BA used to (or maybe still) do practice approaches in the 747 sims with all engines inoperative, I understand thay have landed an aircraft after all four engines have quit. Air Canada also used to do "shuttle" approaches in the B767/B727 for all engines inoperative landings. Ansett Australia also has an incident with a BAE-146 with all engines becoming inoperative.
To my understanding this practice is not carried out anymore by Air Canada.
The Air Canada B767 after a glide approach
The full story can be found here
The Gimli Glider
Another account of the same incident can be found here
The Crash of Flight 143
