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My favorite features of the United Airlines app are:

  • you are able to see what specific aircraft will be operating your flight
  • you are able to trace that aircraft back indefinitely via the "Where is this aircraft coming from?" button

I noticed that each aircraft has a specific number aside from the aircraft model (e.g Boeing 737-900 #3815), that references which specific aircraft will be operating your flight.

My question is how far ahead does an airline plan which specific aircraft will be operating a flight?

I'm not asking if the airline knows that a B737 model will be operating a flight, but I'm asking more specifically how far ahead do they know which specific aircraft in the fleet will be operating a particular flight?

This question may be broad as I realize that it may differ between airlines. I'm just curious as I like to see where my flights have been and will go, I find this very interesting.

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It depends on the airline and their planning capacity. The simple answer is that they can't be sure of the tail number "of record" for a flight until the plane pulls into the gate at its destination. It's rare for a plane that departs the gate for a flight to not run that flight (it's pretty bad business for it to be common for flights to depart the gate and then come right back, or to be diverted, or otherwise not finish the route they left the gate to fly); it indicates a mechanical issue exists that did not develop and/or was not discovered until taxi or takeoff, and at some point in the flight (probably when departure control hands them off the the ARTCC) the plane could be said to have "run" the flight even if it ended up diverted.

Most of the equipment schedules (which tail number will fly which route at exactly what departure and arrival times) are generated by computer and can be set up months in advance if they really wanted to. However, that's usually an exercise in futility along the lines of predicting the weather more than a few days ahead. Airline scheduling, like weather, is susceptible to "chaos theory", because small deviations from the initial plan tend to compound over time instead of factoring out.

Ticket sales, rebooks and cancellations will affect the schedule days and weeks in advance of the flight. It's technically illegal to cancel a flight less than a certain number of days before its scheduled departure if it has even a single paying customer on the roster, but if a route is underperforming, the scheduler can reduce the scheduled daily flights along that route which could bump people who bought a ticket far in advance. Also, an underbooked flight can legally be cancelled if the plane is needed for an equipment change on a fuller flight, as cancelling the underbooked flight would cause less overall disruption than cancelling the full flight because that plane can't make its next hop.

I just mentioned mechanicals; that's a big reality check for a computer-generated schedule. If a plane cannot continue because of a mechanical defect, it gets pulled for maintenance. Airlines typically have a few planes parked strategically on the tarmac of various airports for a day, which can be activated to run a route, but they don't have too many of these since a parked aircraft isn't making money, and there isn't always one of these standby aircraft readily available as a substitute (though if a defect is found in flight the replacement can be on the way before the defective one lands to reduce the delay). Meanwhile, mechanicals are typically such a localized problem that the schedulers can't get away with just delaying flights systemwide, like they can often do with severe weather. They have to run the rest of their fleet on time, and that means either getting a replacement plane to the gate, pronto, or else sending that replacement plane empty to the cancelled flight's next destination, so the new plane can get those passengers on to their destination.

Obviously, severe weather can cause major ripples through an airline's schedule for days to come. If ORD or DFW has a "ground stop" due to severe precipitation or high winds, that will delay not only the flights scheduled to arrive or depart in that time, but every flight those tail numbers were scheduled to run for the rest of the day, as well as various scheduled connectors (those connecting flights, if they're not also under severe weather, often have to depart with whatever passengers they have otherwise there will be even more delays; then the passengers who have missed that connectot will have to be rebooked, on other airlines if need be).

That delay time also counts as "active" time for a flight crew; if a flight is delayed beyond a crew's end of shift, they often must get off the plane to comply with FAA regs and union CBAs. The scheduled replacement crew, however, was supposed to get on at the next stop hundreds of miles away; so, it's not just the planes that have to be replaced on a route when a flight is cancelled. In cases of a severe delay extending beyond a normal shift change, the crew either flies the plane if it's legal and are paid overtime, or if it would be illegal for that crew to continue, an on-call crew is brought in, flies the plane to the scheduled changeover point, then rides a standby seat (or in the extreme an empty plane) back to their home terminal or somewhere else they'll be useful when their shift really starts.

All of these considerations and more are weighed by the sophisticated algorithms now used to generate flight schedules. American Airlines calls theirs the "cancellator", and when a major hub like DFW, ORD or ATL sees a weather shutdown that's pretty much exactly what it does, sacrificing flights to and from that hub and some of its connectors to try to save what it can of its passenger load for the day (and thus the airline's profitability for the quarter).

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  • $\begingroup$ Thank you for your very detailed answer! So here's another question based on the original: how does an airline know that a specific type of plane will be operating a flight if they don't know which specific one will? Don't they need to know that one will be available at the time of the flight in order to use it on that flight? $\endgroup$ – woakley5 Jul 14 '15 at 3:03
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    $\begingroup$ It's easier to know the type of plane in advance versus the actual plane, because the distance to be flown along the route and the typical passenger count at each stop will call for a particular model of plane to be used. Airlines don't typically have just one or two of a model of plane; they'll buy several dozen of a model that works for a specific route type, and they'll try to replace like-for-like because that's the best use of equipment. $\endgroup$ – KeithS Jul 14 '15 at 3:12
  • $\begingroup$ Now, even this isn't set in stone; an airline might have to pull a 737 for maintenance at the last minute, and run the rest of its route with an A319 instead. There are only two situations in which an airline can 100% guarantee even the type of airplane; (1) when the airline operates no other type of aircraft (e.g. Southwest; it's run only 737s for decades, though you could still get a 737-800 instead of a 737-700), or the airline has no other type of aircraft that could do the job (ultra-long-hauls, for instance, are flights that the airline may operate only one model capable of running). $\endgroup$ – KeithS Jul 14 '15 at 3:16
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    $\begingroup$ Replacing a 319 with another 319 means the original crew can keep on doing what they were scheduled to do. Replacing it with a 737 means the 319 pilots now have to be replaced with 737 pilots for the next legs. Less of a deal if their day was hub to somewhere to hub, more of a complication if it was hub to A to B (overnight) to C to D to home. But you do what you have to do, sometimes. $\endgroup$ – Ralph J Jul 14 '15 at 13:51
  • $\begingroup$ There are many cases where a plane returned after two hours and another plane was pulled to do the flight instead and there are many cases where a plane broke long before a flight and the flight was cancelled and everybody rebooked. Which is done depends on which is easier to arrange, not on when the problem occurred. $\endgroup$ – Jan Hudec Jul 14 '15 at 16:18
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How far ahead they formulate their plan will vary for each airline, but plans can change even as late as hours or even minutes before the flight, in case of things like broken planes or inoperative equipment that makes a swap necessary. Say an aircraft that was going to operate a couple of long-haul flights & overnight at an out-station develops a problem with a center-tank fuel gauge. It might get swapped with an aircraft operating shorter legs (so the center tank would remain empty) that's scheduled to overnight at a maintenance base. This would get the gauge fixed sooner, and make for fewer issues for the long-haul crews.

But you'd have a different tail number show up at your gate than was planned before the swap was put on.

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    $\begingroup$ Also note that the minutes before flight can come even hours after scheduled departure. It happened to me once (I'm not a frequent flyer!), that other passengers and I watched intensive service efforts on our scheduled plane for half an hour after our scheduled departure, to be moved to the neighbouring gate as soon as a replacement plane became available. The captain then apologised for the delay saying "We are going to be some twenty minutes late, which happens when you take someone else's plane." $\endgroup$ – Pavel Jul 14 '15 at 9:13

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