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Tuesday, January 18, 2022

What Is The Coffin Corner In Aviation?

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The language of the aviation industry is one that is full of fascinating but complex terms. While some terms are obvious enough for people to be able to work of their meanings from the word(s) alone, others aren’t so explicit. One such term that, at face value, doesn’t seem to have an explicit meaning is ‘coffin corner.’ Let’s take a look at what this refers to.

Air France Airbus A330
The crash of Air France flight 447 in 2009 has links to the coffin corner. Photo: Pawel Kierzkowski via Wikimedia Commons

What is the coffin corner?

The ‘coffin corner’ refers to the intersection of a given aircraft’s stall speed and critical Mach number. While stall speed is enough of a self-explanatory term, critical Mach number may be a less well-known term. Skybrary reports that it refers to the lowest Mach number at which the airflow over any part of the aircraft reaches the speed of sound.

An aircraft’s altitude at a given time defines the speed at which these two variables intersect. The margin between them will decrease as a plane climbs towards this defining altitude. Bold Method reports that the real name for this is the ‘Q corner.’ The reason for this is the fact that Q is recognized as an official abbreviation for dynamic pressure.

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As stall speed and critical Mach number converge, the coffin corner emerges. Image: RosarioVanTuple via Wikimedia Commons

However, the phenomenon has also taken on the name ‘coffin corner’ due to the triangular region of a flight envelope chart where stall speed and critical Mach number are close together. More morbidly, it also refers to the danger of death that can arise as a result.

What can happen?

When an aircraft is in the coffin corner, it can be more difficult to retain stable flight. This is because reducing speed can cause the aircraft to stall, whereas increasing speed can reduce its lift. Even the slightest of movements can prove dangerous, as Skybrary explains:

In the most critical case, simply turning the aircraft could result in exceeding both limits simultaneously. In a turn, the inside wing slows down, whereas the outside wing increases speed. Likewise, encountering turbulence could result in a ‘beyond limits’ change in airspeed.”

F-16 Stall
Military aircraft sometimes perform intentional stalls as part of displays. Photo: Peter Mulligan via Flickr

In any case, both a stall and a loss of lift can cause the aircraft to fall from its existing altitude. Such situations can be extremely perilous to an aircraft as the g-forces encountered when falling can cause structural failures, leading the plane to break apart.

Case study: Air France flight 447

Air France flight 447 crashed in the Atlantic Ocean in June 2009 due to a high altitude stall while flying from Rio de Janeiro to Paris. All 228 passengers and crew perished in the tragic accident, which was the deadliest involving both Air France and the Airbus A330.

Air France flight 447 Getty
Debris at the crash site suggested that the aircraft hit the water largely intact. Photo: Getty Images

The crash has been linked to the coffin corner by publications such as Scientific American. This is because the accident happened after ice crystals obstructed the aircraft’s pitot tubes. This led to inconsistencies in speed readings and a failure to report stall conditions.

Amid confusion in the cockpit due to the incorrect readings, the flight’s crew pitched the aircraft’s nose upward to negate a roll caused by turbulence. This caused the aircraft to deviate from its cruising altitude of 35,000 feet, climbing to FL380. At this point, it lost lift, and experienced the aerodynamic stall that caused the tragic accident.

Did you know about the ‘coffin corner’? What other aviation phenomena and vocabulary would you like to see us explain? Let us know your thoughts in the comments.

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