Comment by loeg | Hacker News Reader

Comment by loeg | Hacker News Reader

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> For all the talk about fighting global warming, I almost never hear about this. It seems like the most inconvenient truth of all: if we want to fight global warming, we have to fly less.

I don't know what circles you hang out in, but I've heard this many times. The focus is on road vehicles and land power consumption because that's where the low hanging fruit is. And airlines do already have a profit motivation to decrease fuel consumption, which equates to reduced emissions. I'd like to see a CO2 tax to help align incentives.

Cleaner burning fuels might be an option at some point.

> But there is no electric airplane.

(I assume you're talking about commercial airplanes. Electric airplanes do exist, but have limited range and/or speed, and thus limited commercial application, due to reduced energy density.)

That could change, if batteries get a lot cheaper and lighter. Or you know, kerosene gets more expensive (government-enforced pricing of the externality of CO2 emission). Some of both.

It's obviously more viable for short-haul flights than long-haul, but even that would help.

Boeing had a choice between a more fuel efficient, durable and quieter airplane design, or a trans-sonic airframe design. The trans-sonic design lost, we got the 787.

Since then Boeing refreshed all their other planes (some might call it "stapled new wings and engines on") and called them -8 and -9 models.

But the problem is that the more efficient the tech is, the more of it people use. An extra 500 miles of range opens up decentralization of routes, which means more planes flying.

>That could change, if batteries get a lot cheaper and lighter.

The batteries could be free. Until they literally burn up so the plane can get lighter and therefore can land, it's pretty much a non-starter for anything over a few hundred miles. The max takeoff weight of a Boeing 787 is 50 tons more than it's maximum landing weight, which will probably damage the gear in some capacity.

> The max takeoff weight of a Boeing 787 is 50 tons more than it's maximum landing weight

I honestly don't know, but could this be part of the design simply because the engineers know the plane will weigh less on landing? Seems like that design would change if so much mass from fuel was no longer lost/burnt.

This, but the reason it's engineered that way is because landing gear face far higher loads on landing. During takeoff, landing gear essentially only have to support the weight of the aircraft. During landing, the gear have to arrest the aircraft's descent (which may be momentarily 3 or 4 g's of acceleration if the aircraft encounters turbulence while landing); and then they need to brake the entire weight of the plane within the remaining length of the runway, which is a high loading in a different direction (along the aircraft's longitudinal axis). So maximum landing weight is set by the landing gear, while maximum takeoff weight is usually set by the wings and engines.

I didn't realize that most of the braking on landing was handled by the landing gear. I'd always assumed that the obvious sound of the engines reversing when we landed did most of the work, but some casual googling suggests that it's only supplementary, to reduce the load on the landing gear.

https://www.pprune.org/archive/index.php/t-379610.html is interesting :-).

Also, https://aviation.stackexchange.com/questions/1219/for-large-... .

The stronger the gear needs to be, the heavier it becomes. Planes are optimized for safety first and foremost, but weight is a very close second. Heavier plane = more batteries = more weight = more batteries etc.

yes and no, because now it's a pain in the butt when there is a takeoff issue. If there is a tail strike, an engine issue on take off, or a passenger emergency early in the flight, now they have to loop around in circles for hours with a damaged airplane before landing. Or they have to dump fuel, but they do it extremely rarely, because most planes can't even do it, and it's a pain in the butt and actually quite slow for the planes that can do it.

here is an example: https://www.youtube.com/watch?v=VSq9r05lVTo

All aircraft landing gear are certified for landing at MTOW, they just need inspected afterwards before the plane can re-enter service. The fact that frequently the captain's judgement is to prefer to land at a lower weight is a separate issue.

and because fuel dump is only allowed above 4000ft/1300m.

It is way more complicated than just altitude, with zones and procedures, incredibly long checklists and a bit of maths. But a plane that can't reach 4000ft is probably in a mayday situation, and there is absolutely no time to try to dump fuel, it's a slow procedure.

Just to put illustrative numbers on it and not to dispute the parent comment in any way, the big 787's (787-10) have 251 Mg max take-off weight and 202 Mg max landing weight.

In comparison the A380 has a max landing weight of 386 Mg. Clearly the 787 is not up against a hard practical limit in terms of landing weight. Yes, stronger gear weighs more.

We agree that weight of batteries impacts their usefulness for aircraft. That's why I mentioned it in the quoted sentence :-).

You're arguing facts not in evidence.

The 787 can operate out of many airports the A380 cannot.

> You're arguing facts not in evidence.

I don't understand what that sentence means.

> The 787 can operate out of many airports the A380 cannot.

Yes? I said batteries would have to be a lot lighter and kerosene a lot more expensive before they are a practical replacement. I think we agree, but I think you think we disagree?

Managing a few hundred miles on electricity would in itself have a huge positive impact. Most of the busiest air routes in the world call under that heading.

be careful with airline motivation, they are looking into their profit margins, and one way to reduce fuel price is to increase the economy of scale, that means more passengers, more pollution, more fuel, but less fuel per passenger. That was the whole point of the A380, a giant gas guzzler, but per passenger it was more efficient, it only works if you destroy smaller planes or increase passenger traffic.

The second aspect of it is that economies of scale are on the last leg, most airline companies use smaller planes, with a hub and spoke model, and a highly energy inefficient system, because it's better in their overall profit making system.

They are there to make money first, carrying passengers is the hoops they have to go through to get people's money (but by cleverly worded passenger carrying contracts, they are more and more getting around the issue), and fuel price is a variable in the system.

Economies of scale is a perfectly viable strategy for legs that already have plenty of passengers. Busses are gas guzzlers compared to mopeds...

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