Ndtyjky

Last night, I had to change planes at LAX because the United Airlines B737 that I was flying on had radio problems and thus could not fly over the ocean. One of the hi-frequency radios had problems, and the pilot said that both radios must function in order for the plane to fly over the ocean. So I flew on another B737 and arrived at Honolulu almost two hours late.

Do airlines keep "spare" planes at major airports in case problems with planes that are scheduled to fly happen? What is the procedure for getting a replacement plane? A plane could not be flown in at a moment's notice, obviously, because it could take hours for it to arrive.

In my experience, having permanent airplanes standing by as spares does not really happen. Based on historical data potential realtime spares, airplanes with potential availability on any given day, can be pinpointed for reschedule or delayed maintenance, should the need arise.

In reality, often the "spare" as you called it, is an airplane that can be pulled from another trip with a subsequent replacement available causing less delay than the flight generating the need for the spare in the first place.

At some airline "hub" locations, maintenance is being performed on aircraft and those aircraft can be used as spares, assuming the checks or maintenance is complete or can be delayed.

Two important points though:

1. It is extremely costly to have "extra" airplanes standing by to replace an aircraft that may go out of service. Swapping aircraft is sometimes quite difficult because the type aircraft scheduled may not be available. An alternate aircraft (different type) poses difficulty because the crew that was going to fly the B737 you were scheduled on, could not fly a substitute Airbus, for example (they either would not be qualified or not current/legal to fly the different type at a moments notice).




2. The airline's SOC (Systems Operations Control) or similar has a difficult challenge on most occasions dealing with maintenance delays or cancellations. It is not uncommon for the flight to just be cancelled and have the passengers put on later flights. However, the airline's Operational management people are experienced at minimizing actual delays and maximizing aircraft utilization.

This is touched on a bit in this travel.se question. Strictly speaking they don't really keep "spares" per se. It's just too expensive.

There are a lot of factors to this and flight planning has gotten much better over the past few decades so planes are rarely flown empty, moved for no reason, or just sitting around any more like they did in the past. The prevailing mentality is that an aircraft is only making money when it's flying, so keep it in the air as much as possible.

However the reality of airline operations requires aircraft to come in and out of service every so often for maintenance and routine checks. This creates a bit of wiggle room in the fleet, so while there are no spare aircraft per se, a fleet of sufficient size may have excess resources. The chances of getting a replacement aircraft are greatly increased if you are at an airport the airline considers a hub or an airport that has large maintenance facilities. Ultimately the demand is not always there to keep all planes in the air all the time so spare capacity is sometime driven by the nature of the business.

In your particular case it's also possible another aircraft was swapped in with the assumption the HF would be fixed fairly fast and thus capable of being dispatched for a flight in fairly short order filling the void it created.

A plane could not be flown in at a moment's notice, obviously, because
it could take hours for it to arrive.

That depends on where the other plane is sitting. Sometimes an aircraft can be flown in if it's sitting at a nearby airfield.

These days it's often cheaper for an airline to cancel the flight, put everyone up in a hotel and book them on the next available flights to the destination. This will obviously be weighed by the airline prior to doing so, but it's a risk they know they have and one they are typically fairly prepared for.

The only operation that I know of, that keeps a full spare on hand 100% of the time, every time, is Air Force One.

HF radios are only used on transoceanic flights, so in that specific case, all the airline had to do was swap your plane for another one at LAX that had a working one but didn't need it, i.e. a continental flight. Ideally, that plane would be on the ground at LAX long enough for them to replace the radio there, but if not, they could prearrange to have it replaced later somewhere else, probably overnight.

More generally, as others have noted, airlines don't really keep spare planes around. However, many of the planes out of service for maintenance could be pressed into service if needed without too much hassle, especially at hubs like LAX, so there is some slack in practice.

My experience is on the Regional side but in that business spare aircraft are common with operators that have large fleets. There are two important metrics: Dispatch Reliability and Schedule Completion Rate.

DR is % of aircraft leaving the gate within 15 minutes. The industry standard is 99% or better, meaning airplanes leave with a delay of over 15 minutes not more than once every 100 departures.

SCR is flights that were completed one way or another, as opposed to being cancelled. SCR should be above 99.5%.

An airline can have a fleet with at DR that is below 99%, sometimes well below, but by having spares they can achieve SCRs close to 100%, which is the critical metric overall. In many cases, especially if the fleet is large and well ammortized, it's worth the cost to keep spares.