1. When I arrived at Heathrow on AA we went through a long sterile corridor above the normal corridor till we got to the transfer bus area to T5. How does one connect to a leaving from the same concourse there? Backtrack? And why did I have to go through security again at T5, wasn't I in a sterile environment from T2?
2. When departing from the T5 satellite, I was looking down at my A319 BA flight (3rd level). Again, how would BA pax transfer to another flight in the same terminal complex if on different levels?
3. When connecting through CDG, LHR,FRA etc. from far flung countries how do pax connect if going through sterile halls?
I have heard of horror stories going through ATL where you have claim your luggage and recheck it and go through security again.
1. Yes, you'd have to backtrack. (Incidentally, if you arrived on AA, it would have been T3, not T2). The UK rescreens ALL connecting Passengers, unless they have just arrived on a domestic flight - these arrive straight into the departure lounge, and then have the option to either exit via baggage reclaim, or proceed straight to the onwards gate (this way, they know that all outbound passengers have been screened to the same standards, irrespective of where they were in transit from. Some other European Countries allow arrivals from certain *sterile* countries to bypass Security directly).
2. I've only ever arrived once at a T5 Satellite, and my connecting flight was from the main Building, so I'm not 100% sure if there's a passageway from 1 level to another via a security check in the Satellite to bypass having to go to the Main building [I suspect not as gates at LHR are announced so late to *allow* people more time in the main building shopping mall], or if you have to go to the main building with the other arriving Passengers before joining transit security, but when arriving at a gate in the Main Building, there's a walkway to follow which then splits into a passageway to Baggage reclaim and to Security (this is actually just the main departure security, but connecting Passengers get fast tracked). The arriving train from the satellites also feeds into this walkway pre-split.
3. As mentioned, LHR rescreens everyone arriving from any other Country. FRA and CDG I *think* allow Passengers arriving from elsewhere in the EU, USA and Canada (and select other Countries?) to follow a corridor straight back to departure, depending on the Terminal/Satellite building (As in, some Buildings like CDG Terminal 1 don't (or at least didn't when I last flew through it 7 years ago) have centralized Security at all, and everyone gets screened at the departure gate).
As for the ATL anecdote, anyone arriving in the USA who hasn't been through pre-clearance has to collect their bags and physically take them through CBP even if they're tagged through to the final destination, although most terminals have facilities to drop the bags as soon as you're through CBP. Some Terminals may indeed require you to go through TSA again - again, my last experience of connecting off an International arrival in the US was 12 years ago, when I was connecting from NZ4 AKL-LAX to NZ2 LAX-LHR (the AKL-LAX NZ2 leg was fully booked, so I'd had to take an earlier flight the same day) - CBP was quick at that time of the morning at LAX Terminal 2, then dropped bag straight afterwards, while still airside but had to go landside to access the departure lounge again, thereby having to reclear TSA.