AirAsia 8501

New England Cable News wants to interview me regarding AirAsia 8501, which motivated me to search Google News to find out what is known about this missing Airbus A320. So far the most significant piece of information is that the pilots were seeking a deviation from their planned route to avoid clouds at 32,000′. In latitudes closer to the Equator there is more energy pumped into the atmosphere by the sun and therefore thunderstorms are more intense and cumulonimbus clouds that generate thunderstorms extend higher in the atmosphere, e.g., to as high as 60,000′. Airliners typically fly no higher than 40,000′ and therefore must divert around, rather than fly over, the most severe thunderstorms. By far the best Web page on this incident that I could find was the Wikipedia page, showing thunderstorms and the flight path on the same map. Also see the video at CNN, which says that the T-storms during that particular flight were forecast to extend up to 52,000′.

What’s bad about flying into a thunderstorm? Turbulence can be severe, exceeding the 2.5G load factor for which airliners are designed (light planes must tolerate up to 3.8Gs by regulation, but heavier airplanes have more inertia and are therefore less likely to experience heavy G loads in turbulence). Lightning can damage the electrical system, without which a modern airplane simply cannot be controlled (you need the electrics to run the hydraulic pumps that actually move the ailerons, elevators, and rudder against the heavy airloads; the Airbus A320 is also a fly-by-wire system that gets rid of the traditional mechanical connections from pilot yoke out to the hydraulic controls near the flight controls (truly light airplanes don’t have hydraulics; there are simply cables or pushrods out to the flight controls and pilot muscle power is used to move them, though sometimes with the help of trim tabs that are powered by air rushing over the flying plane)). Hail can slam into the airplane and damage windshields, wing leading edges, etc. (most hail-damaged airplanes remain flyable, however) Thunderstorms can also generate airframe icing, which, if severe, may exceed an airplane’s anti-ice/de-ice capabilities. An airplane covered in ice cannot climb and cannot fly at slower airspeeds without entering an aerodynamic stall. (De-icing on a heavy turbojet-powered airplane such as the Airbus A320 is generally accomplished with compressed (“bleed”) air from the engines fed into metal tubes on the leading edges of the wings, tail, and engine cowlings.)

How does a pilot avoid dangerous weather like this? It is relatively easy flying over heavily settled regions such as North America and Europe where ground-based RADAR can see the rain and that turns into a map (example). If the airplane has a datalink of some sort, a slightly delayed version of the map can appear on a multi-function display along with the airplane’s planned course. There are some limitations of such maps, starting with the fact that the map is two-dimensional and there is no fine-grained information on cloud or thunderstorm tops. I have been at 20,000′ in clear New York air flying over a line of solid red and yellow (heavy rain) but the same flight in Texas over the same map image might have resulted in being in clouds/turbulence/rain/ice/etc. Airliners also have on-board weather RADAR that can look ahead and see if there is rain in a cloud, but these images require a lot of experience to interpret. Someone who flies at low altitudes around the Caribbean and Florida would be great at this. A modern jet pilot usually isn’t because, most of the time, jets climb out of the bad stuff so quickly. If you’re out in the middle of the ocean and can’t get an accurate map from ground-based stations, you might have to rely on the on-board RADAR (not sure if that was a factor here; the plane was never all that far from land, though I am not sure if Indonesia has invested in as many RADAR stations as we have (air traffic control RADAR is not set up to paint an accurate weather picture)).

So the specifics of the incident remain a mystery but even a modern airliner is no match for a real thunderstorm and there is some evidence that this flight encountered one.

Philip Greenspun’s Weblog

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