What characterizes the term 'coffin corner' in aviation?

The term 'coffin corner' in aviation refers to the region at high altitudes where the stall speed increases while the maximum speed limit decreases, creating a narrow margin between these two speeds. This situation arises when the aircraft is nearing its absolute ceiling, where the performance characteristics begin to converge.

At this altitude, the stall buffet—the aerodynamic phenomena experienced at the critical angle of attack—and the Mach buffet—which occurs at high speeds near the transonic range—can coincide. This means that the aircraft is flying at its absolute limits; if it exceeds these limits, it risks entering a stall or experiencing instability due to excessive speed. Therefore, flying at such altitudes requires careful management of speed and altitude to maintain safe operation.

The other options do not encapsulate the essence of 'coffin corner' as they address separate concerns related to aviation performance. For instance, the maximum fuel threshold is about fuel management and weight rather than aerodynamic performance, and the ceiling of cabin pressure pertains to passenger comfort and safety rather than flight dynamics. Similarly, the threshold altitude for engine performance addresses the operational capabilities of the aircraft engines but does not relate to the aerodynamic principles encapsulated in the concept of coffin corner.