The aim of airframers is to make sure all maintenance is “planned” maintenance – where operators can schedule the maintenance ahead of time, to make sure parts and technicians are available etc. “Unplanned” maintenance – often when something has gone wrong with the aircraft – can be very costly to aircraft operators, as they will often have to wait for technician availability or key parts, the lead time for which can be many months if parts required are not in stock. In the worst cases, “un planned” maintenance can also cost a company in terms of image and corrective action if there was a major incident on the helicopter. Legal authorities may be involved, and design safety can be questioned.
Maintenance procedures fall under one of two categories – “mandatory” and “recommended” (different OEMs will have different names for the two categories, but they are fundamentally the same). Within mandatory maintenance are all the checks and procedures required to keep aircraft in an airworthy condition. “Recommended” checks are those that will help keep the aircraft functioning at its best, but are not critical to safety.
Within the scope of SKF products, maintenance typically means one of three things:
- Inspection – end users are required to check the product for any signs of damage, deterioration or any other visual changes. This will most commonly apply to easy-to-access parts, such as main-or tail-rotor components (for example self-lubricating bearings and rod ends, elastomeric or composite rods).
- Measurement – end users are required to measure the parts’ functionality. Some examples are play or “looseness” measurement, torque measurement, or vibration measurement. All SKF products will be subject to some form of measurement checks.
- Replacement – end users will be required to replace a part either after a set number of flight hours, or due to some criteria found through inspection or measurement.
Calculation of maintenance intervals is complex and depends on OEM requirements, but generally can be divided into two categories based on the part in question. For parts which are impossible to measure or inspect, a replacement interval is specified which is typically the average life of the part with a large safety factor applied. For parts which can be inspected or measured, an inspection interval is typically based on the time between the rejection criteria of a part and the full failure of a part.
With the advent of condition monitoring, airframers are able to propose some form of intelligent maintenance. Systems already exist to make known, system performance problems, for example, through vibration analysis, which in itself is a significant safety system. However, this approach is less useful for maintenance management, as it is a relatively late stage measurement.
SKF is working to integrate sensing within its supplied components, which when combined with SKF knowledge on product performance and reliability, will enable better scheduling of maintenance, and a prognostic approach to aircraft systems.