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"On-condition" appears to mean that a component should be "fit until failure" or that the maintenance should only be performed upon failure of the component.

This seems problematic.

Can someone help me to understand why some components are handled this way and not as part of a regular inspection schedule of some kind?

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I'm not a maintenance guy, but you may be mixing 'inspection' and 'maintenance' together here.

An inspection helps you decide if maintenance is needed or not. For example, inspecting the tire tread before every flight, or checking the cylinder compressions every 100hrs. If the inspection shows that the condition of the item is still within acceptable limits, then you don't need any maintenance yet. This is the 'on condition' or 'condition-based' approach, at least as I understand it.

Another approach is to do planned maintenance at fixed time intervals whether it's 'necessary' or not. For example, replacing engine oil every 50hrs, or replacing the entire engine every 2000hrs. In this case it doesn't matter what the item's physical condition is, you perform the maintenance even if everything looks good.

FWIW, some people strongly prefer the 'on condition' approach, especially for piston engines (see this book, for example). But even they probably replace their oil at more or less fixed intervals.

If you want something more official, I found an FAA definition:

On Condition Item (Oc)

A primary maintenance process requiring repetitive inspection or test to determine the condition of units, systems or portions of structure to assure continued serviceability. Corrective action is taken when required by item condition as determined by analysis of inspection and/or test results.

The key point as I understand it is that you base the decision to do maintenance (or not) on the physical condition of the item.

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    $\begingroup$ Additionally, there may not be a discrete "inspection" task. A jet engine under an Engine Condition Monitoring program would have, for example, vibration and exhaust temperatures automatically transmitted and checked against thresholds and trends in near-real time. If a problem is detected, maintenance control knows about it in minutes. $\endgroup$ – user71659 Dec 8 '17 at 22:22
  • $\begingroup$ @user71659. Correct, the aim is for no (or at least fewer) scheduled inspections, based on fixed intervals such as engine hours, or number of flights, or engine throttle cycles. The last sentence of the answer is the key point. $\endgroup$ – Penguin Dec 9 '17 at 4:03
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"On-condition" appears to mean that a component should be "fit until failure" or that the maintenance should only be performed upon failure of the component.

This seems problematic.

Only if there isn't a second unit of equal capability which can take over. This is called redundancy and is the guiding principle in aircraft design. You will not find an airliner with fewer than two engines or two tires per landing gear leg. If one of them fails, the other is capable of maintaining the core functionality of both to bring the craft to a safe landing.

In case of single-unit designs (most GA aircraft use only one engine, after all), the safety concept is a low wing loading and minimum speed of 61 knots or less so an emergency landing can be performed easily. In systems design, the goal is to have a gradual failure ("graceful degradation") so the flight can be continued even if that means temporarily a higher pilot workload or less than ideal flight characteristics.

The philosophy of aviation certification is to prove that fatal consequences occur less than once per

  • 10⁵ flight hours to persons directly involved with control of the aircraft (pilots, in other words),
  • 10⁷ flight hours to persons accepting the risk of flight travel but not directly involved with control of the aircraft (passengers, in other words) and
  • 10⁹ flight hours to innocent bystanders (occupants of the house the aircraft is crashing into, for example).

If you can prove that on-condition maintenance will keep you above those numbers, the procedure is fine with the authorities. You keep the unit running and monitor its condition in regular intervals. The intervals are chosen such that the probability of deadly consequences due to the failure of both parts of any redundant system will be lower than the numbers cited above, and in case of single-component systems that the probability of failure with deadly consequences for one airplane relative to the flying hours of the whole fleet is lower than the numbers above.

The accident tolerance has been higher in the past for some aviation organisations, but in general we have managed quite well lately to surpass the required minimum level of safety.

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  • $\begingroup$ Read this long after it was posted but what an amazingly detailed answer! $\endgroup$ – curious_cat Oct 29 '18 at 17:46
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An "On-condition" inspection is a non technical term, in other words it is not defined under definitions by the FAA in FAR 1.1 (Federal Regulations). However, FAR part 147 Appendix A Par(a)(1) defines non description "inspection" (on-condition is a inspection) as "to examine by sight and touch". By exclusion from other types of inspection such as "destructive", "magnetic resonance", etc - "On-condition" is an inspection able to be accomplished by an A&P using their "sight and touch". [jz pilot/A&P]

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On Condition are those items which, maintenance depends on certain Conditions being met. In simple terms, maintenance depends on the condition of the item, which can be determined by regular inspections, hence the 'On-Condition' term. The maintenance required can be determined based on the conditions being met, for eg. If a part is being lubricated regularly, there would be less wear, and hence less replacement. On the other hand, Condition Monitored items are those, on which we cant do any inspection to determine if it is wearing down or not. Hence we monitor them for failures, and get them replaced when failed. Thus called fit until failure items. Common eg are avionics items.

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