Did a Mega-Storm Doom Air Asia 8501?

What exactly happened to Air Asia Flight 8501 over the Java Sea on December 28? Indonesian officials are not helping to make the picture any clearer.

They are releasing piecemeal details that lack connection and do not add up to a coherent account of what the investigators have found.

As they are obliged to under the international rules for reporting air crash investigations, the Indonesian National Transportation Safety Committee has submitted its preliminary report to the International Civil Aviation Organization in Montreal.

The chief of the NTSC, Tatang Kurniadi, told a news conference in Jakarta that the report included information from the airplane’s flight data recorder and cockpit voice recorder but was factual and “contained no analysis.” He said that it would be months before more details would be made public.

At the same time the head of the investigation said the black box provided “a pretty clear picture” of what happened, but did not give details.

The investigators did confirm that the Airbus A320 was being flown by the copilot, Remi Emmanuel Plesel, a French national. There was nothing exceptional in this—copilots normally fly the airplane for most of a trip. The difference in experience between the captain, Iriyanto, and Plesel, was also not unusual: Iriyanto had more than 20,500 flying hours while Plesel had 6,000.

But the most crucial detail that has now been stated by investigators several times is that the jet began suddenly to climb at an extreme rate—3,000 feet in 30 seconds—and that as a result it went into a high speed stall. And the inference has been that this was as a result of the airplane being sucked into a powerful updraft that was part of a storm system.

One problem with this is that the information comes from two different sources, one reliable and the other not so much. The cockpit data recorder captured the loud siren warning of an imminent stall—this was another fragment released by investigators, and it is invaluable. But the information about the extreme climb has so far been attributed to radar records, not the flight data recorder—and radar does not provide a reliably precise record of violent changes in an airplane’s altitude (misleading radar returns confused the picture of what happened to Malaysian Airlines Flight 370 when it changed course before disappearing into the southern Indian Ocean).

It is therefore odd that the account of the jet entering a potentially fatal high altitude stall depends on radar readings, not data that must now be available from the black box.

Data from the black box and cockpit voice recorder provides the investigators with the ability to drill down into every detail of the last minutes of the flight. Although a large part of the fuselage is proving difficult to raise from the floor of the Java Sea, most of the wreckage will be retrieved. An experienced investigator can “read” every piece of wreckage and put together a picture of how the airplane broke up. Wreckage already brought ashore, some of it substantial, will already have told a story, possibly whether the jet was intact when it hit the water or had already suffered a structural failure.

Nonetheless this crash involves understanding something beyond the normal forensic scope of an air crash investigation: the role of weather. Here the science is far less well established.

To begin with is the question nobody can yet answer: How can four airliners fly on the same route into the same storm system at the same time and only three of them fly out of it safely?

Early that Sunday morning there were two other flights ahead of AirAsia Flight 8501 and one behind it.

The first was another AirAsia Airbus A320 flying from Bali to Kuala Lumpur at a cruise height of 34,000 feet and behind it was an Emirates Airlines A380 (the world’s largest jet, able to carry at least 550 passengers) on a flight from Melbourne, Australia to Kuala Lumpur and, at 30,000 feet, already beginning its descent into Kuala Lumpur.

Air Asia 8501 followed the A380 at 32,000 feet and had just requested an altitude change to 38,00 feet. Behind Flight 8501 was another Air Asia A320 flying from Bali to Singapore cruising at 33,000 feet.

The line of thunderheads across the route had grown rapidly. This is normal in the intertropical convergence zone in the monsoon season. As it towered as high as 55,000 feet the storm line would have shown up on the forward-scanning radar of all the airplanes on that route, and been familiar to the pilots. This is a heavily traveled air corridor—similar in distance and density, for example, to the one between the U.S. northeast airports and Florida.

If pilots see a risk of high turbulence in the huge, boiling bank of cumulonimbus clouds typical of this region they will frequently make a request to alter course to avoid what they judge to be hot spots. These decisions cannot be made earlier when the flight plan is first made because the conditions change rapidly. The pilot’s judgment is as important to this decision as his instruments. It’s impossible for radar to analyze the complex dynamics of these clouds, where there are extremes of hot and cold air and strong convection currents moving vertically.

This was the situation faced over another section of the intertropical convergence zone over the South Atlantic, near the Equator, by the pilots of Air France 447 on the night of June 1, 2009. To avoid severe turbulence they altered course to go a little west of their planned route. An Iberia flight twelve minutes behind them and another Air France flight 37 minutes behind them both altered course more decisively, going 70 to 80 miles further east, not west.

But what doomed Flight 447 (while the other flights close to them experienced nothing more than a bumpy ride) was not the turbulence but ice crystals that jammed their speed sensors, called pitot tubes. The jet’s flight management computers, faced with garbled air speed data, shut down. Left to fly the Airbus A330 manually the Air France pilots mishandled the controls and began a high speed aerodynamic stall from which they never recovered.

This specific chain of events—the impact of the storm itself, the failure of the instruments to indicate the most crucial single piece of information a pilot needs to retain control, the air speed, and the consequent failure of the pilots to fly out of a situation as they should have been able to—was scrupulously analyzed by the French investigators. And they had to confess that these gigantic cumulonimbus clouds were not a fully understood peril: “The precise composition of the cloud masses above 30,000 feet is little known, in particular with regard to the super-cooled water/ice crystal divide.”

There is another recent crash that fits this pattern in the intertropical convergence zone, in the same season and at a similar latitude as the Air France disaster, this time over the African state of Mali. Last July, not long after takeoff, the pilots of an Air Algerie flight from Burkina Faso to Algiers told air traffic controllers that they were changing course to avoid a towering storm formation.

Shortly afterward the airplane lost contact. A preliminary report by the French investigators said that the jet, carrying 116 people, spiraled out of control and hit the ground at very high speed, being more or less pulverized on impact.

The airplane involved was an 18-year-old McDonnell Douglas MD-83. Unlike the Airbus jets this airplane did not have a highly automated fly-by-wire cockpit, and it cruised at lower altitudes than the newer jets—when it encountered the storm it was at 31,000 feet. The crash is still under investigation by the French.

As they flew over the Java Sea at 32,000 feet the pilots of AirAsia Flight 8501 asked for a significant change of altitude, wanting to climb another 6,000 feet to 38,000 feet, close to the operational ceiling of the Airbus A320. (The air traffic controller declined their request, limiting the change to 34,000 feet because of the other traffic in the area, but by that time contact with the flight was lost.)

Pilots I have spoken to point out that a change of altitude on its own won’t usually get you out of turbulence in cloud that rises another 12,000 feet or more above you. Course changes to the left or right are the usual way of avoiding trouble. Until we get a more detailed analysis of the jet’s trajectory and its relation to the forces inside the storm we won’t know if there was an unusually localized force like a super cell strong enough to destabilize the airplane whatever the pilots did.

There is no recent record of an airliner being grabbed in this way by such a violent hidden hand that the pilots are helpless to counteract the force as they soar upward—in the case of Air France 447, for example, the fatal stall was induced by the pilots, not a violently rising or falling column of air.

But this whole scenario raises the question of what exactly the intertropical convergence zone is. The idea that such a force existed goes back to the early days of global marine navigation and to the concept of trade winds—the prevailing winds that sailors used to take them across the oceans. Sailors knew that to the north and the south of the Equator there was some kind of convergence of winds—and this included the dreaded doldrums where there was no wind at all and ships would become becalmed, motionless under a relentless equatorial sun. By the 18^th century, with more science being applied to the study of weather, it was clear that upper atmosphere winds were influenced by polar temperatures and seasonal changes.

Meteorology is, of course, now far advanced from that point, but the behavior and nature of weather systems that ring the globe is a lot more complicated than can be explained by any general, shorthand, term like intertropical convergence zone. The term itself dates from the time in World War II when, for the first time, pilots were becoming as numerous as sailors in these regions and were aware of highly unpredictable and dangerous weather systems that for the airplanes of that era, restricted to lower altitudes, were potentially lethal.

The usual graphic representation of the intertropical convergence zone is misleading. Rather than being an organized entity it’s really a combination of different storm systems of different intensity whose only common feature is the way they move in parallel to the Equator, shifting to the north in our summer and to the south in our winter—embracing, for example, the Java Sea in the current monsoon season and shifting north across the South Atlantic and sub-Saharan Africa in our summer.

What can we take away from this with any sureness? Just that in three separate parts of the world—the south Atlantic, sub-Saharan Africa and the Java Sea—violent storms have played a role in crashes and reminded us that given certain circumstances weather remains a threat to air safety that cannot be underestimated and is still not well understood.

This is not a “Bermuda Triangle” fantasy in which some mysterious force snatches airplanes from the sky. The shape, power and ubiquity of the force is clear but its complexity makes it particularly difficult to deal with and beyond any usable computer modeling. We have been warned that climate change will increase the power of storm systems across the globe. The intertropical convergence zone, whatever it really is, is one of those systems.