How can airplanes on short
Answered by: Anas Maaz,Airline pilot. Airbus A320/A321.
I will start with a real life example from a pilot's point of view. Two days back, I did a 55 minute flight in the Airbus A320. With the weights and the flight plan inserted, the flight management system calculated an optimum altitude of 38000 ft.
Because we felt that climb to an altitude of 38000 ft is too high for such short a flight, we decided to climb to an altitude of 36000 ft. You will get almost the same efficiency as at the optimum altitude as long as you are within plus or minus 2000 ft of the optimum. So, we were quite happy to climb to 36000 ft.
We took off as planned and climbed to 30000 ft as per the initial ATC clearance. After a while, we requested to climb to our planned 36000 ft. Unfortunately we got denied further climb and we had to continue our flight at 30000 ft. Our cruise was only about seven minutes, but because we cruised at 30000 ft, we burnt 300 kg of fuel more than we should have.
How much is 300 kilos, you say? Well, the A320 burns 2.4 tonnes every hour. So, 300 kilos is about eight minutes of fuel. With Jet-A fuel prices in our country, that costs about 400 dollars.
I think this example says a lot. Jet engines are very inefficient at low altitudes where the temperatures and air density is higher. The longer you cruise at lower levels, the more you will burn. With higher altitudes the compressibility effect also increases. This creates something called the ram effect which increases the pressure ratio through the compressor stages. This increases engine efficiency.
Higher altitudes also increase the True air speed (TAS) of the aircraft. This is particularly important to increase the range, or in layman terms, the fuel mileage of the aircraft. The higher the TAS at a given engine thrust and Indicated air speed (cockpit indicated speed), the better will be the range of the aircraft. It is also worthwhile to note, that the airplane calculates a best economical cruise speed based on pilot entered cost index, weight, present wind conditions, temperature and the altitude.
The lower you cruise, the lower will be this economical speed. Hence, it will have negative effect on your trip time. Long story short, it is far better to spend most of the times climbing in a short flight in a high flying jet rather than leveling off at a low altitude.
Jet engines are most efficient at high altitudes, where the temperatures are low and RPM is high while burning less fuel.