Line Maintenance generally refers to minor, unscheduled or scheduled maintenance carried out on aircraft that includes:
- Any unscheduled maintenance resulting from unforeseen events
- Scheduled checks that contain servicing and/or inspections that do not require specialized training, equipment, or facilities.In service; and that is preparing for its first flight in service
- Maintenance performed on aircraft after a period of being out of service (such as aircraft in storage)
- Maintenance on en route aircraft that are stopped before their next flight including Servicing or repair between successive flights
- Preparing and readying an aircraft for flight during a period of service
- Maintenance activities being performed to ensure that the aircraft is airworthy and fit for flight.
The three common areas where Aircraft Maintenance is performed are:
Line or Flight Line Maintenance
Occurs at or near the gate or terminal (Tarmac), launch area, ready area, hardstand or alert area. The level of dis-assembly is usually limited to what can be reassembled and restored within a period of less than a shift or two. A limiting factor is usually the level of Ground Support Equipment required such as Electrical Carts, Hydraulic Mules, work stands and lifts. Various safety regulations limit the amount of work that may be practically performed outside of a hangar facility
Hangar Maintenance or Intermediate Maintenance
Any level of Maintenance from Servicing to Overhaul may be performed inside a hangar. Typically most organizations utilize a facility where either the complexity, length of repair, number of staff, support equipment, tools and parts required dictate a covered, dedicated facility. Most repairs conducted at this level are usually “on-wing” or simple parts replacement.
Heavy Maintenance, MRO or Overhaul facility
The highest level of Maintenance with capabilities of disassembling Inspecting, repairing, refurbishment, Overhaul and restoration. Often the capabilities of the facility will include a repair station certificated under part 145 where off wing repair and overhaul of individual components may also be performed.
In fact by virtue of being certificated under part 145 requires the entity to maintain a controlled facility where the work is being performed and when working outside they must have facilities that are acceptable to the FAA which is essentially advance approval. See 14 CFR 145.103 (c) below
145.103 Housing and facilities requirements.
(a) Each certificated repair station must provide—
(1) Housing for the facilities, equipment, materials, and personnel consistent with its ratings.
(2) Facilities for properly performing the maintenance, preventive maintenance, or alterations of articles or the specialized services for which it is rated. Facilities must include the following:
(i) Sufficient work space and areas for the proper segregation and protection of articles during all maintenance, preventive maintenance, or alterations;
(ii) Segregated work areas enabling environmentally hazardous or sensitive operations such as painting, cleaning, welding, avionics work, electronic work, and machining to be done properly and in a manner that does not adversely affect other maintenance or alteration articles or activities;
(iii) Suitable racks, hoists, trays, stands, and other segregation means for the storage and protection of all articles undergoing maintenance, preventive maintenance, or alterations;
(iv) Space sufficient to segregate articles and materials stocked for installation from those articles undergoing maintenance, preventive maintenance, or alterations; and
(v) Ventilation, lighting, and control of temperature, humidity, and other climatic conditions sufficient to ensure personnel perform maintenance, preventive maintenance, or alterations to the standards required by this part.
(b) A certificated repair station with an airframe rating must provide suitable permanent housing to enclose the largest type and model of aircraft listed on its operations specifications.
(c) A certificated repair station may perform maintenance, preventive maintenance, or alterations on articles outside of its housing if it provides suitable facilities that are acceptable to the FAA and meet the requirements of § 145.103(a) so that the work can be done in accordance with the requirements of part 43 of this chapter.
Depot Level Maintenance (US Air Force Definition)
Depot-Level Maintenance. The level of maintenance consisting of those on and off-equipment tasks performed using
highly specialized skills, sophisticated shop equipment, or special facilities of an ALC, contractor facility, or, by field teams
at an operating location. Maintenance performed at a depot also includes those organizational- and intermediate-level tasks
required to prepare for depot maintenance, and, if negotiated between the depot and the operating command, scheduled
field-level inspections, preventative maintenance or TCTOs which come due while equipment is at the ALC for PDM.