Coffin corner (aviation)
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The coffin corner or Q-Corner is the altitude at or near which an aircraft's stall speed is equal to the critical Mach number, at a given gross weight and G loading. At this altitude the aircraft becomes nearly impossible to keep in stable flight. Since the stall speed is the minimum speed required to maintain level flight, any reduction in speed will cause the airplane to stall and lose altitude. Since the critical Mach number is maximum speed at which air can travel over the wings without losing lift to flow separation and shock waves, any increase in speed will cause the airplane to lose lift, or to pitch heavily nose-down, and lose altitude. The "corner" refers to the triangular shape at the top of a flight envelope chart where the stall speed and critical Mach number lines come together. Some aircraft, such as the Lockheed U-2, routinely operate in the "coffin corner", which demands great skill from their pilots.[1]
[edit] Aerodynamic basis
It is basic aerodynamics that an aircraft remains in level, constant-airspeed flight when its lift is equal to its weight, and its thrust is equal to its drag. In most circumstances this equilibrium can occur at a range of airspeeds. The minimum such speed is the stall speed, or VSO. At this speed the aircraft's wings are at a high angle of attack.
At higher altitudes, the air density is less, while the area of the wing is unchanged. In order to provide the same lift, the wings must move faster through the air. Thus the stall speed, in true airspeed terms, increases with altitude. (Measured in indicated airspeed terms the stall speed is constant at any altitude.)
Air conducts sound at a certain speed. This speed of sound becomes lower as the air becomes cooler (such as at high altitude). A given airspeed, divided by the speed of sound in that air, gives a ratio known as the Mach number. A Mach number of 1.0 indicates an airspeed equal to the speed of sound in that air.
As an aircraft moves through the air faster, the airflow over parts of the wing will reach speeds that approach Mach 1.0. At such speeds, shock waves form on the wings, drastically increasing the drag due to drag divergence, causing mach buffet, or drastically changing the center of pressure resulting in a nose-down force called "mach tuck". The aircraft Mach number at which these effects appear is known as its critical Mach number, or Mach CRIT. Since Mach number is a ratio with the local speed of sound, which decreases with pressure as altitude increases, the aircraft's critical Mach number, in true airspeed terms, decreases as altitude increases.
The flight envelope is a plot of various curves representing the limits of the aircraft's true airspeed and altitude. Generally, the top-left boundary of the envelope is the curve representing stall speed, which increases as altitude increases. The top-right boundary of the envelope is the curve representing critical Mach number in true airspeed terms, which decreases as altitude increases. These curves typically intersect at some altitude. This intersection is the coffin corner, or more formally the Q corner.[1]
The above explanation is based on level, constant speed, flight with a given gross weight and G-force of 1.0. The specific altitudes and speeds of the coffin corner will differ depending on weight, and the load factor increases caused by banking and pitching maneuvers. Similarly, the specific altitudes at which the stall speed meets the critical Mach number will differ depending at the actual atmospheric temperatures that day.
[edit] Consequences
When an aircraft slows to below its stall speed (or more properly, when the wing exceeds its critical angle of attack), the airflow over the top of the wing separates from the wing surface, and lift decreases dramatically (the wing "stalls"). Because the lift reduces while the aircraft's weight does not, the aircraft loses altitude. When the aircraft exceeds its critical Mach number, then drag increases or Mach tuck occurs, which can cause the aircraft to upset, lose control, and lose altitude. In either case, as the airplane falls, it could gain speed and then structural failure could occur.
As an aircraft approaches its coffin corner, the margin between stall speed and critical Mach number becomes smaller and smaller. Small changes could put one wing or the other above or below the limits. For instance, a turn causes the inner wing to have a lower airspeed, and the outer wing, a higher airspeed. The aircraft could exceed both limits at once. Or, turbulence could cause the airspeed to change suddenly, to beyond the limits.
Aircraft capable of flying close to their critical Mach number usually carry a machmeter, an instrument which indicates speed in Mach number terms. As part of certifying aircraft in the United States of America, the Federal Aviation Administration (FAA) certifies a Mach number for maximum operation, or MMO.
Some aircraft, such as the Lockheed U-2, routinely operate in the "coffin corner", which demands great skill from their pilots. The FAA is concerned that as jet aircraft become more common, less experienced pilots will be flying those aircraft closer to the altitude of their coffin corners, and that catastrophic accidents will occur as a result.[1]
