Coffin Corner: Where a Jet Is Too Slow and Too Fast at the Same Time

Coffin corner is the altitude where a jet’s slowest safe speed and its fastest safe speed meet. Fly any slower and the wing stalls. Fly any faster and shock waves form on the wing, shaking the airplane and pushing the nose down.

The FAA puts numbers on how fast that gap closes. For a typical jet in level flight, the buffet-free speed margin is 135 knots at 35,000 feet and just 26 knots at 45,000 feet.

Pilots also call it the aerodynamic ceiling or Q corner. Airliners almost never reach it, because their flight computers keep a margin from it built in, but the airplanes that have strayed close show exactly why it earned the name.

What Is Coffin Corner in Aviation?

Every airplane has a flight envelope: the combinations of speed, altitude and load it can fly safely. “Pushing the envelope” means flying at its edges, and coffin corner is the top edge for a high-flying jet.

The FAA defines it as operations at high altitude “where low indicated airspeeds (IAS) yield high TAS (as indicated by Mach number) at high angles of attack.” In plain terms, the wing is working hard to hold the airplane up in thin air, while the airplane is already moving close to the speed of sound.

The point where the two limits finally merge is the airplane’s absolute or aerodynamic ceiling. There, says the FAA, the airplane “can neither be made to go faster without activating the design stick puller at Mach limit nor can it be made to go slower without activating the stick shaker or pusher.”

The FAA also notes that this aerodynamic ceiling sits higher than the altitude limit printed in the flight manual. So the corner itself lies above the altitudes the airplane is approved to fly.

Why the Stall Speed Climbs With Altitude

A wing stalls at its critical angle of attack, not at a fixed speed. In indicated airspeed, which measures the air pressure the wing actually feels, the stall speed hardly changes with altitude.

The catch is that thin air makes indicated airspeed fall further and further behind true speed as the airplane climbs. To stay above the same indicated stall speed at 40,000 feet, the airplane has to move much faster through the air, so measured in Mach, the stall limit keeps rising.

Weight and G-load raise it further. A heavier airplane needs more lift at every speed, and so does one that is turning or riding a gust, which is why a high-speed stall is possible at all.

Why the Mach Limit Comes Down to Meet It

Air speeds up as it flows over the curved top of a wing. So an airliner cruising below the speed of sound can have air flowing over parts of its wing that is already supersonic.

Where that local flow reaches Mach 1, a shock wave forms, and the air behind it separates from the wing. The FAA calls the resulting shaking Mach buffet.

Push further and the airplane can enter Mach tuck. The center of pressure moves aft and shock-induced separation weakens the downwash over the tail, which together pitch the nose down.

To keep pilots clear of this, every jet has a certified MMO, its maximum operating Mach number. MMO is a fixed Mach number, but the speed of sound falls as the air gets colder, up to about 36,000 feet, so the true speed that MMO represents drops as the jet climbs.

How Narrow Is Coffin Corner?

FAA Advisory Circular 61-107B, the agency’s guidance for flying above 25,000 feet, gives two worked examples for a typical jet. The first is the 135-knot margin at 35,000 feet that shrinks to 26 knots at 45,000 feet.

The second shows why a margin in level flight is not the whole story. At 51,000 feet and 1.0 G, the same kind of jet would meet Mach buffet just above its MMO of Mach 0.82 and low-speed buffet at Mach 0.60.

Add only 0.4 G, from a turn or a gust, and buffet arrives at Mach 0.73, the jet’s optimum cruise speed. In the FAA’s words, any further change in airspeed, bank angle or gust loading “may reduce this straight and level flight 1.4 G protection to no protection.”

A level turn makes the point concrete. A 40-degree bank loads the wing to about 1.3 G, enough to swallow a thin margin without the pilot doing anything unusual.

How Airliners Stay Out of Coffin Corner

Airline jets do not cruise at their certified ceiling as a matter of course. The flight management computer calculates a recommended maximum altitude, which Airbus calls REC MAX, and it sits below the certified ceiling.

According to the French accident investigator BEA, REC MAX must leave a climb rate of at least 300 feet per minute at maximum climb thrust and guarantee a margin of at least 0.3 G before buffet. In other words, the airplane can take a 1.3 G load at that altitude without shaking.

That altitude is not fixed. It falls as the airplane gets heavier and as the outside air gets warmer, which is why crews keep an eye on it over the course of a long flight.

For anything sharper, pilots read the manufacturer’s cruise maneuver and buffet limit charts. The FAA tells crews to pick a cruising altitude that leaves enough buffet margin for the maneuvering and the turbulence they expect.

The U-2: The Airplane That Lives in the Corner

One airplane was designed to work in coffin corner every day. The Lockheed U-2 Dragon Lady is routinely flown above 70,000 feet, according to the U.S. Air Force.

At that height, the margin between the U-2’s stall warning buffet and its Mach buffet is commonly reported as small as about 5 knots. That leaves the pilot almost no room either way.

The Air Force says its handling earns it “a widely accepted title as the most difficult aircraft in the world to fly.” Its pilots wear full pressure suits similar to astronauts’, and each landing is normally chased by a second U-2 pilot in a high-performance vehicle, calling out altitude and alignment.

Pinnacle 3701: The “410 Club” Flight

On October 14, 2004, two Pinnacle Airlines pilots were repositioning an empty Bombardier CRJ-200 to Minneapolis. They asked for 41,000 feet, the airplane’s maximum operating altitude, and the NTSB found that the climb was made for personal, not operational, reasons.

Pilots at the airline later told investigators that an informal “410 club” existed for those who had flown the jet that high. On the cockpit recording, the captain told a controller, “we don’t have any passengers on board so we decided to have a little fun and come on up here.”

The airplane had climbed from 37,000 to 41,000 feet with its speed bleeding away, from 203 knots (Mach 0.63) to 163 knots (Mach 0.57). The NTSB noted it could have held 41,000 feet had the crew kept its speed at Mach 0.7.

Instead the speed kept falling, and at about 150 knots the stick shaker fired. Over the next seconds the shaker went off five times and the stick pusher four, and each time the pilots pulled back against it until the jet stalled.

The disturbed airflow flamed out both engines, which then would not restart. The CRJ came down in a residential area about 2.5 miles (4 km) south of Jefferson City Memorial Airport, Missouri, killing both pilots, the only people aboard.

This was the stall side of the envelope closing in, not a Mach event. It is also the plainest lesson in the topic: at the top of the envelope, speed is the margin, and an airplane that is allowed to slow down runs out of it.

Was Air France 447 a Coffin Corner Crash?

Many articles on this subject, including an earlier version of this one, uses Air France 447 as its example. The final report from the BEA tells a different story.

On June 1, 2009, ice blocked the airspeed probes over the South Atlantic and the autopilot disconnected. The pilot flying made nose-up inputs and climbed the airplane to about 38,000 feet, where it stalled and fell at roughly 10,000 feet per minute, with the stall warning sounding continuously for 54 seconds.

The link to coffin corner is real, but it is psychological. The BEA found it possible that the pilot flying believed the airplane was overspeeding, misreading the noise and the buffet as signs of high speed.

The report explains why that fear came so easily. In cruise, the red MMO band on the A330’s speed tape sits about 10 knots above the current speed, and the copilots’ training had linked buffet with both stall and overspeed, although on the A330 buffet occurs only on the approach to a stall.

Captain Chesley Sullenberger walks through the final report’s findings in the video above. Watch the “Air France 447: Final report on what brought airliner down” video on YouTube.

How Pilots Get Out of Coffin Corner

The way out is the same whichever edge is closing in: take load off the wing and give it denser air. Both margins widen as the airplane descends, which is what the FAA’s 135-knot and 26-knot figures show.

On the stall side, that means lowering the nose to reduce the angle of attack and accepting a loss of altitude, the step the Pinnacle crew fought against the stick pusher. On the Mach side, it means reducing thrust smoothly, since the FAA warns against over-action of the thrust levers at high altitude.

In both cases, shallow bank angles and gentle pitch changes keep the G-load near 1.0. The better answer is never to arrive there, by respecting REC MAX, cruising with a margin below it, and keeping the airspeed up in the climb.

Coffin corner is close kin to a jet’s service ceiling, but not the same thing. The service ceiling is a performance limit, where the airplane can no longer climb at 100 feet per minute, while coffin corner is an aerodynamic one.