AVIATION MAINTENANCE

Choosing of engine and aircraft maintenance generally influences on flight safety. The number of engine inspections and diagnostic methods being applied during maintenance influences on the probability of determination of failures.

During maintenance of modern engines following techniques are applied:

­­– Maintenance by operating time.

– Maintenance by state, with the control of reliability level.

– Maintenance by the state, with parameter control.

    The method of maintenance by operating time is recommended for application to the aggregates failures of which directly influence on flight safety. To the units, which don’t have good controllability and interchangeability level, units’ good state of which decreases with operation time and aggregates for which there are no methods of technical state estimation. 15 percents of all aggregates is maintained by statistical data requirements.

            The method of maintenance with parameter controlling is the most progressive method. 30 percents of all aggregates maintained by this method. This technique is recommended to be applied for elements with good controllability, and these elements must have methods of non-destructive control. This category of maintenance is divided into following strategies:

– With periodical parameter controlling.

– With uninterrupted parameter controlling.

Aggregates which are maintained with the help of method of periodical parameter controlling: inlet vanes, rotor and nozzle blades. Application of modern electronic systems on the engine significantly increases the number of aggregates which are maintained by the uninterrupted parameter controlling.

 When failure of some element was detected during operation, element will be in operation till the limits of its pre failure state.

Engine maintenance consists of operative, periodical and special maintenance.

Operative engine maintenance includes following kinds of works:

– Visual inspection for determination of technical state of engine structural elements.

– Disassembling  and checking of oil and fuel systems filters.

– Fuel and oil systems inspections on the criteria of leakage presence.

– Oil level determination in the oil tanks.

Periodical maintenance consists of the following works:

–       Visual inspection with the help of the optical control methodic.

–       Cleaning of engine filters

–       Checking of correspondence of power lever position which is located in the cockpit, with levers on the engine.

–       Cleaning of oil tanks and heat exchangers.

–       Lubrication of engine details, which work with friction.

–       Changing of oil in the system.

During special maintenance, which executed after some accidents, works with special optical tools are performed at the engine.

Engine testing and starting

When engine was returned from repair plant, after some kinds of maintenance which was performed on the engine it is necessary to perform test starting of the engine in order to determine possible defects or deviations from normal operating mode.

Peculiarities of engine maintenance in low temperature conditions  

When the temperature of air is +5 and below, in the possibility of icing or in rainy and snow conditions it is necessary to perform following actions:

– To switch on engine anti icing system before its starting. Engine operation without help of this system is forbidden. It is allowed to switch off this system for short time in order to measure air parameters at the engine inlet.

–       To install engine protection screens immediately after engine shut down.

When air temperature is 0°С or less it is necessary:

1. To determine absence of ice on the engine inlet.

2. Check the possibility of fan rotor rotation. If fan rotor rotation is impossible it is necessary to blow air-gas channel with hot air of temperature not greater than 80°С till all ice on fan blades will disappear.

        When air temperature is – 40°С and lower, and engine was not in operation for the time greater than two hours it is necessary:

1. To heat engine by hot air with temperature not greater than 80°С, also air starter, oil-fuel heat exchangers, gear box, and oil tank must be heated  by this air.

2. Engine cold idling must be done.

         Possible failures of engine during operation and ways of their solution

         As the engine prototype was chosen engine of foreign manufacturer, we have no any data about possible failures. But statistical data of failures of our engines show that main reasons of engine failure practically don’t change for all engines. Failures of engine main units appear practically by technological defects, bad operating conditions and special cases of aircraft flight.

       For the analysis of failures special table with data was made. This table contents all possible failures , engine, which is the analog of projected engine. 

Conclusion

Because many aspects of maintenance are subject to the approval of a recognized authority, it should be fully understood that the information given in this part is of a general nature and is not intended as a substitute for any official instructions.  The comprehensive instructions covering the actual work to be done to support scheduled maintenance and unscheduled maintenance are contained in the aircraft maintenance manual. The maximum time an engine can remain installed in an aircraft (engine life) is limited to a fixed period agreed between the engine manufacturer and airworthiness authority. On some engines this period is referred to as the time between overhaul (T.B.O.) and on reaching it the engine is removed for

complete overhaul.

Because the T.B.O. is actually determined by the life of one or two assemblies within the engine, during overhaul, it is generally found that the other assemblies are mechanically sound and fit to continue in service for a much longer period. Therefore, with the introduction of modular engines and the improved inspection and monitoring techniques available, the T.B.O. method on limiting the engine’s life on-wing has been replaced by the ‘on-condition’ method.  Basically this means that a life is not declared for the total engine but only for certain parts of the engine. On reaching their life limit, these parts are replaced and the engine continues in service, the remainder of the engine being overhauled ‘on condition’. Modular constructed engines are particularly suited to this method, as the module containing a life limited part can be replaced by a similar module and the engine returned to service with minimum delay, The module is then disassembled for life limited part replacement, repair or complete overhaul as required.

M.moghadasi