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0 pointsReason0772mo ago0 comments

> ”the 767 burns about a tonne taxiing”

This seems incredibly inefficient. Is there a future for hybrid aircraft, which would feature both traditional turbofans and large batteries for energy storage?

Batteries would eliminate the need for an APU and power the aircraft during taxi, allowing the engines to be started just before actual takeoff, and shut down immediately after landing.

Either the batteries could power wheel motors directly during taxi, or the aircraft could mix turbofans with e-fans (which could also allow energy recovery during descent and help power the aircraft during cruise, reducing fuel consumption further).

0 comments

tetromino_2mo ago

> This seems incredibly inefficient.

Very inefficient but good for safety: if an engine is failing, you hopefully might discover that while taxiing rather than when you are in the death zone 25 meters up in the air.

Reason077OP2mo ago

Not an expert, but intuition suggests this probably isn’t true.

If an engine is going to fail spontaneously it’s almost certainly going to happen at high thrust, not while at idle or very low thrust values during taxi.

philipwhiuk2mo ago

Your intuition is wrong.

Engines experience issues when changing speeds (especially start-up) not when at steady thrust output.

1 more reply

masklinn2mo ago

Electric taxiing (on APU) has been in development for over a decade, but it's mostly intended for single aisles (the shorter the flight the more the taxi overhead), and the relatively low fuel prices has led to these projects mostly dying off: L3 shuttered their effort in 2013, Honeywell and Safran's EGTS joint venture was dissolved in 2016, and wheeltug... apparently still lives (with no support from either boeing or airbus), though it was initially supposed to enter service in 2018.

Reason077OP2mo ago

Fuel saving would be only one of the benefits.

Airlines would also significantly reduce engine operating hours, reducing engine wear and thus maintenance costs. I’ve been on flights out of Heathrow that seem to spend almost as much time taxiing as they do in the air (due to weather or ATC delays or whatever), so for short-haul operations this seems really significant.

Local air quality is also a concern for airports: the air in the neighbourhoods around Heathrow often stinks of jet exhaust, sometimes you can smell it from miles away. Presumably, much of those emissions come from taxiing aircraft.

labcomputer2mo ago

The limiting factor for most turbine engines isn’t really operating hours, but “cycles”, which is to say starts and stops. From a maintenance perspective it’s not terribly important whether you start the engine at the gate or the runway.

Also, as far as maintenance goes, engine hours are weighted by operating power. So, an hour at idle doesn’t count as much as an hour at cruise power. One of the reasons airlines started using not-full power on takeoff when conditions allow it is because of “power by the hour” maintenance contracts, which incentivize that.

1 more reply

kamarg2mo ago

> This seems incredibly inefficient. Is there a future for hybrid aircraft, which would feature both traditional turbofans and large batteries for energy storage?

I would assume the extra weight would make it not really worth the added cost and complexity.

elmomle2mo ago

Honestly it sounds like the "right" way to do it would be electric ground vehicles pulling the planes into position, as with tugboats in water. Plane never need carry batteries into the sky and saves a literal ton of fuel.

masklinn2mo ago

IIRC towing to and from the runway has two major issues:

- standard towing tractors are really slow when towing, nowhere near taxiing speed, so you need a fleet of heavier duty "fast tow", possibly dedicated (depending on price)

- more traffic around the runway, which creates more airport complexity

Taxibot does exist tho, and is certified, and used in a few airports. Though I think it's only hybrid not electric.

1 more reply

fragmede2mo ago

Because jet engines are notoriously simple devices to begin with.

toast02mo ago

Airport tugs might be a better fit to improve ground operations efficiency?

tim3332mo ago

I think they've looked at that kind of thing but not found if practical so far. One innovation has been airbus jets taxiing with just one engine which cuts fuel use a lot as it mostly goes to just spinning the engines.

jelder2mo ago

eTAXI is just one such solution

https://www.safran-group.com/videos/e-taxi-safran-unveils-it...

j / k navigate · click thread line to collapse

0 comments

tetromino_2mo ago

> This seems incredibly inefficient.

Very inefficient but good for safety: if an engine is failing, you hopefully might discover that while taxiing rather than when you are in the death zone 25 meters up in the air.

Reason077OP2mo ago

Not an expert, but intuition suggests this probably isn’t true.

If an engine is going to fail spontaneously it’s almost certainly going to happen at high thrust, not while at idle or very low thrust values during taxi.

philipwhiuk2mo ago

Your intuition is wrong.

Engines experience issues when changing speeds (especially start-up) not when at steady thrust output.

1 more reply

masklinn2mo ago

Reason077OP2mo ago

Fuel saving would be only one of the benefits.

labcomputer2mo ago

1 more reply

kamarg2mo ago

> This seems incredibly inefficient. Is there a future for hybrid aircraft, which would feature both traditional turbofans and large batteries for energy storage?

I would assume the extra weight would make it not really worth the added cost and complexity.

elmomle2mo ago

masklinn2mo ago

IIRC towing to and from the runway has two major issues:

- standard towing tractors are really slow when towing, nowhere near taxiing speed, so you need a fleet of heavier duty "fast tow", possibly dedicated (depending on price)

- more traffic around the runway, which creates more airport complexity

Taxibot does exist tho, and is certified, and used in a few airports. Though I think it's only hybrid not electric.

1 more reply

fragmede2mo ago

Because jet engines are notoriously simple devices to begin with.

toast02mo ago

Airport tugs might be a better fit to improve ground operations efficiency?

tim3332mo ago

jelder2mo ago

eTAXI is just one such solution

https://www.safran-group.com/videos/e-taxi-safran-unveils-it...

j / k navigate · click thread line to collapse