Table of Contents
19.2.4 Risik minimising by use of the maintenance.
It's not seldom, that failures respectively problems occur which can not be ruled out at further aeroengines. Such potential risks can be reduced to an acceptable degree in many cases during the maintenance. The necessary measures (Fig. "The Art of risk minimizing") can be carried out, according to the failure/damage mechanism and the supposed failure progress during the normal maintenances and/or in shorter, failure specific time periods. The measures normally are assigned in so called AD's (airworthiness directives, Fig. "Maintenance instructions and specifications") by the responsible aeronautical authority.
Requirement for such a measure is the sufficient sure clarification of the failure. To this belong cause, origin, mechanism, time flow and noticeable features of the failure.
Figure "The Art of risk minimizing": After a failure the question after avoidance respectively remedies arises. Those normally are prepared by the OEM and approved by the responsible authorities. The instruction takes place in the appropriate form according to prescribed procedures.
The picturew shall schow typical measures how the risks of failures can be minimised. Expecially in many cases so a further or a short term use is possible. Measures are primarily supported by informations in instructions respectively overhaul manuals and maintenance mauals. Are here definitions and specifications of approved faults respectively specific procedures unsatisfying, the OEM must be consulted.
Monitoring measures can to be considered, if the damage has not yet reached unacceptable consequences or a not bearable risk. Examples are slowly propagating or stagnant thermal fatigue cracks in stationary hot parts (e.g., combustion chamber or turbine guide vanes, Fig. "Risk potential of brazing repairs").
Borescope inspections allow precautionary checks, e.g., for foreign object impacts in the blading.
Non destructive tests like penetrant inspection, eddy current or ultrasonic testing (Ill. 220.127.116.11-9) can be carried out at a suitable position and size of the failure in the in the assembled condition or after temporary disassembly (e.g., at blade roots).
Chip control (Fig. "Material specific particle content in oil") of filters and magnet plugs (oil, fuel) can as well show the beginning of a failure as also allow conclusions at concerned components.
Operation constrictions normally will get necessary when it is possible „to live with the failure“ at least over a determined time period. That is the case if the time is not available or the lack of spare parts. The constrictions comply with the mechanism and the propagation of the failure.
If there are fatigue problems in the LCF region like under cyclic load of rotor disks or thermal fatigue at hot parts, frequently the number of starts till an exchange is necessary, will be limited. Thereby the crack propagation (volume 3, chapter 12.2.1) referred to starts and/or operation time must be known (volume 3, Ill. 18.104.22.168-2). This requires an analysis of the fracture surfaces from the failed part. Also a decrease of the thermal stresses and temperature peaks with an optimised respectively „more careful” start procedure of the aeroengine can be promising. (volume 3, Ill. 22.214.171.124-7 and Ill. 126.96.36.199-8).
Problems with unacceptable heavy oxidation may rather require a limitation of the operation lifetime.
Increased temperatures of the turbine blading by clogging of the cooling air passages or an unsuitable temperature distribution of the hot gas can require a limitation of the performance.
Repairs on site are limited by the manual specifications. To this belongs the removing of mechanical damages (FOD) at the blading (Fig. "Rework instruction for fan blades"). Cracks in the gas ducts behind the aeroengine frequently can be repaired by welding. In some cases where is no unacceptable leakage to expext, the crack propagation can be delayed by a stop-drill (volume 3, Ill. 12.6.2-18).
With additional measures maintenance can prevent failures. To this count especial efforts for the cleaning of the compressor blading (Fig. "Cleaning military aeroengines") and/or turbine blading (Fig. "Negligence can lead to failures"). Further frequent checks of the oil (oil analysis) can ensure the operation (Fig. "Specifications for oil contaminations").
Also an improved or adapted lubrication to the problem can ease difficulties with sluggish or firm stuck parts.
Exchange and rework of deteriorated/damaged components. To those belong blades with alarming fretting wear or hot parts (combustion chamber, turbine blading) with degradations by thermal fatigue or oxidation.
Are not yet fatigue cracks occurred at potential endangered components the risk can be minimised by shot peening.
19.2.4-1 Transportation Safety Board of Canada, TSB Report A9500232, „Engine Failure Air Canada Airbus A320-211 C-FFWJ, Montreal International (Dorval) aiiport, Quebec, 14.November 1995“, page 1 -6.
19.2.4-2 „Tay Engine Service Bulletin Manual, Revision 3 to Service Bulletin TAY-72-1442”, Oct. 31/97, page 3.
19.2.4-3 „Tracking Footprints from Engine 011“, Zeitschrift: „Aviation Engineering & Maintenance Magazin”, 1979, Sept, page 30-33.