If a passenger jet flies too high, it risks entering a dangerous aerodynamic situation known as "Coffin Corner," where it can lose the ability to maintain altitude.
Understanding Coffin Corner
Coffin Corner is not a physical location, but rather an area in the flight envelope where the aircraft's stall speed and critical Mach number converge. This convergence creates a narrow margin for safe flight.
What is Stall Speed?
Stall speed is the minimum speed at which an aircraft can maintain lift. Fly slower than this, and the wings can no longer generate enough lift to keep the plane airborne, causing it to stall and potentially descend rapidly.
What is Critical Mach Number?
Critical Mach number is the speed at which airflow over parts of the aircraft reaches the speed of sound (Mach 1). Exceeding this speed can create shockwaves that disrupt airflow, leading to a loss of lift, increased drag (known as "buffet"), and control problems.
The Risks of Flying Too High
When a plane flies too high, the air becomes thinner. This has several consequences:
- Increased Stall Speed: The thinner air requires the aircraft to fly at a higher true airspeed to maintain sufficient lift, increasing the stall speed.
- Decreased Critical Mach Number: The thinner air also lowers the critical Mach number.
As the aircraft approaches its maximum altitude, the stall speed increases, and the critical Mach number decreases. They get closer and closer together.
Coffin Corner Explained
The "Coffin Corner" occurs when the stall speed and critical Mach number meet. At this point, the aircraft has virtually no safe operating margin. Any slight increase in airspeed can cause buffet, while any decrease in airspeed can cause a stall.
Consequences
According to the provided reference, at Coffin Corner, the aircraft can no longer maintain altitude. This means it will likely descend. Furthermore, because of the narrow operational window, the pilot has very little margin for error to recover.
Preventing Flight in Coffin Corner
- Altitude Limits: Aircraft have certified maximum operating altitudes that are carefully calculated to prevent flight in Coffin Corner conditions.
- Monitoring Instruments: Pilots use instruments such as airspeed indicators, Mach meters, and altitude gauges to monitor the aircraft's performance and avoid exceeding safe limits.
- Flight Planning: Flight plans take into account weather conditions, aircraft weight, and performance characteristics to determine the optimal altitude for a safe and efficient flight.
Summary
| Risk | Explanation |
|---|---|
| Stall | Flying too slowly at high altitude leads to a loss of lift and a rapid descent. |
| High-Speed Buffet | Flying too fast at high altitude causes shockwaves, leading to loss of control and increased drag. |
| Inability to maintain altitude | At Coffin Corner, the aircraft lacks the power to stay at its current height. |
