Comment by gsnedders

My guess is higher air density means more wind resistance, which acts as negative forward acceleration.

With 100mph winds I could easily see the 30 min reserve being eaten up by the flight from Edinburgh to Manchester. It's 178 miles! It takes a good 15-20 minutes to cross that distance when flying normally, add ascent & descent time and the landing pattern and you're easily at 24 minutes.

Edit: in other comments here, it seems like Edinburgh to Manchester is a 45 minute flight. So yeah, they could easily have been outside of reserves when they did the go-around at Edinburgh and still had only 6 minutes left at Manchester.

4 replies and 3 are dismissing even the idea..

Yes, you get "some" back, and its not negligible amount. Typical modern airliner can descend on 15-20:1, giving you over 150-200km (90-120mi) range from typical cruising altitude of 33 000 feet even with engines off. Most everyday descents are actually done by maintaining altitude as long as possible, and then iddling the engines fully for as long as clearance allows. (Ofc you then use engines as you geat nearer, because its safer to be a little low when stabilizing on approach, than a little high)

Thanks to turbofans(edited from turboprops) better efficiency + less drag at higher altitude its actually more fuel economical to command full thrust and gain altitude quickly, than slower climb, or maintaining altitude (which goes against our intuition from cars, where if you wanna get far, you never give full throttle).

But theres still some drag, so you dont get everything back, so you generally want to avoid murking in low altitudes as long as possible. Full thrust repeatedly at lowest altitudes (from failed go arounds) is the least economical part of flight, so you want to avoid those if possible. But its true that the altitude you gain is equivalent to "banking" the energy, just not all of it.

Wow this has a lot of replies!

Yes, you get a lot of the energy back, BUT there is a huge problem!

Large airliners incur a LOT of additional drag to slow down while landing. Some of that is entirely intentional, some is less intentional.

It is highly preferred to deploy the landing gear before touching down. Failure to do so may lead to a hard landing and additional paperwork, so airlines do not allow the captain to exercise their own discretion.

Extending the flaps maintains lift at lower speed, and higher flap settings allow even lower speed. The highest flap setting generally also deploys leading edge slats.

If the wheels of the airliner touch down and detect the weight of the plane then spoilers kill the lift of the wings, air brakes fully deploy, as well as thrust reversers.

All of these things add drag, which uses up all that energy you've been converting.

The upshot is that each landing attempt uses a LOT of energy, and you have to use fuel to replenish that energy after every attempt.

In other words, yes you get it back, but only for one landing attempt.

As someone who has ridden a bike up a big hill, and then down it, I don't think you get it back.

No, and you don't want it. You want to be on the ground and stopped. In the lowest energy state.

It's not currently feasible to harvest it into fuel. It's (very very nearly) all lost to drag, on purpose.

How? On descent you can trade some of your altitude (potential energy) for kinetic energy, but then you can’t land the plane. For descent on an approach you’re going from low energy to even lower energy. In emergencies and with enough runway you can futz around with this some, but wiggle room on an airliner is not great, negligible to what will be expended on a go around.

Some of it. The air density is an important part of efficiency at higher altitudes, so every moment spent under like FL320 is wasted fuel.

So the entire climb "up", you are also wasting energy fighting the thick air. On the way back "down", that air again fights you, even though you are basically at idle thrust.

Your fuel reserves are calculated for cruise flight, so time spent doing low altitude flying is already at a disadvantage. "Two hours of reserves" is significantly less than that spent holding at a few thousand feet. Fuel efficiency while climbing is yet again dramatically worse

The problem isn’t getting the energy back, it’s doing so more slowly than gravity. Planes are somewhat limited in their ability to glide.

Some of it, but much is lost to drag. They do have to limit speed at all times.

Not really. While you have a large potential energy buildup at a higher altitude, you cannot "bank it" / "save it" on descent. There is no way to store it in batteries or convert it back into fuel.

One of the challenges of aeronautics is the efficient disposition of the potential energy without converting it all into kinetic energy (ie speed) so that the landing happens at an optimally low speed - thus giving you a chance to brake and slow down at the end.