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If a 777 pilot wanted to fly further than with both engines running, how many more miles could the plane go with just one engine running, at high altitude?

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    $\begingroup$ I really like the proposed edits. While the OP 'really' wants to know about the lost airliner, questions like this are seemingly 'fishing for conspiracy theories.' $\endgroup$ – CGCampbell Jun 23 '14 at 0:01
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    $\begingroup$ While the answer on this question is correct that it wouldn't go farther, I'd be interested to hear the actual answer to the title question. That is, if it lost an engine and the flight crew didn't have to care about ETOPS limitations, how far could it fly? I'm assuming the answer is "a really long way," but I'm curious approximately what that distance is. $\endgroup$ – reirab Apr 16 '15 at 4:36
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    $\begingroup$ On one-engine, a multi-engine plane can always make it to the crash site. (or so the joke goes). $\endgroup$ – abelenky May 4 '15 at 22:39
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    $\begingroup$ @abelenky -- the joke only goes for CAR 3/FAR 23 aircraft -- a FAR 25 bird like the triple-seven can do V1 cuts all day without ever leaving the pilot wondering if they'll make it. $\endgroup$ – UnrecognizedFallingObject Jul 15 '15 at 4:32
  • $\begingroup$ How high of an altitude are we thinking here? $\endgroup$ – Sean Feb 23 at 5:51
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Flying with only one of two engines will not increase range, but reduce it. A gas turbine consumes fuel in proportion to the mass of air moving through it, so with increasing altitude consumption decreases proportional with air density. Thrust goes down with density as well, so thrust-specific fuel consumption is roughly constant (actually, the engine becomes more efficient with altitude due to the lower air temperature, see Carnot cycle for more on that).

Generally, a jet transport wants to fly as high as possible for maximum range. Its cruise altitude is limited mainly by the thrust available from all engines. If the pilot shuts one down, the aircraft needs to fly much lower until thrust has increased sufficiently to overcome drag. Since friction drag is also proportional to air density (and the square of flight speed), the aircraft has to fly much slower at that lower altitude, which cuts range since the fuel consumption of the single engine is now roughly what both consumed at altitude, but now the aircraft covers less distance in the same time. In addition, the pilot has to compensate asymmetric thrust with rudder deflection, which again increases drag.

Short answer: The aircraft will not go MORE miles, but FEWER.

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    $\begingroup$ Excellent answer thank you. So, I assume it is NOT possible to use both engines (777), get to cruise altitude and then shut one engine down and NOT lose altitude using just the one engine? The drag as you explain of the non running engine and deflection of the control parts does make sense. I was just thinking IF the running engine was powerful enough to not drop the airspeed or altitude, seems at altitude, a jet plane could go further? (Or does the math just not work that way with aerodynamics for any amount of single engine thrust?) $\endgroup$ – user2667 Jun 22 '14 at 16:22
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    $\begingroup$ In any case, cutting one engine will also cut thrust in half, and will slow the plane down. If an airliner has thrust reserves, it will (ATC permitting) climb higher to cruise more efficiently. The engines run close to maximum thrust during cruise, so there is no reserve to compensate for the switched-off engine. If there were, the designers would have made a grave mistake. $\endgroup$ – Peter Kämpf Jun 22 '14 at 17:01
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    $\begingroup$ @PeterKämpf What percentage of engine power do they actually use at cruise? I was under the impression that close to full power was only used during TO/GA. And just from personal observation, I know the engines are at least much louder on a 777 when it's climbing than when it's cruising (even if it's just climbing a couple of thousand feet to increase cruise altitude after burning some of the weight off.) $\endgroup$ – reirab Sep 19 '14 at 16:29
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    $\begingroup$ @reirab: The primary display usually shows N1, but my 96% figure would be for N2, because the wear happens in the HP turbine. N1 in cruise is typically 85% to 92%, depending on altitude, temperature and aircraft mass. $\endgroup$ – Peter Kämpf Sep 20 '14 at 23:04
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    $\begingroup$ Just as a practical example of reduced range with an engine out, We departed Santiago, Chile in a 747 freighter with enough fuel to reach Miami. During the climb out we lost an engine. We elected to continue rather than dump fuel and return. However, because we could not reach our intended cruising altitude, the increased fuel burn meant we only had enough fuel to reach Panama City, Panama. $\endgroup$ – Terry Jul 15 '15 at 4:55
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It depends on the airliner design. If one engine is inside the fuselage tail section and can close side intake scoops and streamline engine bypass cowling, then range will increase as drag is a lot lower with one engine out of the air steam, and the remaining outside engine will have a higher engine rpm, hence higher efficiency using less fuel for the needed thrust. Chances are the needed thrust will be available as engines spool down quite a lot at cruise (oversized to take off). However, the higher thrust will decrease the life of the single cruise engine. Of course, the single cruise engine could be sized to take advantage of this condition to maximize fuel efficiency at normal wear.

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    $\begingroup$ This seems to address the possibility of designing an aircraft to cruise on fewer engines, but the question is about a regular 2-engine aircraft cruising on one engine. $\endgroup$ – fooot Jul 14 '15 at 22:20

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