The operational ecosystem of a commercial aircraft relies on its near-continual use. 

The average lifetime of a jet can span anywhere between 20 to 30 years, where it will experience up to 50000 flight hours and approximately 75000 pressurization cycles before operational fatigue begins and the aircraft is eventually retired. 

During its commercial lifespan, an aircraft will undergo several scheduled inspections, known as maintenance checks. These checks, which are tied to Maintenance Planning Data (MPD) issued by Original Equipment Manufacturers (OEMs), are a varied application of base and line maintenance procedures that need to be performed at regular or certain time intervals. 

Ultimately, these base and line maintenance packages are regulated and overseen by aviation authorities, such as The European Union Aviation Safety Agency (EASA) or the Federal Aviation Administration (FAA). These regulatory bodies ensure that all airline operators perform routine maintenance on its fleets to maintain a standard of airworthiness.

However, in some more unconventional cases, aircraft are temporarily decommissioned for a designated period of parking or storage and maintenance tasks are then carried out. But these tasks are not part of the MPD issued by OEMs and do not fall into the category of scheduled maintenance checks performed during the operational life of an aircraft.

Parking: what does it mean? 

A parked aircraft is categorized by OEM terminology to determine its physical state. While technical aspects of these parking states are similar (if not almost identical), terminologies differ. 

Operators encounter terminologies such as immobilization, active storage, short-term storage, prolonged and long-term storage and deep storage. These terms can be reduced to four main sections: normal parking, active short-term parking, prolonged long-term parking and storage.

Normal parking is a period where the aircraft is on the ground and between flights or maintenance events, which can span a few hours to a few days. Here, no specific maintenance action, besides servicing tasks such as refueling, is carried out. The aircraft is immediately ready-to-fly.

In comparison, active short-term parking (which can be from a few days to several weeks) is when an aircraft is grounded but considered out of operation. The aircraft does not undergo any scheduled maintenance checks, but minimal preservation procedures are installed, including safety pins, covers and plugs. However, the aircraft will go through periodical maintenance tasks such as APU and weekly engine runs. While not immediate, the aircraft can be returned to flight on short notice, normally within 24 hours.

For parking beyond an active short-term period, major preservation tasks are implemented and reactivation on short notice is not possible.

Prolonged long-term parking can span from a few weeks to several months. During this period, the aircraft is out of operation and does not undergo any scheduled maintenance checks. In addition to installing aircraft safety pins, covers and plugs, preservation measures performed at the beginning of the parking period are now increased and may also include extensive landing gear lubrication, APU/engine fuel circuit preservation. While the aircraft undergoes these periodical maintenance tasks, only a few are continued on a weekly or shorter-term basis. 

During routine maintenance, some aircraft LRUs, such as engines and APU batteries, may be removed from the aircraft. But the aircraft configuration is otherwise preserved and has no missing parts or components. The aircraft will not be able to return to flight on short notice.

During storage, a grounded aircraft is kept out of commercial operation for a medium to long period (three to six months). In most cases, the aircraft is stationed in a location with limited means and gaining timely access is difficult for qualified technical personnel. 

Many aircraft systems are kept in a preserved condition, which doesn’t allow for immediate operation, and major parts, such as batteries, oxygen bottles, fire bottles and in some cases even the APU and engines, have been removed. A return to service (RTS) for this aircraft state would require advanced notice.

When returning an aircraft to service many variables must be considered and analysts suggest that there is no “one size fits all” procedure. The different ways that aircraft have been parked or stored will have a profound impact on when it is deemed safe to return to service. This can vary from parking an aircraft in an environmentally and temperature-controlled hangar to exposure to weather elements.

Aircraft reactivation

Most aircraft reactivation procedures are designed for aircraft that have been parked for a relatively short period. When this parking period is extended - as we witnessed at the height of the COVID-19 pandemic - aircraft require major maintenance of issues like corrosion, blockages, leaks or faulty systems.

It can total 72 hours to correctly park an aircraft and it can take just as long to restore its airworthiness. The average reactivation time for a Boeing 737 can take up to 1,000 working hours over seven days.

During the pandemic, many unorthodox parking options were introduced, but the reactivation procedures used on the grounded fleets were not drastically altered, rather they were augmented. 

According to a spokesperson from US-based American Airlines (A1G) (AAL), most carriers had minimal exposure to long-term aircraft storage, including American Airlines (A1G) (AAL). However, this did not spark any major restructure. Instead, the pandemic created an opportunity where procedures could be enhanced, while still following regulatory standards set by authorities. 

American Airlines (A1G) (AAL) added: “In anticipation of travel demand returning, our Tech Ops team kept our stored aircraft in excellent condition with regular care and maintenance as part of a comprehensive storage program that went above and beyond FAA requirements.” 

According to Steffen Glombitza, Head of Maintenance Planning at Lufthansa Technik AG, “the practices for reactivating stored aircraft are already laid down in principle in the manufacturers' reactivation manuals”. 

He continues: “Even before [COVID-19], these were repeatedly updated to reflect the latest findings.

“In fact, the huge number and increased duration of parked aircraft during the [COVID-19] crisis also brought some new experiences to owners, manufacturers and MRO operations, some of which have also led to adjustments in the corresponding manuals and thus have an impact on periodic inspections and reactivation.”

Issues discovered during an extended parking period can include blocked air valves, leaks, corrosion, reduced tire and brake pressure, faulty engine reignitions and erroneous instrument readings. 

For aircraft parked outside and in an area exposed to the elements, technicians reported finding insect infestations, hives, rodents and owls. 

Due to the changing factors that have been found to cause irregular aircraft behaviour and instrument readings, optimizations have been introduced in various maintenance cycle activities.

Glombitza continues: “Some maintenance cycle activities, such as the servicing of certain components, have been optimized. Certain adjustments were also made to the topic of sealing and closing, because certain surface corrosion problems, for example in the engine area, occurred more frequently due to the formation of condensation water.  Here, the processes were adapted to optimize keeping such sealed areas dry and to prevent unpleasant findings during reactivation.”

Despite adaptations and enhancements to the original maintenance manuals and procedures, Glombitza confirms that the original processes are still reliable in successfully reactivating aircraft in this new environment. 

He says: “The original processes have proven themselves very well in our view so that there have been no major changes in the processes and specifications here.” 

There is a possibility that maintenance, logistic and economic problems could still arise if parking and storage procedures are not fulfilled, or are done so only partially. However, the evidence suggests that the industry and its regulators are moving towards enhancing already existing manuals.