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 (figure 3), 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.
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.