I know the MEL and CDL are the minimum equipment list is provided by the manufacturer, if component missing or damaged means the aircraft must pass the MEL and CDL, I need some suggestions how it was actually working in aircraft maintenance
The MEL lists the limitations of the aircraft when some equipment (autopilot, transponder...) are inoperative.
The CDL deals only with non-structural external parts (wheel fairing...) that may be missing without grounding the aircraft.
From this website, emphasis mine:
The CDL is a listing of regulator-approved non-structural external parts that may be missing but the airplane remains airworthy.
To qualify an item onto the CDL, a restrictive set of conditions must be met, e.g.:
- The effect of the missing part upon adjacent structure and systems must be evaluated.
- The effect upon airplane performance must be measured.
- The combined effect upon the aircraft when more than one CDL item is present must be determined (i.e. the effect of a combination of items missing).
The CDL should not be confused with the MEL (MEL). While the MEL describes the limitations of aircraft operation in case of a system being inoperative/having malfunctioned (e.g. transponder failure), the CDL deals with situations where external parts of an aircraft are missing/fallen off (e.g. fairings, aerodynamic seals or panels).
The practical applications of MELs and CDLs happen when a snag over a defect is raised over something broken or missing, by somebody examining or operating the aircraft. The exceptions allowed are intended to allow the airplane to dispatch with something inoperative or missing, for some interval of time and possibly subject to some mitigating action by maintenance and/or flight crew.
The objective is to avoid grounding the airplane for repairs right away. A lot of MEL items allow operation for up to 10 days with a redundant subsystem inoperative for example, usually subject to a maintenance deactivation procedure, or an additional flight crew action, that mitigates the risk of the inop redundant subsystem.
So a typical scenario is a flight crew observes a status message that one of the two channels of some system is inoperative, maybe inbound, or at the gate. They will consult with the airline's maintenance control organization and look up the subsystem in the MEL and if the subsystem is covered, follow whatever maintenance action (like deactivating an actuator by removing power to it or locking it out) or crew action (like doing an ops test every flight instead of first flight of the day, or at routine check), continue flying, and schedule the repair within the MEL item's time allowance.
When a component has good MEL coverage, a significant reliability problem with it becomes a cost issue, related to the need to do something about it eventually, instead of a causer-of-delays/cancellations, which is a much worse problem.
When an airplane enters service, the OEM makes a huge effort to make its MEL/CDL as expansive and comprehensive as possible, as this has a huge effect on dispatch delays and the overall dispatch reliability of the fleet. They will also grow it over time, and there is normally a permanent working group set up between the OEM and regulator to manage and improve the MEL/CDL over the fleet's operating life.