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Tue, Nov 02, 2004

CASA AD: Textron Lycoming

Applicability: All owners, operators and maintainers of Textron Lycoming piston engines. Background: CASA has developed a database to review and analyse bearing failures on piston engines. This Bulletin captures the underlying reasons for bearing failures and recommends steps to minimize recurrence.

Bearing Construction

A plain bearing, largely used in piston engines, consists of two semicircular steel shells, providing a closed circular housing for crankshaft main and piston rod big-end journal. The shell has lubrication holes for oil flow and dowel locating holes for correct alignment and retention. The outermost steel shell provides strength while the inner layers provide other desirable features such as "fatigue strength" and the ability to form itself.

Plain bearings supplied prior to 1995 were "copper cast." While all specifications are indicative and actual composition remains proprietary formula, a typical 'copper cast' bearing would have a steel outer shell, deposited with a thick layer of an alloy of copper (80%), lead (15%) & tin (5%). The copper-lead layer will then have a very thin (1 to 2 micrometer) nickel dam to stop the flow of tin, followed by a Babbitt layer.

The Babbitt layer (also called white metal) is a 25-30 micrometer thick sacrificial material layer comprising lead (88%), tin (9%) and copper (3%). White metal is relatively soft and has a low melting point. Babbitt "forms" itself to a perfect-fit with respect to crankshaft and crankcase by "settling-in." It also allows embedding of small debris present in oil due to its softness and reduces scoring of crankshaft main and piston connecting rod journals.

"Aluminum-cast" plain bearings were introduced in the late 1990s. They consist of a steel shell deposited with a layer of aluminium (95%) and tin (5%). The nickel dam and Babbitt layers are then deposited on an aluminium layer and are similar to that on 'copper cast' bearings.

Available data indicates that "aluminium cast" bearings have lower durability when compared to "copper cast" bearings. KS Bearing Inc of Indiana in the USA mostly supplied aluminium cast bearings. Textron Lycoming switched back to copper-cast bearings in 1999-2000 with a modified composition and has made a gradual shift to these new bearings since then. (Reference Lycoming Service Bulletins 544, 544A, 547, 547 supp 1, Special Advisory 59-800 and SB 561 and SI 1512.) Other PMA bearing suppliers have also confirmed their switch to copper cast/trimetal bearings.

Engine OperationsFuel–air ratio, ignition timing, charge temperature and BMEP (Break Mean Effective Pressure) (BMEP) are four critical variables that can adversely affect bearing durability.

Operating with the lean fuel–air ratio may mean give better economy but it may also mean engine deterioration. Lean mixture burns more slowly, has a lower maximum temperature and may cause valve overheating and deformation. Consequently engine malfunction can adversely affect bearing durability.

Assuming that the ignition is correctly timed, many other factors like deposition of combustion products inside the combustion chamber may also cause "pre-ignition." Engines operating on leaded fuel form deposits of "lead oxy-bromide" inside cylinders. Lead oxy-bromide and/or unburnt carbon deposits inside a combustion chamber create 'hot spots' and may cause pre-ignition.

Pre-ignition and detonation may cause can lead engine to engine overheat and which may burn the holes in pistons or the combustion chamber. The rapid increase in pressures associated with detonation can cause lubrication breakdown and subsequent bearing failure.

Maintenance

Plain bearings operate with a hydrodynamic lubricating film provided by oil supplied under pressure. When the oil film breaks down, metal-to-metal contact generates excess heat, which may cause the white metal portion of the bearing to melt and then resolidify in "cooler" parts of the bearing. Re-deposited bearing material does not form a metallurgical bond with the underlying material and can be easily peeled off. Circumferential scoring of such bearing surface usually occurs. This phenomenon will also occur if the temperature in the oil film rises above the melting point of the white metal bearing material.

There are a number of possible factors that eventually lead to bearing failure:

  • Inadequate supply of oil due to malfunctioning oil pump, low oil level or restriction in lubrication lines.
  • Inadequate bearing clearances.
  • Use of inappropriate lubricant.
  • Blockage or restriction in oil-coolers or any factor leading to ineffective cooling.
  • Loss of bearing area and stress concentration due to pre-existing defects.
  • Unapproved operating procedures.
  • Engine baffles not correctly sealed or insufficient amount of cooling-air to cylinder cooling fins.
  • Incorrect ignition timing.
  • Incorrect adjustment of mixture for the engine power setting during engine ground runs and in operation.
  • Propeller strike.
  • Failure to 'break-in' the engine.

The purpose for engine break-in is to set the piston rings, stabilize the oil consumption and maintain bearing clearances after piston rings / cylinders replacement or honing (as in field overhaul) or after an engine installation on the aircraft for the first time following shop assembly. Failure to pre-oil the engines before the first start, whenever there is an engine change, oil cooler replacement or draining or any prolonged period of inactivity. Keeping an eye out for the signs of impending bearing failure through proper inspection of oil, filters and engine general condition may help identify signs of an impending engine failure.

Assembly/Overhaul Practices

The crankcase bearing locating diameters, crankshaft main diameters and piston connecting rod internal diameters must be within manufacturer's dimensions and tolerances with no planar discontinuity in the journal surface. Any high spots, eccentricity or axial asymmetry of major diameters will result in uneven bearing loading, lower contact area, higher stress concentration, leading to white metal overheat and displacement resulting in bearing failure.

Similarly "lack of bearing crush" may result in bearing rotation, fretting and failure. Conformance to manufacturer's recommended overhaul procedures, materials and standard practices can never be over-emphasized.

Recommendations

For Engine Overhaul Shops:

  • Incorporate Lycoming Service Bulletins 544A, 547 supp 1 , and 561 and SI 1512 (or later Service literature Bulletins related to bearings), as applicable.
  • During overhaul, ensure a high degree of cleanliness of the bearing areas.
  • Ensure proper retention of bearing inserts by correct dowelling.
  • Ensure correct bearing crush between the shell and the crankcase or the connecting rod.

For Maintenance Personnel:

  • Change oil and filter and carry out inspection per provisions of Lycoming SB 480D. Cutting the filter open and inspecting contents may provide valuable information on impending engine failure. CASA recommends that when carrying out the engine oil and oil filter change procedure detailed in Textron Lycoming SB 480D, drain the oil whilst the engine is still hot and strain the hot oil through a fine mesh screen filter. Draining the oil whilst it is hot will assist in flushing out bearing material flakes.
  • Ensure correct sealing of engine baffles and keep cooling-air passages to cylinder cooling fins clear. If there is any sign of engine, cylinder or piston overheat, carry out a full inspection of the piston and cylinder assembly.
  • Perform engine break-in per provisions of Lycoming Service Instruction SI 1427B (or the latest revision), if applicable.
  • Pre-oil the engine per provisions of Lycoming Service Instruction SI 1241C or the latest revision, before the first start, whenever there is an engine change, oil cooler replacement or draining , or any prolonged period of inactivity.
  • Ensure correct functioning of engine lubrication systems.
  • Ensure correct ignition timing.
  • Ensure correct adjustment of mixture for the engine power setting during engine ground runs and in operation.
    Use clean, clear and contamination-free fuels, oils and additives as per aircraft flight manual (AFM).
    In case of doubt, contact your overhaul facility or manufacturer.

Pilots/Flight Operations:

  • Use fuel-air mixture as per approved POH/AFM.
  • Report all instances of engine malfunction, exceeded parameters and propeller strike, even if they are minor.
FMI: www.casa.gov.au/avreg/aircraft/AWB/index.htm

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