If you have ever flown in a commercial jet, the first thing you would ever notice in the cabin is the ambience and serious nature of the cabin preparation. Take off and climb session of the aircraft is considered one of the most critical phase of the flight and the silence and caution that prevails in cabin  will surprise you till  reach the height of safe altitude FL250 and above. Why the aircraft need to climb this high why cant it fly at low altitude.

There are few concerns on why the aircraft  has to fly at high altitudes like 36000 feet above ground. As we know atmosphere is divided into different layers based on the property of Pressure, Density and Temperature.

Most of the weather phenomenon occurs in the troposphere, the atmospheric layer that is closest to the earth’s surface. The average height of the troposphere is estimated as 36000ft, however, it varies depending on the region from which measurement is made (eg. polar, equatorial) and temperature of the atmosphere.

By flying above clouds, aircraft can avoid bad weather and turbulence. Flight above the troposphere helps avoid a considerable amount of weather phenomenon like thunderstorms and heavy rain clouds etc.

Misconceptions that prevail why aircraft fly high:
  • Air is less dense higher up, so the aircraft experiences less drag.  The airplane has to produce enough lift to carry its weight, regardless of air density. It compensates for the lower density by flying faster, which increases drag again. Due to the decreasing Reynolds Number with altitude, drag even increases a little with altitude.
  • The high-flying aircraft flies faster, and ram pressure will increase thrust for free. No, wrong again. While it is true that faster speed increases the pressure ratio between outside air and the air in the engine, to produce thrust the engine first needs to speed up the air in the nozzle to more than its entry speed. Remember that thrust is proportional to the velocity ratio between entry and exit? To produce any thrust, the engine needs to accelerate the air more the faster it flies. This costs energy. There is no free lunch, not even in aviation. In fact, the thrust-specific fuel consumption of a modern turbofan is twice as high in cruise than at take-off.

Jet engine produces maximum thrust when they are static for a given air density.

Thrust = Air mass × (Final velocity of gases—Initial Velocity of air)

When the aircraft  is not moving , the initial velocity of air mass is zero, so we have maximum thrust. So the thrust of jet engine starts to decrease as the aircraft starts to accelerate from zero speed because the difference between two velocities starts to decrease. Theoretically, speaking if the initial velocity of air becomes equal to the final velocity of gases exiting engine, then the thrust should become zero. This never happens because air is slowed down before entering the engine.

The Final velocity of gases or exit velocity  is increased by accelerating the airflow through the engine, and the acceleration is achieved by heating the air.

The amount of thrust produced by an engine is also a function of air mass (density). As we go higher the air starts to thin out, reducing mass.Therefore, thrust at higher altitudes is lesser than thrust at lower altitude. Air density also reduces with increase in ambient air temperature, resulting in lower engine thrust.

In Turbo Fan engines a large mass of air does not pass through its core. It rather flows around it and mixes up with exhaust gases at the tail end of engine to increase thrust.

Since air mass is also a critical factor in the lift, hence take offs from hot and high altitude airports require longer runways because both aircraft wings and engines become less efficient.

It is a fact that thermal efficiency increases with increase in the difference of ambient temperature and gases temperature. But it should take more fuel for a given mass of cooler air to heat it up to required minimum Turbine inlet temperature and Exhaust Gas Temperature (EGT). A turbine converts heat energy into mechanical energy, therefore, the difference in the turbine inlet temperature and the turbine itself should improve its efficiency.

So it is never a single reason why aircraft is flying higher rather a combination of facts.

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