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Crankshaft Failures

Crankshaft Failures

Several years ago there was a spate of aircraft engine crankshaft failures in Australia. There are many possible reasons for this so I will have a brief look at these and provide some suggestions that may stop you experiencing a failure.

Although a high level of safety is provided in the design and testing of engine crankshafts, failures can and do occur. Reports indicate that crankshaft failures can mainly be attributed to:

•      Material defects – very rare due to the very rigid material testing and procurement procedures imposed on the manufacturers by the relevant airworthiness authorities.

•      Manufacturing defects – similarly also very rare due to the rigid quality control and inspection procedures required of the manufacturers.

•      Overstress due to improper operating techniques, out of balance condition, or undetected damage following an accident or incident.

•      Overheating caused by improper operating/maintenance techniques.

Of course, aircraft owners, operators and maintenance personnel have no control over material or manufacturing defects, but with proper attention to engine operation and maintenance techniques, crankshaft failures related to overheating and overstress can be minimised.

OVERSTRESS  -  Firstly let’s look at what the pilot can due to minimise the probability of a crankshaft failure. Crankshafts are very susceptible to overstress. The design, testing and certification of the crankshaft takes into consideration all normally expected loads but if additional vibratory stress or increased forces are imposed, a failure can be induced. Common causes of overload are:

•      A propeller strike resulting in sudden engine deceleration or stoppage. Special consideration should be given to crankshaft damage if a propeller strike, severe vibration, or impact damage is involved. Pilot solution – be careful when taxiing and don’t land with the wheels up!

•      A hydraulic-lock that occurs when the combustion chamber is filled with oil or fuel and is usually encountered at initial start-up. This situation is most common with radial engines when oil tends to collect in the lower cylinders during periods of engine inactivity. As a result of hydraulic-lock, the piston, connecting rod, cylinder and crankshaft all suffer very high stresses if any attempt is made to operate the engine with fluids in the cylinders. Pilots should turn the engine over by hand for several revolutions prior to start to check and/or clear any fluid in the cylinders. This is particularly the case with radials.

•      A detuning of counterweights on balance weight-equipped crankshafts is another 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 become ‘detuned’, the normally expected torsional 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 power, operating at high engine speed and low manifold pressure and improper feathering procedures. Proper operation of engines with counter balanced crankshafts is vital for safety. Usually this type of crankshaft is fitted to higher power piston engines.

•      Overboost of supercharged engines applies high stresses to the crankshaft. Clearly this is something the pilot can control. While most modern engines are fitted with some form of overboost protection, this may fail. The pilot should always monitor the manifold pressure when increasing power, particularly during take-off.

•      Detonation or pre-ignition caused by improper fuel leaning procedures, excessive cylinder head temperatures or improper ignition.

•      Operation at critical vibration conditions that may occur during a one cylinder-out situation or when operating in a restricted (yellow arc) speed range.

•      A propeller out-of-balance or out-of-track condition. Any abnormal vibration observed by the pilot should be entered on the maintenance release so the maintenance personnel can investigate the problem.

OVERHEATING  – The following may cause crankshaft damage due to overheating:

•      Bearing spin causes heat and refers to a connecting rod or crankshaft main bearing rotating in its mounting. Crankshaft damage then results from oil restriction to the bearing.

•      Fillet ride creates heat and refers to a condition where the crankshaft rides a curved fillet area on a bearing. The condition results in inadequate lubrication, excessive heat being generated in a local area of the crankshaft. The condition usually develops from wear on engine parts.

•      Engine oil over-temperature may result in improper lubrication by thinning the oil. Improper operating techniques are most often the cause of oil over-temperature.

•      Contaminated oil resulting in improper lubrication or oil starvation.

•      Improper engine pre-oiling when new, overhauled, or after a long shutdown period.

•      Overboost of supercharged engines contributes to overheating as well as overstress.

RECOMMENDATIONS

1.      Operating procedures. Pilots should adhere to manufacturers’ operating instructions and avoid overheating, detuning, hydraulic-lock and overboost. Observe engine speed and power output limitations. Observe and avoid any critical RPM ranges.

2.      Maintenance procedures.

•      Maintenance personnel should consider crankshaft damage if a propeller strike, sudden stoppage, or hydraulic-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. 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.

•      The engine’s worst enemy is contaminated oil that eventually clogs the engine oil passages resulting in oil starvation and deterioration 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.

•      Before starting an engine that is new, overhauled, or has been inactive 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.

•      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 eventual crankshaft failure. Particular attention should be given to specified grinding limits.

•      Present day aircraft engines with high cylinder pressures require greater accuracy in ignition timing. Improper engine performance, burned pistons and failures of crankshafts have been traced directly to improperly timed ignition and inadequate ignition system maintenance. Follow the recommended procedures provided by the manufacturer.

•      When an engine experiences an overspeed, overboost or a connecting rod failure, the crankshaft is subjected to abnormal stresses. A detailed inspection may be necessary to locate any possible crankshaft distress.

The bottom line is- look after your crankshaft and it will look after you.

 

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