Lax termnals
 

Looking at a flight from phx to mso via lax.

Concerned with 42 m connect time from aa to amer egle or sky west. What terminal to what terminal? Time to make it?

On certain days of the week Allegiant has direct flights from Phoenix/Mesa to Missoula MT.. $80/pp instead of over $300. I saw some flights that made a connection in SLC instead of LAX. Try to find flights out of either Phoenix airport that have a longer layover in an airport other than LAX.

I don't connect at LAX because I live in L.A., but I would find that connection too tight for comfort. American and its regional affiliates operate domestic flights out of two terminals which are connected by a tunnel on the airside. It can be a long walk if the two flights happen to be at the far ends of different terminals. If the arriving flight is even a few minutes late or there is a delay in deplaning, you'll be running. You could get lucky with the two flights arriving and departing within the same terminal, but I think you can only find out this detail by speaking with someone at American Airlines.

When I'm worried about how close gates are, I look up the flight on the same day of the week and see what gates they use. . I don't think 42 minutes is a workable connection. Can it even be booked on a single ticket?

You can find your flight at aa.com and see if it is flagged:
Change terminals

If it has that then I would not book it. I probably wouldn't book it either way.

flight times are usually padded these days so you probably actually will have more like 52 or 57 minutes.

There are so many flights to Phx all day that I wouldn't worry much about missing a connection. Unless you're on the last flight of the day or are traveling at a busy time like 4th of July weekend or something.

Dealing with Terminal 4/5 and Terminal 7, at most you'd have a ten minute walk and wouldn't have to go through security or anything again.

They want to end up in Missoula, not Phoenix.

There are 2 Phoenix airports: Skyharbor and Phoenix/Mesa (AZA) Allegiant has direct flights from Phoenix/Mesa for 100's less.

The OP wrote ;phx, taken to mean PHX, the airline code for Phoenix Sky Harbor International Airport

The OP wrote aa, taken to mean AA, the airline code for American Airlines. AA does not uise AZA (Mesa)

The OP wrote Amer Eagle on Skywest. American Eagle is the brand name for regional carriers operating flights for AA. In this case the LAX TO MSO flight is an AA flight operated by Skywest.

I found the OP's itinerary at aa.com on June 23 (randomly chosen date, there was no LAX-Missoula flight tomorrow). It is flagged with the note: Please note connection time

There is only one AA flight that day from LAX to MSO, departing 8:20 AM.
The OP has to take the first flight of the day from PHX to LAX to have any chance of connecting with the MSO flight. That is the 6:10 departure arriving 7:38 AM

First leg is AA 1311 on an A320 operated by American Airliknes
Second leg is AA 2996 on a Canadair RJ 700 operated by Skywest for American Eagle

I smell a terminal change between T4 and T5.because of the change of aircraft type to a regional jet.

Allegiant Air is IATA airline code G4. Their route map shows nonstop flights between AZA (Phoeniz-Mesa Gateway Airport) and MSO (Missoula, Montana) and between LAX (Los Angeles International Airport - NOT Union Station LAX the Amtrak code) and MSO. But there is no AZA <-> LAX line on the route map at the G4 website.

Looks like Mesa to Missoula is on Sundays and Thursdays on Allegiant. Their website accepted the IATA code AZA in the search but at seat selection it showed the FAA code for Mesa which is IWA. That is a first for me (only have seen IATA codes), maybe because I don't use airlines like that one.

I've only flown through LAX a couple times, but that seems like you're cutting it pretty close, especially if there's any delay with your connecting flight. It's a big airport and those gates could be pretty far apart. I'd definitely want more leeway than that for a layover there.

If the OP could make the Allegiant schedule work, that would probably be better. The big issue with this type of carrier is that if the flight is delayed or cancelled they can be terrible.
Of course we all know about the baggage fees but this type of carrier probably has a few that American doesn't. Not that American is great, either.

Well, Mesa is an important airport for Allegiant. The Thursday flight (e.g. June 24) departs at 6:10 AM. The plane will have (almost surely) spent the night there. If the plane had issues the night before then they had a chance to fix it or replace it and be ready for that first flight of the day. Or not. But they will have all day to fix it.

The Sunday flight to MSO is at 6:05 PM. The have all day to mess that up and less time to fix it.