Aircraft Maintenance Schedules
All private and commercial aircraft operations in Canada are subject to regularly scheduled maintenance in accordance with an Approved Maintenance Schedule (sometimes referenced as a Maintenance Schedule Approval or MSA). The MSA is based upon the aircraft manufacturer’s requirements and/or the corresponding regulations concerning the type of aircraft and its systems. As an Air Operator, the Brantford Flying Club are required to have maintenance schedules in place for the C-152, C-172 M/N/P/R and the C-172RG. The four C-172 models have been included on a single schedule since the features and systems of these aircraft are quite similar between the models shown. Each schedule is subject to approval by Transport Canada before implementation by commercial operators. Whenever another aircraft is added to the fleet, and fits the description of an existing schedule, then the operator need only comply with the familiar requirements of that schedule as applicable to the incoming aircraft. If a new type of aircraft is added to the fleet, then a new schedule will be required.
The purpose of each schedule is to ensure that each of the applicable maintenance activities are completed (and released) according to the operating time or calendar time as indicated. This structured method of planning this preventative work is necessary to maintain the aircraft to “like new” condition and to conform to its flight authority (the Certificate of Airworthiness or C of A). The responsibility for controlling the maintenance activity on an aircraft rests with the owner/operator of that aircraft. The owner tells the maintainer what activity they would like done. Even if the maintainer is aware of items not specifically requested by the owner, these items must first be authorized by the owner before being carried out (unless a specific approval process is already in place). The owner must also indicate the next maintenance event due after each maintenance event is completed. This is how an owner/operator controls the technical dispatch of their aircraft. The Maintenance Manager (or other signing authority) will refer to the maintenance tracker when determining the next items due (both hourly and calendar items) as part of the inspection release entry. This forms part of the technical dispatch process. Approved maintenance schedules for the BFC can also be referenced in section 12 of the company Maintenance Control Manual (MCM).
The basis of the MSA starts with the 100-hour inspection interval as a minimum requirement which can be based upon either the aircraft manufacturer’s program (and corresponding check sheets) or it can follow the more general check list provided in Standard 625, Appendix B (of the Canadian Aviation Regulations or CARs). Many private and small commercial operators (including BFC) use the general checklist to allow for some degree of flexibility in determining the appropriate maintenance activities based on experience with these types. The owner/operator may also just include references to the manufacturer’s list and or other alternatives based on supporting technical data. Private aircraft are required to have a 100-hour/Annual inspection (aka the Annual) every 12 months and commercial aircraft require the 100-hour inspection at least once per year even though they are generally busy enough for several recurring inspections within that same time frame. Each Club aircraft may end up flying in excess of 500 hours per year. That’s five 100-hour inspections over the course of 12 months.
When referencing a 100 hour maintenance event in the aircraft journey log, the opening statement may read, “100 hour inspection completed as per MSA P1652LON…” which is a direct reference to the TC Approval for the C-152 series operated by the Brantford Flying Club. This MSA reference must also be clearly documented in the front of the journey log for both private and commercial aircraft to the effect of “This aircraft is being maintained in accordance with MSA XX…” so that anyone can reference the specific maintenance schedule for that aircraft. In the case of a private aircraft, the reference would be made to Standard 625, Appendix B and C.
Other periodic maintenance items contained in the schedule (beyond the inspection) include various manufacturer’s recommendations and mandatory replacement items. This includes both hourly and/or calendar items such as the 50-hour oil change, 500-hour vacuum filter replacement, 1500-hour engine control replacement, and 12-month wheel bearing grease, to name only a few. All of these items are either recommended or required as a result of the manufacturer’s technical requirements.
The CARs also require that certain tasks be performed (as per Standard 625, Appendix C) as applicable to each aircraft. This includes both hourly and/or calendar items such as the 12-month compass calibration (aka Magnetic Direction Indicator or MDI), 24-month altimetry re-certification, and the 5-year fixed pitch propeller corrosion inspection, to name just a few. All of these additional items beyond the 100-hour inspection are referred to as out-of-phase items since they do not always come due in alignment with the inspection itself. This is where strategic monitoring and scheduling of these items is critical to being completed on time and part of an inspection where possible.
One of the Appendix C items includes engine overhaul as determined by a maximum number of operating hours recommended by the engine manufacturer. The exception to this rule is a quite common scenario where commercial aircraft operate according to an established “engine on-condition” program. This program utilizes additional inspection criteria through a combination of oil analysis, cylinder compression and output performance monitoring. Continued adherence with this program may permit engine operation beyond the published Time Between Overhauls (TBO) until such a point that the on-condition inspection requirements can no longer be met. Engine TBO may be extended well beyond the typical 2000 hours, for example, so that operators can safely manage their operating budget. Private aircraft are not bound to an engine hard-time limitation but would surely benefit from an on-condition program as well.
The aircraft operator also has the option to review MSA items to determine reliability trends for certain tasks and apply to have the prescribed intervals extended after a data driven risk analysis. Like the engine example, operators can more effectively manage these tasks based on their experience with the aircraft type. In some cases, the reliability monitoring may also indicate where certain intervals may need to be reduced as well so that known problem areas can be addressed more often to minimize the risk of failure. This quality analysis is largely dependent upon the aircraft operating environment, number of hours flown, and proven experience with the aircraft types and related tasks. These are all good considerations for a Flight Training Unit (FTU).
The BFC maintenance control system includes an electronic maintenance tracker which provides a comprehensive list of all MSA items and their corresponding due times and/or dates as reviewed daily by the Person Responsible for Maintenance (PRM aka the Maintenance Manager). This is how we determine which items will form part of the next 100-hour inspection work order. The operator must also track other requirements outside of the MSA items such as recurring Airworthiness Directives (ADs) and Service Bulletins (SBs) or any other Instructions for Continued Airworthiness (ICAs) pertinent to the installed equipment. These tasks are included in the same maintenance tracker as well so that the Maintenance Manager can see a complete picture of items to be scheduled for upcoming maintenance events. The maintenance tracker is a critical administrative tool for the Maintenance Manager to use on behalf of the Air Operator. The status of each aircraft depends on current information on a daily basis. Private owners/operators can develop their own tracking system to identify the items that they want done to ensure compliance.
In a commercial operating environment, it can become a challenge to coordinate all of the necessary out-of-phase items with a scheduled inspection amidst other daily scheduling demands. So there are inspection tolerances approved for each MSA item to provide some flexibility in scheduling. These tolerances are also in place to permit aircraft to return to base for maintenance, especially when involved with more active charter operations and/or sub-bases. Recognition of these tolerances is primarily to deal with unforeseen circumstances more so than as a scheduling tool, However, Cessna also builds in a tolerance to their progressive inspection schedule to serve that purpose. Typical BFC operations sometimes require us to implement a tolerance to work around extended flight bookings or for scheduling of sub-contracted services. In this case, the Maintenance Manager will make an entry in the journey log stating that certain identified maintenance tasks are now operating on a tolerance limiting the aircraft to “x” hours or days before they must be completed (according to the limitations of the approved tolerance).
I hope that this overview explaining the purpose of aircraft maintenance schedules has provided you with a little better understanding of the basis for controlling the maintenance on your aircraft. Some of my previous articles on aircraft ownership and airworthiness also reference maintenance schedules. In summary, commercial operators may get more involved with the contents of their maintenance schedules whereas private operators tend to remain much simpler. Either way, it still makes sense to take an active role in controlling the maintenance of your aircraft.