Dear Captain Lim,
This is great site with very interesting topics. I enjoy reading it very much.
Well, Captain, maybe this is a silly question, but let*s start with this statement: No oxygen, no fire. Right?
If there is very little oxygen at 10 000 meters (35000 feet), can a turbine engine burn fuel with the same efficiency as it would during take-off or at lower altitudes?
And what limits a plane to fly higher than the normal cruising altitude?
I haven*t had the time to go through all the topics yet, maybe these questions have been asked before.
Kindly refer me to where this has been discussed on your site.
It is true that the amount of oxygen decreases (to support the combustion) in a jet engine as altitude increases. Yes, they are unlike rockets that carry their own source of oxygen instead of getting them from the air. However, there are other factors that make the jet engine more efficient at a higher cruising altitude when compared to the turbo-prop (propellers) engines.
At higher altitude, the temperature decreases. The colder the air temperature, the better it is for the jet engine as it convert fuel more efficiently at low outside air temperatures. Basically, the efficiency is directly proportional to the temperature differential. The greater that temperature differential is, the more efficient the engine.
Colder air is also good for jet engine operation. Colder mass means more air. Remember the principle of a gas turbine engine - it requires air to be sucked at the front, compressed, combusted then expanded and blasted out through the jet nozzle (a more simplistic explanation - suck, squeeze, blow and go!)
Further, when a plane climb higher, the air outside gets thinner and less dense. This has the effect of reducing the forward resistance.
So, overall, a gas turbine engine is more fuel efficient when operated at higher altitudes than nearer to the surface.
Now that I have explained why it is more efficient for a plane to fly at a higher cruising altitude, what then limits a plane to fly higher than this normal cruising altitude?
On a Boeing 777, the maximum service ceiling (altitude at which the maximum rate of climb is reduced to around 100 feet/min) or aircraft*s certification altitude is 43,100 feet. But, most of the time, the plane cruises at its optimum altitude of between 35,000 to 39,000 feet (this depends mainly on the weight and the environmental conditions.) If the flight were on a 12-hour journey with a full load where it would take off near to its maximum take off weight (approximately 286 tons), then it could only cruise initially at around 29,000 feet.
What other factors that limit the height? Cabin pressure differential is one. All airplanes are pressurized to maintain a comfortable living environment for human beings. Off course, if it were possible, a plane would be pressurized to ground level pressure but this is not practical, as the fuselage of a plane would have to be incredibly strong. Therefore, modern airliners are only pressurized to an altitude of around 8000 feet or a cabin pressure differential of 8.5 psi (pounds per square inch). Yes, at 43,000 feet actual cruising altitude on the Boeing 777, you are only experiencing an altitude of only 8000 feet in the passenger cabin!
Then there are the technical performance limitations - mostly the wing area and engine thrust available of the particular plane. On a Boeing 747-400, the maximum ceiling is 45,000 feet and on the Airbus A320, it is 39,800 feet. So, it varies with the different type of planes concerned. Generally, most airliners could fly at higher altitudes, but they are usually certified to an altitude that gives them a good safety margin.