Why do you have to make calculations for different ETP (equal time point)?

Question

I thought that you just have one single ETP (so the point where it is equal if you decide to continue or decide to return because of an emergency, because you are in the exact middle of the flight length).

Answer 1

there are 3 ETP's you calculate per airport pairs:

1. decompression

2. one engine fail

3. one engine fail with decompression (worst case scenario of the three).

Obviously these three conditions give rise to different speeds and hence different ETP's.

Answer 2

If for instance you were flying over the atlantic to europe, you would be flying south of Keflavic, Iceland. So you would have two ETPs. One from say Gander, Newfoundland to Keflavic and another from Keflavic to Shannon, Ireland. If you can make it to land you plot an ETP to the sucker regardless of whether or not you plan on landing there or if it's even near your course. That's how ETOPs works from SFO to Tahiti for instance. A twin engined aircraft would have to "bend" it's route toward Hawaii because it needs somewhere to make the ETOPs rule so you'd have two ETPs there too.