Engine Maintenance And Operation Flashcards
What is the probable cause of hydraulic lock and how is it remedied?
Whenever a radial engine remains shut down for more than a few minutes, oil or fuel may drain into the combustion chambers or intake pipes of the lower, downward extending cylinders. This is known as hydraulic lock. As the piston moves toward top center in these cylinders, it will collide with these incompressible liquids. Severe damage can be caused. The liquid must be removed to remedy the hydraulic lock and make it safe to start the engine. This is done by removing either the front or rear spark plugs of the affected cylinders and pulling the engine through by hand in the normal direction of rotation. Then the spark plugs are reinstalled and the engine is started.
What checks are necessary to verify proper operation of a reciprocating engine?
A ground check or power check is performed to evaluate the functioning of the engine by comparing power input as measured by manifold pressure with power output as measured by RPM (or torque) and comparing these to known acceptable values. It also includes checking the powerplant and accessory equipment by ear, visual inspection, and by proper interpretation of instrument readings, control movements, and switch reactions. Fuel pressure and oil pressure checks must verify that these pressures are within established tolerances. A cylinder compression test can be performed if it is suspected that there is a problem with valves, pistons or piston rings. A magneto safety check exposes problems with the ignition system. Idle speed and idle mixture checks can also be performed although these relate more to the proper functioning of the fuel
system than the engine itself just as the magneto check focuses more on ignition system integrity.
Explain the checks necessary to verify proper operation of propeller systems.
The propeller must be checked to ensure proper operation of the pitch control and pitch change mechanism. The operation of a controllable pitch propeller is checked by the indications of the tachometer and manifold pressure gauge when the proper governor control is moved from one position to another. Each propeller requires a different procedure. The applicable manufacturer’s instructions should be followed.
What is involved with the correct installation of piston rings and what results if the rings are incorrectly installed or are worn?
Piston rings prevent leakage of gas pressure from the combustion chamber while lubricating the cylinder walls and reducing to a minimum the seepage of oil into the combustion chamber. Worn or broken piston rings can cause excessive oil consumption and loss of compression. Oil blow-by into the combustion chamber can lead to sticking valves, spark plug misfiring, as well as detonation or preignition due to carbonization of the oil. During installation, the rings are place in the proper grooves facing the correct direction according to the engine manufacturer’s instructions. The ring gaps are staggered around the piston. They are compressed with a ring compressor to the diameter of the piston as the cylinder is slid down around the piston making sure that the cylinder and piston plane remain the same. As the cylinder is lowered around the piston with a straight,
even motion, it displaces the ring compressor as the rings slide into the bore. Rocking or forcing the cylinder
over the piston and rings could cause a ring to escape from the ring compressor and expand or it could crack
or chip a ring or damage the ring land.
What are some procedures for inspecting various engine components during overhaul?
There are 3 basic categories of inspection during overhaul: visual, structural Non-Destructive Testing (NDT) and dimensional. The first inspection to be done is the visual inspection. A preliminary visual inspection should be performed before cleaning the parts since indications of failure may often be detected from the residual deposits of metallic particles in some recesses of the engine. Then, parts can be cleaned and visually inspected. Structural inspections can be performed on parts by methods such as magnetic particle inspection, dye penetrant, eddy current, ultra sound, and x-ray as specified by the manufacturer. Finally, using very accurate measuring equipment, each engine component can be dimensionally evaluated and compared to the service limits and tolerances set by the manufacturer.
What are the procedures for reciprocating engine maintenance as they pertain to overhauling an engine?
Aircraft engine maintenance practices, including overhaul, are performed at specified intervals established by the manufacturer. For an overhauled engine to be as airworthy as a new engine, worn and damaged parts must be detected and replaced. This is done by completely disassembling the engine. Visual, non-destructive, and dimensional inspections are performed. The manufacturer publishes inspection criterion and a new minimum and serviceable dimension for all critical component parts. Parts that do not meet these standards must be rejected for use in the engine. A major overhaul of an engine consists of the complete reconditioning of the powerplant. This includes disassembly of the crankcase for access and inspection/rework of the
crankshaft and bearings. It is not a major repair and can be performed or supervised by a certified powerplant technician as long as the engine does not contain an internal supercharger or propeller reduction other than spur-type gears. At the time of an engine overhaul, all accessories are removed, overhauled, and tested in accordance with the accessory manufacturer’s instructions.
What are some checks necessary to verify proper operation of a turbine engine?
The manufacturer’s operating instructions should be consulted before attempting to start and operate any turbine engine. Checking turbofan engines for proper operation consists primarily of simply reading the engine instruments and then comparing the value with those known to be correct for a particular operating condition. Be sure the engine and instrument indications have stabilized. Idling speed must be checked (tachometer) as well as oil pressure and Exhaust Gas Temperature (EGT). Engine Pressure Ratio (EPR) measures thrust and is used to set takeoff power. It varies with ambient temperature and pressure. Takeoff thrust is checked by adjusting the throttle to obtain a single, predicted, indication on the Engine Pressure Ratio (EPR) gauge. This
can be computed from the takeoff thrust setting curve in the operations manual. It can be done at full power or when the throttle is set at the part power stop. If an engine develops the predicted thrust and if all the other engine instruments are indicating within their proper ranges, engine operation is considered satisfactory. On newer aircraft, performance is a function of the onboard computer. FADEC engines have means for checking the engine and displaying the results on the flight deck.
What are some turbine engine troubleshooting procedures?
Turbine engine troubleshooting should be performed in accordance with the engine manufacturer’s instructions. Trouble shooting charts exist to guide the technician. Engine analyzers are manufactured that can assist with calibration and the accuracy of important engine parameter indicators such as RPM and EGT. Follow the analyzer manufacturer’s instructions.
What are some turbine engine maintenance procedures?
The detailed procedures recommended by the engine manufacturer should be followed when performing inspections or maintenance on a turbine engine. Some common functions include periodic inspection, cleaning and repair of compressor components which can have performance reduction due being dirty and Foreign Object Damage (FOD). Compressor blades, inducers, and guide vanes all may be damaged. The extent of the damage must be ascertained. Repairable damage limits set by the manufacturer must be adhered to and various NDT methods may need to be employed. The possibility exists to rework or blend out damage using stones usually blending parallel to the length of the component. Combustion section inspection and cleaning is very important since the serviceability of the combustion section is a controlling factor in the service life of a turbine engine. Inspection for hot spots, exhaust leaks and distortions can be done without opening the case. After the case has been opened, inspection for localized overheating, cracks,
and excessive wear are important. Evidence of FOD can be found even in the combustion section. Hot section inspections, which include the turbine section components, usually are required at regular intervals. Follow the manufacturer’s instructions for procedures and damage limits. This is applicable for fuel nozzles, turbine disks, blades, and guide vanes as well as the exhaust section components.
What is the possible problem with an engine that indicates high Exhaust Gas Temperature (EGT) for a particular Engine Pressure Ratio (EPR)?
The engine is likely out of trim. The accuracy of the EGT gauge/sensors could also be in question and should be checked.
What are the typical parameters sought for oil pressure when starting a reciprocating aircraft engine?
Oil pressure indication should occur within 30 seconds of start-up. Normal oil pressure indication should occur within 1 minute of start-up or the engine should be shut down.
Explain the operation of a temperature indicating system on a turbine engine.
Temperature of turbine engine gases entering the first stage turbine inlet guide vanes is the most critical of all engine variables. In many engines, it is impractical to measure the temperature exactly at this point. Relative temperatures that are proportional to this temperature are measured instead. EGT, TGT, TOT, and TIT are all such indications. Accuracy of temperature indication in this portion of the engine is critical because if it is too high, the engine could be destroyed. If is too low, insufficient power will be developed. A series of individual thermocouple probes located around the engine section where the temperature is measured are connected to a gauge on the flight deck. The gauge shows the average temperature measured by the thermocouples. This assists in producing an accurate indication. This indication is so important that testing of the individual
thermostats, the indicator, and the entire indicating system is done periodically. An analyzer test box capable of measuring the accuracy of all thermocouple components and the entire system is used.
What is the operation of a turbine engine tachometer?
Gas turbine engine speeds are measured by the RPM of each turbine-compressor spool and are displayed on the flight deck. Percent of RPM is used rather than the actual RPM so that various types of engines can be operated on the same basis of comparison. Also, turbine engine speeds are very high and percent RPM simplifies monitoring. Some tachometers measure RPM using a rotating tachometer generator that is geared to the engine. The frequency of the generator output is proportional to the engine speed. Another (newer) type of tachometer uses a magnetic pickup that counts passing gear teeth edges, which are seen electrically as pulses of electrical power as they pass by the pick-up. By counting the number of pulses, the shaft RPM is obtained. Clearance between the gear teeth and the magnetic pickup must be maintained for accuracy.
How is an EGT System checked for proper operation?
A calibrated test analyzer unit is used to check the EGT system and components for proper operation. The unit contains heater probes and built-in thermocouples against which the accuracy of the aircraft thermocouples is compared. The analyzer also is capable of checking the continuity of the system and the accuracy of the EGT indicator on the flight deck. Resistance and insulation checks are also made. Follow the instructions that come with the test analyzer unit being used.
What is EPR and how is it instrumented on a gas turbine engine?
EPR stands for Engine Pressure Ratio. It is an indication of thrust developed by a turbofan engine and is used to set power for takeoff on many types of aircraft. It is instrumented by total pressure pickups located in the engine inlet (P2) and in the turbine exhaust (P7). The indication is displayed on the flight deck on the EPR gauge, which is used in making engine power settings.
