General Maintenance (MX) chart for airline aircraft
| Type |
Description |
| A | This is performed approximately every 100-150 flight hours. This check is usually done overnight at an airport gate. The actual occurrence of this check varies by aircraft type, the cycle count (takeoff and landing is considered an aircraft "cycle"), or the number of hours flown since the last check. The occurrence can be delayed by the airline if certain predetermined conditions are met. |
| B | This is performed approximately every 3 months. This check is also usually done overnight at an airport hangar. A similar occurrence schedule applies to the B check as to the A check. B checks may be incorporated into successive A checks, ie: A-1 through A-10 complete all the B check items. |
| C | This is performed approximately every 12–18 months or a specific amount of actual Flight Hours(FH) as defined by the manufacturer. This maintenance check puts the aircraft out of service and requires plenty of space - usually at a hangar at a maintenance base. The schedule of occurrence has many factors and components as has been described, and thus varies by aircraft category and type. |
| D | This is most comprehensive check for the airplane, also known as a Heavy Maintenance visit (HMV). This check occurs approximately every 4–5 years. This is the check that, more or less, takes the entire airplane apart for inspection. This requires even more space and time than all other checks, and must be performed at a maintenance base. Often, older aircraft being phased out of a particular airlines' fleet are stored or scrapped upon reaching their next check due to the high costs involved in comparison to the aircraft's value. Many Maintenance, Repair and Overhaul (MRO) shops believe it is virtually impossible to perform a D check profitably at a shop located within the United States, and thus do not offer D checks. |
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