Originally planes were designed and developed to fly and operators are adjusting their flight plans for each aircraft to get as much utilisation as possible. From a maintenance perspective – the longer aircraft is on the ground more maintenance it will require to bring it to flight-ready condition. On the other hand, from the revenue perspective, aircraft generates income only when it is flying. Nevertheless, there are occasions when operators or lessors would need to park aircraft, the good example was when COVID 19 emerged and all airlines were pushed to park their fleet as travel restrictions significantly affected the travel demand.
Several factors may raise the decision to park the aircraft such as:
- Force Majeure situation
- Bankruptcy
- Unsuccessful remarketing of the asset
- Long lead times
- Age of the asset
- Other…
Fig1. Alice Spring Terminal
Fig2. Heathrow Airport
Fig3. Frankfurt
Fig4. Pinal Airpark
Fig5. Paris-Charles de Gaulle
Unfortunately, the fact that the planes are not flying does not mean that their expenses are zero. While the calendar-based maintenance requirements continue, parking and protection costs are added. Below we will go through parking and storage periods and requirements for Airbus aircraft. For Boeing these will be the same, however, periods and maintenance requirements will slightly change.
Before parking the operator should consider the following:
1.The parking or storage period starts at the end of the last flight cycle.
2.The operator should start to apply one of the four parking or storage protections on the day of the last flight or on the next day:
a.Parking period of not more than 1 month in flight-ready condition
b.Parking period of more than 1 month in flight-ready condition
c.Storage period of not more than 1 year
d.Storage period of more than 1 year.
For parking period of "Not more than 1 month" or "More than 1 month" in the flight-ready condition is applicable if:
1.The aircraft will stay in flight-ready condition
2.The aircraft will be out of operation for a short period to a period of some months
3.There is a risk that the aircraft must do a return to the operation procedure
4.The operator does a preservation procedure with small initial costs. It must regularly maintain the aircraft with important periodic ground checks (frequent engine runs, etc.)
5.The operator must do a non-revenue flight at intervals of 3 months during a parking period in flight-ready condition.
6.The operator must not remove primary parts (e.g. engines) before or during parking procedure.
7.If there is cannibalization or removal of non-primary parts from the aircraft during the parking period, the operator must install them again before:
a.The periodic ground-check tasks
b.The return-to-operation procedure
For Storage period of "Not more than 1 year" or "More than 1 year", aircraft storage is applicable if:
1.The operator will remove primary parts (e.g. engines)
2.The aircraft will be out of operation for a period of some months to a long period
3.The operator does a preservation procedure with optimized total cost for a longer period
4.The operator can do the maintenance of the aircraft with light periodic ground checks most of the time
5.The operator must do a non-revenue flight every 2 years during a storage period.
The return-to-operation procedure is longer after a storage period than after a parking period. It is necessary to do a flight to start a new parking or storage period. The steps the operator must do for the parking or storage procedure are given in table 1, below. The operator must do each step identified with a cross in the column related to the selected period of:
1.Not more than 1 month in flight-ready condition, or
2.More than 1 month in flight-ready condition, or
3.Not more than 1 year, or
4.More than 1 year.
Looks like quite a lot of specific requirements should be met even during parking and storage procedures, however, let's have a look at what checks must be accomplished while the aircraft is on the ground.
During each parking or storage period, the operator must do specific periodic ground checks. For the ground checks during the parking periods of "Not more than 1 month" and "More than 1 month" the following applies:
1.At 7-day intervals: parking, 7-day periodic ground check
2.At 15-day intervals: parking, 15-day periodic ground check
3.At 1-month intervals: parking, 1-month periodic ground check
4.At 3-month intervals: parking, 3-month periodic ground check
5.At 1-year intervals: parking, 1-year periodic ground check.
For the ground checks during the storage periods of "Not more than 1 year" and "More than 1 year", the following applies:
1.At 7-day intervals: storage, 7-day periodic ground check
2.At 15-day intervals: storage, 15-day periodic ground check
3.At 1-month intervals: storage, 1-month periodic ground check
4.At 3-month intervals: storage, 3-month periodic ground check
5.At 6-month intervals: storage, 6-month periodic ground check
6.At 1-year intervals: storage, 1-year periodic ground check
7.At 2-year intervals: storage, 2-year periodic ground check.
Table 1. Parking and Storage Chart
There are possibilities to replace parking procedures with storage procedures, to extend parking and storage procedures and return to operation procedures at any time following the applicable AMM (Aircraft Maintenance Manual) procedures.
The general procedure for a period of less than 1 year of storage is as follows;
Putting the applicable protection equipment on all the probes and the sensors (pitot probes, static probes, Total Air Temperature (TAT) sensors, Angle Of Attack (AOA) sensors and ice detection probes etc Ref Fig 6.
Fig6. Protective Equipment
- If the aircraft is parked or stored in high wind conditions, aircraft stability must be checked and moored if necessary.
- Similarly, if there is also cold weather conditions, special cold-weather maintenance procedures must be applied such as draining all waters from water/waste lines etc.
- Cleaning of the aircraft externally and internally
- Parking the aircraft on a flat surface, making sure that the wheels of the NLG are on the aircraft axis and that the aircraft points into the wind.
- Installing the safety devices on the landing gears and putting the chocks in position
- Retracting all the flaps, slats, spoilers and thrust-reverser cowl doors
- Setting the Trimmable Horizontal Stabilizer (THS) pitch-trim control-wheel to two degrees aircraft nose up.
- Putting the warning notices in position.
- Electrically grounding the aircraft.
- Applying protection of the Engines Ref. Fig 7 to 9
Figure 7. Protective Equipment
Figure 8. Protective Equipment
Figure 9. Protective Equipment
- Applying protection of the Integrated Drive Generator (IDG), APU, Fuel System, Landing Gears and the Wheels/Brakes, Water and Toilet System, Flight Controls, Cabin and the Cockpit, Doors, Windows, Oxygen System, Hydraulic System
- Flushing of the Air Data System
- Inspecting of the drains visually to make sure drain holes are not clogged by debris, dirt, bird nests, insects etc.
- Removing or Protecting of Some Critical Equipment such as Digital Flight Data Recorder (DFDR), Cockpit Voice Recorder (CVR) aka Black Box, if they are not solid-state design, Emergency Locator Transmitter (ELT), Emergency Power Supply Unit (EPSU) and disconnecting batteries
- Removing the fire-extinguishing bottles from engines, APU, cargo, cabin and cockpit if the Outside Air Temperature (OAT) is more than 50 DEG celsius. This is because some aircraft internal areas can reach around 70 DEG celsius.
- Protecting the engines and the APU with specially adapted protection covers and equipment Ref. Figure 10.
Fig10. Protective Equipment
- Putting a cover on the top of the rudder to give protection from bird excrement.
- Sealing surge-tank overpressure protectors, ailerons to give protection from bird nests, air conditioning inlets and outlets, avionics ventilation inlets and outlets and battery venturi.
- Applying Corrosion Preventive Compound (CPC) to all the surfaces exposing the air.
- For more detailed information please refer to Appendix 1 of this article.
Taking into consideration available information it can be seen that there is a lot of work involved even when aircraft is parked and it is not taking into consideration maintenance that is required to get aircraft into flight-ready condition. This is why many airlines were burning millions of dollars per day during the first months of COVID and unfortunately, at the moment the full return to service to pre-pandemic levels is far out and very hypothetical.