? If the starter turns but the propeller
sometimes doesn't, then the starter adapter is slipping and needs
to be repaired.
On Lycoming engines if the starter turns but the propeller
doesn't then the starter Bendix is starting to stick. Usually
cleaning and silicone spraying the starter Bendix shaft fixes
Does the engine kickback when starting? If it does, then you
have a problem with the magneto impulse couplings, engine timing,
or the starter vibrator.
If the engine's getting hard to start then your magnetos probably
Many engine problems are first noticed during idle. Engine
roughness, caused by carbon fouled spark plugs, lead fouled spark
plugs, a sticky valve, or a hydraulic lifter not operating properly
are more common at idle.
A carbon fouled spark plug clears when you increase power,
a lead fouled spark plug does not clear when you increase power.
A carbon fouled spark plug indicates a spark plug that is not
firing constantly or that the engine is operating at a too rich
fuel/air mixture. Lead fouled spark plugs indicate a rich fuel
mixture or that the power is being increased too rapidly at takeoff.
Bad hydraulic lifters are more noticeable during idle then
during flight. A worn hydraulic lifter that leaks oil causes
rocker arm to valve clearance. The rocker arm strikes the valve
tip instead of pushing the valve open, resulting in a tapping
noise. The noise goes away as the cold engine oil flows into the
hydraulic lifter. Cold oil, being more viscous, doesn't leak out
the hydraulic lifter as fast as hot oil. This causes the hydraulic
lifter to pump up, closing the tappet clearance and causing the
tapping noise to go away. This is fine and should not be a concern
if the noise goes away shortly. If tappet noise occurs regularly
then replace the hydraulic lifters. Worn or defective lifters
cause the valve to pound against the seat, possibly causing valve
Is the oil pressure at its normal position? Low oil pressure
at idle and high oil pressure during flight is caused by leakage
in the oil delivery system and cannot be fixed by adjusting oil
Is takeoff rpm lower then normal? If takeoffs are getting
longer and climb performance is getting worse, then suspect that
a camshaft lobe is flattening out. Damaged camshaft lobes cause
a gradual decrease in takeoff rpm in an otherwise smooth engine.
If takeoff rpm is low on a constant speed engine then the
problem may be in the governor and not in the engine. Check to
see if you can reach redline rpm in cruise flight. If a constant
speed propeller airplane won't reach redline rpm in cruise, then
the propeller governor is holding back the propeller and your
problem is not low engine power. In cruise flight or descent,
even an engine with low power will turn a propeller past red line
because of the low engine loading.
Monitor for engine smoothness and power.
Is vacuum pump pressure normal? As the vacuum pump starts
to fail it often produces lower suction for a flight or two before
Magneto problems often cause a slight roughness as you climb
to altitude. The roughness may go away when you reduce power to
cruise. High manifold pressure requires more voltage from the
magneto to spark the plugs then lower manifold pressure. Therefore,
if you can turn the engine roughness on and off by changing the
manifold pressure, then the magneto is at fault.
The higher the altitude the less resistance to arching within
the magneto. Therefore, a marginal magneto often causes slight
engine roughness during the climb, only to clear up when you reduce
power or descend to a lower altitude.
To some degree oil pressure follows oil temperature and oil
temperature follows cylinder head temperature. As oil temperature
goes up, oil pressure goes down. As cylinder head temperature
goes up, oil pressure goes up. This can be used as a crosscheck
of proper gauge operation. The relationship is not linear and
sometimes may not exist. For example, increased heat transfer
from the cylinders to the oil occurs when the piston rings start
leaking hot combustion gas into the oil. This causes oil temperatures
to rise without a corresponding increase in CHT temperature.
If the propeller has more than 100 hours on it and is starting
to sling oil onto the windshield then its time to send it off
to a propeller shop.
Does the engine cutoff evenly? If not, the idle cutoff circuit
You should get no more than a 100-rpm increase when going
to idle cutoff. Any more than 50 rpm means that idle mixture is
too rich.The optimum idle setting is one that is rich enough to
provide a satisfactory acceleration under all conditions and lean
enough to prevent spark plug fouling or rough operation. A rise
of 25-50 rpm will usually satisfy both conditions.
Check the aircraft belly again.