AC NO: 

DATE 8 

-- 8 

20-103 

3/7/78 

ADVI SORY 

CIRCULAR 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

SUBJECT: AIRCRAFT ENGINE CRANKSHAFT FAILURE 

- 

1. PURPOSE. This advisory circular provides information and suggests 

procedures to increase crankshaft service life and to minimize crankshaft 

failures. 

. 

2 .’ GENERAL. 

a. Although a high level of safety is provided in the design of 

engine crankshafts, failures do occur. Reports indicate that crankshaft 

failures can be attributed to: 

(1) Material defects, which are few in number. 

(2) Manuf ac t uring defects that are minimal due to the application 

of quality control and inspection procedures employed by the manufacturers. 

(3) Overheating caused by improper operating/maintenance techniques. 

(4) Overstress due to improper operating techniques, out of balance 

condition, or undetected damage following an accident or incident. 

b. Aircraft owners/operators and maintenance personnel have no control 

over material or manufacturing defects that cause crankshaft failures. 

However, with proper attention to engine operation and maintenance techniques, 

crankshaft failures related to overheating and overstress can be reduced. 

3 . m. 

a. The following may cause crankshaft damage due to overheating: 

(I) Bearing spin causes heat and refers to a connecting rod or 

crankshaft main bearing rotating in its mounting. Crankshaft damage is 

usually due to oil starvation. 

I ni ti ated by: A.~+830 
. AC 20-103 3/7/78 

(2) Fillet ride creates heat and refers to a condition where 

the crankshaft rides a curved fillet area on a connecting rod, thrust, or 

main bearing. The condition occurs without presence of an oil film, and 

excessive heat is generated in a local area of the crankshaft. The condition 

usually develops from wear on engine parts. 

(3) Engine oil overtemperature may result in improper lubrication 

by thinning of the oil. Improper operating techniques are most often the 

cause of oil overtemperature. 

(4) Contaminated oil resulting in improper lubrication or oil 

starvation. 

(5) Improper engine pre-oiling when new, overhauled, or after a 

long shutdown period. 

(6) Overboost of supercharged engines contributes to overheating as 

well as overstress of parts. 

-b . Repair following any of those conditions should include inspection 

and consideration of crankshaft damage which could lead to complete 

crankshaft failure. 

4 OVERSTRESS. Crankshafts are very susceptible to overstress. 

original design of the crankshaft provides for normal loads; 

The 

however, if 

additional vibratory stress or increased force are imposed, structural 

failure can be induced. Common causes of structural overload are: 

a. A propeller out-of-balance or out-of-track condition. 

b. A propeller strike resulting in sudden stoppage. 

c. A liquid-lock that occurs when the combustion chamber is filled 

with fuel or oil and is usually encountered at initial start-up and is 

caused by overpriming or oil filling the cylinders during long periods 

of engine shutdown. As a result of liquid-lock, the piston, connecting rod, 

cylinder, and crankshaft suffer very high stress if any attempt is made tc 

operate the engine. 

d. A detuning of counterweights on balance weight-equipped crankshafts 

is a source of overstress for the crankshaft. Many engines are fitted with ’ 

balance weights mounted on pins running in precision ground bushings as an 

integral part of the crankshaft. The counterweights are designed to position 

themselves by the inertia forces generated during crankshaft rotation and 

effectively absorb and dampen crankshaft vibration. If the counterweights 

are detuned (allowed to slam on mounts), the vibrations are not properly 

dampened and crankshaft failure can occur. Counterweight detuning can occur 

from rapid opening and closing of the throttle, excessive speed, excessive 

Paf,E: 2 Par 3 
3/?/78 AC 20-103 

power, operating at high engine speed and low manifold pressure, and improper 

feathering procedures. 

e. Overboost of supercharged engines applies high stress to the 

crankshaft. 

f. Accident/incident damage. Special consideration should be given to 

crankshaft damage, if a propeller strike, severe vibration, or impact damage 

is involved. 

& Detonation or pre-ignition caused by improper fuel leaning procedures, 

excessive cylinder head temperatures, or improper ignition. 

h. Operation at critical vibration conditions that may occur during a 

one cylinder-out situation or a restricted speed range. 

5 . RECOMMENDATIONS. 

a. Operating procedures. Adhere to manufacturer’s operating instructions 

and avoid overheating, detuning, liquid-lock, and overboost. Observe engine 

speed and power output limitations. Observe any special placards to avoid 

critical RPM. 

b, Maintenance procedures. 

(1) Maintenance personnel should consider crankshaft damage if a 

propeller strike, sudden stoppage, or liquid-lock occurs. Crankshaft 

damage should also be considered when repairing piston or connecting rod 

failure, gear train failure, loss of oil pressure, and other engine malfunctions and problems. Frequent checks of propeller track and balance will 

reduce undesired.vibratory stress. Concern for proper quantity and quality 

of oil will aid in reducing friction heat. Proper rigging and operation 

of oil cooler and cowl flaps will reduce heating problems. 

(2) The engine’s worst enemy is contaminated oil that eventually 

clogs the engine oil passages resulting in oil starvation that starts the 

deteriorations of parts, creating overheating and crankshaft distress. 

Follow the manufacturer’s recommendations for changing the oil. Keeping 

the engine oil clean will help increase engine life and reliability. 

(3) Before starting an engine that is new, overhauled, or ha-s been 

shutdown for a long period of time, particular attention should be given 

to pre-oiling, especially during cold weather. The pre-oiling procedure 

recommended by the engine manufacturer should be followed to preclude 

the operation of an engine without sufficient lubrication. 

(4) Most crankshafts are nitrided to increase the bearing journal 

surface hardness. The removal of nitride during overhaul or repair of a 

crankshaft can result in a stress riser and eventual crankshaft failure. 

Par 4 Page 3 
AC 20-103 3/7/78 

Accordingly, particular attention should be given to the grinding limits 

recommended by the engine manufacturer. 

(5) Present day aircraft engines designed with higher cylinder 

pressures for increased efficiency and horsepower require greater accuracy 

in ignition timing. Improper engine performance, burned pistons, failures 

of engine crankshafts, and engine failures have been traced directly to 

improperly timed ignition and inadequate ignition system inspection. The 

degree of accuracy necessary to correctly time the ignition can be obtained 

by following the recommended procedures provided by the manufacturer for 

each engine model. 

(6) When an engine experiences an overspeed, an overboost, or a 

connecting rod failure, the crankshaft is subjected to abnormal stress 

levels. A detailed inspection may be necessary to locate possible crankshaft distress. The inspection should be accomplished in accordance with 

the engine manufacturer’s recommendations. 

hf l 

. L 

/ 

/,/JAMES M. VINES 

Ll". Acting Director 

Flight Standards Service 

Page 4 Par 5