The primary reason airplanes fly at high altitudes revolves around fuel efficiency and speed. The air at higher altitudes is significantly less dense than air closer to the ground. This thinner air creates less drag, allowing the plane to travel faster and further with the same amount of fuel. Think of it like swimming – it’s much easier and faster to move through water than through thick mud. The thinner air also allows the plane to maintain lift with less effort.
Another crucial factor influencing flight altitude is the speed of sound, or Mach number. The speed of sound doesn’t change drastically with altitude; it actually decreases slightly due to falling temperatures. As an aircraft approaches the speed of sound, shockwaves can form on areas where airflow accelerates, like the top of the wing. These shockwaves create a significant increase in drag, drastically reducing efficiency. By flying higher, the aircraft can achieve a higher ground speed and true airspeed (TAS) for the same indicated airspeed (IAS), allowing it to fly faster without exceeding the critical Mach number where these shockwaves begin to form.
The limiting factor for altitude is called the “Coffin Corner.” This occurs when the true airspeed (TAS), which reflects the actual speed through the air, approaches the critical Mach number while the indicated airspeed (IAS), which represents the aerodynamic force the plane experiences, nears the stall speed. At this point, the plane has very little margin for error in terms of speed. Increasing speed would result in dangerous shockwaves, while decreasing speed risks a stall.
Private jets, designed for shorter runways, typically have lower stall speeds due to their larger wing area relative to their weight. While their maximum Mach number is often similar to larger airliners, their ability to maintain lift at lower speeds allows them to reach a higher altitude before encountering the Coffin Corner. To clarify, TAS is similar to ground speed, adjusted for wind, while IAS reflects the aerodynamic force on the aircraft. IAS decreases relative to TAS as air density decreases with altitude.
