7 ms·

The A340, A380, 727, 747, and L-1011 existed because of the hub-and-spoke model, and the requirement for a minimum of three engines for transoceanic flights. Th

by nickff 6y ago

The A340, A380, 727, 747, and L-1011 existed because of the hub-and-spoke model, and the requirement for a minimum of three engines for transoceanic flights. The 777 was the harbinger of their doom, as it brought ETOPS with it. Somewhat tangentially, there is a (strong) argument that twin-engine aircraft are actually safer than three or four engine aircraft. First, they are at lower risk of having an engine problem (as they have fewer engines). Second, the requirements all focus on having sufficient performance with a single engine out, which means that a twin engine aircraft normally has at least 200% of the power required to keep it aloft, whereas the four-engine aircraft only has 133% of what it needs. Given that many problems can be solved with a sufficient application of power, the twin-engine airplane is actually at an advantage! https://en.wikipedia.org/wiki/ETOPS https://en.wikipedia.org/wiki/ETOPS

jrockway 6y ago
What is interesting to me is that having 200% power onboard is more cost efficient than only having 133% power onboard.
- nickff 6y ago
  Lots of reasons for this, here are a couple: Having extra power helps you climb to cruising altitude quickly. This is a significant consideration, as a great deal of fuel expenditure takes place during the climb through dense, drag-inducing atmosphere. Engines and aircraft structure are non linear in both up-front cost, maintenance, and economy. Two larger engines have about half as many parts as four smaller ones, impacting production costs and service requirements. Larger fans are also more efficient than smaller ones for a variety of reasons. Fuel systems and aircraft structures are also simpler, lighter, and less expensive with fewer engines.
mannykannot 6y ago
Where do you get that 133% figure from? you appear to be suggesting that a four-engined aircraft could only tolerate the loss of a single engine, but if that were the case, four-engined aircraft would always have been more vulnerable to loss on account of an engine failure over the ocean than twin-engined ones. The introduction of additional requirements (in terms of stringent maintenance, ancilliary equipment redundancy and proximity of alternative destinations) before ETOPS was permitted strongly suggests that was not the case.
- 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.
        6y ago
dingaling 6y ago
> The 777 was the harbinger of their doom, as it brought ETOPS with it. The A300 brought ETOPS, quickly followed by the 767.
- nickff 6y ago
  My understanding is that the A300 brought the so-called 'early ETOPS', A.K.A. ETOPS-90 and ETOPS-120; 777 brought ETOPS-180.
supernova87a 6y ago
Well, a related item is that airlines simply got tired of maintaining 4 engines (I'm putting it somewhat jokingly but not too far from the truth). For the not much better fuel efficiency and capacity, the complexity of the 747 4 engines are that much more of a headache to maintain properly.
kd913 6y ago
Looking at Maximum Take off-weight it appears that a 747 has almost double the MTOW of a 767. https://en.wikipedia.org/wiki/List_of_airliners_by_maximum_takeoff_weight https://en.wikipedia.org/wiki/List_of_airliners_by_maximum_t... I can imagine this being the case mainly because the calculation is assuming 1 engine failed. Given this, and assuming an almost maximum payload, would there necessarily be scenarios here for the 747 to be more fuel efficient?

by nickff 6y ago

jrockway 6y ago

What is interesting to me is that having 200% power onboard is more cost efficient than only having 133% power onboard.

nickff 6y ago
Lots of reasons for this, here are a couple: Having extra power helps you climb to cruising altitude quickly. This is a significant consideration, as a great deal of fuel expenditure takes place during the climb through dense, drag-inducing atmosphere. Engines and aircraft structure are non linear in both up-front cost, maintenance, and economy. Two larger engines have about half as many parts as four smaller ones, impacting production costs and service requirements. Larger fans are also more efficient than smaller ones for a variety of reasons. Fuel systems and aircraft structures are also simpler, lighter, and less expensive with fewer engines.

mannykannot 6y ago

Where do you get that 133% figure from? you appear to be suggesting that a four-engined aircraft could only tolerate the loss of a single engine, but if that were the case, four-engined aircraft would always have been more vulnerable to loss on account of an engine failure over the ocean than twin-engined ones. The introduction of additional requirements (in terms of stringent maintenance, ancilliary equipment redundancy and proximity of alternative destinations) before ETOPS was permitted strongly suggests that was not the case.

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.
      - 6y ago

dingaling 6y ago

> The 777 was the harbinger of their doom, as it brought ETOPS with it. The A300 brought ETOPS, quickly followed by the 767.

nickff 6y ago
My understanding is that the A300 brought the so-called 'early ETOPS', A.K.A. ETOPS-90 and ETOPS-120; 777 brought ETOPS-180.

supernova87a 6y ago

Well, a related item is that airlines simply got tired of maintaining 4 engines (I'm putting it somewhat jokingly but not too far from the truth). For the not much better fuel efficiency and capacity, the complexity of the 747 4 engines are that much more of a headache to maintain properly.

kd913 6y ago

Looking at Maximum Take off-weight it appears that a 747 has almost double the MTOW of a 767. https://en.wikipedia.org/wiki/List_of_airliners_by_maximum_takeoff_weight https://en.wikipedia.org/wiki/List_of_airliners_by_maximum_t... I can imagine this being the case mainly because the calculation is assuming 1 engine failed. Given this, and assuming an almost maximum payload, would there necessarily be scenarios here for the 747 to be more fuel efficient?