If a four engined aircraft had to land with three operating at an outstation with minimal maintenance facilities, could it be signed off to take off with the remaining three engines?
Or would it be a case of repairing it in-situ regardless?
Three engine ferries certainly exist, but they aren't simple events, and have a nasty habit of ending up in the crash comics.
The failed engine would have to be made safe for the flight. The work for that could be just about anything, but would have to be approved by the manufacturer of the aircraft and the engine. Then the regulator of the country you're in, and the aircraft's home. And the airline would actually want to do it. Just as likely easier to fix/replace it.
But, if you actually wanted to do it, and the engine was made safe, any sensible regulator would want the crew doing the ferry to be specifically trained in the simulator beforehand. It would not be a case of the line pilots who are there doing it (back to the crash comics).
There are a number of issues. For a start, it's not a takeoff with the performance of 3 engines. It's quite a bit less than that.
If you line up, and spool up all three engines as you do normally, you will immediately depart the runway, and no amount of control input will stop the aircraft turning. The issue is that there is a thing called Vmcg, which is basically the minimum speed you need, to have enough rudder authority to be able to keep the aircraft straight after the failure of an outboard engine. That's quite a high number, in the order of 120 knots. So, the upshot of that is that your take off procedure will have to be bring up all of the engines to about 30% (that's the pause you always feel at the start of a takeoff), then taking the symmetrical pair up to TO/GA. Then bringing up the 3rd engine, but matching its power to about half of the rudder input. As the rudder isn't effective at less than 60 knots, there's quite a pause between the first pair being pushed up, and the 3rd moving at all. You can't take it all the way until you've passed Vmcg.
There's unlikely to even be a valid V1 (you cannot continue at any speed with another engine loss), and that's why no sensible regulator would ever allow it.
We used to have the occasional play with it in the simulator. In fact I've even done a single engined takeoff in the 767 sim. But it was done as an exercise in Vmcg, and lateral aircraft control, not as a procedure to actually be used.
I'm tipping with 3 engines operating it'll be able to fly to the nearest full service airport.
As long as you had a clean shutdown in flight, there'd be no pressure, or requirement, to land. Your range is reduced, but in many cases you could quite legally fly to your destination. A rough rule of thumb from the 747 was that within 2,000nm of destination, and as long as there was fine weather forecast, you'd most likely be able to continue.
