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Comment by jetrink | Hacker News Reader

> As for CO2, the plane flying slower is more fuel-efficient.

It's counter-intuitive, but an aircraft can be more efficient at higher airspeeds. The reason for this is that, unlike a ground vehicle, an aircraft in level flight must expend energy generating lift to balance its weight. This loss of energy shows up as lift-induced drag [1]. As airspeed increases, the losses to lift-induced drag decrease, while the losses to aerodynamic drag increase. Where the point of minimum total drag lies depends on several factors including weight and altitude. See [2], which contains a chart for a generic aircraft which is 5% less efficient at Mach 0.72 than it is at Mach 0.80.

1. https://en.wikipedia.org/wiki/Lift-induced_drag

2. http://atmseminar.org/seminarContent/seminar11/papers/481-Je...

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Sure, but nowhere do I see any indication that the airplanes are purposely being flown below their maximally efficient speed? Most of my flights are arriving early these days (I guess because of the padded schedules). They're certainly not lingering in the air any longer than they have to, because lingering in the air costs money that they don't want to unnecessarily burn.

As another commenter pointed out, even if the gate isn't ready yet, there's always room to wait somewhere on the taxiway (this has happened to me a few times recently). They'd rather wait on the ground with the engines idling than circling around in the air with the engines running at speed. Plus, it's not like gate availability is affected by schedule padding anyway; there's only so many gates at an airport, and they always are trying to turn over planes as fast as possible.

It’s likely that part of the reason you’re arriving earlier (especially if flying west to east) is due to higher jet stream velocities in the winter. This increases the ground speed of the plane even if the optimal indicated airborne airspeed is being maintained.

A planes lifespan is measured in flight hours. As soon as you're on the ground, it isn't really depreciating much.

Sure, aircraft can be more efficient at higher speeds, but then when speeds pass a certain threshold, drag starts to dominate and efficiency drops like a rock. There's a sweet spot, and it's certainly less than the plane's top speed.

...this is exactly what the commenter you’re replying to said, except also with nice charts/graphs showing the sweet-spot for various aircraft...

Except the comment was making a point that was irrelevant for the domain in consideration (typical aircraft speeds for commercial flight).

It's like if someone said, "going a little slower on the highway [typically ~60 mph+] saves on fuel" and someone pedantically ButAkshully'd that cars are more efficient at 40 mph than 30 mph because of engine performance.

Edit: Ironically, I'm the one normally promoting my "Scylla-Charybdis heuristic", that you should be able to identify when tweaking a parameter goes too far. But here I think the context of the tweak was clear enough that this caveat was a distraction from the core point.

That’s a theoretical aircraft, not an actual aircraft.

In practice they save fuel by flying slightly slower. But, their are many other considerations like weather that cause airlines to fly slightly outside of optimal altitudes and speeds.

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