AWB 02-1 Issue 1 - On-condition maintenance
AWB 02-1 Issue 1, 27 November 2001
Some C of R holders and LAMEs believe that "on-condition" means fit and forget, or don't do anything until a failure occurs. The above interpretations of "on-condition" maintenance may cause operational surprises, which could not only prove very costly, but also jeopardise the safety of an aircraft and its occupants.
The majority of aircraft mechanical components do not fail abruptly, but give some warning or sign of the fact that they are about to fail. These warnings or signs are called Potential Failures, and are defined as identifiable physical conditions which indicate that a functional failure is about to occur or is in the process of occurring. The amount of warning given by different potential failures varies from microseconds to decades. Longer warning intervals mean greater maintenance task intervals.
Maintenance tasks (inspections/checks) used to detect potential failures, and consequently to avoid a total functional failure, are called "on-condition" maintenance tasks. This is because items are left in service on the condition that they "continue" to meet a desired physical condition and performance standards.
The process of "on-condition" maintenance is applied to items on which a determination of their continued airworthiness can be made by visual inspection, measurements, tests or other means without disassembly inspection or overhaul. The condition of an item is monitored either continuously or at specified periods. The item's performance is compared to an appropriate standard to determine if it can continue in service. These appropriate standards may relate to, but are not limited to, cleanliness, cracks, deformation, corrosion, wear, pressure or temperature limits, looseness or even missing fasteners, and are published in the applicable approved data of the aircraft or aircraft component.
"On-condition" maintenance means an inspection/functional check that determines an item's performance and may result in the removal of an item before it fails in service. It is not a philosophy of fit until failure or fit and forget.
For example, an upper or lower limit of an indicated parameter such as a fluid pressure (continuous monitoring) or monitoring of upper limits of solids content in a lubricant may indicate a component's wearing condition, etc. Failure of an item to continue to meet the specified standard will indicate that further maintenance actions are necessary.
At specific periods, the condition of an item may require a sound technical judgement to determine that a malfunction or failure of the item will not occur prior to the next scheduled inspection.
According to an extensive international study conducted in the aviation industry, as much as 89 per cent items can benefit from "on-condition" maintenance tasks. That is why, at present, the majority of maintenance tasks listed in the manufacturer's maintenance schedules/checks or CASA maintenance schedule (Schedule 5 of CARs 1988) are "on-condition" tasks.
Depending on the maintenance system in force, compliance with "on-condition" maintenance tasks is mandatory if listed in the manufacturer's maintenance schedules/checks or Schedule 5 of CARs 1988.
Manufacturers Recommended TBO
Aircraft and component manufacturers can make "Hard Time" recommendations (i.e. removal of items from service at a specified period for overhaul or replacement indifferent of the items current performance condition), usually referred to as Time Between Overhaul (TBO), which specify how long they consider their product should remain in service. These recommendations are based on average utilisation and conditions and usually recommend that the item be fully stripped and returned to the original specifications. TBO's do not normally involve a condition check being done during the items life. The ability to escalate these hard time limitations however, comes from effective condition monitoring - the real basis for "on-condition" maintenance.
C of R holders should utilise the philosophy of "on-condition" maintenance to detect the onset of failures of such items, particularly when time in-service of these items are in the vicinity of the manufacturer's recommended TBO.
Provided that a component continues to meet the documented standard, at the appropriate frequencies, it is considered satisfactory to remain in service. TBOs that are not included in the manufacturers Airworthiness Limitations or in Airworthiness Directives issued by CASA should still be considered, unless substantiation has been collated to show the outcome of "on-condition" inspections are still appropriate for the safe operation of the aircraft or equipment.
Where alleviation is permitted beyond the manufacturer's TBO, an example of which would be AD/ENG/4, C of R holders and LAME's must ensure at the completion of the aircraft periodic inspection the "on-condition" maintenance inspection requirements are included on part 1 of the aircrafts maintenance release as "maintenance required".