I’ve always wondered the specifics of how diversion airports work.
I know that pilots select an alternate airport and add fuel accordingly before every flight. How are these chosen? What factors are in play?
Also, for flights from, say San Francisco to Honolulu, there is a point of no return over the Pacific Ocean- if you decide to go forward with your flight, you’re committed to going to Hawaii no matter what.
Are there similar systems for other destinations like this? Like in Bermuda, for example. The nearest other airport is on the East Coast of the US.
Your question is something I’ve been wondering for quite some time now. If I were to guess, ATC would most likely give radar vectors to the nearest suitable airport if it’s within their control range. If not, I would assume the same procedures apply when a plane enters a new airspace.
For example, if an aircraft wasn’t able to land at KJFK, and KBOS was the diversion airport, then I would assume New York TRACON would vector the plane out of NY airspace and hand control off to Boston Center or TRACON.
Oceanic alternates for us are chosen on a couple factors. But proximity to our route of flight being the main factor. In the private jet world, we don’t have ETOPS. We instead utilize something else called “ETPs” or “Equal Time Points”. Rather than being the physical/geographical ½ way point, this is the ½ point in time. This is indicated by the “squared S with arrows on the end”.
Another factor that’s taken into account when choosing an alternate would be the services the airport has to offer. It wouldn’t make sense to choose an uncontrolled airport in the boonies should we have a medical emergency. Instead larger airports with crash fire rescue, maintenance and other personnel who may be able to assist us in the event something should go wrong in flight.
Below are a couple screenshots that I pulled from the archives. Anytime we’re flying over water we need to document these flights. The airports depicted in pink are our alternate airports.
In the world of just any other typical airport where you need to file for an alternate we follow something called the 1-2-3 Rule.
1hr before and after time of arrival
2000ft ceilings
3sm visibility
If our destination has any one of the following at the time of arrival, we need to have an alternate filed.
Some airports like KASE (Aspen) an alternate is always required. Even if the airport is forcast to be VFR for the entire day. All Jet aircraft regardless of size have a strict 10kt tailwind maximum limitation. Because ASE is a one way in, one way out airport, its common to have a 10kt+ tailwind on the field thus restricting a lot of the aircraft that can get in.
I’ve done some exercises on choosing a proper alternate airport in given conditions. It’s pretty theoretical but still valuable for reference.
First you’ll check the METAR and TAF report of a list of available airports, and compare them with the landing minimum as in their respective charts. When comparing, you need to add a safety margin to the landing minimum. For example, if the visibility minimum is 800m, typically we’ll add another 800m (this value may be different), so that’s 1600m, and compare this with the current, or forecasted weather.
Then we’ll look up the NOTAM for some key information. Such as:
The runway being closed at the time of arrival
Navaids (like ILS) unserviceable (this will result to the safety margin being larger)
The airport not receiving certain categories of aircraft (such as class D and above)
If available, SNOWTAMs (to check the runway condition and braking actions, etc)
A thing to note is that if the weather of the departure airport is below its landing minima, you’ll need to choose a “departure alternate” so you have a place to land if something goes wrong shortly after departure.
