Jetrader - November/December 2011 - 27

planning becomes very critical for the short delivery times and the high service levels required by an operator.

Wiring
Wire bundles are everywhere; they are critical in any aircraft and the engine has its share. Shielding contributes signiﬁcantly towards an aircraft’s continued airworthiness by maintaining electromagnetic protection over the lifetime of an aircraft. Aircraft aging and exposure to environmental conditions affects aircraft wiring harness shielding. The quality of the electrical bonds between the cable shield, backshells, connectors and metallic structures can deteriorate with time. Most aircraft wiring harnesses are 24-inch-long wire bundles with end connectors and connector backshells, termination boxes and bulkhead receptacles. Wire bundles can consist of a combination of unshielded wires and shielded wires, secured along their length with plastic tie wraps. This is a standard wire bundle conﬁguration. In the engine environment, temperature, altitude, vibration, salt spray and humidity can all cause mechanical degradation of the aircraft’s wiring shielding. This degradation can lead to secondary issues with extensive troubleshooting due to false indications induced by this condition.

Borescope inspections are important maintenance procedures useful for maintaining good engine condition. longevity and total cost of ownership. These are “Operate to Failure,” “Fixed Interval Maintenance” and “On-Condition Maintenance.” In the Operate to Failure method, all maintenance is unscheduled and can have some signiﬁcant impacts on operations and budgets. The Fixed Interval Maintenance model establishes mandatory overhaul at established times between restorations and can cause unnecessary removal of serviceable units and parts which can be costly both in the shop visits and partial use of a life limited part. While Operate to Failure and Fixed Interval Maintenance programs have distinct ﬁnancial advantages in the short term, the old adage “pay me now or pay me later” may apply in more than just engine maintenance dollars. As an engine degrades over time, its efﬁciency also degenerates, leading to higher fuel burn. The On-Condition Maintenance program maintains serviceability if the engine is meeting ﬁeld limits and corrective actions are scheduled. This helps optimum utilization of parts and can lead to a cost-effective maintenance program.

Electrical Wire Interconnect Systems
As an aircraft ages, so does the integrity of its Electrical Wire Interconnect Systems (EWIS), which can lead to aircraft system malfunctions and in some cases, arcing, which can result in more signiﬁcant damage. A breach in the insulating material and contact with a grounded structure or foreign ﬂuids can result in electrical arcing. Polyimide insulation is susceptible to environmental deterioration due to hydrolysis that can reduce the polymer’s molecular weight. Attacks on wiring insulation can come in many types: Extreme heat and cold temperature swings can accelerate hardening (or softening), humidity and chemical spills or leaks can create brittleness, and mechanical strains can fatigue the wire conductor creating a gap that can arc and generate heat. Over-voltage can cut holes in electrical insulation leading to ﬂashover. Mechanical damage that occurs during the wire installation, often in the form of metal chips lodged in wire bundles, can go undetected for years. Tight bend radiuses can chafe the insulation and also fatigue the conductor.

Parameters for an On-Condition Maintenance Program
When establishing this program, you need to use eight parameters for data considerations. These are: Exhaust Gas Temperature (EGT), Fuel Flow (FF) or (WF), Low Pressure Rotor (N1), High Pressure Rotor Speed (N2), Airborne Vibration Monitoring (AVM), oil pressure, oil temperature, and throttle position on some none EEC. In addition to inﬂight monitoring of engine parameters, several other maintenance procedures are useful for maintaining good engine condition and health such as oil ﬁlter checks, oil analysis, and borescope inspections, as seen in the photo above. This method is designed as a proactive engine maintenance management system and is based on actual engine health, not time intervals. It requires monitoring of engine conditions and overall performance with a combination of scheduled on-wing inspection programs and engine-trend performance. Engine health monitoring involves comparing ﬂight record data to a baseline. Engine baseline values are obtained from curves for EPR versus corrected fuel ﬂow N1, N2, and EGT for a given engine model. Adjustments may be made to baseline values to account for certain modes of ﬂight. Since monitoring assesses the engine performance and health, it reduces unexpected problems and unscheduled maintenance by detecting abnormal trend shifts in the engine performance. Data is the key to the success of this program and may be recorded either automatically by means of the onboard data acquisition units or manually via log book entries. Due to variations in aircraft operating conditions, this data must be normalized to detect abnormal shifts and trends. Jetrader 27

Choosing the Right Program
When considering the management of engines under an Engine Maintenance Program (EMP), one constant you will face is that as an engine ages, its performance will deteriorate. Causes include physical distortion of engine parts due to corrosion, the ingestion of foreign objects, the buildup of deposits (fouling), erosion of parts and general wear. Degradation internal to the engine begins to manifest as changes in measurable engine parameters such as exhaust gas temperature (EGT), fuel consumption (as speciﬁc fuel consumption, SFC, or fuel ﬂow, FF), turbine inlet temperature (TT4), low or high pressure spool speeds (N1 or N2 respectively), and/or engine pressure ratio (EPR). Also, there are changes in engine performance bounds such as the stall margin. There are three basic models for their EMP from which an operator can choose. Each has a positive and negative side to both engine

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Jetrader - November/December 2011

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