hi there
This does assume that not even the slightest snag occurs that could take longer than 20 mins to fix and that there's a spare aircraft on standby,just in case.It also assumes that any snag that does occur will not prevent boarding and can be carried forward(MEL'd)until nighttime.The pressure is on the engineers to be as compliant as the pilots/cabin crew.Perhaps the MEL/CDL is being tweaked to it's elastic limits!?
regards
TDD