4 engines, one fail. Go, no go?

[Checkboard]

The performance for FAR 25 (or equivalent) aircraft is based upon a guarantee that the aricraft can keep flying if an engine fails at any point of the flight.

This performance guarantee only became possible with the advent of jet engines, and their excess of power at low (i.e. take-off) altitudes. Compare the regs for FAR 25 aircraft with FAR 23 - no guarantees for an engine failure in a Piper Navajo!

If you have a FAR 25 aircraft with more than two engines then continuing is always possible - provided you can give the passengers the same guarantee. That is: if you now lose the most critical remaining engine, you can guarantee the aircraft's performance - the big advantage is that (being airborne already) the most performance critical time (take-off) doesn't have to be taken into consideration.

SO, the engine fails (and you are making these decisions airborne of course):

1. Satisfy yourself that the failure is an isolated one. If the techlog mentions that all of the oil pressure sensors were changed over night, and you have just shut down an engine for low oil pressure, it is time to land!
2. On the decision to continue - check that you can have enough fuel to complete the trip at the lower level and new winds, with the reduced number of engines.
3. Check that you have sufficient fuel to divert, following a failure of the most critical engine at the most limiting CP [critical point]
4. Check that you can maintain above the lowest safe altitude, following a failure of the most critical engine at any point along the route, or perform a safe drift-down.
5. Check the reduced Go-Around weight limit, assuming a critical failure on the approach, and increase the approach minima as required.

Provided all of the above is completed, then the aircraft is perfectly safe to complete the route, flying as a "three engined" aircraft instead of as a four engined one.

Now those performance calculations should all be within the ability of any competent crew on board the aircraft, however it would always pay to radio the raw data to the airline performance department (should you have one!) to have the calculations cross-checked by someone relaxing on the ground.