What is coffin corner in aviation?

Coffin corner in aviation specifically refers to the altitude range where the effects of decreased air density and increased speed cause the stall speed and the onset of compressibility effects to converge. This situation occurs at high altitudes, where the aircraft is approaching its critical Mach number, which is the speed at which airflow over certain parts of the aircraft begins to reach supersonic speeds, leading to a phenomenon known as Mach tuck.

At this point, the stall speed increases with altitude due to the reduced air density, and the critical Mach number also becomes a concern as the aircraft approaches its maximum efficient speed. As a result, the aircraft may experience a buffet due to either stall or compressibility effects at the same airspeed, which is why the term "coffin corner" is used; it indicates a dangerous situation where the pilot has very little margin to safely operate the aircraft without risking stalling or experiencing control issues.

This definition clarifies why the correct answer focuses on the convergence of these two critical aerodynamic phenomena, highlighting the inherent risks associated with flying in this altitude range. Other options, while they may speak to different aviation aspects, do not capture the essence of the unique aerodynamic challenges associated with coffin corner.