Explained: What Is the Coffin Corner?

DALLAS – Aviation terms are full of surprises One such term is the coffin corner. In simple terms, the name refers to the flight region where slight altitude and speed changes can result in woeful mishaps.

Let's examine the terminology and the requirements pilots must follow to maintain their course and avoid the infamous coffin corner.

Definitions

In aviation terms, the coffin corner or Q corner, as it was originally known (where "Q" is the abbreviation for dynamic pressure), refers to the point at which the Flight Envelope boundary defined by a high incidence stall intersects with that defined by the critical Mach number (M_cr or M_crit) of an aircraft, which is the lowest Mach number at which the airflow over any part of the aircraft reaches the speed of sound.

The Bold Method explains the term as at a jet's operating ceiling, as its Maximum Mach Number (MMO) is often intensely close to its stall speed.

Flying within the Coffin Corner

It seems intuitive that if a pilot decreases the flight speed, the aircraft will stall, but not if the pilot increases it above the critical Mach number, the air traveling over the wings will start to separate following the formation of shock waves, which will cause the aircraft to lose its lift, pitch the nose down heavily and lose altitude.

It is therefore tricky for pilots to maintain a stable flight as it climbs toward the coffin corner because the margin between stall speed and critical Mach number becomes smaller and smaller until the Flight Envelope boundaries intersect. At this point, any change in speed would result in exceeding either limit.

If an aircraft is powered by a piston engine, the pilot will never have to worry about this phenomenon. But if it's a subsonic Turboprop, Turbojet, or Turbofan aircraft, pilots generally avoid altitudes approaching the coffin corner.

Let's dive deeper into the aerodynamics involved in flying the Q corner, starting with the Mach Number.

Mach Number

According to SKYbrary.com, The Mach Number (M) is the ratio between the true air speed (TAS) and the local speed of sound (LSS). This ratio, which equals one when the TAS is equal to the LSS, is known as M and is very important in aircraft operating at high speed.

For example, if an aircraft is flying at 0.1 Mach, it is flying at 10% of the speed of sound. If the aircraft achieves the Mach 1 mark, it is considered supersonic. Modern airliners cruise at a speed that generates regions of subsonic airflow, i.e. 75% to 85% of the speed of sound.

Commercial airliners are restricted from flying over Mach 1, and some countries like the US, Canada, Malaysia, and India have opposed supersonic flights.

True Air Speed

As an aircraft climbs, the reduced density causes an increase in its True Air Speed (TAS). Additionally, the speed of sound decreases proportionally with temperature, which also decreases as altitude increases. Consequently, M rises as it climbs higher.

So the aircraft cruising at a transonic speed at a higher altitude has a risk of becoming supersonic. This is crucial for aircraft that are not designed to fly at this speed. So how do they prevent it?

Enter the MMO or Critical Mach Number.

Critical Mach Number

In aerodynamics, the Critical Mach Number of an aircraft is the lowest Mach number at which the airflow over any part of the aircraft reaches the speed of sound. The wing of an airplane reaches Mach 1 way before the plane itself, so the pilot knows that the aircraft is flying at a much higher speed and can slightly pull back the throttle.

True stall speed increases as the aircraft gains altitude, so there is a risk of stalling if the pilot reduces the speed, but there is also the risk of becoming supersonic if the pilot increases the speed.

As the aircraft flies higher, the high and low stall speeds get closer. At this point, they become a single speed, which we call the ceiling, i.e. the coffin corner endpoint.

How Pilots Avoid the Coffin Corner?

To summarize, the intersection between the TAS with the MMO leads to the junction of the coffin corner; beyond this point, the aircraft cannot stay airborne, stalling or buffeting and losing altitude. But how do commercial pilots avoid the coffin corner and always know if they're flying close to it?

Commercial airliners must comply with certification standards requiring maneuverability of at least 0.3 gs at their highest operating altitude. This assures that there is an adequate margin for the pilot to maneuver without encountering the high-speed or low-speed buffet, which refers to the shaking of the aircraft during a stall when detached airflow hits the tail surfaces.

Aircraft manufacturers provide flight manuals that include buffet onset charts to guide pilots about the limits. Using these charts, the flight deck crew can determine the altitude, airspeed, and weight at which lower and higher speed buffets take place.