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It seems that turbofans for civil airliners are improving fuel efficiency by building larger and larger fans that spin slower. For example, the RR Trent 800 introduced in 1993 had a diameter of 2.8m, while the RR Trent Ultrafan expected in 2025 may be 3.56m in diameter.

In the case where both engines fail in flight, I imagine that wider engines would create much more drag, reducing glide distance. Is this true, and if so, what do aircraft manufacturers and airlines do to mitigate it?

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    $\begingroup$ Trent 800 MTBF > 120 years! and Trent 1000 is 2 IFSD (in-flight shutdown) per million flight hours. $\endgroup$
    – mins
    Feb 12, 2021 at 15:56
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    $\begingroup$ @mins, I was thinking of both engines failing because of a common cause, like volcanic ash, fuel exhaustion (Gimli glider scenario), or bird strikes (miracle-on-the-Hudson scenario). Perhaps those cases play an inordinately large role in the public imagination. $\endgroup$
    – AshleyZ
    Feb 13, 2021 at 22:26
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    $\begingroup$ The question is valid. Most cases of the last 40 years result from errors (lack of knowledge for ashes), except bird strikes, but for them considering the low height, the glide ratio difference is likely not very relevant. Still there are new surprising shutdown causes. $\endgroup$
    – mins
    Feb 14, 2021 at 1:51

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Both-engine-out glide performance isn't accounted for and isn't a certification requirement (of any airplane that I'm aware of). Airliners have good glide performance as a happy side effect of design optimization for cruising at high altitudes.

What does have to be accounted for is single engine performance issues; things like departure performance with an engine failure at V1, single engine ceilings, etc. Gliding ability comes into it in a secondary way, in terms of what is called "driftdown". Driftdown is when you are forced to descend from a both engines cruising altitude to the single engine ceiling following an engine failure or shutdown when your power is cut in half, and could be thought of as "semi-gliding" or "power assisted" gliding performance.

Driftdown speeds are speeds to use to get the flattest descent with maximum continuous thrust on the good engine, and you may use these speeds where terrain clearance means you need to stay at high as possible for as long as possible. Driftdown performance is measured during certification for the purpose of providing data for the pilots in the QRH, but there is no minimum driftdown "flatness" or glide ratio requirement per se.

As far as designing around geared TFs, well, the effects are what they are, and since nobody makes feathering fan blades, you are stuck with whatever the drag penalty is. A windmilling GTF is going to have more windmilling drag than a direct drive TF. But a direct drive TF has more windmilling drag than a traditional turbojet. So what did the manufacturers do when TFs came out?

They made allowance for the differences, and maybe an airliner would need somewhat more thrust from a given engine in TF form, all else being equal, to accommodate the higher windmilling drag of a failed engine on the other side, compared to the same airplane with pure turbojets. The drag penalties are not as bad as windmilling propellers, and don't necessitate radical solutions like feathering fan blades.

Geared TFs just take the same problem a little farther, but still get away without needing things like feathering fans (which would have killed the entire GTF concept) by making sure the thrust reserve is there to deal with a windmilling fan. But if a GTF equipped airplane ends up with a both-engines-out glide ratio of 12:1 instead or 14:1 because of the drag of its super-size fans, that's neither here nor there.

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    $\begingroup$ Maybe one more aspect: A failed GTF engine also causes some loss of lift on the wing and a yawing moment (due to asymmetric thrust and drag), which the tailplane must be able to counter, plus additional trim drag. In new aircraft designs, there's a balance between engine size, the distance between engines and fuselage and VTP size. The associated compromises are part of the technological "cost" of using GTF engines but usually much smaller than the efficiency benefit. In re-engined aircraft, it mostly just limits engine size. It also pushes research on improved nacelle designs :) $\endgroup$
    – Zak
    Jun 7, 2023 at 10:17
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    $\begingroup$ Also: note that the Rolls-Royce Ultrafan does include a variable-pitch rotor! It probably won't be able to be "feathered" in the same way a propeller can, but some pitch settings will create less windmill drag, and I'm sure there will be some mechanism to pitch it to "minimum windmill drag" whenever needed. That should lessen the impact of windmill scenarios quite a bit, too (although the "stuck rotor" scenario after a shaft or bearing failure might still remain...) $\endgroup$
    – Zak
    Jun 7, 2023 at 10:21

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