4 ms·

If the aircraft is required to fly at a certain performance level with one engine out, a 4 engine aircraft must have 4/3x100% of minimum required power when in

by nickff 6y ago

If the aircraft is required to fly at a certain performance level with one engine out, a 4 engine aircraft must have 4/3x100% of minimum required power when in nominal operating condition, a 2 engine aircraft must have 2/1x100%... I first heard this from a lecture by Henry Pohl (a truly brilliant engineer): https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-885j-aircraft-systems-engineering-fall-2005/video-lectures/lecture-9/ https://ocw.mit.edu/courses/aeronautics-and-astronautics/16-...

mannykannot 6y ago
I think you are missing the point here, or at least misapplying it. These are minimum figures, but if the four-engined airliners used in transoceanic flights were actually built to have no more reserve power (above the minimum necessary to keep flying) than the 133% you are quoting, they would have always have been less safe from engine failure than contemporary twin-engined airliners (which, in practice, always achieved the 200% reserve), as they, just like the twin-engined aircraft, could not have tolerated any more than a single engine failure - but then, ETOPS would either not have been necessary, or should also have been applied to four-engined airliners. ETOPS is primarily about the risk of two engines failing (and secondarily about range with one engine out.)
- nickff 6y ago
  Maybe the people at the FAA in the 1950s who wrote the so-called 60-minute rule missed your point, but I don't know what to tell you.
  - mannykannot 6y ago
    The 60-minute rule does not explain or justify your 133% claim with respect to four-engined aircraft - in fact, it is evidence against it: in practice, four-engined airliners, unlike twins, could tolerate more than a single engine failure.
    - nickff 6y ago
      I just looked at a few airliners, and the 2-engine aircraft all had significantly higher power to weight ratios than the 4-engine aircraft.
      - mannykannot 6y ago
        That does not justify your 133% figure, nor the explanation you gave for it. That explanation implied that four-engined airliners could only tolerate the loss of a single engine.
mr_toad 6y ago
A 747, once airborne, can climb on 3 engines. Fully loaded, with gear down. A jet requires a lot less power to cruise than to take off. Normally they’d operate at 80-90% power when cruising, but they can stay airborne with much less power. How much power it takes to stay airborne depends on many factors, such as weight, altitude, wind conditions and temperature, but 747s have been known to fly and land safely with just two engines.
7952 6y ago
The minimum power is what is needed to fly to the closest diversion airport. This could be at a lower altitude and reduced speed. It is substantially less than the power used in take off or go around. Airplanes work in a variety of conditions and do not need 100% to get home in an etops situation. The rule you talk about is more clearly applicable to rockets that run at 100% for most of the flight.

by nickff 6y ago

mannykannot 6y ago

I think you are missing the point here, or at least misapplying it. These are minimum figures, but if the four-engined airliners used in transoceanic flights were actually built to have no more reserve power (above the minimum necessary to keep flying) than the 133% you are quoting, they would have always have been less safe from engine failure than contemporary twin-engined airliners (which, in practice, always achieved the 200% reserve), as they, just like the twin-engined aircraft, could not have tolerated any more than a single engine failure - but then, ETOPS would either not have been necessary, or should also have been applied to four-engined airliners. ETOPS is primarily about the risk of two engines failing (and secondarily about range with one engine out.)

nickff 6y ago
Maybe the people at the FAA in the 1950s who wrote the so-called 60-minute rule missed your point, but I don't know what to tell you.
- mannykannot 6y ago
  The 60-minute rule does not explain or justify your 133% claim with respect to four-engined aircraft - in fact, it is evidence against it: in practice, four-engined airliners, unlike twins, could tolerate more than a single engine failure.
  - nickff 6y ago
    I just looked at a few airliners, and the 2-engine aircraft all had significantly higher power to weight ratios than the 4-engine aircraft.
    - mannykannot 6y ago
      That does not justify your 133% figure, nor the explanation you gave for it. That explanation implied that four-engined airliners could only tolerate the loss of a single engine.

mr_toad 6y ago

A 747, once airborne, can climb on 3 engines. Fully loaded, with gear down. A jet requires a lot less power to cruise than to take off. Normally they’d operate at 80-90% power when cruising, but they can stay airborne with much less power. How much power it takes to stay airborne depends on many factors, such as weight, altitude, wind conditions and temperature, but 747s have been known to fly and land safely with just two engines.

7952 6y ago

The minimum power is what is needed to fly to the closest diversion airport. This could be at a lower altitude and reduced speed. It is substantially less than the power used in take off or go around. Airplanes work in a variety of conditions and do not need 100% to get home in an etops situation. The rule you talk about is more clearly applicable to rockets that run at 100% for most of the flight.