Dan wrote:
unless all airliners were to be electrified or hydrogenated by then
I somehow doubt this will happen soon…
maxbc wrote:
I do believe there is a warmup time but it’s a lot faster than piston engines because starting up the engine (which can take half a minute) already sets up the thermodynamic flow, close to thermal equilibrium.
Off the A320 normal procedures: Operate the engine at or near idle for at least 2 mins before advancing to high power. Taxi time at idle may be included in the warm up period.
So, as I wrote above there is practically no delay between last engine start, checklists done, PEDs briefed, line-up, and then advance the levers to the FLX/TOGA position.
maxbc wrote:
the savings may not be that great
Yep, the savings are not that great, proof being all these brilliant ideas being on the back burner. For now…
Should the price of oil one day reach its true value as a finite ressource, or stop being subsidised, the following increase by say tenfold or much more would probably lead to the resurgence of those ideas… unless all airliners were to be electrified or hydrogenated by then 
This would be most useful for airports where pushback to threshold is already really long (like LFPG). So this would definitely make sense as a provided infrastructure (where it’s needed) instead of an aircraft system.
I do believe there is a warmup time but it’s a lot faster than piston engines because starting up the engine (which can take half a minute) already sets up the thermodynamic flow, close to thermal equilibrium. It’s very much unlike a piston engine where mechanical parts (engine block, shaft, oil, gearbox) have to heat up by conduction or oil circulation, and idle power is so low (no continuous combustion, just a small explosion from time to time).
We are, however, no longer in the Concorde era which could easily burn 2t of fuel on taxi, so the savings may not be that great.
I remember back in 08 when oil was 140 dollars a barrel there was talk of this – Branson/Virgin were advocating for it but Boeing and Airbus both made noises about how they had not modelled the nose gear for trundling along at 30km/hour(being pulled) and the increased load and stress, if you were limited to the 5km per hour that a tug normally operates, the time to threshold would be painfully slow.
This was an idea I had years ago and is now practical would be 3 electrical skid carts to position themselves on all 3 gears and basically lift(you only have to lift the plane 3 cms) and drive the plane to the threshold – the wireless positioning is a doddle with the technologies we have nowadays.
Interesting that somebody patented it in 2006.
Actually Dan with the development of electric hubs such as some new electric cars, I think that would be the perfect solution. Very little in the way of infrastructure modifications and possibly even retrofit options could be developed. Maybe fit to the main gear rather than the nose wheel. (Possibly less CofG issues too.
Obviously rather than batteries, a larger output APU would possibly be required. (Safer than carrying lithium )
dublinpilot wrote:
You’d also need ground power available adjacent to the runway, and it might not be ideal to have personnel working so close to the threshold.
Not really since the APU will provide elec + air once undocked.
dublinpilot wrote:
warm up period? Or are you good for takeoff once you’ve started the engine?
Once the engine is stabilised, and all the flight checklists done, you can just go.
GA_Pete wrote:
Balance those savings against our tiny output of emissions.
The idea was not geared towards being green, but rather centred on economics. Yes, it could easily flip nowadays 
GA_Pete wrote:
Wouldn’t the crew be able to start the engines at the appropriate point while being towed?
Of course the crew could. Same is being done during every pushback, but on slippery surface.
IIRC towing trials were performed a few years ago, but the savings were too small to offset the whole concept. OTOH some airports, thinking about LGW, LHR, JFK, ORD, etc, easily offered 1/2h or more taxi to the threshold, and the standard 200kg taxi fuel (for the 320 series, more on bigger types) upped on those destinations. In latter years a few airlines, including mine, started with the introduction of one engine taxi (for twins…) prior to takeoff, and after landing. That came along with the use of lower flap settings for takeoff and landing.
Another idea that was studied and trialed was having an E-motor inside the nose wheels. Also this idea has been put on the backburner.
But who knows… should the price of finite oil reach its real cost one day might see a resurgence of those and other ideas..
It would probably cause delays waiting for aircraft to start up. But that’s probably manageable.
Wouldn’t the crew be able to start the engines at the appropriate point while being towed?
Wouldn’t take much to figure out where in the taxiway works best.
I’m imagining an unmanned tug releasing the aircraft and turning away somewhere near the final hold.
In a WWII RAF bomber pilot memoir, possibly No Moon Tonight by Don Charlwood or Pathfinder by Don Bennett, there was something similar. For range-critical flights the engines would be warmed up before shutting down and topping up the tanks again, then the aircraft towed to the runway by one of several dedicated tractors.
I did read somewhere that Total sells more fuel into 1 A380 at Heathrow than its entire UK Avgas market (or possibly UL91).
