Do planes fly higher to make up delayed time?
 

Was on an MU flight today (A340) and saw on the overhead monitor that we were at 12,000 meters. Seemed a little high to me as i usually see the monitors showing ~10,000 meters. We were behind schedule due to a delay, so i thought they were flying at a higher altitude to try catch up on lost time.
Is this possible?

It's not going to help much at all.

Jetliner speed is limited by the need to stay well clear of Mach 1--a subsonic airframe that busts it might very well go down. Even if you don't actually exceed it you have to be careful of nasty effects if the airflow over any part of the aircraft exceeds it. (And remember that wings are based on making the air flow faster over the top than over the bottom--that's what generates the lift that keeps the plane in the sky.) A U2 at cruising altitude flies so close to the edge that if it turns too fast the inner wing can enter a stall buffet at the same time the outer one enters a mach buffet.

Lately some airlines have flown a tad slower to save fuel.

In the northern hemisphere, going west to east, higher will be faster up until around the tropopause. So, yes, if the conditions are right you can make up time by flying higher due to a tailwind. Loren's comments are not valid, since aircraft at the same airspeed will go faster across the ground with more of a tailwind. Mach implications only apply to airspeed, not groundspeed.

what's a MU flight? MUC, from USA? or China Eastern? from somewhere

one can go higher/lower to take advantage of headwind/tailwind.

the speed of sound drops as altitude increases, and planes must fly slower at higher altitudes. this presumes they can fly near speed of sound at lower altitudes, also.

i do not know the exact numbers, but the window between mach1 and stall for say a 747 is very narrow at 35-40.000 feet.

I've certainly been on long-hauls where they've made up time by going higher and faster.

But then I've also been on long-hauls which are showing a speed exceeding that of the speed of sound due to the tail wind....it's not technically mach though.

This isn't a matter of flying higher, it's a matter of taking advantage of tailwinds--something planes do when they can anyway.

Isn't that saving negated by having to fly longer?

MU...China Eastern

Was just wondering what the reason was for flying at such high altitude. It was HKG-PVG, we were delayed, but flight was shorter than usual.

You fly as close as you can (considering weight, turbulance, icing and ATC limitations) to the most favorable winds. If wind doesn't change much with altitude then you'd want to fly at the altitude where the air is coldest.

No, when aircraft fly at a slower more efficient cruise speed, the flying equivalent of the MPG (Miles Per Gallon) increases, which is independent of time, so the total fuel burned per trip is reduced.

Wirelessly posted (Opera/9.51 Beta (Microsoft Windows; PPC; Opera Mobi/1718; U; en))

Air resistance curve is exponential at speeds closer to mach 1, so flying mach .83 vs mach .85 wil make a big difference in overall fuel consumption on a longhaul, but will only increase flying time very insignificantly.

They could also avoid bad weather/turbulence by flying higher.