3 ms·

Do current aircraft have a backup in-case their fuel tanks run dry?

ben-schaaf 1mo ago
There's reserve tanks or more generally reserve fuel for emergencies. Other than that there's no propulsion backup for when the fuel runs out, instead redundancy comes from multiple independent fuel tanks.
- SideburnsOfDoom 1mo ago
  > redundancy comes from multiple independent fuel tanks. And in larger aircraft, multiple independent engines. See for instance ETOPS https://en.wikipedia.org/wiki/ETOPS https://en.wikipedia.org/wiki/ETOPS From the pictures, I see that this new aircraft has 4 engines. So it should be adequately resilient to single engine failure.
dmitrygr 1mo ago
Current aircraft have * MULTIPLE HOURS* of fuel tanks. Compared to dozens of minutes of battery that this thing has.
- tonyarkles 1mo ago
  Typical reserve requirement is 30 min for a turbine aircraft (45 for piston). Given the 125 mile battery-only range and the probability that it flies at at least 200 kts (230 mph), it’s pretty unlikely to hit even a 30 min reserve on only batteries. I like the design, I think they get the problem and have a decent solution for it
  - Kaliboy 1mo ago
    The most common flight on my island is to the island about 50 miles away from us. We pay about $120 for the privilege, which takes 20 minutes of flying. So I think even in this case the reserve capacity is "needed" (for the reserve requierement), but I can see them realistically being able to fly between the islands purely on electric power.
  - dmitrygr 1mo ago
    > and have a decent solution for it Unless we ignore the additional maintenance cost of having to completely different proportion systems to maintain (and it is worse than a twin engine plane, because the propulsion systems are different, so you have to stock up on more different kinds of parts and possibly have different kinds of mechanics on staff to understand them)
    - tonyarkles 1mo ago
      Something that wasn’t super clear to me was whether the turbines are actually used for propulsion or if they’re used solely as generators to recharge the batteries/power the electric motors. I could honestly see it going either way. Having the turbine, electric motor, gearbox, and potentially clutches to isolate the turbine or electric motor from the transmission seems pretty viable though. This would waste a little bit of power keeping the turbine spinning (unless there are clutches) but would also ensure that the turbine’s been spun recently, the oil pumps are primed and the bearings are lubed and ready to go. Not that I’m saying a turboprop turbine like a PT6A is a simple device, but compared to a high-bypass turbofan like you’d see on a 737 or A320, they are considerably simpler. Edit: looking quick, the time-before-overhaul (TBO) on modern PT6As can be up to 6000 hours, with a ~2000hr Hot Section Inspection. I’m not sure how “spinning but not burning fuel” counts on TBO, but it should at least mean the HSI is simple.
      - labcomputer 1mo ago
        Yea, that wasn't super clear to me, either. But... given that turbine APUs already exist for ground power and are already engineered to be as light as practical, my naive guess would be that it's easier, simpler and faster to just use a COTS APU as a series hybrid rather than trying directly couple a turbine to the propeller. On inspections and overhauls: I'd expect that not needing to start the turbine at all on a given flight massively helps the economics of the airplane. Turbines often have parts that must be replaced after a given number of (startup) cycles, and that's going to be especially important for a turbine that is only expected to run for a short time (if at all) at the end of a flight. If you only start the turbine on, say, 5% of flights, that means you get 20x as many flights before overhaul/inspection. I could see that being incredibly attractive to airlines.
  - labcomputer 1mo ago
    > Typical reserve requirement is 30 min for a turbine aircraft (45 for piston). For private, yes. For airlines, the requirement boils down to basically enough fuel to "fly to the intended destination as scheduled, then loiter for XX minutes, then fly at normal speeds to a pre-designated alternate, land". For short flights, that means the fuel loaded at departure can be more than twice what is actually needed to get from A to B... Which is why the hybrid approach is fairly clever: You can use it for ~125 mile flights, and remain 100% electric on ~95% of flights. Whereas, without the reserve turbine the practical limit would be ~60 miles. On the ~5% of flights requiring more range, you're still mostly operating on electricity.
    - throwaway2037 29d ago
      > You can use it for ~125 mile flights, and remain 100% electric on ~95% of flights. The video says that in the United States about 1 in 1000 flights are diverted from their intended destination airport.
- deleted 1mo ago
  [deleted]
mmanfrin 1mo ago
Yes, they also have ramjet turbines which generate electricity from the movement of the plane on a backup circuit.
ZiiS 1mo ago
Yes the are strict regulations requiring them to carry extra fuel; despite its weight having a real cost. Carrying extra battery would be too heavy even compared to having a whole extra engine.

There's reserve tanks or more generally reserve fuel for emergencies. Other than that there's no propulsion backup for when the fuel runs out, instead redundancy comes from multiple independent fuel tanks.

SideburnsOfDoom 1mo ago
> redundancy comes from multiple independent fuel tanks. And in larger aircraft, multiple independent engines. See for instance ETOPS https://en.wikipedia.org/wiki/ETOPS https://en.wikipedia.org/wiki/ETOPS From the pictures, I see that this new aircraft has 4 engines. So it should be adequately resilient to single engine failure.

dmitrygr 1mo ago

Current aircraft have * MULTIPLE HOURS* of fuel tanks. Compared to dozens of minutes of battery that this thing has.

tonyarkles 1mo ago
Typical reserve requirement is 30 min for a turbine aircraft (45 for piston). Given the 125 mile battery-only range and the probability that it flies at at least 200 kts (230 mph), it’s pretty unlikely to hit even a 30 min reserve on only batteries. I like the design, I think they get the problem and have a decent solution for it
- Kaliboy 1mo ago
  The most common flight on my island is to the island about 50 miles away from us. We pay about $120 for the privilege, which takes 20 minutes of flying. So I think even in this case the reserve capacity is "needed" (for the reserve requierement), but I can see them realistically being able to fly between the islands purely on electric power.
- dmitrygr 1mo ago
  > and have a decent solution for it Unless we ignore the additional maintenance cost of having to completely different proportion systems to maintain (and it is worse than a twin engine plane, because the propulsion systems are different, so you have to stock up on more different kinds of parts and possibly have different kinds of mechanics on staff to understand them)
  - tonyarkles 1mo ago
    Something that wasn’t super clear to me was whether the turbines are actually used for propulsion or if they’re used solely as generators to recharge the batteries/power the electric motors. I could honestly see it going either way. Having the turbine, electric motor, gearbox, and potentially clutches to isolate the turbine or electric motor from the transmission seems pretty viable though. This would waste a little bit of power keeping the turbine spinning (unless there are clutches) but would also ensure that the turbine’s been spun recently, the oil pumps are primed and the bearings are lubed and ready to go. Not that I’m saying a turboprop turbine like a PT6A is a simple device, but compared to a high-bypass turbofan like you’d see on a 737 or A320, they are considerably simpler. Edit: looking quick, the time-before-overhaul (TBO) on modern PT6As can be up to 6000 hours, with a ~2000hr Hot Section Inspection. I’m not sure how “spinning but not burning fuel” counts on TBO, but it should at least mean the HSI is simple.
    - labcomputer 1mo ago
      Yea, that wasn't super clear to me, either. But... given that turbine APUs already exist for ground power and are already engineered to be as light as practical, my naive guess would be that it's easier, simpler and faster to just use a COTS APU as a series hybrid rather than trying directly couple a turbine to the propeller. On inspections and overhauls: I'd expect that not needing to start the turbine at all on a given flight massively helps the economics of the airplane. Turbines often have parts that must be replaced after a given number of (startup) cycles, and that's going to be especially important for a turbine that is only expected to run for a short time (if at all) at the end of a flight. If you only start the turbine on, say, 5% of flights, that means you get 20x as many flights before overhaul/inspection. I could see that being incredibly attractive to airlines.
- labcomputer 1mo ago
  > Typical reserve requirement is 30 min for a turbine aircraft (45 for piston). For private, yes. For airlines, the requirement boils down to basically enough fuel to "fly to the intended destination as scheduled, then loiter for XX minutes, then fly at normal speeds to a pre-designated alternate, land". For short flights, that means the fuel loaded at departure can be more than twice what is actually needed to get from A to B... Which is why the hybrid approach is fairly clever: You can use it for ~125 mile flights, and remain 100% electric on ~95% of flights. Whereas, without the reserve turbine the practical limit would be ~60 miles. On the ~5% of flights requiring more range, you're still mostly operating on electricity.
  - throwaway2037 29d ago
    > You can use it for ~125 mile flights, and remain 100% electric on ~95% of flights. The video says that in the United States about 1 in 1000 flights are diverted from their intended destination airport.
deleted 1mo ago
[deleted]

mmanfrin 1mo ago

Yes, they also have ramjet turbines which generate electricity from the movement of the plane on a backup circuit.

ZiiS 1mo ago

Yes the are strict regulations requiring them to carry extra fuel; despite its weight having a real cost. Carrying extra battery would be too heavy even compared to having a whole extra engine.