Judging distance and times - aircraft on final

Possibly a bit of a daft question, but how do you know how to judge how long it’ll take for an aircraft to get from Point A to Point B when you’re controlling?

Example:

I was controlling last night and had a reasonable backlog of planes on the ground waiting to takeoff. I also had a few planes approaching to land. I found it difficult to judge how much time it was going to take a plane to touchdown once on the glide slope and whether I had enough time to clear the planes on the ground to takeoff. I ended up with a queue of (very patient!) planes on the ground as I faffed trying to figure whether I could let them go. Compounded was the fact that the airport was one like London City and they had to back taxi(?) to get to the start of the runway.

Suggestions and piss-taking all gratefully received :-)

The line infront of an aircraft shows where they will be in 30 seconds continuing at there present speed

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Well generally if a aircraft has just landed and you can barely make out the next aircraft on final you can squeeze a take off in between. This is if the airport has a good taxi system with no back taxi

Thanks, it was the back taxi that was throwing me off. I think I’ll stick to more standard setups for the time being!

I usually use the nm that shows up on the incoming aircraft on tower view. Depending on length of back taxi, you should add at least 2-3 more miles to your already needed 3-4 miles for safe departure. For airports that don’t require back taxi then 4 miles out is a safe distance.

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