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So what do regular aircraft do when they run out of fuel? They require a forced landing. Aircraft fly with a buffer of fuel though to avoid such situations, so

So what do regular aircraft do when they run out of fuel? They require a forced landing. Aircraft fly with a buffer of fuel though to avoid such situations, so why can't an electric aircraft fly with a similar buffer on it's range? ie. Always fly less than the current charged range of the aircraft.

luma 29d ago
Simply put, batteries are heavy.
- gblargg 29d ago
  Something like 1/30 the energy density of liquid fuel. Plus unlike fuel, a discharged battery weighs virtually the same as a charged one.
marcosdumay 29d ago
Regular aircraft fly with enough fuel to change into at least one other airport in an emergency (ideally two). That aircraft would need a very large battery to do that, and it's way more efficient to have some burnable fuel there that you'll never actually use.
MobiusHorizons 29d ago
Regular aircraft are required to have enough fuel to be able to divert to the alternate airfield in their flight plan plus all the margins for taxiing and waiting for a landing clearance. If a plane is low on fuel they can declare an emergency and get priority over other traffic, but that obviously is disruptive and should not happen on well planned flights unless something pretty significant happens after takeoff.
- phire 29d ago
  > and should not happen on well planned flights You make it sound like it is acceptable for a badly planned flight to have a fuel emergency. It is not. If there is a fuel emergency (or the plane lands with less than 30min of fuel) there will be an incident investigation that treats the situation as serious as if the plane had crashed. If the investigation discovers it was nothing more than bad planning, (at minimum) the planning procedures will be changed to ensure it never happens again.
  - bluGill 29d ago
    Badly planned flights should not happen. Pilots who don't carefully plan their flights end up dead quickly. Lack of fuel is one of the easier to handle things that can go wrong from bad flight planning.
    - throwaway2037 28d ago
      Can you give an example of a "badly planned flight"? And did local regulators agree with your opinion?
      - bluGill 28d ago
        My dad dreamed of being a pilot and when I was a kid he subscribed to the flying magazine (or something like that I don't remember that name.) every single issue had the last story being "I learned about flying from that" and nearly every single one was a case of bad planning, usually a case of flying into bad weather because of the bad planning.
      - phire 28d ago
        LaMia Flight 2933 - https://en.wikipedia.org/wiki/LaMia_Flight_2933 https://en.wikipedia.org/wiki/LaMia_Flight_2933 The pilots took exactly enough fuel to reach their destination. They didn't have enough fuel to divert to an alternative airport. They didn't even have the mandatory 30min of reserve fuel. They did not account for a minor holding pattern just before landing (ironically, because another plane had a fuel leak). The crew failed to declare a fuel emergency, and crashed 18km from the airport. They had only been in the hold for 10 min when their engines ran out.. they were very short of fuel, the pilots knew they were short. They should have diverted (or declared a fuel emergency) almost an hour earlier, but they didn't want anyone to know how close they were cutting it.
  - kccqzy 29d ago
    > there will be an incident investigation that treats the situation as serious as if the plane had crashed No the investigation isn’t as serious as if the plane had crashed. In July 2026 nine planes simultaneously had a fuel emergency in London. You bet it’s different from nine planes simultaneously crashing in London. It helps to understand different kinds of fuel emergency. Declaring a fuel emergency to get to a diversion airport is very very different from having 30 minutes of fuel left.
jmward01 29d ago
Lots of down votes on this but it is a legitimate question. The goal is likely to have the majority of flight off of batteries. That being said, the total weight of the emergency fuel plus generators is likely far less than the weight of the extra batteries they would haul around for the safety margin they need. So they end up lighter and more capable AND don't need to burn fuel. That being said, I wouldn't be surprised if they didn't start the generators during critical phases so they may end up using some fuel for a flight, but not much. Total guess on that though.
- phire 29d ago
  > That being said, I wouldn't be surprised if they didn't start the generators during critical phases I somewhat doubt that. Part of the advantage of the design is that the generators don't need to be sized as big enough to power take off, climb and a potential go-around on landing. They only need to be sized as big enough for cruising. So powering them up during critical phases wouldn't help with safety. If anything, normal operating procedures might actually require shutting them down during critical phases. What this does mean is that the batteries need to be reasonably full when it comes into land, possibly as high as 50%. And most go arounds will require immediately powering up the generators, so it probably needs to be fuelled for all but the shortest flights.
- labcomputer 29d ago
  > That being said, I wouldn't be surprised if they didn't start the generators during critical phases so they may end up using some fuel for a flight, but not much. I would guess the opposite. Many parts in a turbine engine are "lifetime limited" by number of engine starts. That is, you are required to tear down the engine and replace certain parts after a certain number of engine cycles. That makes the economics of the turbine hybrid radically different if you need to start it every time you land vs. only the rare cases where you need to dip into fuel reserves. For example, the PT6A (a common 500-1000hp turboprop) requires the turbine and compressor disks to be replaced every 16,000 cycles. That's about 5 years of commercial service at 4x round trips per day. But if you only start the engine once in every 10 flights, now those components (theoretically) last for 50 years of flying.
Toutouxc 29d ago
> Aircraft fly with a buffer of fuel though to avoid such situations, so why can't an electric aircraft fly with a similar buffer on it's range? Because the required buffer is HUGE and we’re at a point where electric planes barely have enough energy for the actual route. So they’re doing the sensible thing here, flying the actual route electrically and falling back to combustion if the plane needs to divert.
- deleted 29d ago
  [deleted]
- tonyarkles 29d ago
  To that point, the battery capacity they’ve got, rough envelope math, is maybe 10 minutes beyond reserve. They’d likely need at least double, probably triple battery capacity to fly useful routes all-electric with enough reserve stored in batteries. The turbines are a great move.
  - labcomputer 29d ago
    > They’d likely need at least double, probably triple battery capacity to fly useful routes all-electric with enough reserve stored in batteries. Yup. And to bring this point home: "Only" doubling the battery capacity likely eats well over half of the payload capacity, in terms of mass. So your 38 seat plane is now a 19 seat (or fewer) plane.

luma 29d ago

Simply put, batteries are heavy.

gblargg 29d ago
Something like 1/30 the energy density of liquid fuel. Plus unlike fuel, a discharged battery weighs virtually the same as a charged one.

marcosdumay 29d ago

Regular aircraft fly with enough fuel to change into at least one other airport in an emergency (ideally two). That aircraft would need a very large battery to do that, and it's way more efficient to have some burnable fuel there that you'll never actually use.

MobiusHorizons 29d ago

Regular aircraft are required to have enough fuel to be able to divert to the alternate airfield in their flight plan plus all the margins for taxiing and waiting for a landing clearance. If a plane is low on fuel they can declare an emergency and get priority over other traffic, but that obviously is disruptive and should not happen on well planned flights unless something pretty significant happens after takeoff.

phire 29d ago
> and should not happen on well planned flights You make it sound like it is acceptable for a badly planned flight to have a fuel emergency. It is not. If there is a fuel emergency (or the plane lands with less than 30min of fuel) there will be an incident investigation that treats the situation as serious as if the plane had crashed. If the investigation discovers it was nothing more than bad planning, (at minimum) the planning procedures will be changed to ensure it never happens again.
- bluGill 29d ago
  Badly planned flights should not happen. Pilots who don't carefully plan their flights end up dead quickly. Lack of fuel is one of the easier to handle things that can go wrong from bad flight planning.
  - throwaway2037 28d ago
    Can you give an example of a "badly planned flight"? And did local regulators agree with your opinion?
    - bluGill 28d ago
      My dad dreamed of being a pilot and when I was a kid he subscribed to the flying magazine (or something like that I don't remember that name.) every single issue had the last story being "I learned about flying from that" and nearly every single one was a case of bad planning, usually a case of flying into bad weather because of the bad planning.
    - phire 28d ago
      LaMia Flight 2933 - https://en.wikipedia.org/wiki/LaMia_Flight_2933 https://en.wikipedia.org/wiki/LaMia_Flight_2933 The pilots took exactly enough fuel to reach their destination. They didn't have enough fuel to divert to an alternative airport. They didn't even have the mandatory 30min of reserve fuel. They did not account for a minor holding pattern just before landing (ironically, because another plane had a fuel leak). The crew failed to declare a fuel emergency, and crashed 18km from the airport. They had only been in the hold for 10 min when their engines ran out.. they were very short of fuel, the pilots knew they were short. They should have diverted (or declared a fuel emergency) almost an hour earlier, but they didn't want anyone to know how close they were cutting it.
- kccqzy 29d ago
  > there will be an incident investigation that treats the situation as serious as if the plane had crashed No the investigation isn’t as serious as if the plane had crashed. In July 2026 nine planes simultaneously had a fuel emergency in London. You bet it’s different from nine planes simultaneously crashing in London. It helps to understand different kinds of fuel emergency. Declaring a fuel emergency to get to a diversion airport is very very different from having 30 minutes of fuel left.

jmward01 29d ago

Lots of down votes on this but it is a legitimate question. The goal is likely to have the majority of flight off of batteries. That being said, the total weight of the emergency fuel plus generators is likely far less than the weight of the extra batteries they would haul around for the safety margin they need. So they end up lighter and more capable AND don't need to burn fuel. That being said, I wouldn't be surprised if they didn't start the generators during critical phases so they may end up using some fuel for a flight, but not much. Total guess on that though.

phire 29d ago
> That being said, I wouldn't be surprised if they didn't start the generators during critical phases I somewhat doubt that. Part of the advantage of the design is that the generators don't need to be sized as big enough to power take off, climb and a potential go-around on landing. They only need to be sized as big enough for cruising. So powering them up during critical phases wouldn't help with safety. If anything, normal operating procedures might actually require shutting them down during critical phases. What this does mean is that the batteries need to be reasonably full when it comes into land, possibly as high as 50%. And most go arounds will require immediately powering up the generators, so it probably needs to be fuelled for all but the shortest flights.
labcomputer 29d ago
> That being said, I wouldn't be surprised if they didn't start the generators during critical phases so they may end up using some fuel for a flight, but not much. I would guess the opposite. Many parts in a turbine engine are "lifetime limited" by number of engine starts. That is, you are required to tear down the engine and replace certain parts after a certain number of engine cycles. That makes the economics of the turbine hybrid radically different if you need to start it every time you land vs. only the rare cases where you need to dip into fuel reserves. For example, the PT6A (a common 500-1000hp turboprop) requires the turbine and compressor disks to be replaced every 16,000 cycles. That's about 5 years of commercial service at 4x round trips per day. But if you only start the engine once in every 10 flights, now those components (theoretically) last for 50 years of flying.

Toutouxc 29d ago

> Aircraft fly with a buffer of fuel though to avoid such situations, so why can't an electric aircraft fly with a similar buffer on it's range? Because the required buffer is HUGE and we’re at a point where electric planes barely have enough energy for the actual route. So they’re doing the sensible thing here, flying the actual route electrically and falling back to combustion if the plane needs to divert.

deleted 29d ago
[deleted]
tonyarkles 29d ago
To that point, the battery capacity they’ve got, rough envelope math, is maybe 10 minutes beyond reserve. They’d likely need at least double, probably triple battery capacity to fly useful routes all-electric with enough reserve stored in batteries. The turbines are a great move.
- labcomputer 29d ago
  > They’d likely need at least double, probably triple battery capacity to fly useful routes all-electric with enough reserve stored in batteries. Yup. And to bring this point home: "Only" doubling the battery capacity likely eats well over half of the payload capacity, in terms of mass. So your 38 seat plane is now a 19 seat (or fewer) plane.