Amount of time to Plan for a change of planes

The big question, is this one through booking on one ticket, or two separate bookings?

• If it is one booking, no problem. The airline would not have allowed the booking if it was not possible, and you will be able to transfer terminals "airside" without going through immigration, and if you miss the connection because the first flight is late, they have to put you on the next flight.
• if you did this as two bookings, then neither airline is responsible for you making the connection, it is your problem.

In the second case, how long does the second airline require as minimum check-in time? Where are you flying from?
11:30 land, taxing, disembark, walk through corridors = 12:00 earliest.
Passports 10 min - 1 hour, customs zero time, change terminals (T2 and T5 are at opposite sides of the airport, ~30 mins-1 hour) = about 13:00 - 14:00.
Then check-in for flight to Rome. 14:40 flight = check in < 13:40, maybe earlier (what does your airline say?)
This is not looking good. And, if you are coming in on an intercontinental flight, it could be 1+ hours late.

ChrisF has basically covered it all, but I will add a link to this article over on TA: https://www.tripadvisor.com/Travel-g41707-c207311/Newton:Massachusetts:Connecting.On.Separate.Tickets.html

At the end of the day, you want to ask yourself what your risk tolerance is, how important it is for you to get to Rome the same day, and how much money and time you're willing to spend, to make that happen. If you must be in Rome, then book a direct or connecting flight (all on one itinerary with one airline) to Rome. If you're flexible, maybe go for the unprotected connection.

But do have an idea of what your Plan B will be, if your first flight is delayed or cancelled. And I'd also read the fine print in your travel insurance carefully, so you know what they will, and will not, pay for.

Connections at Heathrow can take time, and it's impossible to predict exactly how long, but there is no way I'd book only three hours between flights.

You can also seek more advice on Flyertalk and/or the Air Travel Forum on TripAdvisor.

Good luck with your travel planning.

To answer the questions:
We will be flying in from LAX. Our connection is a separate booking. No I don't want to take any major risks so if about 4 hours isn't enough to make the connection then what is? We will take whatever advice makes this easier.

The bigger question is what is the dollar risk if you miss the flight? We have done this frequently over the years with a four to six hour window. We finally were burned this last year when our international flight was delayed 24 hours because the the first flight was canceled because of a machinal problem. Obviously we missed the connecting flight. Fortunately we were able to talk the airline into re- routing to our finally distination so we didn't have to buy a replacement ticket. But what if you have to buy a replacement ticket? That is your money risk. There is no guarantee safe window. Just the degree of risk you are willing to assume. Four to six hours should be enough.

If you have carry-on only and onward boarding passes in hand, you do not have to go through passport control; you can stay airside and transfer between terminals on the dedicated bus. (We have done this; the transfer took 45 minutes gate to gate). You will go through security at T5.

I would still allow a minimum of 4 hours, and even then there is no guarantee you will make the flight. (Your flight from LA could be delayed).

I am curious why you are not booking this as one ticket?

To answer the last question, it just worked out that way. We were unable to get a code share airline that had a direct flight from London to Rome without having to change planes.

Thanks everyone for your thoughts. I now know there can be a million and one things that can go wrong trying to make connections flying from the US to Europe and connecting from one European city to another without a through booking (lesson learned). At this point that doesn't matter anymore. Thanks to your advice, and recommendations I know what I need to do to ensure we make our connecting flight or a later one if the LA to London flight is delayed.

I think three hours is plenty. That's the recommended time to arrive at the airport for international flights. You will have to get your bags and go through customs, then check back in but I think three hours is plenty. Some people ask if an hour is enough and I think that's too risky.

No matter how long you schedule, there's always a chance of a delayed or canceled flight. You can never eliminate any possibility of missing a connection - even if the entire flight is purchased on one ticket with one airline.