by Palmer, Bill
At 01:34 the crew contacted Atlantico and successfully completed a SELCAL check, which verified that the controller could call the flight. That was the last radio voice contact with Air France 447.
At 01:35 the Atlantico controller asked the crew three times for its estimate for passing TASIL. The crew did not answer, nor was the call noted on the voice recording.
One disadvantage of both HF with SELCAL, and CPDLC is that you cannot hear what the pilots of other aircraft are doing and saying. To somewhat make up for that, pilots will often communicate on a designated air-to-air frequency to exchange such information. The night of the accident other flights along the route were diverting around the worst areas of weather, but no air-to-air calls are in the transcript. The crew of AF447 was on their own to determine if a deviation around weather was warranted.
Chapter 4: Intertropical Convergence
As the flight progressed north of the Brazilian coastline, it entered a band of weather fueled by global circulation patterns known as the intertropical convergence zone (ITCZ).
Soon after the accident occurred, it was a reasonable guess that they had flown into a large thunderstorm and suffered severe damage, which brought the airplane down. After all, the airplane flew through an area of known severe thunderstorms and was lost minutes later.
A detailed meteorological analysis of the flight was done by Tim Vasquez, a meteorologist who did weather route forecasting for the US Air Force in the mid 1990’s. His excellent analysis can be found at his Weather Graphics website: http://www.weathergraphics.com/tim/af447/. If you are at all interested in the more technical aspects of the weather, do not miss this site. Several graphics in this publication are provided courtesy of Mr. Vasquez.
Weather does play a part in this accident. The crew flew into an area of heavy weather, with only a slight deviation from course. The pitot tubes became clogged and shortly thereafter the crew was unable to maintain control of the airplane.
But airplanes have been dealing with thunderstorms and icing for the better part of 100 years, and in high altitude jets for over 50. So what happened here?
The evidence points to a unique combination of the characteristics of the inter-tropical convergence zone, and the specifics of particular models of pitot tube used by Airbus.
The A330-200 and -300 models were launched in 1994 and 1998 respectively. It would be 2008 before this pitot tube issue was known and well after the accident before it was understood.
The ITCZ
The ITCZ is a region that circles the globe near the equator. The zone moves slightly north of the equator in the northern-latitude summer (May-September), as it was on the night of the accident.8 Unlike land-mass thunderstorms which are often driven by convection from below or by frontal action, the thunderstorms of the ITCZ are driven by global circulation patterns with warm moist air coming from the equatorial region. The prevailing easterly trade winds of the Northern and Southern hemispheres converge provide the lifting action required to create a storm. As with all thunderstorms, when the air rises it expands and cools leading to cloud formation.
The tropopause is the dividing line between the troposphere and stratosphere and acts like a ceiling on vertical storm development. It is the point at which an anvil head forms on many thunderstorms, as they cannot grow any higher. Near the equator, where the ITCZ is, the tropopause is typically in the 50-60,000 foot range, as opposed to 30-40,000 feet typical for the mid latitudes. The high tropopause at these low latitudes means that the storms can grow to great heights leaving little prospect of flying over them.
The significance of the ITCZ for aviators is that the oceanic thunderstorms within it show up poorly on weather radar. These equatorial storms also tend to produce less lightning than higher latitude storms, which may tend to mask their severity - especially at night. A moonless night and lack of lightning makes it difficult to make a visual evaluation of the storm. For AF447, the half moon was setting in the west, off the aft left of the airplane. The storm tops, by some estimates, towered another four miles above them.
Studies of storms in this region have shown a weakening of updrafts in the 20,000 foot range, which may account for the lesser amounts of lightning produced. Above approximately 20,000 feet ice crystals form. This shift to a lower energy state of matter (water to ice) gives off a small amount of heat which then adds to the updraft’s upward velocity to reach, and often penetrate, into the stratosphere.
A meteorological analysis of the flight 447 theorizes that the thunderstorm tops reached 56,000 feet, with updrafts strong enough to penetrate the stratosphere by about 6,000 feet.
The final accident report states, “The Captain appeared very unresponsive to the concerns expressed by the PF about the ITCZ. He did not respond to his worry by making a firm, clear decision, by applying a strategy, or giving instructions or a recommendation for action to continue the flight. He favored waiting and responding to any turbulence noticed. He vaguely rejected the PF’s suggestion to climb, by mentioning that if “we don’t get out of it at three six (36,000 ft), it might be bad”.
An enhanced satellite image, courtesy of Tim Vasquez’s site, shows the track of AF447 in relation to the storm. The image was captured about 5 minutes after the airplane entered the storm. The flight path is noted as a yellow line. The small deviation from their course did little to avoid the worst part of the storm.
An animation is also available from the BEA which shows the deviation of other aircraft through that area, including AF459 which was on the same assigned track as AF447, but about 37 minutes in trail.9
The graphic below is an annotated snapshot from the animation. The deviation paths of other aircraft can be seen in blue, purple, and orange. AF447’s track is in yellow.
In the minutes prior to the accident, the crew discussed the appearance of St. Elmo’s fire. St. Elmo’s fire appears as a glowing static discharge, often accompanied by small lightning like discharges on the radome and windscreen. On the A330, St Elmo’s fire is often seen as a hazy glowing ball on the ice probe that protrudes forward between the windshields. The ice probe is where pilots check for airframe ice accumulation because it is very difficult to see any other part of the airframe. A YouTube.com search for “St. Elmo’s fire cockpit A330” will yield numerous examples of this for you to see. In my experience whenever I have seen St. Elmo’s fire, turning on the exterior lights has revealed snow conditions, which can lead to the static charge build-up causing the phenomenon. At 01:37 the captain remarked, “it’s snowing.”
The following illustration, from weathergraphics.com, is the result of analysis of satellite and other data on the storm’s profile and the flight’s progress through it. Light shading is precipitation near the surface, medium shading is cloud material, and dark shading is suspected updraft areas. The green line (not part of the original image) is an approximate vertical path through the storm.
Air France 447 encountered conditions that clogged its heated pitot tubes with frozen material. The question is-“How could heated pitot tubes ice over?”
One early theory, and the subject of a NOVA television production, The Crash of Flight 447, was one of supercooled water - water cooled to below its normal freezing temperature, yet remaining as water. When disturbed, the water freezes almost instantly.
Supercooled water is not difficult to produce in your own freezer at home. Take a bottle of purified water and put it in the freezer. Hours later you may find the water still in liquid form, but if you agitate it, the ice crystals will grow and it will freeze solid within seconds. This can happen in the air too. If an airplane encounters supercooled water, significant ice accumulation will rapidly occur.
Many experts discounted the likelihood of supercooled water in this type of storm. Supercooled water in the atmosphere would not only have iced over the probes, but the entire airplane, and that did not happen. We know this because the A330 is equipped with two very sensitive ice detectors, and the flight data recorder revealed that at no
time during the flight did they detect any ice accumulation.
One commenter on the Weather Graphics website’s AF447 article provided this interesting observation: “ I'm an aircraft icing specialist and wanted to point out a factor that hasn't been discussed much...high ice crystal concentrations. I've seen flight test data from power rollbacks due to flight in high ice crystal environments … In our case, the crystals collected within heated, aspirated Ram Air Temperature sensors, forming a 0°C slush…”
Seconds before the pitot tubes clogged, ice crystals hitting the exterior or the airplane are heard on the voice recorder. Ice crystals bounce off the exterior of an airplane and cause no visible ice accretion, but they can enter the probe inlets. When highly specific climatic conditions exist in combination with certain combinations of altitude, temperature, and Mach, the concentration of ice crystals entering a probe can exceed its capacity to melt and evacuate the moisture through its drain holes. The result is that the ice crystals form a physical barrier within the probe that disrupts the measurement of total pressure.
The final report states, “As soon as the concentration of ice crystals is lower than the de-icing capacity of the probe, the physical barrier created by the accumulation of crystals disappears and measurement of the total pressure becomes correct again. Experience and follow-up of these phenomena in very severe conditions show that this loss of function is of limited duration, in general around 1 or 2 minutes.10”
The type of particle that has been suggested is graupel. Graupel forms when tiny super cooled water droplets adhere to snow crystals to the point that they engulf the snow crystal itself.
11
The graupel theory is supported by the following evidence from the accident investigation:
No airframe icing. The supercooled water theory is discounted by the non triggering of the A330's icing detectors.
Graupel has large enough particles to be audible on the voice recorder. It takes a particle with enough mass and inertia (a given density) to hit the fuselage with a sound, instead of flowing around it with the relative wind, like snow.
Graupel has enough mass to temporarily overwhelm pitot anti-icing when concentrations are high enough. The pitot tubes are hot. But even if you put a snowball on a hot skillet it does not melt instantaneously. If there is enough mass in the blockage, and in combination with new particles being added to the blockage as the first ones melt, it may exceed the pitot tubes capability to melt the obstruction as fast as it is introduced. Graupel is of significantly higher density than snow.
Graupel has sufficient blocking properties to prevent efficient transmission of dynamic pressure within the pitot tube. For example, water can flow and transmit pressure within the pitot tube, though it too can alter pitot-static readings, a physical non-fluid blockage could shield the pressure sensing port.
The likely presence of snow or similar form, as evidenced by the St. Elmo's fire discussed by the crew. The accident report stated that the sound of ice crystals hitting the aircraft can be heard about 20 seconds before the airspeed loss and autopilot disconnect.
The detailed inner workings of the ITCZ thunderstorms are not well known, and the specifics of high concentrations of ice crystals within them is one of the unknown factors. Pilots routinely try to avoid flying through thunderstorms, so it is no wonder there is not a great deal of experience of flying through them in this area.
In regards to testing pitot tubes, the final report states:
There are many wind tunnels around the world in which this type of test can be performed. Each wind tunnel nevertheless has its limits and its own utilization envelope in terms of speed, minimum temperature possible and water or ice crystal concentration.
It is important to note that there are no wind tunnels capable of reproducing all the conditions that the crew may be confronted with in reality.
Furthermore, some scientific studies are under way to characterize the exact composition of the cloud masses above 30,000 ft. They show in particular that not all the phenomena are known with sufficient precision. This is particularly true concerning the nature of ice crystals (size and density) as well as the dividing level of supercooled water and ice crystals.
Chapter 5: Into the Weather
The crew was aware of the storm before they entered it, but probably not its severity. At 01:46, about 24 minutes before the pitot tubes clogged and the autopilot disconnected, First Officer Bonin dimmed the cockpit lights to see outside and noted that they were entering the cloud cover. At 01:50 the captain and First Officer Bonin discussed their desire to climb higher to get above some of the weather, but that the airplane was too heavy for the outside temperature to do so.
At 01:51 the captain remarked, “All we needed was Mr. St. Elmo,” obviously referring to St. Elmo’s fire. Bonin said, “I don’t have the impression there was … much … storm … not much.”
At 01:59 First Officer Robert returned to the cockpit from his rest break. First Officer Bonin briefed him on the weather saying, “Well the little bit of turbulence that you just saw, we should find the same ahead. We’re in the cloud cover unfortunately we can’t climb much for the moment because the temperature is falling much more slowly than forecast.”
After the captain left, Bonin specifically mentioned the Inter Tropical Convergence Zone to First Officer Robert and the location where they would soon encounter it.
At 02:06:05, four minutes before the autopilot disconnected, Bonin called the flight attendants and said, “in two minutes there, we ought to be in an area where it will start moving around a bit than now you’ll have to watch out.”
But First Officer Bonin, who was the pilot in command at the time, apparently had no thought of deviating around the weather.
At 02:08 First Officer Robert changed the gain on the radar to MAX. That will often significantly increase the displayed weather on the screen - both in quantity and intensity. It must have depicted parts of the storm not previously displayed or noticed. He suggested, “Do you maybe want to go to the left a bit? You can possibly go a bit to the left. I agree that we’re not in manual, eh? Well, you see at twenty with the …” Then “It’s me who just changed it to max.”
They then turned 12° left.
At 2:08:17 there was a change in the background noise of the precipitation striking the airplane. Shortly thereafter Bonin commented on a change in the cabin temperature. “Did you do something to the A/C?” He also noticed a smell, apparently concerned, “What’s that smell now?”
Robert recognized the smell and answered, “It’s ozone, that’s it, we’re alright.” Then explained that ozone is, “the air with an electrical charge.”
02:09:20 Robert commented, “It’s amazing how hot it is all of a sudden.” Twenty seconds later, the background noise changed and then intensified. The sound was identified by investigators as similar to the typical sound of ice crystals striking the airplane.
The turbulence intensified and they slowed the aircraft from Mach .82 to the turbulence penetration speed of Mach .80, and the engine anti-ice was selected on. Then at 02:10:02 the autopilot disconnected and within 7 seconds the indicated airspeed fell from 274 knots to 55 knots.
The discussion among the two first officers was ignored in the accident reports. However, it bares a striking resemblance to a first-hand account of a Northwest Airlines A330 pilot who encountered a loss of airspeed event in the South Pacific, also in the ITCZ12. The airplane’s air conditioning system, which extracts its air supply from air coming through the engines, became overwhelmed by the amount of water in the air. It is indicative of the conditions in the updraft they were flying through. The crew reported:
Outside air temperature was -50C SAT -21C TAT (you’re not supposed to get liquid water at these temps). We did.
As we were following other aircraft along our route. We approached a large area of rain below us. Tilting the weather radar down we could see the heavy rain below, displayed in red. At our altitude the radar indicated
green or light precipitation, most likely ice crystals we thought.
Entering the cloud tops we experienced just light to moderate turbulence. (The winds were around 30 kts at altitude.) After about 15 sec. we encountered moderate rain. We thought it odd to have rain streaming up the windshield at this altitude and the sound of the plane getting pelted like an aluminum garage door. It got very warm and humid in the cockpit all of a sudden.
Five seconds later the captain’s, first officer’s, and standby airspeed indicators rolled back to 60 kts. The auto pilot and auto throttles disengaged. The Master Warning and Master Caution flashed, and the sounds of chirps and clicks letting us know these things were happening.
The Capt. hand flew the plane on the shortest vector out of the rain. The airspeed indicators briefly came back but failed again. The failure lasted for THREE minutes. We flew the recommended 83% N1 power setting. When the airspeed indicators came back. We were within 5 knots of our desired speed. Everything returned to normal except for the computer logic controlling the plane. (We were in alternate law for the rest of the flight.)
We had good conditions for the failure; daylight, we were rested, relatively small area, and light turbulence. I think it could have been much worse. The captain did a great job flying and staying cool. We did our procedures called dispatch and maintenance on the SAT COM and landed in Narita. That's it.
The Air France 447 investigation concluded that ice crystals had clogged the pitot tubes. But the similarity in sounds between and interior air conditioning effects between the above account and AF447 indicates that water ingestion should not be completely discounted.
Alternate law is locked in for the remainder of the flight, and the autopilot cannot be engaged, in cases where the airspeed does not return to within 50 knots of the original airspeed within about 10 seconds.