Miles O’Brien, pilot and blogger, has a fascinating two part blog called “The ‘Coffin Corner’ and a ‘Mesoscale’ Maw”, which has been widely reprinted by Reuters. Its broad subject is what happens inside the narrow aerodynamic envelope that a modern jetliner inhabits for economic reasons. It is near its maximum altitude where it must fly fast to maintain lift, but at which it has little or no reserve power in its engines, where the distance between stalling and overstressing the airframe can be only 25 knots wide. This was the situation that Air France Flight 447 must have been on the fateful night it disappeared. O’Brien writes:
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). … AF 447 was flying through the eye of a speed needle only about 25 knots (28 mph) wide.
But don’t worry. Passengers fly through this needle-eye all the time. All is well, O’Brien says, as long as the instruments are functioning on the aircraft and no extreme weather is encountered. But what happens if neither of those two conditions hold true? The first factor he examines is the weather, quoting Tim Vasquez’s meteorological analysis which the Belmont Club had also linked to earlier. That far out into the ocean the pilots would have been operating in an information-poor environment with respect to the weather. Their pre-takeoff weather briefings will have been outdated. They would be reliant on their instruments “akin to a blind man with a cane” to know what they were up against.
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…”
He will continue in the second part, which I suspect, will focus on the other factor: the airplane instruments, the length and reliability of the cane. O’Brien’s wonderful piece brought home to me how unavoidably dependent aviation is on information. Information about the plane’s airspeed, altitude, attitude and condition; information about the outside environmental variables. Whether derived from the pitot tubes, the onboard radar or downlinks, an airplane flies on information as much as lift. That should be especially true for a fly-by-wire aircraft whose automated systems play such a large role in controlling the plane. Unless the data flight records are retrieved we’ll never know what the doomed pilots realized in the last four minutes of their lives. What did they realize, but realize too late?
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