Flying Altitude
 

Hello all,

Not sure if I've posted these questions to the right forum.

Can anyone please educate me why all the flights need to stay at either 30000ft or 33000ft? Would it fly faster at that altitude or for security reason?

Also, why does it require to open up all the windows when landing?

Thank you!!

I'll move your post to TravelBuzz for answers.

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Craig6z
Buzz & United Moderator
[email protected]

It is my understanding that flying at higher altitudes conserves fuel.
I'm not sure why the window shades have to be up. It can't be that important, because I've been on plenty of flights where they haven't even bothered telling people to do it.

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"I just wanna wish you good luck, we're all counting on you"
-Dr. Rumack

"Why do I have to open my window shade for landing? And why are the cabin lights dimmed?

You are asked to raise your shade so you can see through the window. Not for the view, but to help you remain oriented (which way is up, etc.) if there's an accident. Further, it lets you see what hazards exist outside (fires, debris and such), which would be important during an evacuation. Additionally it lets light into the cabin and makes it easier for rescuers to see inside.

Dimming the lights helps your eyes adjust to darkness, so if anything happens and it goes dark, you're not suddenly blind while dashing for the exits. Also it makes the emergency path/exit lights more visible. These might be the only lights you see in an emergency. And as with the shades, it allows you to see outside for orientation. With the cabin lights burning brightly, the glare would make this impossible."

http://archive.salon.com/tech/col/sm...15/index1.html

30,000ft is not a usable altitude in most airspace.

Flights flying up to 29,000ft are separated by 1000ft with even altitudes being Westbound flights and odd altitudes being Eastbound. Above 29,000ft the spacing shifts to 2000ft increments so the next useable altitude above 29,000ft is 31,000ft which is Westbound altitude.

There is a new system called Reduced Vertical Separation Minimum (RVSM) which is in effect in certain oceanic airspace and will eventually be phased in for domestic airspace. RVSM uses 1000' increments both above and below 29,000ft. In order for an aircraft to operate in RVSM airspace it must have upgraded RVSM equipment (altimeters, pitot/static system, etc.) and the airline must be authorized for RVSM operations. It is the conversion of the fleet to RVSM specs which is slowing the transition to RVSM in the domestic airspace.

Once you've determined the legal altitudes for your flight you find the optimum altitude for your flight.

Jet engines are more efficient at colder temperatures so, in general, you want to go as high as possible until you reach the point where the temperature stops dropping with increased altitude (tropopause) which typically occurs at roughly 36000ft (though that can vary considerably with season and latitude). Another factor is absolute temperature (vs relative temp) and aircraft weight. A very heavy airplane, in a warmer than normal atmosphere, may not be able to reach the coldest altitude so it's optimal altitude may be lower.

After all of that the last big variable is winds aloft. The wind speed and direction can vary considerably with altitude so it may be more efficient to pick a less efficient (warmer) altitude in order to take advantage of more favorable (or avoid less favorable) winds.

Oh, one more... Turbulance. The more efficient altitudes may have a rougher ride so a less efficient altitude may be choosen for passenger comfort.

cblaisd, thanks! and also for the site.

LarryJ, thank you for the detail explanation! Obviously I remembered the figure wrong, as most of the time when I look at the flight path screen, the plane stays on 3x,000 ft. As I was wondering why it's 3x but not higher or lower. Now I can understand.

Another factor that I left out is length of the flight. On flights of less than an hour, it isn't uncommon for ATC to limit your maximum altitude due to traffic. I frequently fly a 45 minute flight (air time) in a DC9 and we are usually stopped in the mid to upper 20,000's regardless of what our requested altitude might be. There simply isn't time to get up and get down while avoiding traffic conflicts during the climb and descent.

On short flights the ideal altitude profile would be to climb until reaching the optimum altitude even if you never reach it. In that case you'd climb until reaching the point where you intercept a flight idle descent profile at which point you would transition from a climb to a power off (flight idle) descent. Rarely happens, though.

You will also often find, on longer flights, that the aircraft moves to a higher altitude during flight. The optimum altitude for fuel consumption is a function of aircraft weight. This goes down as fuel is burned, increasing the best altitude for flying. The ideal flight path is a very gradual climb, which ATC won't let them do, but one or two altitude changes aren't unusual if there's no other traffic in the way.

Hi,

On flights from the UK to US (East/West Coast) (usuallly with VS but occasionally with BA - they have sky maps that point out the altitude of the aircraft). We have usually started the cruise at 33,000ft before climbing during the flight to 35,37,39 and once 41,000 feet for the last part of the cruise before decending to land.

Regards
TBS

Well, all the flights don't fly at that altitude. Some fly higher, some lower. But that general range is pretty much the optimum operating altitude for jets to get a good combinates of true air speed and good fuel burn.

Other factors are traffic, weather, and wind.

Wind, especially, is an important point that nobody mentioned earlier. The jetstream in the northern hemisphere blows from the west. Eastbound planes want to be in it. Westbound planes want to fly above or below it, since the increase in fuel consumption from flying at a non-optimal altitude is smaller than the increase from flying a longer distance through the air. If you check schedules for longer east-west routes in both directions, you'll typically find that the eastbound flight is noticeably shorter.

Does the Concorde fly at 52,000' then to get out of the way of slower planes, or is it more efficient up there?

Nobody mentioned?

Concorde doesn't fly at 52,000', it cruise climbs reaching nearly 60,000', depending on weight, prior to starting it's descent. At lower altitudes it step-climbs until climbing above other traffic. At that point ATC gives them a block altitude allowing them to cruise-climb slowly throughout the flight staying at the most efficient altitude for it's current weight.

A supersonic airplane's speed is limited by skin temperature. Higher altitudes have thinner air which produces lower skin temperatures.

Efrem wrote:
As a rule, ATC doesn't care if they want a gradual climb. There is a minimum required climb rate per FARs, however. Each airline has their own thoughts on what they feel is most efficient. Some want to get up to crusing altitude as quickly as possible, where they figure they'll then save more fuel than they burned on the way up; others take the opposite approach. ATC works with them all to do what they want.

Commerical airliners also regularly use FL410 and sometimes FL430. Citations get up into FL510.

LarryJ wrote:
Right, there are a lot of routes that have capped altitudes : SFO/OAK-LAX and SFO/OAK-RNO to name a couple.

Omni for LarryJ: Was it you I was talking to about CPDLC? I have a bunch of new info from the NATCA convention. It's on track to be fully online domestically sooner than I thought - 2007. MIA is using it domestically now, but only for some serivces. If you're interested, I can send you info.



[This message has been edited by letiole (edited 10-25-2002).]