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Cruising altitudes!

Pro Member First Officer
Mohit (Mc_GaNgStA) First Officer

I've always had this question in my mind that why do aircrafts actually cruise very high when doing short-haul or long-haul destinations (both in real life and in flight simulator). What I mean to say is that from about 4'000 feet, or maybe a bit more or less, buildings (perhaps not mountains) don't become much of an obstacle for aeroplanes yet they cruise at 33'000 feet to about 40'000 feet, which is what I don't understand if they can be happily at about 10'000 feet or even lower. What is the actual reason for cruising very high and what are the actual advantages for staying at that altitude (as getting to those altitudes uses up a considerable amount of thrust and fuel)?

21 Responses

Pro Member Chief Captain
Solotwo Chief Captain

The air is thinner up there so you can go faster and use less fuel, plus the jet streams up there can be real helpful if you have a 100knot tail wind.

Pro Member Chief Captain
CRJCapt Chief Captain

The reason is that at high altitudes, in thinner air, jet engines work more efficiently by burning a lot less fuel. The ground speed of the aircraft is significantly higher than the airspeed the aircraft is measuring, we call what is being displayed to the pilot IAS (Indicated Airspeed) and the actual speed of the aircraft through the air TAS (True Airspeed).
For example at 30,000 feet at 275 KIAS, the actual speed of the aircraft is 440 knots, and at 40,000 it would be 495 knots. Jet aircraft also have a lower speed limit at lower altitudes due to the stress that denser air imposes on the airframe. 🙂

Pro Member First Officer
TimH First Officer

Thats a great explanation CRJCapt couldnt have put it better myself

Pro Member First Officer
Mohit (Mc_GaNgStA) First Officer

Ah ok. Thanks for explaining.

SidRST Guest

I would have thought the noise from a 747 tearing along at 500mph at just over 4000ft would play a significant factor too ! 😂

Pro Member First Officer
Sico2 First Officer

It is always the air pressure, at 41000ft it is very low, so it causes less air drag (is that how you say it?). It is like cutting through butter with warm knife. Pressure where we live 0-3000ft is very high and thick and aicraft needs more power to fight itself against and through the so called 'own wind' which appears while moving in envoirnment surrouding you without any wind. (with nose wind is even harder)

Try to move an object in vacuum. It will need very little force to move, do the same at sea level 0m. Results will vary. Less drag air and pressure, less force needed to move. Look at fuel consumption indicator in your aircraft let us say at N2 90% few seconds after starting and the same at FL350.It will be 5-6 times less or more. High altitude = best way to save fuel and money.

Pro Member First Officer
mossy First Officer

this is a brilliant explanation, thanks. it is something i had always wondered about. 🙂

Pro Member First Officer
Dan Young (dannyboy2005) First Officer

The air is thinner up there so you can go faster and use less fuel, plus the jet streams up there can be real helpful if you have a 100knot tail wind.

[Ahh he sighs] What if you get a 100kts head wind, the plane uses far more fuel and slows aircraft down. What happens then? How much would he decend by? (We're talking of a comercial jet crusing at 35,000ft)

Jamie4590 Guest

Where a pilot has less fuel than they calculated or are losing fuel what factors decide if they should asscend to use less fuel or descend to land at a closer airport?

Pro Member Captain
Bindolaf Captain

Ahh he sighs] What if you get a 100kts head wind, the plane uses far more fuel and slows aircraft down. What happens then? How much would he decend by? (We're talking of a comercial jet crusing at 35,000ft)

Actually that happens *all* the time on transatlantic flights. Usually winds blow one way (I think from the east, correct me). So flights going to the US do great time, while flights coming back have a nice 100 kts headwind =p They don't descend really (sometimes you're lucky to get ANY flight level to cross the pond, it can get really crowded), you got wind all the way.

I think they try to fly closest to the tropopause, but that's not always feasible.

Pro Member Chief Captain
CRJCapt Chief Captain

Jamie4590 wrote:

Where a pilot has less fuel than they calculated or are losing fuel what factors decide if they should ascend to use less fuel or descend to land at a closer airport?

In a situation of low fuel or fuel value lower than expected, the real question is range. If you climb, you will use more fuel(during the climb) and that will shorten your range. At the new altitude, consumption will be less, which increases your range but your ground speed will also be less(.80 Mach is slower at FL 340 than FL 370) and this will lower your range. It's possible to power back so that your getting the most distance from each pound of fuel, this would give you max range. The question has a few considerations. Winds aloft, altitude, power setting(airspeed) and of course the distance to the destination and/or alternate. Modern FMC/FMS equipped aircraft will continuously show the crew the estimated fuel upon arrival at destination and how many nautical miles per pound of fuel used(specific range). If this fuel level indicates an unacceptable value(less than 45 minutes of fuel), the crew can consult with Airline dispatch on changing one or more of the parameters above to land safely. 🙂

Guest Ed Guest

Bindolaf wrote:

Ahh he sighs] What if you get a 100kts head wind, the plane uses far more fuel and slows aircraft down. What happens then? How much would he decend by? (We're talking of a comercial jet crusing at 35,000ft)

Actually that happens *all* the time on transatlantic flights. Usually winds blow one way (I think from the east, correct me).

Well, since you ask, I will. . . the jet stream always blows from west to east. So you have a tail wind from North America to Europe, and a headwind going the other way.

Ed

Pro Member First Officer
Mohit (Mc_GaNgStA) First Officer

What is the worst possible headwind a commercial aircraft can face at around 35'000 feet?

Pro Member Chief Captain
Manuel Agustin Clausse (Agus0404) Chief Captain

A jet stream.

Pro Member First Officer
Mohit (Mc_GaNgStA) First Officer

I'm sorry but I don't get what a jetstream is. Is it that what jets leave behind them at the cruising altitude, like that "smoke-like" line?

Pro Member Chief Captain
Jonathan (99jolegg) Chief Captain

No, they're strong, narrow streams of Westerly wind that occur in the Northern Hemisphere.

This will give you a lot of information on it:

http://en.wikipedia.org/wiki/Jet_stream

😉

Pro Member Chief Captain
Manuel Agustin Clausse (Agus0404) Chief Captain

Is it that what jets leave behind them at the cruising altitude, like that "smoke-like" line?

That is called a contrail.

Jon, jet streams also occur in the Southern Hemisphere and some of them are Easterly jet streams.

Kuvem Guest

Hello, I have a question regarding this Topic.

The reason is that at high altitudes, in thinner air, jet engines work more efficiently by burning a lot less fuel

Is it the same case with Propelled Powered Planes?

My favourite Planes are the Douglas DC3 and the King Air 350. What is the best altitude for long range flights?

Thanks for any Tip

Guest

surley it would say in the kneeboard?

Kuvem Guest

I just took another look at the kneeboard, I dont see any Info about what is the best altitude for long distance flights.

Pro Member First Officer
ARD-DC First Officer

Kuvem wrote:

Is it the same case with Propelled Powered Planes?

Have a look at the Learning Center in FS, look for the article "Turbine Engines". It gives some superficial easy-to-understand information about Turboprop and Turbofan engines, along with some pro's and con's for each. I found it an interesting article when I first started reading about the different engine types.

Anonymous wrote:

surley it would say in the kneeboard?

I think you're thinking of Service Ceiling, which could/should be listed on the reference tab of the kneeboard. Haven't seen a reference page with suggested cruise levels though (at least not yet).

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