why do jet airliners fly at 36000 ft?

Question

What is the reason why pilots fly as high as they can? Is flying at 20k ft good enough?

Answer 1

So two questions, why so high and why a nice round even number of thousands?

Why so high? Basically because colder thinner air allows the aircraft to be more efficient. But it's not quite as simple as that.

Up to about 28,000 feet the aircraft is limited by indicated air speed (Vne), so say you can do 350 kias at ground level, you can do that all the way up to 28,000 too. Above that the limit is by Mach number (Mmo), as you continue to climb your indicated air speed and your ground speed now decline as your Mach number remains constant.

However, even though you are slowing down you engines are burning less and less fuel, part of this is from the decreased IAS, but part is from the cold air. Colder air gives greater charge weight, it can be compressed more and the engines get greater thermodynamic efficiency.

Eventually you reach the lowest clean IAS for your current weight, and that's as high as you are going. It doesn't make any difference how much power you could add, you still can't climb because to do so you would need to slow down or break up, and to slow down you'd have to start deploying high lift devices which increase drag and reduce efficiency. This is what's called the coffin corner, your engines are probably running near max power, you are near or at max speed and you are just above the stall.

Typically the service ceiling is actually a function of rate of climb, so in a C5 your service ceiling is defined to have been reached when your rate of climb can't exceed 500 feet per minute at maximum continuous power.

Then there's the complications of weather and routing, odd altitudes are one direction and even the other.

So why the nice round even number? Separation.

First you don't want to meet someone coming the other way, so they want your altitude to be at least 1000 feet different from anyone else. They need to interleave east and west and give each direction a reasonable set of altitudes. So naturally altitudes for one direction are spaced 2,000 feet appart.

By convention, in the US, above 18,000 feet, Eastbound flights with courses from 0-179 are on odd altitudes and Westbound from 180-359 are on even altitudes. So at 36,000 feet youd be heading west, or westish, give or take 90 degreees or so.

Some jets can fly very high, this gives them tremendous freedom because they can always get a routing at the level they want. Learjets can often fly at around 50,000 feet and Concorde used to fly at 65,000 feet. High altitude jets can either run at high mach numbers or have very low stall speeds, which explains the SR71 and the U2.

Answer 2

This Site Might Help You.

RE:
why do jet airliners fly at 36000 ft?

Answer 3

It depends on the aircrafts weight, and the out sdie temperature etc...

Aircraft engines are more efficient at higher altitudes and burn less fuel. However climb limit is determined by the factoprs stated above. Once an aircraft can climb up to a higher altitude, the pilots will usually do so. This of course is limited to ATC restrictions etc etc...

Also the climb altitude is determined by the direction the aircraft is heading. In many places unless otherwise cleared, aircraft heading East bound fly at odd flight levels and Aircraft westbound fly at even flight levels. In other countries, aircraft heading North bound will fly at odd flight levels and southbound will be at even (It is one of the two conditions in any country, not both).

This is for Safety and ATC purposes.

Answer 4

36000 Feet To Miles

Answer 5

Mostly all of the routes that pilots fly inside the united states for commercial airliners are given via the FAA.The FAA determines who gets what routes. These routes are distributed throughout airlines flying inside,into and out of the united states.To make things alot easier for the airlines all routes are given the direction and hiegth to travel at,giving an invisible "highway in the sky" as the airliners are aproaching the destinations, most airports have beacons to lead airliners in.There are current modication to aircraft guidance systems that will revolutionalize airline travel.Its a technology that is changing as we talk. And as to the real question as to "why they fly that high?"..its because ...they can.

Answer 6

Being higher in the air can help the plane become a lot more fuel efficient. Since the air up their is so thin, the drag on the airplane is reduced, therefore making it a lot more fuel efficient. Flying at 20,000 feet is the stupidest thing one could do, because all the drag would make a 747 only able to do a Los Angeles-Chicago, when its range is about 6000 miles.

Answer 7

I personally have found this amazing discount store and bought one there. Not totally awesome regarding the shipping speed but did save money around 80% and delighted for this. The quality is actually superior and also right until now not find any problems.

Answer 8

Economics. Fuel flow is lower. If the ride's good, why go to another flight level? If you can pick up a decent tailwind, sure, go for it.

There are any number of reasons to pick that flight level, or leave it as well.

Answer 9

The air is thinner the higher you go. So less resistance againt the aircraft.

Engine efficency improves.

And you fly above most of the weather found in the Troposphere.

Answer 10

We fly as high as we can for the weight that we're at. The higher we fly the more fuel efficient. 41,000 ft is not uncommon for some of us.