The Covid-19 pandemic has led to an unparalleled decline in air traffic demand as countries around the world entered lockdowns and closed their borders. In response, airlines grounded large parts or in some cases all of their fleets, which brought the dual challenges of how and where to efficiently place their aircraft into storage programs. With some signs of recovery now on the horizon, airlines must also now start planning for returning their aircraft to service. Sandra Everest, Director of Maintenance and Engineering at Aircraft Analytics provides a summary of some key considerations for maintenance planners in terms of aircraft storage and return to service maintenance requirements.
The instructions for aircraft prolonged parking, periodic storage checks, and aircraft return to service checks are found within the Aircraft Maintenance Manual (AMM) ATA Chapter 10 “Parking and Mooring”.
Prolonged parking storage instructions vary per Original Equipment Manufacturer (OEM) and aircraft type. However, as a general rule, if the aircraft is to be parked for more than seven days a set of procedures to help prevent the deterioration of the aircraft structure and systems must be followed. In most cases this includes entering the aircraft into a storage program with some aircraft types having the option to maintain the aircraft in a ready-for-flight condition.
One key decision facing airlines in recent months will have been whether to put aircraft into short term or long-term storage (using such terms generically). There are different procedures to prepare airframes, engines, and aircraft systems for storage. These procedures are determined by the length of time the aircraft is planned to be stored. For example, short term storage may be up to 60 days, with long term storage then considered to be greater than 60 days. Programs can also have a staged entry in to deeper storage such as at 30 days, and again at 180 days.
While handling pre-planned aircraft storage requirements is not uncommon at many Airlines and Maintenance Repair and Overhaul organisations (MROs), the current unplanned mass storage of aircraft for an unpredictable length of time will have introduced many airline maintenance planners to the complexity of storage programs. The request for an aircraft to enter storage is normally accompanied with a reasonable estimate of the pre-determined length of prolonged parking required. This in turn would be used to set the initial parameters of the “into storage” and “in storage” criteria. In other words, it is a planned event, not an urgent reaction to a global event.
Storage programs are not an exact listing of “to do” tasks. There are options to follow in areas such as the initial protection of the exposed metal surfaces, fuel level and treatments, interior furnishing removal/protection, plus the potential removal of engines to consider and plan for.
Interior furnishing protection requirements are guided by the AMM and are in large part left up to the operator to plan in relation to the environment the aircraft is stored in. For example, seats and carpet may in some cases remain in the aircraft if the humidity is controlled to below 70%. It can be difficult to store aircraft on a large scale with dehumidifiers continually powered on each airframe. If the process is not followed however, the interior furnishings can become damp and grow mouldy coverings! Therefore, removing the soft furnishings in some environments is the best option. Finding room to store them is another issue to plan for.
Other storage program variables are more easily managed but make fixed-pricing storage work at third-party MROs exceedingly difficult. For example, the AMM may not require external aircraft protection if there are no unusual weather conditions and the storage period is known to be less than two months.
Under more pre-planned circumstances, aircraft destined for storage are often relocated to maintenance bases away from the major hubs. Long term storage can be lucrative for third-party MROs with dedicated experienced storage teams, and a facility to capture any required return to service maintenance due.
The prolonged parking instructions within the AMM contain a list of references (often AMM section/sub-section numbers) for actions to be performed. A matrix type table containing task instructions at the initial “into storage” stage and then the periodic inspections while “in storage, is also provided.
Periodic inspections include: “Service and protection” (Boeing term – terms differ per OEM) at seven days, 14 days (some aircraft use 15 days), 30 days, 60 days, 90 days, 180 days and often “as required”. The term “as required” can include tasks for inclement weather conditions. If the engines are being stored on-wing, additional preservation requirements will have to be met such as a 10 day repeat engine ground idle run.
Alternatively, some OEMs have a read-to-fly prolonged parking procedure. Examples of these periodic inspections are: Ready-to-fly – not more than 15 days, seven day ground checks, 15 day ready-to-fly including 15 day ground checks, then return to operation. Often these ready-to-fly prolonged parking procedures will have limitations. For example, the parking procedure in a flight ready condition is for not more than 12 weeks. You can perform the 0-12 weeks ready-to-fly prolonged parking once again but must be reset with a flight. Additional criteria to monitor include such tasks as landing gear free fall checks required on the fourth consecutive ready-to-fly renewal.
Stored aircraft around the world are being maintained against the matrix applicable to the aircraft type. For maintenance planners it is important to look at the notes following the matrix tables as work options selected (and performed) will impact future scheduled storage program work. As an example, within one storage program cycling all flight control surfaces once every one-to-two weeks eliminates the need for the lengthy lubrication/servicing steps (up to a set time period). Additionally, credit can be given within storage programs if the flight control cables within the body of the aircraft were lubricated shortly before storage in accordance with an operator’s scheduled maintenance plan. There is a lot to monitor within aircraft in storage programs.
Aircraft Return to Service.
As aircraft begin to return to service, depreservation checks will by underway. This includes performing a set list of procedures to put the aircraft back into a serviceable condition in accordance with referenced AMM tasks following on from the storage matrix. These include servicing, lubrication, function checks and of course numerous inspection tasks of the aircraft and aircraft systems.
The full list of work varies per OEM instructions, the environment in which the aircraft is stored, operator selected options while in storage, and time in storage. Within some return to service programs the removal of all rack-mounted electronic packages is required for the inspection of their condition, cleanliness, and signs of corrosion, if the relative humidity during storage was not controlled below 70%.
Other tasks, depending on aircraft maintenance history, include entry into the fuel tanks to inspect for corrosion and deterioration if they have not been examined in the last 60 days. Return to service programs can also require gear retraction tests which then ideally require a hangar slot.
Engines have their own depreservation requirements and can include component replacements. Many operators will have a history of known reliability issues on previous return to service engines to evaluate and determine the applicable preventive actions.
Man-hours (MH) for the return to service maintenance requirements can easily reach 250-450 MH and more. That is a 10 to 15 man crew working 10 hour days taking two and a half to three days to accomplish the checks and paperwork. It is important to note that hours will vary depending on cosmetic requirements, time in storage, the environment where the aircraft was stored, and the various options selected for aircraft protection listed in the program.
Two main additional work considerations on top of the depreservation requirements are the due scheduled maintenance (A Checks, C Checks, Structural Checks), and the defects arising from all inspections. Looking for defects as a result of an extended period of inactivity will be a new thought process from the standard operational defects’ engineers come across. Some aircraft play along nicely and want to get back to work. Others will put up a fight and need a little extra care to get flying again.
On the routine scheduled maintenance planning front, it is important to note that inspection task thresholds and repeat intervals are Flight Hour (FH), Flight Cycle (FC) and/or calendar based. For example, a single task or even a large grouping of tasks may be due at 6,600 FC/36 Month (MO), whichever comes first. Letter checks, if still in use within the maintenance program, will often be linked with a FH or calendar backstop interval. While in storage, the FH and FC utilisation intervals have stopped but calendar time is still ticking over. This will currently be being monitored by the planning engineers via each aircraft’s maintenance due list. Any due maintenance will need to be performed prior to the next flight. This may impact which aircraft return to service first.