The 'Coffin Corner' and a 'Mesoscale' Maw

The A330 that crashed a week ago - from JetPhotos.net

The Air France 447 mystery may never be solved beyond a shadow of doubt, but there are some telling, tragic clues to consider based on what we know about the airplane systems and the extreme weather and aerodynamic conditions it encountered before it went down a week ago.

First, a bit of aerodynamics: The doomed Airbus A-330-200 was flying ever so close to its maximum altitude – in a zone pilots call the “Coffin Corner”. It refers to the edge of so-called “flight envelope” of an aircraft. At this altitude, the air is much thinner and that significantly narrows the swath of speed at which the airplane can safely operate.

Because there are relatively few air molecules passing over the wings, they need to be moving faster to generate enough lift to keep the plane at altitude. They will stop flying (stall) at a much higher speed (true airspeed) than they would on approach to an airport at sea level.

At the other end of the safe speed spectrum is the sound barrier. The wings on an airliner like the A-330 are not designed to break the speed of sound. Venture toward Chuck Yeager country and an airliner will begin buffeting. And as altitude increases, the buffet speed (the sound barrier) decreases (once again the dearth of air molecules is to blame).

So you see the squeeze play as a plane flies toward the Coffin Corner: the margin between the between the high and low speed limits gets thinner and thinner (along with the air).
Matter of fact, given its estimated weight, altitude and the outside air temperature (which also affects air density), AF 447 was flying through the eye of a speed needle only about 25 knots (28 mph) wide.

And one more important point: as jet engines fly higher, they steadily lose their oomph (you know, thin air). Matter of fact, the maximum altitude a plane can safely fly is partially determined by the point where the engines can no longer maintain a minimum rate of climb. In other words, you are supposed to level off just before they go into “Scottie” mode (“No more power, Captain!”).

So while you are napping, eating or watching a movie on that flight to LAX, you should know the plane you are flying is cruising along at the ratty edge of its capabilities. Why? Money. The higher an airliner flies, the better gas mileage it gets.

But rest easy, white-knucklers; flying in the “Corner” is routine and safe – so long as the weather is benign, the air is smooth and the sensors, avionics, computers and autopilot are all doing their job.

But of course that was not the case for Air France 447.

The weather where and when the plane went down was horrible – the storms among the meanest weapons in Mother Nature’s arsenal. On their nose, the crew would no doubt have seen the outline of a towering wall of cumulonimbus clouds – illuminated in strobe-light fashion by lightning.

Seeing this would not have been a big surprise to them. Their pre-flight weather briefing would have included satellite imagery clearly depicting the “Cb’s” – as pilots call them. Besides, big storms are a common occurrence over the Atlantic along the equator – where the airflows of two hemispheres collide.

Satellite Image of MCS that Air France 447 flew into - From Tim Vasquez

Meteorologists call the storm AF 447 flew into a “Mesoscale Convective System” – a large complex of multiple thunderstorms where the sum total is greater than the individual parts. MCS storms are obviously much bigger than your garden-variety thunder-bumper – and they last a lot longer. [More about the weather at Tim Vasquez’ insightful, detailed blog.]

Precisely because big thunderstorms are common there, airliners are constantly threading their way through the nastiest cells – deviating at the pilot’s discretion. But no professional pilot would knowingly auger into the heart of a thunderstorm this potent. A pro knows no airliner is designed to survive those conditions – no matter how advanced it is technologically and structurally.

Hard to believe in this day and age, but when you are flying over the pond, you are pretty much on your own. You are not talking to air traffic controllers or being painted by their radar – and of course there are no weather reporting stations beneath you. By definition, thunderstorms are unstable, dynamic and fast-moving. So by the time they reached the storms – more than four hours into the flight – what they learned in the pre-flight briefing was yesterday’s news.

Weather Radar - from NASA

As a result, flight crews rely heavily on the weather radar bolted onto the nose of the airplane. It is a very useful safety device but interpreting its display is a bit of a black art. A lot of pilots, frankly, do not fully understand the intricacies of its capabilities and limitations. It is akin to a blind man with a cane; he can tell something is in his way, but he doesn’t see it.

For instance, the radar mostly detects rain and hail – and if that first layer of storm cells was particularly heavy, it might have acted like a curtain – hiding the reinforcements from radar beams. With the benefit of hindsight (and satellite imagery captured at the time of the crash), we know now there were at four more layers of strong storms behind the first line of cells. And radar cannot detect the strong updrafts of warm air that feed a thunderstorm.

Infrared image of storm - from Tim Vasquez

Did the Air France crew spot a gap in that first line of storms that turned out to be a “sucker hole” – sending them into a box canyon of violent storm cells? Maybe. If they could have seen the full depth and intensity of those storms, would they have changed course to avoid it? Hard to imagine they would say, “Steady as she goes…”

No matter how they made their decision to fly into the maw, it was likely not long before they would have known they made a big mistake.

The last message from the crew – a text – indicated they were flying through thunderstorms with “fortes turbulences” (strong turbulence). Hard to know exactly what he meant. In the US, we define turbulence as “Light”, “Moderate”, “Severe” or “Extreme”.  The FAA defines the latter as “turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage.”

Extreme turbulence is precisely what an airplane would be apt to encounter inside an MCS.

Flight Profile created by Tim Vasquez

So why wouldn’t they just make a speedy U-turn at that point? They might have, but attempting a maneuver like that in severe or extreme turbulence would likely have made things worse. Remember, the engines were close to maxed out and the speed margin was minuscule before the plane pierced the storm clouds. Simply banking the wings could be enough to trigger an aerodynamic stall.

And consider this: those updrafts bring warm moist air to higher altitudes – feeding the storm. That also might have increased the air temperature where the Airbus was flying. Warmer air is less dense – with fewer molecules – meaning the airliner might have suddenly been flying above its maximum safe altitude.

Bottom line: if all sensors and systems on the Airbus kept working the aircraft might have been hard pressed to stay aloft. But of course, the systems started crashing before the airplane did.

So were those failures contributing causes of the crash – or simply the upshot of an aircraft taking a beating it could not withstand? Maybe it is a little bit of both.

More on that tomorrow…

Tags: Air France, Airbus, Aircraft, Chuck Yeager, Fixed-wing aircraft, Mesoscale Convective System, Technology, Thunderstorm