The following paragraphs outline the most common reasons for removing and replacing an engine. Information to aid in determining engine conditions that require removal is included; however, in every case, consult applicable manufacturer’s instructions as the final authority in establishing the basis for engine replacement.
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Engine or Component Lifespan Exceeded
Engine life is dependent upon such factors as operational use, the quality of manufacture or overhaul, the type of aircraft in which the engine is installed, the kind of operation being carried out, and the degree to which maintenance is accomplished. Thus, the manufacturer sets engine removal times. Based on service experience, it is possible to establish a maximum expected time before overhaul (TBO) or span of time within which an engine needs to be overhauled. Certain critical components of turbine engines such as turbine blades, turbine disks or combustion liners may have shorter life limits established by the manufacturer due to the stresses imposed on those parts during operation. The FAA requires that the manufacture identify and establish which parts have mandatory replacement times.
Sudden Stoppage
Sudden stoppage is a very rapid and complete stoppage of the engine. It can be caused by engine seizure or by one or more of the propeller blades striking an object in such a way that revolutions per minute (rpm) goes to zero in less than one complete revolution of the propeller. Sudden stoppage may occur under such conditions as complete and rapid collapse of the landing gear, nosing over of the aircraft, or crash landing. Sudden stoppage can cause internal damage, such as cracked propeller gear teeth, gear train damage, crankshaft counterweights becoming detuned or misalignment, or damaged propeller bearings. When sudden stoppage occurs, the engine usually requires replacement or disassembly and inspection as per manufacturer’s instructions.
Sudden Reduction in Engine Speed
Sudden reduction in engine speed can occur when one or more of the propeller blades strike an object at a low engine rpm. After impact, the foreign object is cleared and the engine recovers rpm and continues to run unless stopped to prevent further damage. While taxiing an aircraft, sudden reduction in speed can occur when the propeller strikes a foreign object, such as a raised section in the runway, a tool box, or a portion of another airplane. When the accident occurs at high engine rpm, shocks are much more severe. When sudden reduction in rpm occurs, the following procedure can be used as a general rule, but you must comply with the manufacturer’s information.
Make a thorough external inspection of the engine mount, crankcase, and nose section to determine whether any parts have been damaged. If damage is found that cannot be corrected by line maintenance, remove the engine. Internal components can be damaged, especially counter weights on the crankshaft.
Remove the engine oil screens or filters. Inspect them for the presence of metal particles. Remove the engine sump plugs, drain the oil into a clean container, strain it through a clean cloth, and check the cloth and the strained oil for metal particles. Heavy metal particles in the oil indicate a definite engine failure, and the engine must be removed. However, if the metal particles present are similar to fine filings, continue the inspection of the engine to determine its serviceability. If there are no heavy metal particles in the engine oil, check again for metal in the oil system after operating the engine. Metal in the screens is a sign that the bearings have been compromised and are in the process of failing.
Remove the propeller and check the crankshaft, or the propeller drive shaft on reduction-gear engines, for misalignment. Clamp a test indicator to the nose section of the engine. Use the dial-indicator that has 1ś1,000-inch graduations. Remove the spark plugs from all the cylinders. Then, turn the crankshaft, and observe if the crankshaft, propeller shaft, or flange turns straight without any bending taking place. If there is an excessive runout (bend in the crankshaft or propeller flange) reading at the crankshaft or propeller-drive shaft at the front seat location, the engine should be removed. Consult the applicable manufacturer’s instructions for permissible limits. If the crankshaft or propeller drive shaft runout does not exceed these limits, install a serviceable propeller. Make an additional check by tracking the propeller at the tip in the same plane, perpendicular to the axis of rotation, to assure that blade track tolerance is within the prescribed limits.
Start the engine to see if operation is smooth, without vibration, and the power output adequate. If the engine operates properly during this ground check, shut the engine down and repeat the inspection for metal particles in the oil system.
Metal Particles in the Oil
Metal particles in the engine oil screens or the magnetic chip detectors are generally an indication of partial internal failure of the engine. Carbon tends to break loose from the interior of the engine in rock-like pieces that have the appearance of metal. It is necessary to consider these possibilities when foreign particles are found on the engine oil screens or magnetic chip detectors.
Before removing an engine for suspected internal failure, as indicated by foreign material on the oil screens or oil sump plugs, determine if the foreign particles are ferrous metal by placing them close to a magnet to see if they are magnetic. If the material is not magnetic, it is not attracted by the magnet. Any ferrous metal in the oil screens is cause for concern. Very small amounts of nonferrous metal, especially after major engine maintenance, can sometimes be normal. If the particles are metal, determine the probable extent of internal damage. For example, if only small particles are found that are similar in nature to filings, drain the oil system, and refill it. Then, ground-run the engine and reinspect the oil screens and magnetic chip detectors. If no further evidence of foreign material is found, continue the engine in service or per the manufacturer’s instructions. However, engine performance should be closely observed for any indication of difficulty or internal failure.
Spectrometric Oil Analysis Engine Inspection Program
Spectrometric oil analysis program allows an oil sample to be analyzed and searched for the presence of minute metallic elements. Due to oil circulation throughout an aircraft engine, every lubricant that is in service contains microscopic particles of metallic elements called wear metals. As the engine operates over a certain amount of time, the oil picks up very small particles that stay suspended in the oil. Oil analysis programs identify and measure these particles in parts per million (PPM) by weight. The analyzed elements are grouped into categories, such as wear metals and additives, and their measurement in PPM provides the data that expert analysts can use as one of many tools to determine the engine’s condition. If certain metals have an increase in PPM, it can be a signal of component wear or pending failure of the engine. The amount of wear metals is recorded and noted each time a sample is taken. If the amount of wear metals increases beyond a normal rate, then the operator can be notified quickly so repair, a recommended specific maintenance procedure, or inspection can be ordered.
The advantage of oil analysis is an increase in safety by noticing an engine problem before engine failure. It also saves money by finding engine problems before they become large problems or complete engine failure. This procedure can be used for both turbine and reciprocating engines. Oil analysis can be used to diagnose impending engine failure, and would be a reason for removing the engine from the aircraft and sending it to overhaul.
Turbine Engine Condition Monitoring Programs
Many turbine engines are monitored by an engine condition program that helps determine the health of the engine in service. This can also be called trend analysis performance monitoring, but it consists mainly of monitoring certain engine parameters daily and watching for trend shifts or changes in the engine parameters. A shift in key parameters (change over time) could be a warning that the engine has serious internal deterioration and should be overhauled.
Engine Operational Problems
Engines are usually removed when there are consistent engine operational problems. Engine operational problems generally include, but are not limited to, one or more of the following conditions:
- Excessive engine vibration; this is especially true with turbine engines.
- Backfiring, or misfiring, either consistent or intermittent due to valve train or other mechanical defect in reciprocating engines.
- Turbine engines that exceed normal operating parameters or life limited components exceeding maximum time in service or cycles.
- Low power output, generally caused by low compression, with reciprocating engines and internal engine deterioration or damage with turbines.