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Prior to the start of the current day, the IOC expects to operate the schedules with all the planned fleet of aircraft, in a serviceable condition and with no operating limitations. In addition, maintenance equipment and spare parts to support the aircraft in appropriate ports are also expected to be serviceable, and labour sufficient to carry out the planned maintenance activity to be available. That’s the theory. However, for many reasons this ideal is not met. In terms of equipment, break-downs do occur, and parts may not be available or in position should they be needed. For example, a GPU needs to be serviceable in the event an aircraft is planned to operate to, or arrives at, a port with an unserviceable APU (auxiliary power unit in the aircraft), in order to be able to start the engines prior to departure.

Aircraft may not be completely serviceable, but are in such a condition that enables them to remain in service. They may, for instance, operate under an MEL which effectively enables the aircraft to fly in specific conditions, but without all components of the aircraft functioning correctly or at all. For example, an aircraft may be authorised to operate with only one of two air conditioning packs serviceable, but may be limited, for example, to 25,000 ft cruise level, which in the case of flying over mountainous terrain, may be an inhibitive factor. Another consideration is that the aircraft will burn additional fuel at this lower level, which may be another factor for the IOC in its preference of where to send the aircraft with this deficiency. Perhaps an anti-ice system is unserviceable, such that the aircraft cannot operate into known icing conditions, or a thrust reverser has been locked out on one engine, which may limit the conditions for landing. In this case, the aircraft may not be able to operate on a wet runway in cross-wind conditions.

In some circumstances, there may be a unique set of procedures that need to be enacted. For example, in what may appear to be a seemingly trivial problem, such as an unserviceable coloured (red/green) navigation light on a wing tip, the aircraft may be able to depart (on an MEL) but in doing so, notification prior to the flight may need to be given to such parties as the airline’s Chief Pilot or Duty Pilot, the aircraft manufacturer, control towers at departure and arrival airports, regulators, and en-route АТС. In addition to an MEL, an aircraft might also operate under a CDL. That is, the aircraft can be authorised to depart a port with an external component missing. An example might be a panel or fairing which may be removed but still permit the aircraft to operate, albeit perhaps with speed and/or altitude restrictions. To enable these operations, the airline’s maintenance procedures must also ensure licensed Engineers/Technicians ‘sign-off’ the aircraft in order to be compliant with the manufacturer, regulator(s) and airline’s own procedures and processes outlined in their operations manuals. Engineering also have to advise the operating crew and any other personnel deemed to need the information. Operational limitations such as these often result in performance degradation, such that additional fuel may be burnt, schedule duration increased, range reduced, or special clearances needed from АТС.

Aircraft Characteristics

Frequent travellers will be mindful of the safety briefing prior to takeoff, which includes reference to differences between each aircraft. This is to alert passengers to take heed of the individual peculiarities of each aircraft, referring in particular to the stowage and use of various safety- related equipment. In an operational sense, aircraft units belonging to a specific fleet also differ. An airline that has purchased, say, 30 B737-800 or A320-200 aircraft (i.e., 30 units of the same model), may request the manufacturer to, or may of itself, configure some of these aircraft to meet specific purposes or missions. In addition, airlines that operate a mixture of domestic and international flights may utilise fleet units to serve both networks. For example, an A330 operating New York to Dublin may also serve New York to Denver. The aircraft is likely to be configured (e.g., arrangement of seats, galleys and toilets) and fitted (with in-flight entertainment (IFE), crew rest facilities and work-stations) to facilitate the desired level of service on the airline’s international routes.

But the airline may also have the need to use this particular aircraft for some domestic operations, at least for a limited time, perhaps to cover the unavailability of another fleet unit. In this circumstance, changing the characteristics of the internationally configured aircraft would be time or cost prohibitive given the short-term nature of the change and the airline would have to accept the nature of operating a ‘non- standard’ aircraft. Customers may well notice a significant difference on board too. Another common practice is the chartering of aircraft from another airline or company (such as a leasing company). This may be due to a number of reasons including the airline’s own fleet shortages or a number of unserviceable aircraft, a significant capacity requirement beyond the capability of the airline’s own resources or may be the result of a major disruption (such as multi-diversions), affecting the airline’s own aircraft. In this instance, one solution may be to lease additional capacity to aid the recovery process. Notwithstanding the short-term or even long-term use of externally sourced aircraft, there may be many inconsistencies in configuration, equipment and performance across the airline’s own fleet, as shown below.

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