Removing an airplane from service and parking it for any period of time requires protecting it from potentially harmful environmental effects. Accordingly, returning a parked airplane to service after a lengthy downtime requires extensive restoration of its systems. While the airworthiness of an in-service airplane is proven, the airworthiness of an airplane after extended downtime must be certified before it resumes operations. As a result, the process for returning an airplane to service after extended downtime for maintenance or parking must include attention to controlled, repeatable maintenance processes.

How the operational status of an airplane is assessed before its return to flight depends on its status as one of the following:

1. Airplane in service.
2. Airplane returning from maintenance downtime.
3. Airplane returning from parking.

The operational status of an in-service airplane is well known, as all airplane systems are verified to be functioning properly. The airplane is performing as it was designed to fulfill regulatory requirements established for airworthiness. The designed redundancy of flight-critical systems, the proven reliability of airplane systems, and regular maintenance checks have also contributed to this standard of airworthiness. Between flights, a line mechanic performs a transit check of the airplane at the airport ramp. This includes a walk-around inspection of the airplane for obvious damage, required servicing, correction of discrepancies, and operational tasks specified for the airplane. Qualified ground personnel or the flight crew performs a visual preflight walk-around of the airplane, and the flight crew completes a preflight checklist from the flight deck. Together, these precautions should help ensure the airworthiness of the airplane.

An airplane may be out of service for an extended period of time, such as for a scheduled maintenance check or unscheduled maintenance, or for a short period of time, such as for overnight maintenance. Whenever an airplane is out of service, uncertainty or risk may be introduced if preparations to return the airplane to service are not properly executed. After extended downtime, the operational status of airplane systems, the airplane configuration, and the airworthiness of the airplane must be verified. This type of verification is necessary because during events such as a scheduled heavy maintenance check, major portions of the airplane may have been disassembled for extensive overhaul. Instrument panels may have been removed, leaving only wire bundles and electrical connectors. Airplane systems may have been dismantled for inspection and refurbishment. Major modifications and structural repairs to the airplane may have been performed.

As another example of where verification is needed, electrical wiring must be properly reconnected and tested after major rework. If a hundred electrical wires were disconnected during the extended downtime, all 100 must be correctly reconnected. Otherwise, system malfunction or nuisance conditions may occur. All airplane systems that were disturbed during maintenance should be operationally tested according to the airplane manufacturers instructions or equivalent operator instructions. Airplane system performance and operation should meet the manufacturer's specification requirements with all components in a serviceable condition and fully functional. The airplane must be in a configuration suitable for flight before the airplane can be returned to service. In some cases, a functional check flight is required before the airplane can begin revenue service.

An airplane may be parked for more than a day because of business or operational factors such as overcapacity, company restructuring, market conditions, or regulatory action. Deterioration of airplane structure, surface finish, airplane systems, and components can occur if preservation procedures to protect the airplane are not followed. If exposed to the outside environment, an airplane can be damaged by heat, humidity, cold, ice, snow, rain, lightning, hail, wind, sandstorms, and insects. The airplane must also be protected from damage or debris contamination of pitot probes, static ports, total air temperature probes and angle-of-attack sensors. External openings on the airplane such as the outflow valve, relief valves, vents, ports, and openings must be closed and sealed against environmental effects.

Because of airplane system inactivity and the lack of regular maintenance checks during parking, the following may also occur: component mechanisms may lose lubrication, batteries may discharge, contamination of potable water systems or fuel tanks may occur, and some systems or components (such as oxygen cylinders, tires, hydraulic systems, and landing gear shock struts) may lose pressure. Although the airplane is inactive during parking, it is important to maintain the engine, auxiliary power unit, and cargo fire extinguishing systems and all portable fire extinguishers in fully serviceable condition in case of a fire. The airplane must be electrostatically grounded while it is parked.

Operator experience shows that dispatch reliability is higher and maintenance problems are fewer for airplanes in regular service as compared to airplanes used sporadically (e.g., infrequent charter flights or parked airplanes). When the airplane is in service, flight crews are monitoring airplane system performance from the flight deck; maintenance personnel are performing preventative maintenance, regular inspection, and repair procedures; and the onboard computing systems and electronics units are performing internal diagnostics to monitor system health. The regular use of an airplane maintains its systems and components in a condition conducive to airworthiness.

Parking creates the risk that an airplane may not be properly protected or that system functionality may not be properly restored. The increased integration and complexity of both hardware and software in airplane systems means greater care and attention must be paid to protecting and restoring the airplane after parking. The procedures established to preserve an airplane during parking and later restore it to in-service condition are extensive and lengthy, but necessary to ensure airworthiness. Specific procedures exist for servicing and protecting an airplane parked for different periods of time. The airplane manufacturer's recommended maintenance practices for parking can be found in chapter 10 of the Aircraft Maintenance Manual (AMM).

"Recommended Resources" includes further information about preparing airplanes for return to service.

(The sidebar "Maintenance-Related Accidents Involving Air Data Instrumentation" offers a detailed examination of two accidents that occurred when airplanes were returned to service after maintenance downtime and prolonged parking, respectively. In both cases, maintenance of the air data instrumentation on the airplanes played central roles in the accidents.)

Summary
Increased emphasis on aviation safety has required operators and maintenance personnel to exercise greater care and vigilance in all aspects of airplane maintenance. Any airplane being returned to service after extended downtime requires additional attention, because in many cases a lengthy period of time has passed since the airplane last flew. Any time an airplane is in extended downtime for maintenance, either system modifications or configuration changes may have been implemented. When an airplane is parked, protective maintenance must be performed to prevent damage to the airplane. The airplane was airworthy prior to either instance of extended downtime, but no assumptions should be made about the airworthiness of the airplane after the downtime, especially if protective measures and the manufacturer's recommended maintenance instructions have not been carried out. When an airplane is returned to service after parking or maintenance downtime, the appropriate checks and tests must be performed to ensure its airworthiness. All airplane systems that were disturbed during maintenance should be operationally tested according to the airplane manufacturer's instructions or equivalent operator instructions. Following these fundamental practices can help maintenance organizations ensure the airworthiness of an airplane returned to service after extended downtime.