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4 replies and 3 are dismissing even the idea.. Yes, you get "some" back, and its not negligible amount. Typical modern airliner can descend on 15-20:1, giving

4 replies and 3 are dismissing even the idea.. Yes, you get "some" back, and its not negligible amount. Typical modern airliner can descend on 15-20:1, giving you over 150-200km (90-120mi) range from typical cruising altitude of 33 000 feet even with engines off. Most everyday descents are actually done by maintaining altitude as long as possible, and then iddling the engines fully for as long as clearance allows. (Ofc you then use engines as you geat nearer, because its safer to be a little low when stabilizing on approach, than a little high) Thanks to turbofans(edited from turboprops) better efficiency + less drag at higher altitude its actually more fuel economical to command full thrust and gain altitude quickly, than slower climb, or maintaining altitude (which goes against our intuition from cars, where if you wanna get far, you never give full throttle). But theres still some drag, so you dont get everything back, so you generally want to avoid murking in low altitudes as long as possible. Full thrust repeatedly at lowest altitudes (from failed go arounds) is the least economical part of flight, so you want to avoid those if possible. But its true that the altitude you gain is equivalent to "banking" the energy, just not all of it.

jacquesm 1y ago
(1) this was a jet, not a turboprop Edit: changed turbofan into turbprop, which is what I meant. (2) fuel burned stays burned, you don't 'get it back' (3) the altitude gained may have been adjusted to account for the low fuel situation (4) the winds are a major factor here, far larger than the fact that 'what goes up must come down', something that is already taken into account when computing the fuel reserve in the first place.
- FireBeyond 1y ago
  1 - a turbofan is a subset of jet engine, and there are no 738s running anything other than a turbofan. Actually, nothing in civil aviation that has a "jet engine" has used anything but a turbofan (or turboprop) since the early 70s with the exception of Concorde and some older business jets. (Turboprops are jet engines, too, to be precise, with the jet of exhaust gases powering the propeller.)
  - deleted 1y ago
    [deleted]
  - devilbunny 1y ago
    > Turboprops are jet engines They are certainly turbine engines, but I thought "jet" was reserved for those engines that propel the vehicle solely by their exhaust stream and bypass air. I am willing to be told I'm wrong, though.
    - mnw21cam 1y ago
      Turbofans are by your own definition jet engines. It's just that the bypass air is much larger.
      - devilbunny 1y ago
        I think you meant turboprop there, but the distinction I notice is that one has all propulsive airflow inside the nacelle, and one does not.
        mnw21cam 1y ago
        Agh. No, I meant turbofan, but I misread your post and actually completely agree with you - turboprobs are not jet engines.
        devilbunny 1y ago
        Ha! It happens. Enjoy your weekend.
- connorlu 1y ago
  The 737-800 uses CFM56-7 turbofan engines. [1] https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-7_series https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-...
- alwa 1y ago
  (1) The turbofan category of jet engine seems to inspire a lot of very pretty animated technical diagrams—here’s one set from a German manufacturer [0]. Now if only we could convince Bartozs Ciechanowski to take on such a subject… [1] (2) I know glider pilots who fly without any fuel at all, once aloft… sounds not unlike the 150-200km glide range that @MaxikCZ mentions at idle from cruising altitude. [0] https://aeroreport.de/en/good-to-know/how-does-a-turbofan-engine-work-the-structure-of-an-engine https://aeroreport.de/en/good-to-know/how-does-a-turbofan-en... [1] e.g. https://ciechanow.ski/airfoil/ https://ciechanow.ski/airfoil/
  - jacquesm 1y ago
    Yes, sorry, meant to write turboprop.
  - devilbunny 1y ago
    Aircraft that are designed as gliders are much lighter and thus have much longer glide range than aircraft that aren't. They're so lightweight that they can climb on thermals. A 737 is not going to be able to do that, but a regular glider can't fly at 400 knots.
    - MaxikCZ 1y ago
      > thus have much longer glide range Im gonna be a little pedantic, but the weight has surprisingly small effect on glide range, actually none of the weight affect the range directly, its all from secondary effects. The glide is given mainly by drag and lift (so body and wing geometry), correlated to certain speed. The weight isnt in the equation at all. What weight does, is increases the speed in which the aircraft achieves this maximum glide ratio, and in higher speed you have higher drag, which finally reduces the range. Thats why many modern gliders have water tanks in wings, to increase the weight of the glider, moving planes speed of best glide ratio higher, allowing for more efficiency at higher speeds. Its worth it if the atmospheric condition provide strong lifts. Pilot can then dump the water in flight to reduce the wing load, allowing them to land with less speed, or just keep in the air longer as thermals get weaker in the afternoon/evening (source, I used to be a glider pilot)
- db48x 1y ago
  No, you don’t magically get the fuel back. But you do get a lot of the _kinetic energy_ back, and that energy keeps you flying without having to burn yet more fuel. You burn a lot of fuel while climbing, but then hardly any at all while descending. And that descent might cover 100 miles across the ground.
  - deleted 1y ago
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- MaxikCZ 1y ago
  1) Yea, sorry, turbofan, not turboprop nor a jet. 2) It stays burned, but the energy is banked in potential energy of the aircraft, namely in a form of altitude. If you run out of fuel 5 feet above ground, you dont get to fly far. When you run out of fuel 35000 feet above ground, you can still choose where to land from multiple options. 3) huh? I dont get what you trying to say, but: Its always more economical to climb, and the faster the better. Ofc you cant climb too high when you intend to attempt to land in 5-10 mins, but nontheless, every feet gained is "banked", and the aircraft is more economical to run the higher you are. 4) I am not saying the winds arent a factor, and in no way I was arguing about how fuel reserves are calculated. My only claim is that: yes, by spending more fuel to gain altitude, you can then "glide" down almost for free later. Its not 1:1, because of constant losses like drag, but its being compensated by higher engine efficiency and less drag at altitude, that its always worth it to climb if you can. There was a flight that was low on fuel diverting to alternate between 2 islands. The pilot panicked and chose slower climb to intuitively save fuel. They had to ditch the plane in water because of it - if they initiated full climb, they would have made the jump.
- dotancohen 1y ago
  > fuel burned stays burned, you don't 'get it back' The it that they get back is not fuel, it's energy. Maintaining flight is energy management. They are getting the gravitational potential energy back, which is converted to velocity on descent, or bled off in drag by slowly losing altitude while maintaining airspeed.
  - jacquesm 1y ago
    So it seems. But because you want to land you then want to shed all that velocity. So you 'get it back' only to have to waste the bigger fraction of it. A go around is much like a mini take-off, you just miss the runway portion of it.
    - MaxikCZ 1y ago
      Nah. You want to land, but you are really not shedding most of your velocity until after touchdown. What you gain by burning fuel is energy, and you can either bank it into altitude, or velocity. You must shed both to land, but not so for go-around. There you shed almost all of your altitude, but you keep most of your velocity -> you still have a lot of energy left. That's why on go-around you spool your engines and start climbing basically right away, unlike typical takeoff, where after spooling up the engines you are still earth-bound until you build enough velocity. So you only ever really lost your "altitude" component of energy, not "velocity" one. You run your engines at TOGA (Take Off / Go Around = maximum thrust), thrust to gain mainly altitude, only increasing speed a little bit. Then on another approach attempt you use both the altitude and excess velocity bank again. In flight, ~all your energy losses go to drag. Doesn't matter if you bank it into speed or altitude, both is exchanged to be at minimums (0 altitude above ground, lowest safe landing speed) at touchdown. If you produce extra energy in your engines, it has to go to either speed or altitude, which you then pull out again, usually by maintaining speed while lowering altitude while having engines at idle.
deleted 1y ago
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jacquesm 1y ago

(1) this was a jet, not a turboprop Edit: changed turbofan into turbprop, which is what I meant. (2) fuel burned stays burned, you don't 'get it back' (3) the altitude gained may have been adjusted to account for the low fuel situation (4) the winds are a major factor here, far larger than the fact that 'what goes up must come down', something that is already taken into account when computing the fuel reserve in the first place.

FireBeyond 1y ago
1 - a turbofan is a subset of jet engine, and there are no 738s running anything other than a turbofan. Actually, nothing in civil aviation that has a "jet engine" has used anything but a turbofan (or turboprop) since the early 70s with the exception of Concorde and some older business jets. (Turboprops are jet engines, too, to be precise, with the jet of exhaust gases powering the propeller.)
- deleted 1y ago
  [deleted]
- devilbunny 1y ago
  > Turboprops are jet engines They are certainly turbine engines, but I thought "jet" was reserved for those engines that propel the vehicle solely by their exhaust stream and bypass air. I am willing to be told I'm wrong, though.
  - mnw21cam 1y ago
    Turbofans are by your own definition jet engines. It's just that the bypass air is much larger.
    - devilbunny 1y ago
      I think you meant turboprop there, but the distinction I notice is that one has all propulsive airflow inside the nacelle, and one does not.
      - mnw21cam 1y ago
        Agh. No, I meant turbofan, but I misread your post and actually completely agree with you - turboprobs are not jet engines.
        devilbunny 1y ago
        Ha! It happens. Enjoy your weekend.
connorlu 1y ago
The 737-800 uses CFM56-7 turbofan engines. [1] https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-7_series https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-...
alwa 1y ago
(1) The turbofan category of jet engine seems to inspire a lot of very pretty animated technical diagrams—here’s one set from a German manufacturer [0]. Now if only we could convince Bartozs Ciechanowski to take on such a subject… [1] (2) I know glider pilots who fly without any fuel at all, once aloft… sounds not unlike the 150-200km glide range that @MaxikCZ mentions at idle from cruising altitude. [0] https://aeroreport.de/en/good-to-know/how-does-a-turbofan-engine-work-the-structure-of-an-engine https://aeroreport.de/en/good-to-know/how-does-a-turbofan-en... [1] e.g. https://ciechanow.ski/airfoil/ https://ciechanow.ski/airfoil/
- jacquesm 1y ago
  Yes, sorry, meant to write turboprop.
- devilbunny 1y ago
  Aircraft that are designed as gliders are much lighter and thus have much longer glide range than aircraft that aren't. They're so lightweight that they can climb on thermals. A 737 is not going to be able to do that, but a regular glider can't fly at 400 knots.
  - MaxikCZ 1y ago
    > thus have much longer glide range Im gonna be a little pedantic, but the weight has surprisingly small effect on glide range, actually none of the weight affect the range directly, its all from secondary effects. The glide is given mainly by drag and lift (so body and wing geometry), correlated to certain speed. The weight isnt in the equation at all. What weight does, is increases the speed in which the aircraft achieves this maximum glide ratio, and in higher speed you have higher drag, which finally reduces the range. Thats why many modern gliders have water tanks in wings, to increase the weight of the glider, moving planes speed of best glide ratio higher, allowing for more efficiency at higher speeds. Its worth it if the atmospheric condition provide strong lifts. Pilot can then dump the water in flight to reduce the wing load, allowing them to land with less speed, or just keep in the air longer as thermals get weaker in the afternoon/evening (source, I used to be a glider pilot)
db48x 1y ago
No, you don’t magically get the fuel back. But you do get a lot of the _kinetic energy_ back, and that energy keeps you flying without having to burn yet more fuel. You burn a lot of fuel while climbing, but then hardly any at all while descending. And that descent might cover 100 miles across the ground.
- deleted 1y ago
  [deleted]
MaxikCZ 1y ago
1) Yea, sorry, turbofan, not turboprop nor a jet. 2) It stays burned, but the energy is banked in potential energy of the aircraft, namely in a form of altitude. If you run out of fuel 5 feet above ground, you dont get to fly far. When you run out of fuel 35000 feet above ground, you can still choose where to land from multiple options. 3) huh? I dont get what you trying to say, but: Its always more economical to climb, and the faster the better. Ofc you cant climb too high when you intend to attempt to land in 5-10 mins, but nontheless, every feet gained is "banked", and the aircraft is more economical to run the higher you are. 4) I am not saying the winds arent a factor, and in no way I was arguing about how fuel reserves are calculated. My only claim is that: yes, by spending more fuel to gain altitude, you can then "glide" down almost for free later. Its not 1:1, because of constant losses like drag, but its being compensated by higher engine efficiency and less drag at altitude, that its always worth it to climb if you can. There was a flight that was low on fuel diverting to alternate between 2 islands. The pilot panicked and chose slower climb to intuitively save fuel. They had to ditch the plane in water because of it - if they initiated full climb, they would have made the jump.
dotancohen 1y ago
> fuel burned stays burned, you don't 'get it back' The it that they get back is not fuel, it's energy. Maintaining flight is energy management. They are getting the gravitational potential energy back, which is converted to velocity on descent, or bled off in drag by slowly losing altitude while maintaining airspeed.
- jacquesm 1y ago
  So it seems. But because you want to land you then want to shed all that velocity. So you 'get it back' only to have to waste the bigger fraction of it. A go around is much like a mini take-off, you just miss the runway portion of it.
  - MaxikCZ 1y ago
    Nah. You want to land, but you are really not shedding most of your velocity until after touchdown. What you gain by burning fuel is energy, and you can either bank it into altitude, or velocity. You must shed both to land, but not so for go-around. There you shed almost all of your altitude, but you keep most of your velocity -> you still have a lot of energy left. That's why on go-around you spool your engines and start climbing basically right away, unlike typical takeoff, where after spooling up the engines you are still earth-bound until you build enough velocity. So you only ever really lost your "altitude" component of energy, not "velocity" one. You run your engines at TOGA (Take Off / Go Around = maximum thrust), thrust to gain mainly altitude, only increasing speed a little bit. Then on another approach attempt you use both the altitude and excess velocity bank again. In flight, ~all your energy losses go to drag. Doesn't matter if you bank it into speed or altitude, both is exchanged to be at minimums (0 altitude above ground, lowest safe landing speed) at touchdown. If you produce extra energy in your engines, it has to go to either speed or altitude, which you then pull out again, usually by maintaining speed while lowering altitude while having engines at idle.

deleted 1y ago

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