6 ms·

As others have said, final fuel reserves are typically at least half an hour, and you shouldn't really be cutting into them. What if their first approach into M

As others have said, final fuel reserves are typically at least half an hour, and you shouldn't really be cutting into them. What if their first approach into MAN had led to another go around? With a major storm heading north-easterly across the UK, the planning should have reasonably foreseen that an airport 56 miles east may also be unavailable, and should've further diverted prior to that point. They likely used the majority of their final fuel reserve on the secondary diversion from EDI to MAN, presumably having planned to land at their alternate (EDI) around the time they reached the final fuel reserve. Any CAA report into this, if there is one produced, is going to be interesting, because there's multiple people having made multiple decisions that led to this.

jacquesm 1y ago
Just reaching altitude again to make it to the first and later second alternate are mostly likely the biggest factors in the extra fuel consumption. That's very expensive.
- dTal 1y ago
  You get that energy back on descent, no?
  - bagels 1y ago
    Some of it, but much is lost to drag. They do have to limit speed at all times.
  - tamcap 1y ago
    Not really. While you have a large potential energy buildup at a higher altitude, you cannot "bank it" / "save it" on descent. There is no way to store it in batteries or convert it back into fuel. One of the challenges of aeronautics is the efficient disposition of the potential energy without converting it all into kinetic energy (ie speed) so that the landing happens at an optimally low speed - thus giving you a chance to brake and slow down at the end.
    - Reason077 1y ago
      > "While you have a large potential energy buildup at a higher altitude, you cannot "bank it" / "save it" on descent. There is no way to store it in batteries or convert it back into fuel." An electric fan aircraft absolutely can recharge it's batteries on descent. The fans simply act as turbines, creating drag to slow the aircraft and electricity to charge the batteries. Large commercial airliners already have a small turbine that works this way, the Ram Air Turbine (RAT) which is used to generate electrical power in emergencies.
      - tamcap 1y ago
        You can use a turbine to generate electricity, so yes, you are converting potential energy into electrical potential. However, no real mass produced passenger plane today can use that electricity for flight (thrust). RAT is only used when sh*t hits the fan. Even then, it can help you power some hydraulics / electrical, not “store” energy for further flight. The OP asked - in a low fuel situation, can the energy spent on a climb get effectively recovered - and the answer is not really. We convert as much as we can into unpowered (low-powered) descent. But once you are at a spot where you make a final decision to land or not, you are by design low and slow - and all that energy you had 15m ago is gone. If you need to keep flying, those engines need to spool back up. And that takes fuel.
        rkomorn 1y ago
        > RAT is only used when sh*t hits the fan. Isn't it when air hits the fan, technically? (Sorry.)
        Reason077 1y ago
        > "no real mass produced passenger plane today can use that electricity for flight (thrust)" Such aircraft do exist. For example, the Pipistrel Velis Electro trainer. And more recently, the Rhyxeon RX4E became the first electric aircraft to be type-certified for commercial passenger operations. It's likely that we'll see many more electric fan aircraft in the coming years/decades, whether powered by batteries and/or hydrogen fuel cells, or hybrids with both conventional turbofan and electric propulsion in order to improve efficiency and environmental performance.
  - epcoa 1y ago
    How? On descent you can trade some of your altitude (potential energy) for kinetic energy, but then you can’t land the plane. For descent on an approach you’re going from low energy to even lower energy. In emergencies and with enough runway you can futz around with this some, but wiggle room on an airliner is not great, negligible to what will be expended on a go around.
  - recursive 1y ago
    As someone who has ridden a bike up a big hill, and then down it, I don't think you get it back.
    - krisoft 1y ago
      That is perplexing. Of course you get the potential energy back. It turns into kinetic energy as you descend. That is why you need not pedal downhill, and often even need to brake to prevent the bike from speeding up too dangerously.
      - anonymars 1y ago
        > often even need to brake to prevent the bike from speeding up too dangerously. Indeed, which is what the airplane would have done on its way down to land. So it's more like riding the brakes on your way down the hill, and now at the bottom when you realize you need to abort the landing, you are at low speed and it's quite an exercise to get back uphill to try again
        howard941 1y ago
        The glider guys would always suggest a forward slip. It's a lot of fun to do. It's not taught often enough during primary training for powered airplanes.
        anonymars 1y ago
        Yes, but that also doesn't get any energy back on descent, quite the opposite, that is "riding the brakes on your way down"
        howard941 1y ago
        Well it's not all lost otherwise it'd be a stall spin accident caused by performing the maneuver with too little airspeed. And that's hard to do. It's a noisy maneuver, the air slamming against the fuselage makes itself heard. Once performed it's not easily forgotten. More dangerous than inadvertently spinning with too little airspeed is the possibility of shock cooling when relying on a forward slip for too much altitude loss. It really does need to be well-controlled.
  - robotresearcher 1y ago
    No, and you don't want it. You want to be on the ground and stopped. In the lowest energy state. It's not currently feasible to harvest it into fuel. It's (very very nearly) all lost to drag, on purpose.
  - MaxikCZ 1y ago
    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
  - mrguyorama 1y ago
    Some of it. The air density is an important part of efficiency at higher altitudes, so every moment spent under like FL320 is wasted fuel. So the entire climb "up", you are also wasting energy fighting the thick air. On the way back "down", that air again fights you, even though you are basically at idle thrust. Your fuel reserves are calculated for cruise flight, so time spent doing low altitude flying is already at a disadvantage. "Two hours of reserves" is significantly less than that spent holding at a few thousand feet. Fuel efficiency while climbing is yet again dramatically worse
  - csours 1y ago
    Wow this has a lot of replies! Yes, you get a lot of the energy back, BUT there is a huge problem! Large airliners incur a LOT of additional drag to slow down while landing. Some of that is entirely intentional, some is less intentional. It is highly preferred to deploy the landing gear before touching down. Failure to do so may lead to a hard landing and additional paperwork, so airlines do not allow the captain to exercise their own discretion. Extending the flaps maintains lift at lower speed, and higher flap settings allow even lower speed. The highest flap setting generally also deploys leading edge slats. If the wheels of the airliner touch down and detect the weight of the plane then spoilers kill the lift of the wings, air brakes fully deploy, as well as thrust reversers. All of these things add drag, which uses up all that energy you've been converting. The upshot is that each landing attempt uses a LOT of energy, and you have to use fuel to replenish that energy after every attempt. In other words, yes you get it back, but only for one landing attempt.
  - throwaway173738 1y ago
    The problem isn’t getting the energy back, it’s doing so more slowly than gravity. Planes are somewhat limited in their ability to glide.
- cpncrunch 1y ago
  The 30 min reserve is on top of the fuel needed to reach the alternate and do a landing there, so only the flight to the second alternate, plus the 2nd and 3rd landings at the initial destination would have cut into the reserve.
  - nostrademons 1y ago
    With 100mph winds I could easily see the 30 min reserve being eaten up by the flight from Edinburgh to Manchester. It's 178 miles! It takes a good 15-20 minutes to cross that distance when flying normally, add ascent & descent time and the landing pattern and you're easily at 24 minutes. Edit: in other comments here, it seems like Edinburgh to Manchester is a 45 minute flight. So yeah, they could easily have been outside of reserves when they did the go-around at Edinburgh and still had only 6 minutes left at Manchester.
    - cpncrunch 1y ago
      Yeah, although it depends what the alternate was in the flight plan. It may have been Manchester. Although I think its more likely it was Edinburgh, which in the circumstances was too optimistic. Too much concern about the minimal costs of fuel tankering to add a bit more gas? Or saving time by not refuelling? Ive never flown on Ryanair and dont intend to.
      - jacquesm 1y ago
        The flight plans I've seen accounted for two alternates, not one, a significant time in a holding pattern and up to three go-arounds. This was for cargo 747s and a while ago so chances are the regulations have changed by now, also, it may have been due to the kind of cargo.
        cpncrunch 1y ago
        From what I can tell, that only seems to apply to EASA since 2022. As it took off from an EU airport and landed in the UK, I don't know if that rule would apply.
MBCook 1y ago
I thought a lot of airlines had rules to limit the number of attempts you could make at a single airfield in an attempt to prevent this exact kind of situation. It sounds to me like they tried harder at their intended destination than maybe they should have, followed by going to an alternate airport that probably wasn’t a good choice in the first place, and then having to divert to the final airport where luckily they could land in time.
heelix 1y ago
Suspect they were IFR. All your points stand. First time flying things with a jet engine, I was shocked how much more fuel gets burned at low altitude. It almost always works out better to max climb to altitude and descend than to fly low and level. On a small jet, things can get spicy fast when ATC route you around at 5000' for 15 minutes or so. Three aborted landings would gobble gas like crazy. § 91.167 Fuel requirements for flight in IFR conditions. (a) No person may operate a civil aircraft in IFR conditions unless it carries enough fuel (considering weather reports and forecasts and weather conditions) to— (1) Complete the flight to the first airport of intended landing; (2) Except as provided in paragraph (b) of this section, fly from that airport to the alternate airport; and (3) Fly after that for 45 minutes at normal cruising speed
- mr_00ff00 1y ago
  Why does it burn fuel so fast?
  - TZubiri 1y ago
    My guess is higher air density means more wind resistance, which acts as negative forward acceleration.
    - alterom 1y ago
      Not just that. Jet engines are efficient at higher speeds because the exhaust of the jet engine is fast. If the plane is going fast as well, that exhaust is more or less stationary relative to the ground. The engine works to exchange the position of the plane with the position of the air in front of it. If the plane is going slow, it's accelerating the air backwards. That's where the work is going, making the engine less efficient. Think about it this way: if the jet airplane is tied to the ground, its engines are running at 0% efficiency, working hard to blow the air backwards. You wouldn't want to stand behind a jet engine when the plane is about to take off, when that's effectively the case. The same applies to propeller-driven planes, of course. But those can vary the prop speed as well as propeller pitch, having more control on how fast the air is being pushed backwards. This allows the engine to be efficient at a wider ranger of speeds, particularly, at the slower range. But the propeller has a limit of how fast it can push the air back. When the prop blades start reaching the speed of sound, weird shit starts happening [1]. So propeller-driven aircraft have a limit on speeds at which they can go efficiently. Jet engines (turbofans when it comes to airliners) trade off low efficiency at low speed / low altitude (where the airplane is spending a small percentage of flight time) for higher efficiency at high speed / high altitude. Variable pitch turbine fans[2] aim to address this tradeoff, but the tech has yet to catch on. [1] https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech [2] https://en.wikipedia.org/wiki/Variable_pitch_fan https://en.wikipedia.org/wiki/Variable_pitch_fan
tstrimple 1y ago
> As others have said, final fuel reserves are typically at least half an hour, and you shouldn't really be cutting into them. This is one of the multiple layers of defense that airlines employ. In theory, no one single failure should cause a major incident because of redundancies and planning. Airlines rely on the "Swiss-cheese" model of safety. Each layer has its own risks and "holes" but by layering enough layers together there should be no clear path between all of the layers. In theory this prevents major incidents and given the commercial airline's safety records I'd say it works pretty fucking well. Landing with minutes of fuel left should be exceptional. But it also shouldn't be fatal or a major risk due to the other layers of the system. ATC will move heaven and earth to land a plane low on fuel or with engine trouble safely. And everyone else in the system having 30+ minutes of extra fuel gives the space for this sort of emergency sorting. I think this also reflects on the "efficiency" that MBA types bring to companies that they ruin. If an MBA sees a dozen landings with an extra hour of fuel, their mind starts churning at saving money. Surely an hour of extra fuel is too much and just wasted. Wasted because every extra gallon of fuel you take off with is extra weight you have to carry throughout the flight. Surely things would be more efficient if we could make sure planes only carry enough fuel to make their trip with very minimal overhead. And when everything goes perfectly according to plan, these decisions work out fine. Money is saved. Bonuses are paid. But the inevitable always happens. That's why it's called inevitable. Lives are lost. Wrists are slapped. Some people at the bottom lose their jobs. The world moves on.
- ashanoko 1y ago
  [dead]
jstummbillig 1y ago
Interesting. To me it does not really make sense to think in terms of fuel left because, no matter the reserves, there can always be a situation so unlikely, so outside the ordinary, that it will drain all fuel reserves before you make it to the planned destination. I have no clue how else to think about it though.
- lwhi 1y ago
  So maybe the thing we can improve is an understanding of likelihood? I.e. prevent the journey from occurring if weather conditions are likely to be adverse above a certain threshold?

jacquesm 1y ago

Just reaching altitude again to make it to the first and later second alternate are mostly likely the biggest factors in the extra fuel consumption. That's very expensive.

dTal 1y ago
You get that energy back on descent, no?
- bagels 1y ago
  Some of it, but much is lost to drag. They do have to limit speed at all times.
- tamcap 1y ago
  Not really. While you have a large potential energy buildup at a higher altitude, you cannot "bank it" / "save it" on descent. There is no way to store it in batteries or convert it back into fuel. One of the challenges of aeronautics is the efficient disposition of the potential energy without converting it all into kinetic energy (ie speed) so that the landing happens at an optimally low speed - thus giving you a chance to brake and slow down at the end.
  - Reason077 1y ago
    > "While you have a large potential energy buildup at a higher altitude, you cannot "bank it" / "save it" on descent. There is no way to store it in batteries or convert it back into fuel." An electric fan aircraft absolutely can recharge it's batteries on descent. The fans simply act as turbines, creating drag to slow the aircraft and electricity to charge the batteries. Large commercial airliners already have a small turbine that works this way, the Ram Air Turbine (RAT) which is used to generate electrical power in emergencies.
    - tamcap 1y ago
      You can use a turbine to generate electricity, so yes, you are converting potential energy into electrical potential. However, no real mass produced passenger plane today can use that electricity for flight (thrust). RAT is only used when sh*t hits the fan. Even then, it can help you power some hydraulics / electrical, not “store” energy for further flight. The OP asked - in a low fuel situation, can the energy spent on a climb get effectively recovered - and the answer is not really. We convert as much as we can into unpowered (low-powered) descent. But once you are at a spot where you make a final decision to land or not, you are by design low and slow - and all that energy you had 15m ago is gone. If you need to keep flying, those engines need to spool back up. And that takes fuel.
      - rkomorn 1y ago
        > RAT is only used when sh*t hits the fan. Isn't it when air hits the fan, technically? (Sorry.)
      - Reason077 1y ago
        > "no real mass produced passenger plane today can use that electricity for flight (thrust)" Such aircraft do exist. For example, the Pipistrel Velis Electro trainer. And more recently, the Rhyxeon RX4E became the first electric aircraft to be type-certified for commercial passenger operations. It's likely that we'll see many more electric fan aircraft in the coming years/decades, whether powered by batteries and/or hydrogen fuel cells, or hybrids with both conventional turbofan and electric propulsion in order to improve efficiency and environmental performance.
- epcoa 1y ago
  How? On descent you can trade some of your altitude (potential energy) for kinetic energy, but then you can’t land the plane. For descent on an approach you’re going from low energy to even lower energy. In emergencies and with enough runway you can futz around with this some, but wiggle room on an airliner is not great, negligible to what will be expended on a go around.
- recursive 1y ago
  As someone who has ridden a bike up a big hill, and then down it, I don't think you get it back.
  - krisoft 1y ago
    That is perplexing. Of course you get the potential energy back. It turns into kinetic energy as you descend. That is why you need not pedal downhill, and often even need to brake to prevent the bike from speeding up too dangerously.
    - anonymars 1y ago
      > often even need to brake to prevent the bike from speeding up too dangerously. Indeed, which is what the airplane would have done on its way down to land. So it's more like riding the brakes on your way down the hill, and now at the bottom when you realize you need to abort the landing, you are at low speed and it's quite an exercise to get back uphill to try again
      - howard941 1y ago
        The glider guys would always suggest a forward slip. It's a lot of fun to do. It's not taught often enough during primary training for powered airplanes.
        anonymars 1y ago
        Yes, but that also doesn't get any energy back on descent, quite the opposite, that is "riding the brakes on your way down"
        howard941 1y ago
        Well it's not all lost otherwise it'd be a stall spin accident caused by performing the maneuver with too little airspeed. And that's hard to do. It's a noisy maneuver, the air slamming against the fuselage makes itself heard. Once performed it's not easily forgotten. More dangerous than inadvertently spinning with too little airspeed is the possibility of shock cooling when relying on a forward slip for too much altitude loss. It really does need to be well-controlled.
- robotresearcher 1y ago
  No, and you don't want it. You want to be on the ground and stopped. In the lowest energy state. It's not currently feasible to harvest it into fuel. It's (very very nearly) all lost to drag, on purpose.
- MaxikCZ 1y ago
  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
- mrguyorama 1y ago
  Some of it. The air density is an important part of efficiency at higher altitudes, so every moment spent under like FL320 is wasted fuel. So the entire climb "up", you are also wasting energy fighting the thick air. On the way back "down", that air again fights you, even though you are basically at idle thrust. Your fuel reserves are calculated for cruise flight, so time spent doing low altitude flying is already at a disadvantage. "Two hours of reserves" is significantly less than that spent holding at a few thousand feet. Fuel efficiency while climbing is yet again dramatically worse
- csours 1y ago
  Wow this has a lot of replies! Yes, you get a lot of the energy back, BUT there is a huge problem! Large airliners incur a LOT of additional drag to slow down while landing. Some of that is entirely intentional, some is less intentional. It is highly preferred to deploy the landing gear before touching down. Failure to do so may lead to a hard landing and additional paperwork, so airlines do not allow the captain to exercise their own discretion. Extending the flaps maintains lift at lower speed, and higher flap settings allow even lower speed. The highest flap setting generally also deploys leading edge slats. If the wheels of the airliner touch down and detect the weight of the plane then spoilers kill the lift of the wings, air brakes fully deploy, as well as thrust reversers. All of these things add drag, which uses up all that energy you've been converting. The upshot is that each landing attempt uses a LOT of energy, and you have to use fuel to replenish that energy after every attempt. In other words, yes you get it back, but only for one landing attempt.
- throwaway173738 1y ago
  The problem isn’t getting the energy back, it’s doing so more slowly than gravity. Planes are somewhat limited in their ability to glide.
cpncrunch 1y ago
The 30 min reserve is on top of the fuel needed to reach the alternate and do a landing there, so only the flight to the second alternate, plus the 2nd and 3rd landings at the initial destination would have cut into the reserve.
- nostrademons 1y ago
  With 100mph winds I could easily see the 30 min reserve being eaten up by the flight from Edinburgh to Manchester. It's 178 miles! It takes a good 15-20 minutes to cross that distance when flying normally, add ascent & descent time and the landing pattern and you're easily at 24 minutes. Edit: in other comments here, it seems like Edinburgh to Manchester is a 45 minute flight. So yeah, they could easily have been outside of reserves when they did the go-around at Edinburgh and still had only 6 minutes left at Manchester.
  - cpncrunch 1y ago
    Yeah, although it depends what the alternate was in the flight plan. It may have been Manchester. Although I think its more likely it was Edinburgh, which in the circumstances was too optimistic. Too much concern about the minimal costs of fuel tankering to add a bit more gas? Or saving time by not refuelling? Ive never flown on Ryanair and dont intend to.
    - jacquesm 1y ago
      The flight plans I've seen accounted for two alternates, not one, a significant time in a holding pattern and up to three go-arounds. This was for cargo 747s and a while ago so chances are the regulations have changed by now, also, it may have been due to the kind of cargo.
      - cpncrunch 1y ago
        From what I can tell, that only seems to apply to EASA since 2022. As it took off from an EU airport and landed in the UK, I don't know if that rule would apply.

MBCook 1y ago

I thought a lot of airlines had rules to limit the number of attempts you could make at a single airfield in an attempt to prevent this exact kind of situation. It sounds to me like they tried harder at their intended destination than maybe they should have, followed by going to an alternate airport that probably wasn’t a good choice in the first place, and then having to divert to the final airport where luckily they could land in time.

heelix 1y ago

Suspect they were IFR. All your points stand. First time flying things with a jet engine, I was shocked how much more fuel gets burned at low altitude. It almost always works out better to max climb to altitude and descend than to fly low and level. On a small jet, things can get spicy fast when ATC route you around at 5000' for 15 minutes or so. Three aborted landings would gobble gas like crazy. § 91.167 Fuel requirements for flight in IFR conditions. (a) No person may operate a civil aircraft in IFR conditions unless it carries enough fuel (considering weather reports and forecasts and weather conditions) to— (1) Complete the flight to the first airport of intended landing; (2) Except as provided in paragraph (b) of this section, fly from that airport to the alternate airport; and (3) Fly after that for 45 minutes at normal cruising speed

mr_00ff00 1y ago
Why does it burn fuel so fast?
- TZubiri 1y ago
  My guess is higher air density means more wind resistance, which acts as negative forward acceleration.
  - alterom 1y ago
    Not just that. Jet engines are efficient at higher speeds because the exhaust of the jet engine is fast. If the plane is going fast as well, that exhaust is more or less stationary relative to the ground. The engine works to exchange the position of the plane with the position of the air in front of it. If the plane is going slow, it's accelerating the air backwards. That's where the work is going, making the engine less efficient. Think about it this way: if the jet airplane is tied to the ground, its engines are running at 0% efficiency, working hard to blow the air backwards. You wouldn't want to stand behind a jet engine when the plane is about to take off, when that's effectively the case. The same applies to propeller-driven planes, of course. But those can vary the prop speed as well as propeller pitch, having more control on how fast the air is being pushed backwards. This allows the engine to be efficient at a wider ranger of speeds, particularly, at the slower range. But the propeller has a limit of how fast it can push the air back. When the prop blades start reaching the speed of sound, weird shit starts happening [1]. So propeller-driven aircraft have a limit on speeds at which they can go efficiently. Jet engines (turbofans when it comes to airliners) trade off low efficiency at low speed / low altitude (where the airplane is spending a small percentage of flight time) for higher efficiency at high speed / high altitude. Variable pitch turbine fans[2] aim to address this tradeoff, but the tech has yet to catch on. [1] https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech [2] https://en.wikipedia.org/wiki/Variable_pitch_fan https://en.wikipedia.org/wiki/Variable_pitch_fan

tstrimple 1y ago

> As others have said, final fuel reserves are typically at least half an hour, and you shouldn't really be cutting into them. This is one of the multiple layers of defense that airlines employ. In theory, no one single failure should cause a major incident because of redundancies and planning. Airlines rely on the "Swiss-cheese" model of safety. Each layer has its own risks and "holes" but by layering enough layers together there should be no clear path between all of the layers. In theory this prevents major incidents and given the commercial airline's safety records I'd say it works pretty fucking well. Landing with minutes of fuel left should be exceptional. But it also shouldn't be fatal or a major risk due to the other layers of the system. ATC will move heaven and earth to land a plane low on fuel or with engine trouble safely. And everyone else in the system having 30+ minutes of extra fuel gives the space for this sort of emergency sorting. I think this also reflects on the "efficiency" that MBA types bring to companies that they ruin. If an MBA sees a dozen landings with an extra hour of fuel, their mind starts churning at saving money. Surely an hour of extra fuel is too much and just wasted. Wasted because every extra gallon of fuel you take off with is extra weight you have to carry throughout the flight. Surely things would be more efficient if we could make sure planes only carry enough fuel to make their trip with very minimal overhead. And when everything goes perfectly according to plan, these decisions work out fine. Money is saved. Bonuses are paid. But the inevitable always happens. That's why it's called inevitable. Lives are lost. Wrists are slapped. Some people at the bottom lose their jobs. The world moves on.

ashanoko 1y ago
[dead]

jstummbillig 1y ago

Interesting. To me it does not really make sense to think in terms of fuel left because, no matter the reserves, there can always be a situation so unlikely, so outside the ordinary, that it will drain all fuel reserves before you make it to the planned destination. I have no clue how else to think about it though.

lwhi 1y ago
So maybe the thing we can improve is an understanding of likelihood? I.e. prevent the journey from occurring if weather conditions are likely to be adverse above a certain threshold?