Maintenance work is divided between that which is necessary to maintain ongoing daily operations (line maintenance) and those procedures that periodically check, maintain and refurbish the aircraft (base maintenance).
Line maintenance comprises en route (between landing and takeoff) and overnight maintenance. En route maintenance consists of operational checks and flight-essential repairs to address discrepancies noted during flight. These repairs are typically minor, such as replacing warning lights, tyres and avionic components, but may be as extensive as replacing an engine. Overnight maintenance is more extensive and includes making any repairs deferred during the day’s flights.
The timing, distribution and nature of aircraft maintenance is controlled by each company and is documented in its maintenance manual, which in most jurisdictions must be submitted for approval to the appropriate aviation authority. Maintenance is performed during regular checks, designated as A through D checks, specified by the maintenance manual. These scheduled maintenance activities ensure that the entire aircraft has been inspected, maintained and refurbished at appropriate intervals. Lower level maintenance checks may be incorporated into line maintenance work, but more extensive work is performed at a maintenance base. Aircraft damage and component failures are repaired as required.
Line Maintenance Operations and Hazards
En route maintenance is typically performed under a great time constraint at active and crowded flight lines. Mechanics are exposed to prevailing conditions of noise, weather and vehicular and aircraft traffic, each of which may amplify the hazards intrinsic to maintenance work. Climatic conditions may include extremes of cold and heat, high winds, rain, snow and ice. Lightning is a significant hazard in some areas.
Although the current generation of commercial aircraft engines are significantly quieter than previous models, they can still produce sound levels well above those set by regulatory authorities, particularly if the aircraft are required to use engine power in order to exit gate positions. Older jet and turboprop engines can produce sound level exposures in excess of 115 dBA. Aircraft auxiliary-power units (APUs), ground-based power and air-conditioning equipment, tugs, fuel trucks and cargo-handling equipment add to the background noise. Noise levels in the ramp or aircraft parking area are seldom below 80 dBA, thus necessitating the careful selection and routine use of hearing protectors. Protectors must be selected that provide excellent noise attenuation while being reasonably comfortable and permitting essential communication. Dual systems (ear plugs plus ear muffs) provide enhanced protection and allow accom-modation for higher and lower noise levels.
Mobile equipment, in addition to aircraft, may include baggage carts, personnel buses, catering vehicles, ground support equipment and jetways. To maintain departure schedules and customer satisfaction, this equipment must move quickly within often congested ramp areas, even under adverse ambient conditions. Aircraft engines pose the danger of ramp personel being ingested into jet engines or being struck by a propeller or exhaust blasts. Reduced visibility during night and inclement weather increase the risk that mechanics and other ramp personnel might be struck by mobile equipment. Reflective materials on work clothing help to improve visibility, but it is essential that all ramp personnel be well trained in ramp traffic rules, which must be rigorously enforced. Falls, the most frequent cause of serious injuries among mechanics
Chemical exposures in the ramp area include de-icing fluids (usually containing ethylene or propylene glycol), oils and lubricants. Kerosene is the standard commercial jet fuel (Jet A). Hydraulic fluids containing tributyl phosphate cause severe but transient eye irritation. Fuel tank entry, while relatively rare on the ramp, must be included in a comprehensive confined- space-entry programme. Exposure to resin systems used for patching composite areas such as cargo hold panelling may also occur.
Overnight maintenance is typically performed under more controlled circumstances, either in line-service hangers or on inactive flight lines. Lighting, work stands and traction are far better than on the flight line but are likely to be inferior to those found in maintenance bases. Several mechanics may be working on an aircraft simultaneously, necessitating careful planning and coordination to control personnel movement, aircraft component activation (drives, flight control surfaces and so on) and chemical usage. Good housekeeping is essential to prevent clutter from air lines, parts and tools, and to clean spills and drips. These requirements are of even greater importance during base maintenance.