Understanding Air France 447 (6 page)

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 ITCZ
¹²
. 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% N
₁
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.

On February 2, 2013, an Etihad Airways A340-600 experienced an unreliable airspeed incident en route from Abu Dhabi, UAE to Melbourne, Australia. The incident’s preliminary report
¹³
states:

While cruising at FL350, just leaving Colombo FIR and entering Melbourne FIR, the Aircraft encountered moderate to heavy turbulence and experienced significant airspeed oscillations on the captain’s and standby indicators. The autopilot, autothrust and flight directors were disconnected automatically. The aircraft’s flight control law changed from Normal to Alternate Law, which caused the loss of some of the flight mode and flight envelope protections. The change from Normal to Alternate Law occurred twice, thereafter the Alternate Law stayed until the end of the flight. Auto-thrust and flight directors were successfully re-engaged, however, both autopilots (autopilot 1 and 2) could not be re-engaged thus the Aircraft was controlled manually until its landing.

Weather Radar

All of these affected flights had operating weather radar, but interpretation of weather radar requires skill, experience, and understanding of the principles involved. It is not unlike a doctor reading an x-ray. We can all find the bone on an x-ray, but it takes training and experience to discern meaning from the subtle shadows and gradients. Airborne weather radar is much the same. The radar control allows the pilot to adjust the tilt of the radar beam, the sensitivity of the receiver (gain), and the range and brightness of the display. The colors and shapes have to be interpreted. Unfortunately the pilot does not have the option to call in a radiologist.

Pilots rely on weather radar to navigate around storms. The objective is to avoid dangerous turbulence. Radar does not directly indicate turbulence. But operating and interpreting it correctly can indicate conditions where varying degrees of turbulence are likely to be.

Radar sends out pulses of radio waves and then listens for those waves to bound back off of water droplets about the size of a raindrop.

Heavy concentrations of water droplets are often associated with the strongest part of storms, and the correlation with turbulence is quite good. The additional use of Doppler signal processing on some radar sets allows the radar unit to measure the movement of different areas of water droplets within a storm for an additional indication of turbulence, but this is generally only available at short range (40 miles).

Radar reflects poorly when liquid water is not present. It does not reflect off water vapor, the micro-sized droplets that form most clouds, and reflects poorly off ice crystals ranging from snow flakes to hail stones. Unfortunately, the upper portions of thunderstorms have fewer water droplets and more ice crystals than the lower portions and therefore do not show up well on radar.

Radar returns on the A330 are displayed in three colors on the Navigation Display (ND), overlaid with navigation and TCAS (traffic) information.

The operation of airborne weather radar is something that is unfortunately not taught very well or formally in a classroom environment. The basic principles of operation are well known by most, if not all, commercial pilots. Pilots are taught some basic tips for interpretation, but for the most part it is on-the-job training in radar operation. Few flight simulators allow for depiction of weather on the simulator’s displays. Those that do are fairly unsophisticated, lacking the realism necessary for in depth training on interpreting the subtleties of the display. It is safe to say that valuable simulator time is not spent learning how to interpret weather radar displays, even if it had the fidelity to do so.

There are many aspects that a pilot must take into consideration when interpreting a radar display. Among them are the intensity of the return (shown in colors), the gradient and shapes of the different intensities, blocking of returns by heavy weather or the curvature of the earth, and differentiating between radar returns reflecting off the ground and actual weather.

Not all areas displayed on radar need to be avoided. It is often impossible to avoid them all. Patterns of the radar display intensity and the gradient of radar returns as well as the shape of those returns provide clues as to the nature of the weather. Additionally, the altitude of the airplane and the curvature of the earth make it impossible to see storms in close proximity yet below the airplane, or those over the horizon. When determining the height of a displayed area of weather a pilot must also consider the width of the radar beam, and that ice crystals and hail (often found in the upper portions of a storm) reflect radar signals poorly.