WHY DO AIRCRAFT FLY AT HIGH ALTITUDES - AND WHY NOT HIGHER OR LOWER

Commercial jet aircraft typically cruise at high altitudes for three primary reasons: fuel efficiency, passenger comfort, and safety.

1.Fuel Efficiency

At cruising altitudes—generally between 33,000 and 38,000 feet—the air becomes significantly less dense. In this thinner air, an aircraft experiences reduced aerodynamic drag, allowing for more efficient flight and lower fuel consumption. Additionally, true airspeed increases at higher altitudes while maintaining a constant indicated airspeed, contributing to faster point-to-point travel with optimal engine performance within design limits.

2. Passenger Comfort

Most weather phenomena such as turbulence, thunderstorms, and cloud systems are concentrated below 35,000 feet. Operating above this weather envelope allows the aircraft to avoid much of the atmospheric instability, resulting in a smoother ride for occupants.

3. Operational Safety

Altitude provides time and options. At cruising levels—e.g., 37,000 feet—a failure such as total engine loss (due to bird strike, volcanic ash ingestion, or other anomalies) does not lead to immediate danger. Modern airliners, such as the Boeing 737, exhibit excellent glide characteristics, typically maintaining glide ratios between 1:15 and 1:20. This means that for every 1,000 feet of altitude lost, the aircraft can glide 15 to 20 kilometers forward. In dense airspace regions such as over mainland Europe, this often places several airports within gliding range, providing ample opportunity for a safe diversion and landing.

Why Not Fly Higher?

Despite the advantages, several limiting factors prevent commercial aircraft from cruising at even higher altitudes:

a. Engine Performance Limitations

Jet engines require a minimum oxygen concentration to maintain combustion. As altitude increases, atmospheric density and oxygen availability decrease. Beyond a certain altitude, engines are no longer able to produce sufficient thrust to sustain a minimum climb gradient—typically 500 feet per minute for commercial jets. This defines the aircraft's maximum certified operating altitude.

b. Aerodynamic Constraints – The Coffin Corner

With altitude, stall speed (in terms of true airspeed) increases, while the speed of sound (Mach 1) decreases due to falling outside air temperatures (e.g., −50 to −60°C at 38,000 feet). These two speeds converge at what is known as the Coffin Corner—a narrow margin between low-speed stall and high-speed (Mach) buffet boundaries.

At this point:

• Flying slower risks entering a low-speed aerodynamic stall.

• Flying faster risks exceeding the aircraft’s Critical Mach Number—the speed at which parts of the airflow over the airframe reach supersonic speeds.

Exceeding this critical Mach causes shockwaves on the wing surfaces, significantly increasing drag and shifting the center of pressure rearward, both of which degrade stability and control. These effects are particularly hazardous in aircraft designed strictly for subsonic or transonic flight, making further climb impractical and unsafe.

Conclusion

Modern jet aircraft operate within a finely balanced flight envelope. Cruising altitudes are chosen to optimize fuel burn, avoid meteorological hazards, and maximize safety margins. Beyond a certain point, both engine limitations and aerodynamic physics impose non-negotiable boundaries on how high an aircraft can go.