Originally Posted by
B772
In the late 1980's...
...I was concerned about the prospect of up to 200 minutes nail biting on a single engine and the resultant engine bearing damage. (When a heavy twin suffers a shutdown both engines are usually replaced)
I suggested the B777 could have a CFM56 engine in the tail and be regarded as a tri-jet.... My suggestion was shot down by the Boeing attendees as being too expensive and adding weight to the aircraft.
What is this bearing damage of which you speak? In a turbojet engine? Its called Maximum Continuous Thrust for a reason, and the rotating elements of the bearing are subjected to slightly higher velocities but still within their design envelope.
Regarding your aux engine idea: I have heard at the bar: “The only reason I prefer four engines is that there are no five-engined planes” . I guess time has shown that our earlier luddite outrage was misplaced.
Thinking back to the original loooong range aircraft, they went from ten engines (B-36) to eight (B-52). All of that was dictated by take-off performance. Their mission hasn’t changed, yet the new bombers are twins.
Engines share more than just fuel. They share air, containing freezing precip, ice crystals, hail, ash, birds, non-TCAS traffic. They also share oil and maintenance. They share manufacturer, including flaws, bad metallurgy, bad design. They also share a common parts stream which may be open to counterfeits. They also share operators, including people who forget anti-icing, screw up the fuel scheduling, etc. All of the foregoing is true regardless of the number of powerplants.
Oh yeah....engines also share the duties of carrying around people with evil intent, pallets of lithium batteries and flights planned through war zones. The entire enterprise is based on odds which I reckon are about 9 to 5 against.