[FabHK]

Also, aircraft are certified to be able to complete take-off with one engine failure.

Thus, twin engines have to have basically 100% reserve, while the 4 engines only need to have 33% reserve (to make up for one engine failing).

[youngtaff]

Norwegian currently have a 787 stuck in Iran with a broken engine, so I'm not sure they can take off with only one engine

[FabHK]

From what I gather, certification requires that they can take off successfully if an engine fails during the take-off run (after V1). I doubt that they're allowed to leave with half the engines inoperational already.

[youngtaff]

With you… like this Thomson plane that had a failure due to bird ingestion during take off at Manchester - https://www.youtube.com/watch?v=9KhZwsYtNDE