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The Failure Process. Embry-Riddle Aeronautical University. Before the Wright Flyer…. The Failure Process. Scheduled Maintenance Failure Summary. Mechanical Advantage. Mechanical Components subject to: Wear Fatigue Possible Corrosion
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The Failure Process Embry-Riddle Aeronautical University
The Failure Process • Scheduled Maintenance • Failure • Summary
Mechanical Advantage • Mechanical Components subject to: • Wear • Fatigue • Possible Corrosion • Wear out may take long time – referred to as “graceful degradation” • Heat is the worst enemy
Role is to cope with the failure process Age at which they will fail rather than how they fail and what happens after they fail Assumption that as items age they become less reliable and fail Led to overall failure rate of item will be reduced by age limit Hard Time (HT) is the removal of an item at a predetermined interval (hrs, cycles, calendar time) Scheduled Maintenance
Hard Time (HT) • Requires item be removed & either completely overhauled, partially overhauled (restored), or discarded before exceeding the specified interval • (calendar time, engine change, cycles, flight hours, specified flights – over water etc..) • Items that can have an adverse effect on safety but no maintenance check for that condition • Rubber seals, bushing etc.. • Structural inspection, landing gear overhaul, & life limited engine parts, mechanical actuators, hydraulic pumps & motors, electric motors & generators • Can be OC as long as not safety related…
Failure • Failure is an unsatisfactory condition • Any identifiable deviation from the original condition that is unsatisfactory to a particular user • Unsatisfactory depends on the consequences of the failure in a given operating context • Loss of oil on short flight or engine on multi-engine aircraft vs long flight and single engine aircraft • Failure determination varies by organization and type / model / series of aircraft (MEL)
Maintenance Management Techniques • Minimum Equipment List (MEL) • List of equipment that the aircraft may be inoperative but still allowed to fly as long as it does not affect the safety & operation of flight – Tailored to mission • Determined by manufacturer & sanctioned by the regulatory authority (FAA)
Maintenance Management Techniques • Master Minimum Equipment List (MMEL) • Lists ALL equipment for the aircraft model – list is tailored to its MEL • Concept of the MEL is to allow the deferral of maintenance without upsetting mission requirements • Maintenance must be performed within some set of guidelines (1, 3, 10, or 30 days or cycles required) depending on operational requirements of the system • Pilot may always require or defer maintenance per the MEL • Maintenance MUST abide by decision…. (his call) • “Takes a college education to break it & a high school education to fix it.”
Types of Failure • Potential/Functional Failure is the inability of an item to meet a specified performance standard • Includes to function at a level that has been specified as satisfactory (i.e. water heater with 2 elements) • Must have clear understanding of its functions • Brakes – anti-skid, modulated braking or total failure
Types of Failure • Particular Functional Failures – identified that failure is imminent – known as potential failures – so we can continue to use until we can repair or replace before total functional failure (when convenient not when the occur)
Functional or Potential Failures • Clear definitions of the functions of items as they relate to the equipment or operating context in which the item is to be employed • A clear definition of the conditions that constitute a functional failure in each case • A clear definition of the conditions that indicate failure is imminent. • Must be able to define not only the failure but the precise way it is recognized
Perception is Reality?? • Mechanics see fuel leak, super sees seep, maint chief sees weep • Aircrew see most dynamic operational failures • Monitoring systems • Master caution/Warning and on board test and recording systems make failure evident before a system is needed • Evident and Hidden functions • Evident – failure is obvious as they conduct normal duties • Hidden – failure only when called upon
Hidden Functions • A function that is normally active but gives no indication to the crew if it fails. • The Fire detection system. • A function that is inactive, so that crew cannot know it will be available until needed. • A fire extinguishing system or emergency egress system.
The Consequence of Failure • The consequence of a failure determines the priority of the maintenance activity or design improvement required to prevent its occurrence.
Failure Consequences • Safety Consequences – involving potential loss of equipment and its occupants. • Operational Consequences – involves an indirect economic loss as well as the direct cost of repair. • Non-operational Consequences – involves only the direct cost of the repair.
Failure Consequences • Hidden-failure Consequences – has no direct impact BUT increase the likelihood of a multiple failure. • Critical failure is any failure that could have impact on safety.
Summary • Role of Scheduled Maintenance • Definition of failure • Hard Time • Potential and Functional Failure • Hidden Function • Four categories of Failure Consequences • Critical Failure
