Aircraft Maintenance C Check
The Heavy C-Check
Unlike the light A Checks, the C Check is considered heavy maintenance. The aircraft is removed from service for an extended amount of time so that inspections, maintenance, and repairs can be made at a maintenance hangar rather than onsite at an airport. Once the aircraft is taken out of service for a C Check, it cannot be returned to service until the C Check is completed.
Routine maintenance and cleaning (A Checks) is performed but more importantly, the structural components of the aircraft are meticulous inspected. Functional and operational system checks are done as well as incorporation of standard (non-emergency) Airworthiness Directives (ADs), Service Bulletins (SBs), Modifications (MODS), and Engineering Orders (EOs).
There are different levels to C Checks similar to different levels of A Checks. Commonly distinguishable are the C1 and C6. While all C Checks are considered heavy checks, the C1 is considered light and the C6 is known as a heavy check. For modern aircraft (roughly beginning with the Boeing 737NG), airlines have a great flexibility in how C Checks are scheduled. Besides the basic C1 and C6, some airlines have scheduled up to C8 with a variety of tasks planned into more checks. For example, an airline can phase some heavy tasks over a number of more frequent C Checks. Because greater flexibility enables multiple task scheduling schemes, C Checks can be better described as a “Base Check” conducted inside a hangar.
All Maintenance Checks
Aircraft Maintenance C Check
Aircraft Maintenance D Check
Common Light C Check Procedures
Although modern aircraft enable flexibility in maintenance schedules, the C Check schedule is still based on a combination of flight hours (FH), flight cycles (FC), and calendar days. A typical B777 C Check happens at intervals of 7,500 FH, 1,250 FC, or 500 days, whichever is reached first. Therefore, C Checks occur roughly every 16 months. This is a longer interval compared to 12 months for older aircraft types.
Each C Check includes system, structural and zonal tasks. The man hour (MH) requirements vary based on different requirements from different airlines but a light C Check (C1, C2, C4, and C5) typically requires 2,400 to 2,800 MH for the B777. This is for routine tasks and is also relatively low compared to older models such as the 747. This includes removing and installing some rotable components.
Rotables are airplane parts or components that have traceable serial numbers. These can be rebuilt and reinstalled or put back in stock. Some must be removed and replaced based on condition but others have fixed hard time intervals when these must be replaced. These parts are common in the flight controls, emergency equipment, pneumatics, and hydraulics. These can require a significant number of man hours. A B777 has about 2,300 rotable components. Approximately 600 are replaced based on hard time intervals while the remainder are replaced depending on the condition.
Airlines almost always have additional maintenance and repair tasks that have been deferred from line maintenance and A Checks. These include out-of-phase tasks, modifications, and service bulletins, engineering orders, and airworthiness directives requiring additional downtime. Besides routine maintenance, the C Check can include deep interior cleaning such as refurbishing galleys, toilets, and panels. It can include stripping and repainting the entire aircraft and other corrosion protection. To reduce down time and control costs, these are often spread out among multiple C and D Checks.
Common Heavy C Check Procedures
A heavy C6 typically requires between 17,000-18,000 MH. The C6 check is typically scheduled at 3,000-days/100-months (but again it varies by airline). The primary heavy components are the landing gear, wheels, and brakes, thrust reversers, and the auxiliary power unit (APU).
Each of the 2 landing gears on a 777 has 6 wheels with 2 more wheels on the nose gear. Tire wear is mostly affected by the number of flight cycles but can be shortened by other causes such as landing weight and hard landings. Nose gear tires are typically changed after 250 FC and main gear after 360 FC. Wheel removal coincides with wheel rim inspections. Rim inspection is a condition based maintenance process that can occur prior to scheduled tire replacement. The inspection is only visual unless it reveals needed repairs or overhaul. Repairs and overhauls occur about every 4 or 5 wheel removals. Brake replacement is also a conditional based replacement. The 777 features lighter carbon brakes rather than steel and have proved to have longer removal intervals. Both tires and brakes are visually checked on a regular basis during daily line checks.
Landing gear overhaul intervals for 777s were originally established at 10 years or 16,000 FC. Individual airlines have since made adjustments based on first and subsequent visits to the hangar.
Thrust reversers are inspected with overhauls occurring on a conditional basis. The use of composite materials on the 777 has significantly increased the overhaul intervals. Some maintenance occurs with the thrust reversers attached to the wing with removal for overhaul after approximately 4,400 FC. Experience is showing these intervals might be significantly extended by advanced maintenance practices with the reversers remaining on the wing.
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The APU is also removed for overhaul based on condition. This is dependent on variations in APU usage during airplane operations and is effected by both FH and FC. The average removal interval is 7,600 to 8,500 APU hours. Airlines can minimize APU use by restricting use to main engine start and air conditioning prior to main engine start. Use during flight can reduce overhaul intervals to between 3,400-5,500 FC.
A Final Certificate of Release to Service has to be issued after each C Check before the airplane returns to service.