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Comment by masklinn | Hacker News Reader

Yes, it sounds like the repetition of a mangled version of the SR71 stories. Burning 45 tonnes of fuel on the runway would be completely insane.

Checking various links on taxiing burn yields about 2 tonnes which is a lot more realistic and reasonable (a previous HN comment indicates the 767 burns about a tonne taxiing: https://news.ycombinator.com/item?id=24283386 concorde burning twice that sounds fair)

The OP might have gotten confused reading articles like https://simpleflying.com/concorde-fuel-consumption/ stating concorde burned half its tank from the gate to cruise (mach 2 at FL600)

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> ”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).

> 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.

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.

Your intuition is wrong.

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

Fair enough. But delaying engine start-up until the aircraft has nearly finished taxiing wouldn’t have any effect on the way the engine operates during start-up. It just means it would spend less time at idle or near-idle speeds during taxi.

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.

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.

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.

> ”engine hours are weighted by operating power”

Interesting - I didn’t know this!

> 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.

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.

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.

Bigger issue is that the engines need to be idled for a while anyway to get up to proper temps, etc. you don’t want to start the engines and jam them into full takeoff thrust 5 seconds later.

True, the engines need to be warmed up and the hydraulics need to be pressurised, but given e.g. airbus recommends single engine taxi without APU (SETWA) warming up the engines probably doesn't take that long in the grand scheme of things. Definitely not the 15~25mn of taxi. From the sources I can find, "normal" warmup takes 2~5mn depending how long ago the engine was shut down, unless outside temps are exceptionally low, and you can do that while reaching the end of your taxi.

> but given e.g. airbus recommends single engine taxi without APU

This is wrong, unless you have a source for it

The software in modern engines wouldn’t let you do that anyway. The engine startup process can be quite long - several minutes in a 737 MAX - while the engine’s ECU brings things to proper temperatures etc.

But with e-taxi, the startup cycle could be performed while taxiing, potentially saving airlines time on pushback as well as fuel/maintenance cost savings.

Because jet engines are notoriously simple devices to begin with.

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

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.

eTAXI is just one such solution

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

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