Transport jet aircraft are designed and certified for a specific mission profile.
For example, the Lockheed TriStar was originally designed as a twin, but engines were not available at that time to enable the aircraft to meet the weight/range/payload requirements required by customer airlines, so the aircraft was re-designed as as a three-engine aircraft, to meet customer requirements.
3 and 4 engine jet transport aircraft have to meet the same engine-out second segment certification requirements.
Twins however, meet another set of guidelines
In each case, three or four engine types CAN
, if the operator so chooses, continue with one engine inoperative, to the scheduled destination, provided certain enroute conditions are met, subject to the driftdown performance specified.
Twin engine aircraft however, should an engine fail in cruise, must divert to an enroute alternate, subject to certain conditions, not the least of which is the weather conditions at the selected enroute alternate.
In addition, the Lockheed TriStar will, if an engine fails enroute, driftdown to a lower altitude (dependant on weight and ambient temperature aloft), will cruise at a slower airspeed (ballpark figure, 430 knots TAS) and burn, on average, an additional 500kgs of fuel per hour.
The Boeing 707 is quite similar.
One incident that I recall many years ago was a B747, enroute BAH
, where one engine failed enroute over Saudi Arabia.
The flight crew decided to continue to ATH
, as would normally be the case.
However, on descent, another engine failed, and the aircraft was successfully landed with the two remaining engines apparently operating normally.
At the stand, just prior to normal engine shutdown, one of the two operating engines flamed out.
Upon investigation, severe fuel contamination was found...the fuel last uplift having occured in BAH
It is noteworthy to keep in mind that engines fail enroute for a variety of reasons, and the cause at the time might not be readily apparent.