I am a current Part 121 dispatcher. Since we plan the alternates, I guess I am probably one of the better primary sources.
The most important criteria is legality. An alternate airport must meet alternate minimums. It must meet or exceed alternate minimums for the entire flight up until the time it is scheduled to arrive at the alternate. Alternate minimums are determined by the one navaid, two navaid rule. If you have an airport with one ILS that is CAT I visibility 1/2 and DH
200 ft you must add 400 ft and 1 mile. This becomes your TAF weather minimum: 600 ft ceiling and 1.5 sm vis. If you have a second ILS to another runway, reciprocal being legal to use, that has landing minimums of 200 and 1/2 you add 200 feet and 1/2 mile to the most restrictive of the two minimums. The lowest it can be is 400 foot ceiling and 1sm visibility. This is same protocol when using VOR, NBD, or circling minimums. Most airlines are not allowed to use RNAV approaches to derive alternate minimums. Winds must within limits including gusts. If the reciprocal has a 11 knot tailwind and your limit is 10 you cant use the reciprocal for alternate mins since the wind exceeds limitations.
Also, the Jepp plates specify that certain approaches need a tower operating to be used for alternate mins. Late at night, it can make finding a legal alternate difficult at times.
For a passenger airline, the alternate airport must be in the airlines C70 authorized city list. Diversions can and do happen to cities not listed in the C70 but this is not optimal and can result in FAA certificate action against the pilot and dispatcher depending on the circumstances. If it is a weather issue where fuel wasn't properly planned then it can be really bad for both the pilot and dispatcher. If it is a mechanical or other type emergency, it isn't optimal but is considered legal under emergency authority. Airlines wont have performance numbers for offline stations so it will take a call to performance engineering to get performance numbers. In IFR conditions, airlines wont normally even have approach plates for offline airports not in C70 so it makes an offline diversion in IFR very treacherous.
For regionals and also majors, fuel costs and weight and balance play a role in alternate choice. Generally, the closest legal alternate is chosen. A smart dispatcher goes against the fuel policy and files the farthest alternate he can fit on with the fuel and payload limits. This gives more hold fuel since you can amend the release for a closer alternate. At a regional, you bump passengers on a full flight when a major storm system moves through a region. It is not unheard of for MSP
to be used for ORD
even for JFK
just to find a legal alternate. Every dispatcher has a story about how far he needed to go to find an alternate that is legal.
The one loophole airlines use is you can change your destination enroute. If you file CMH
as your YUL
alternate due to weather constraints, you can divert to BTV
simply by amending the destination on the dispatch release from YUL
. As long as SYR
has landing minimums and you still have the gas to get to CMH
it is legal.
The actual diversion airport can in many times be an operational decision enroute made between the pilot and dispatcher that is not pre-planned on the original dispatch release. When actually diverting, the first choice is the closest. If unavailable you go to the next closest. Sometimes, you will pick the actual diverting point and divert before you run out of hold fuel so you can be the first in line for a gas n go diversion. When alternates fill up, you go to the one with the least amount of planes unless you have no choice.
There is a big difference at an airline between an alternate and a diversion. An alternate can be a simple paper alternate whether it is required or not. The only time an alternate is required is if the forecast ceiling is less 2000 ft and the visibility is forecast to be less than 3 sm one hour before and one hour after the scheduled arrival time. Some companies require an alternate for crosswinds and braking action. If a flight is longer than 6 hours, an alternate is required. Ironically, many diversions happen due to thunderstorms but many times the actual alternate is not legally required even though the chance for a diversion is very high. There is no legal requirement for an alternate with forecast thunderstorms. Thus, you will see many alternates given even when not legally required but logically, a good dispatcher with good judgment is putting an alternate one when he sees TSRA, VCTS, or CB
in a TAF or METAR.
Most diversion events don't occur when the weather is forecast to be bad. When the weather is forecast to be bad, typically extra hold fuel is given. You may get a diversion here and there but the most widespread diversions happen when the weather is worse than forecast or unexpectedly bad. You aren't carrying as much extra gas so when you get anything more than one or two turns in holding with an EFC 40 mins away, you aren't going to be able to hold so you divert. When the weather is forecast to be bad, it may take 6-10 turns of holding before diverting. Typically, holds wont last that long so most will be able to get in.
In convective weather, you get re-routes from ATC around the weather. This can cause diversions because you need extra gas for the longer route and if the same weather hits the arrival airport, your hold fuel can be cut down to as much as 100% leaving you with not a lot if any time to wait the weather out at a holding point.
Another factor in choosing diversion point is fuel stop. If the headwinds are too strong, you may have a pre-planned diversion which is in actuality two destinations legally but to the passengers it is a diversion. Occasionally, winds aloft will be stronger than the flight planning computer we use in dispatch forecasts and unplanned diversions can result due to the unplanned increase in enroute burn.
Most airlines have lists of alternates they don't want dispatchers diverting to. Typically, these are airports with limited customer service options or logistics or they can be an airport that is undergoing construction and doesn't have enough parking spots.
Some diversion points are MEL
driven. If the APU is deferred, you can just divert anywhere. If your APU is deferred and the divert point cant hot re-fuel (most places cant), the plane is pumpkin without a working airstart. This will effectively ground the plane until maintenance comes to fix the APU.
For crossing mountains and oceans, you have driftdown and ETOPS alternates. These are single engine alternates in case of the loss of an engine.
For maintenance diversions, some are by the aircraft manual dictated by nearest suitable airport. The closest C70 city is used though it sometimes is offline if it is serious enough an issue. Customer service availability isn't a consideration in these diversions and even the availability of maintenance isn't though this depends on how the airlines POI or DSI interprets the regulations.
The alternate choice is only a small portion of the dispatch job. All these decisions to make for poverty level wages lol. Its the life of a dispatcher.