Airline Scheduling

Tue Nov 09, 1999 12:56 pm

Hi. I have a question about airline scheduling. I'm not talking about Point to Point versus hub and spoke. What I want to know is how airlines schedule their planes because there doesn't seem to be any rhyme or reason to it. For example, one aircraft does not continuously fly the same route. It might fly from say ATL to BOS to MCO to ATL to SLC to LAX to HNL to LAX and back to ATL. It seems so complex as to how aircraft are scheduled to fly their routes that I'm wondering how exactly airlines put together schedules. I, like many people on here, like to create my own fictional airlines and put together schedules and paint schemes, etc, and if anyone can provide more information about airline scheduling, it would be most helpful. Thanks in advance!

Jim in Boston
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RE: Airline Scheduling

Tue Nov 09, 1999 11:35 pm

At FedEx which aircraft takes each specific flight is decided by the aircraft routing deptartment, which is a division of maintenence. What may look pretty hapazard is usually the result of a lot of planning. Here is a very basic explanation of how aircraft get put on those seamingly random routes.

1. Airline Scheduling creates a schedule specifying where and when a flight will go and what type of equipment (DC-10-10, DC-10-30, B-727-200 etc.) will take the the flight based on market needs.

2. Aircraft Routing then 'fills' those empty flights with specific tail numbers five to seven days in advance of the flight, taking into consideration such things as scheduled maintenance on the aircraft, cat status requirments, avionics and navigational equipment requirements (TCAS, RVSM capability), and other operational restrictions.

3. This routing 'plan' is then constantly tweaked and changed right up until departure time ensuring that the aircraft fleet is used in the most economic way.

Generally, most of the routings the aircraft take are driven by maintenance needs. For example, lets say that tail N601FE is scheduled for a 'B' check in TPE in 5 days but only has 35 hours of flight time left on the check (B-checks are done about every 250 hours). This aircraft would have to be put on short flights (MEM-MSP-MEM or MEM-ATL-MEM) for the rest of the week before making the trek over to TPE in order to conserve the time on the airframe.

I hope this helped you out and didn't just confuse you, because I think I may have confused myself!
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RE: Airline Scheduling

Wed Nov 10, 1999 4:08 am

Is that the same for passenger planes? Could my plane beswitched at the last minute tomorrow??
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RE: Airline Scheduling

Wed Nov 10, 1999 4:38 am

First of all, AC_A340, yes your aircraft could be switched at the last moment tomorrow, but it depends on what route you're flying. Changes between similar aircraft types - 330/340 or 319/320 would be much easier to organise than those between 767s and 320s.

Excelsior767, it's heartening to know I'm not the only one who does that!I've had a fascination with airline schedules for 15 years and I still enjoy planning and reworking schedules and trying to work out how a new aircraft might be deployed. Of course, these days it's all done on computer, but even the likes of us - who are relying on pen and paper - can try to see how things are done. Can I suggest you pick an airline with a fairly small fleet of a particular type and try and break down that schedule to see how each type is operated. A schedule for a fleet of 3-5 aircraft can be "deconstructed" to see what flies where. (Try Austrian A330s/A340s, for example) You can also work out other issues, such as hubbing and daily utilisation (a major factor) and also, by studying routes to and from the "home" airport, see how the aircraft's utilisation ties in with a hub.

Now, I know you said "point to point" and in a way, that's easier, in that you don't have to worry about that aspect of scheduling. I.e. you just keep the planes flying for as long as possible from dawn to dusk on the optimum routes (as the airline's planning department dictates) and you maximise use. BUT, where a hub is concerned, you need to consider that sacrificing ultilisation for "hubbing" can help revenue, in that you provide maximum feed opportunities through the hub. It also has to be said that many airlines with very good hubs still have very high utilisation (take Sabena, Austrian - the masters of hubs and connectivity, and Swissair). It just illustrates that there is something else to be taken into account.

Is this a help? Hope so!
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RE: Airline Scheduling

Wed Nov 10, 1999 6:31 am

Not only can we switch your aircraft before departure. But we have in cases switched aircraft after loading, fueling and boarding. Creating a mad rush to swap everything down to the food over.
Last case was a 763 that developed an APU problem..thus cant fly 180etops and cant operate the flight.
Dispatcher describes how most airlines do it.
Allthough sked's are market driven, maintainance tends to be our biggest decider.
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Nice To See...

Wed Nov 10, 1999 6:56 am

...some dispatchers around here..  

To the original poster asking the question, SpUd is correct about MX driving things. Aircraft routings are usually designed so that the aircraft ends the flying day at a MX base every so many days for routine inspections and checks. When it departs the MX base the next morning, it could end up anywhere on the map, thus allowing other aircraft in the fleet to be cycled in on the same pattern.

What gets fun is when aircraft have problems during the day, and have to be either fixed where they are, or re-routed into MX that night for fixes. Some profblems can be deferred, say, like a windshield wiper. You can go with it INOPerative as long as there's no rain at the takeoff or landing airports, and if there's a chance there's gonna be, we dispatch types will route the aircraft from the offending weather and send it someplace where the weather is good. THis sometimes means unscheduled plane changes for passenger and crew alike, but we're looking at the airline as a whole (keeping it safe, legal, and on-time) and keeping passenger inconvenience to the minimum possible.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
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Another Question

Wed Nov 10, 1999 7:31 am

Very interesting topic! I too have wondered about this form time to time (especially when I'm sitting at a hub airport waiting for a connecting flight). It seems to be a very complicated process, and it's not until you examine it at great detail can you see the logic in it all.

The thing I DON'T understand though, is what happens when an aircraft is taken out of service for a couple days (unscheduled, of course). There was a DL 757 sitting here @ SAN a little while back for a couple days - I think it ingested a bird into an engine or something. Anyway, it sat idle for almost two days. Now, I can't imagine an airline (no matter how big) having spare airplanes sitting around to take over in situations like this? How do they do it? Wouldn't something like this disrupt the ENTIRE Delta schedule for a few days? Talk about a major domino effect...

San Diego, CA
My name is Scott, and I am addicted to writing obnoxiously-detailed trip reports.
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RE: Another Question

Wed Nov 10, 1999 7:55 am

Re: the DL 757 @ SAN, it probably had to have the engine changed.

In general, engine changes don't take all that long (depends upon the aircraft type) but what usually takes more time is getting the spare engine to where it's needed. Spare engines cost $$$, and most airlines have some at their major MX facilities. What makes things fun is when the aircraft is grounded in Gopher Gulch, Nebraska, and the nearest available engine is in Atlanta. Spare engines are almost always trucked in, which of course, takes time. The engine for the DL bird may have come in from LAX, or even SLC, or ATL. Had the aircraft been a 3- or 4-engined one, they could have ferried (no pax) to somplace where an engine was, but with a twin, the engine has to come to you.

As far as the aircraft being unavailable for a couple of days, assuming there were no spares, lightly-booked flights were probably cancelled, and the aircraft redeployed to minimize the inconvenience to the fewest number of pax possible.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
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RE: Airline Scheduling

Wed Nov 10, 1999 8:07 am

It is a little different for cargo companies than people movers when an aircraft goes out of service but here is basically what happens with us. Lets say that an MD-11 takes a bird strike into ORD and will be down for at least 12 hours but was scheduled to continue on to LAX-HNL-SYD-SFS. This is much too important of a flight to take that kind of delay so the system controllers immediatly begin looking into what can be done to 'recover' the flight. If the cards have fallen right for you that day you may actually have a spare sitting in MEM that you can blast off to ORD to continue the trip on schedule. More likely, you will pull an MD-11 off of a domestic flight (MEM-SEA or something) and send that aircraft up to ORD to continue the flight on time or close to ontime. That leaves you with a MEM-SEA flight open. Maybe you didn't have a spare MD-11 but you did have a spare DC-10-10, and B-727. If there was enought high priority freight to be moved to SEA you would then schedule those two aircraft out to SEA with the original MD-11 load. After the broken airplane in ORD comes up, it would usually just be ferried back to MEM or some other place where it might be useful. This, of course is all complicated by the maintenance already scheduled on the airplanes that must be complied with. Sometimes there are simply no solutions and the flight must take the delay which can cost us a lot of money, make customers very unhappy, and lead to some interesting meetings the following morning :-(

RE: Airline Scheduling

Wed Nov 10, 1999 8:29 am

We schedule planes according to the equipment type and market. The dispatcher is responsible for setting up the flight lines for the next day (ex. one a/c might fly hpn-cle-mdw-cle-stl-cle-mdw) Changes can occur throughout the day due to events such as mx, wx, etc. We base decisions on cancellations or equipment substitutions on availability of aircraft and passenger loads. So it is basically the dispatcher controlling the a/c assigned that decides on daily routings.
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RE: Airline Scheduling

Wed Nov 10, 1999 8:55 am

Dispatcher and SpUd are fairly well on the button.

For those that are interested, there are a couple of tools and techniques that are used in scheduling aircraft.

A schedule is primarily driven by demand between two given ports. You try to cater for this demand with frequency and aircraft size within the constraints of your fleet. As well as this there are a number of other constraints that need to be looked at. Some of these are:

- Some airports can only handle aircraft up to a certain size (this could include runway length / width, gates, stairs and conveyer belts for baggage, etc..).
- The number of gates at the airport restricting the number of aircraft you can have there at one time.
- Maintenance requirements.
- Crewing requirements (this can be more complex than the schedule itself at times).
- Time it takes to turn-around the aircraft type at the terminal.
- Many more which don't come to mind at the moment.

Using a green-fields approach, generally a schedule is built first ignoring a lot of these constraints and is based purely on demand.

Modern scheduling systems are quite user friendly these days and use visual displays in the form of a Gantt chart to display the schedule on the screen. Once the basic schedule is put together it is then "rotated" to see if it works. This is a term used for a program that allocates the fleet to the schedule in order to see if it is logical.

When allocating, the rotation program can use various distribution methods like LIFO (last in first out), FILO, LILO and FIFO for aircraft at an airport. Normally LIFO is used because this exposes any aircraft that spends lengthy periods on the ground in a port. About 7 days out, the schedule is tail-assigned where actual registrations replace just the aircraft type in the schedule.

Another visual display is a spider plot. This has ports on the Y axis and time on the X axis, and uses a continuous line from left to right to indicate one individual aircrafts activities for a day. It is a good device to look at all of an airlines activities for one day on one (rather big - about 2.5' by 3') piece of paper.

After much tweaking the schedule is in place and is only subject to crewing, maintenance requirements, etc.

So as you can see, using a number of constraints input by humans, computers have a lot to do with where aircraft end up. This is typical of "point to point" scheduling (I have never experienced Hub and Spoke).

I hope this 20-cent tour of scheduling helps a little. If there are any specific questions please post them and I will try and answer them (also, I am currently surrounded by 8 airline schedulers to hassle whilst sitting at my desk).


B727-200 :-)
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RE: Airline Scheduling

Wed Nov 10, 1999 10:04 am

You folks did a very good job of explaning,
thank you, in my part of the job they
just tell us it's part of the big picture.
But my question has always been; has
anyone really ever seen this picture?
And second I still think its ( PFM ).

Thank You

Wed Nov 10, 1999 1:58 pm

Thank you for all your responses. I was getting worried at first as my thread slowly slipped to the bottom with no responses!! I found everyone's response helpful, and I figured that computers had some role in scheduling. Kaitak I like your idea of breaking down a small fleet and working out its schedule. Thanks! If anyone has any other helpful additions, please continue to post! Thanks again.

Jim in Boston

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