Why do airliners fly at 30000-40000 ft?

Question

Do they go faster up there? why cant they fly from LA to Seattle at 7,000 ft? or from london to LA at 15000? I don't get it?

Answer 1

First of all, every aircraft has a prime altitude at which it gets optimal fuel efficiency.  For most jetliners that's around 36,000 ft.  Another reason is traffic.  No other planes except military aircraft can reach that altitude so there's less civilian traffic to deal with.  Also, planes aren't allowed to fly over 200 knots below 10,000ft.  Yet another reason, is noise reduction.

Answer 2

To utilize the jet stream,high speed winds blowing at that altitude.This gives the 'planes speed a boost,uses less fuel,is more economical and environmentally friendly as exhaust vapours are reduced.

Answer 3

biggest reason:

the air is less dense up there, which means less air resistance, they'll save gas. And also, yes they do go faster up there for the same reason and another: regulations do not allow aircraft to fly faster than 250 knots under 10,000 feet.

It's higher up, so less noise for the people on the ground

It higher up, so most of the weather is below the aircraft, so a smoother ride is in order for everyone aboard.

There are no mountains/geographical features as high as 30,000 feet+

At 7,000 feet (or even 15,000 feet), some mountains will be in the way and the aircraft will need to fly around or go higher in order to avoid them.

by the way, just so you know, there are aircraft flying at lower than 30,000 feet.

Short haul flights commercial or otherwise, civilian aircraft, etc, do cruise at 18,000 feet and up.

Aircraft flying under VFR fly even lower. (your example of 7,000 feet)

airliners generally do not exceed 41,000 feet because their cabins are pressurized for the comfort of the passengers and any higher, the pressure difference will cause too big a stress on the fuselage. (not to mention fuel inefficiency at altitude higher than what the aircraft/engines were designed for)

Answer 4

Simple.

1. Air is less dense - so less resistance in-turn better  fuel efficiency, better speed.

2. Higher terrain clearance.

3. Higher reception of radio signals and navigation signals.

4. Less noise pollution.

5. Avoid weather.

6. Efficient usage of jetsreams.

these are major reasons they fly higher.

Hope this helps.

Answer 5

It is highly fuel  efficient for a plane to fly at that altitude. Air is a lot thinner thousands of feet high, so the plane flies easier. Planes will never fly at very low altitudes because the air is denser. The airplane would also have to use a tremendous amount of fuel to make it travel 550 mph+ at low altitudes. Have you heard that rolling down your windows in a car wastes fuel. it is because the engine has to work more to fight more air resistance. Same idea for an airplane

Answer 6

Your out of the weather (most of the time anyway) at that altitude, fuel economy is better and you could catch the jet stream also less traffic up there

Answer 7

The thinner the air, the less fuel is required for the engines to produce power efficiently.  Also, at higher altitudes, you are above most weather, the ride is usually smoother, and there is more time to take action in case of an emergency.

Many medium and short range flights operate at lower altitudes.  25,000 to 28,000 feet is not at all unusual.

Have a nice flight!

Answer 8

Saves on Gas, less noisy, less congested, safety

Its funny you say LA to Seattle. 7000 feet would run you right into the side of Mt. Shasta, Mt. Hood, Mt. Adams, Mt. St. Helens, and Mt. Rainer, as well as half of the Cascade range.

Answer 9

That is the calculated BEST fuel efficiency.

Any lower and higher the the fuel economy goes down. Any lower the air is thicker and so it is harder to fight through the air. Any higher and the air is thinner and the wing can't LIFT the plane so the jet uses most of it power to LIFT instead of pushing (like the plane flying in 45 degree).

Good Luck...

Answer 10

The main reason is that the engines are more efficient at that altitude.  The speed has nothing to do with it, although at altitude the speed they can fly at is reduced. (to do with reduced air pressure and lower speed of sound)

Answer 11

Speed has nothing to do with it?? WRONG,   The faster you go, the less time your engines are running and the less time you put on the airframe, the sooner the a/c is ready to turn and go somewhere else. It costs thousands of dollars PER MINUTE to operate a 150 seat+ transport category a/c. There was an old joke about a  747 coming into O'Hare that was instructed to perform a left 360 turn for spacing, and the old Captain answered back "do you know that it costs my company $5,000 to perform that turn?" and the controller answered him, "no, but since you are that much closer in ... you can give me $10,000 to the left now"

Thinner air means faster speeds, and less fuel required to obtain it. As to being "over most of the weather" I assume we are talking clouds and turbulence because Tstorms routinely reach 65,000 in Texas, and can build in excess of FL 450 in an HOUR. If you have a wimpy thunderstorm topping out at FL 350 would you want to take YOUR passengers over the top of it?  They are going to be WEARING their sodas and coffee in the back. You can get over a lot of Summer time turbulence above 250 and the ride is much smoother as you get further away from the Earth and the surface heating that causes the problems.

Also, the law of diminishing returns,  the higher you go, the harder and longer it takes to get there. One major reason for going high as well is taking advantage of the jetstream this time of year. You can pick up as much as 250 kts of free push just by swimming downstream West to East (for the most part) ... so... the same thing applies when you turn around to head back to SFO, you are bucking a huge headwind then...  that is when you pull out your handy dispatch release that was printed out and handed to you in Ops... and it has your entire flight plan, route, altitude *everything* pre-planned for you based on all the previous information in the company data base. They are so accurate that they can predict your fuel burn of 60,000 lbs accurate to within 500 lbs. Your ETA to the minute,  all based on previous flights and their performance.  They are so accurate now, that recurring delays due to ATC and traffic are built in as well. Pilots don't fly these machines any longer, they program them.

  Taxi out, turn onto the runway push up and engage auto-throttles, rotate, retract the gear and autopilot ON at 1,200 feet. The Flight Management System (FMS) climbs, cruises, navigates, sets power, descends, joins the arrival, lines up the approach  the CREW gets to configure the airplane by lowering the gear, and setting the flaps, the auto pilot is never interupted while it shoots the approach, flares, lands, tracks the centerline of the runway and applies brakes.  THEN, the crew switches off and taxi to the gate.  Sure took the fun out of it eh?  (Oh.. the company doesn't want YOU flying the plane, because the computer is more accurate with the throttles and saves fuel)

Answer 12

They can move faster, less traffic, above the weather for most parts - The only speed limit barrier is the sound barrier which cannot be broken over US .  LA to Seattle can't be done by airliners because it will be terrible on fuel consumption plus they cannot fly faster than 250 knots, and 200 knots within busier airspace.  London- to LA - too much weather - rougher flight, weaker winds aloft, altitude is your friend, the higher you are the more options you have if an emergency occurs.

Answer 13

Air is much thinner at altitude, therefore the aircraft can move through the air more easily. Saves a lot of fuel.

Answer 14

The question is well answered above.  But not all airliners have time to fly that high.  Flights between Ottawa Ont. and Montreal fly at betwen 9,000 and 14,000 feet.  It's a short haul so there's no point in flying higher.  It's neat to fly low sometimes, you do see more!