Is 40 minutes enough time to transfer at Newark (EWR)?

Personally, I would contact your travel agent and see if an earlier connection from MCO is available. You can also try the airline, although it may not be easy to change to an earlier flight without paying a change fee. On the day of travel, you can also attempt to get a Same-day Change (SDC) to an earlier flight.

Assuming you are booked on a single United ticket and are flying UA aircraft to the bitter end, you should be able to obtain your boarding passes in Orlando and have your baggage checked through to your final destination.

(On the other hand, as far as I know, United has no domestic airline partners in Britain since the demise of BMI, so if you have an onward connection, all bets are off. UA can interline baggage with BA, but not Flybe, for instance.)

Published minimum connection times… are nearly useless

Each airline maintains its own standard minimum connection times (MCTs) for various airports. The MCTs are only a standard measure for the itinerary construction process, however, to prevent people from booking truly impossible connections. Their relationship to reality is… aspirational for anyone other than a fit adult with light hand luggage taking no toilet breaks who's arrived on time and had no delay deplaning, walking briskly with boarding pass in hand for the connecting flight. For the rest of us, you may want to pad it by 25-50%.

Of course, United makes it nearly impossible to find out their MCT anyway (in pre-merger United it was 40 minutes at EWR), so perhaps the point is moot.

Terminals and gates

United's primary terminal at EWR is Terminal C, and as far as I can tell, all international flights would normally arrive and depart from C. As Terminal A is mostly used for United Express (commuter/feeder) flights, I suspect your Orlando flight would also use Terminal C. Wikipedia is a surprisingly reliable reference on such matters, having an army of aviation geeks keeping the pages for major hubs and major carriers pretty well updated.

Gate assignment is unpredictable, though a flight may use the same gate for a number of days or weeks at a time. United's own flight status tool is helpful here, as you can see the arrival and departure gates for your inbound and outbound connections over a period of days.

Ontime performance

It is extremely unlikely that any flight on a trunk route such as EWR-LHR will be held for connecting passengers, so an awareness of the reliability of your inbound flight is helpful.

FlightStats is a free tool that lets you compare flight delays and other information across flights, airports, airlines, and time. Newark is a highly congested airport, and delays accumulate throughout the day; this is borne out in stats that show that only around 72% of MCO-EWR flights are on time between 1500 and 1800, with an average delay of almost 45 minutes.

When looking at connections there's always 2 things you should keep in mind :

• What are the odds of you not making the connection, and
• What's the impact of not making it?

40 minutes at Newark for a Domestic -> International connection is plenty of time, presuming that your inbound flight is on time, and you're within the same terminal.

Flights from Orlando will almost always arrive in the C terminal, and flights to London will (almost?) always leave from terminal C - so you're covered there.

As for on-time - who can say? There are websites that will give you historic information, but history isn't going to control whether your specific flight is on-time or not.

So all up, the odds of you making the connection are good. Not excellent (if your inbound is more than 10-20 minutes late your odds drop significantly), but still good.

So what happens if you miss the flight?

Newark is a very major United hub. They run multiple flights to London from there every day - especially in the evenings when flights depart as frequently as 1-2 hours apart! Depending on the day, there will likely be 2 other direct flights to London after yours - and the airline will happily move you to one of those flights presuming there are seats available.

Yes, it's possible there won't be seats available - but odds are there will be, especially as it's likely someone from one of the later flights actually went "standby" on your original flight.

Yes, it's possible that you'll end up separated from your traveling companions, and in a middle seat.

But you'll almost certainly get on!

So what should you do? Presuming you've already booked, I'd say go with it. Changing flights is not going to be cheap, and the odds of them letting you do it for free due to the short connection are almost zero.

If you haven't booked, then it comes down to cost. If you can get a later flight to Heathrow for the same price, I'd probably do it. It'll get you in an hour or two later, but it'll be worth it for the piece of mind. But if it's going to cost more, then I'd stick with the 40 minute connection - worst case you miss the flight and end up on a later one anyway!

If you were connecting to a location with less frequent services (eg, one-a-day rather than every few hours) then this would be a very different story! In that case, make sure to allow yourself enough time as having to wait an extra 24 hours for the next flight isn't fun...