I use to deal with this when I worked at an airline in reliability, so I will speak from experience. It all depends on fleet size. There are two types of spare aircraft. There is a hot spare sitting on the ground and there is a virtual spare.
A hot spare is an airplane that is on the ground for an extended sit that is available to take over and fly any route. Typically an aircraft might spend 24 hours after scheduled maintenance as a spare before it enters the fleet. With airplanes continuously coming out of most rotate as a spare aircraft. These are usually positioned where maintenance is done. At DL
is the focal point. At AA
is likely. At UA
are where spares often are.
The second type of spare is a virtual spare. Some airlines use this concept. It consists of routing an airplane specifically between destinations so that it can cover a more important flight. For example, UA
had a 747 flight from SFO
in the morning. That plane could be a spare for any of the afternoon 747 departures out of ORD
. If it is not needed, then the plane flies back to SFO
so it is available to cover evening SYD
flight or the morning departures to Asia the next day. The SFO
flight is willing to be sacrificed or downgauged so that they have a spare 747.
To get the real answer, for an international fleet (of less than 40 planes), most airlines have about 2 spare aircraft during peak season. Winter season has more spares since the schedule usually reduces flying. The spares can be used to cover charters or to cover in case an airplane goes out of service. They are also used to cover special maintenance visits such as cabin refurbishments or painting during slow season. For narrowbodies, the spare count usually varies a lot. For a large sized narrowbody fleet, spares will range 2~6 airplanes. It depends on how maintenance is being performed. Some days have more spares than others. Narrowbody spares counts are comparatively less than widebody since a narrowbody cancellation causes less pain to an airline. Reliability analysis staff at airlines try to match the spares count to the out of service count with a little buffer. If the out of service count gets higher than the spare count on a regular basis, it is time to change the schedule or find a way to get more planes.
The final way to get spare aircraft is to use swaps. At a hub, planes can be continually swapped between flights to minimize delays if one plane goes out of service or if it is restricted (for example if the plane goes out of ETOPS limits due to some mechanical reasons). This actually can allow flights not to get canceled when one plane is out. The airline can essentially operate a full schedule with fewer aircraft than required. The sacrifice is delays. For this reason, you never want to be on the last flight out of a hub at night, especially to an international destination. The airline might have expected a plane to get fixed during the day, but if maintenance could not fix it, the last flight will cancel.
|Quoting HAWK21M (Reply 3):|
If 100% fleet is flying & getting the minimum load for that sector....The Airline is doing well.
While I typically respect your opinion, that's not necessarily a good thing. You want a little gap between typical out of service and spares. Continental in the US typically schedules its widebodies tighter than anyone else and as a result has a higher airplane reliability cancellation rate.
If the fleet is scheduled too tight, a long duration damage event such as an airplane clipping another can cause rippling cancellations since it eats away at the spares. Airlines can do all they want to ensure airplanes don't get damaged and taken out of service for weeks, but it does and always will happen.
If you have never designed an airplane part before, let the real designers do the work!