Air France Flight 447 from Rio de Janeiro to Paris crashed into the Atlantic Ocean about 725 km (450 miles) northeast of Brazil at about 2:30 a.m. local time, Monday, June 1. The accident occurred three hours into the 11 hour flight; 228 people were aboard the twin-engine Airbus A330-200 jet. While flying at 521 mph (839 kph) at 35,000 feet (10,671 m) at 2:15 a.m., the plane encountered heavy turbulence. An automated communications system in the airplane began an exchange of data with Air France maintenance computers on the ground that totaled four minutes and indicated that multiple electrical and pressurization failures had occurred. The last contact was at 2:33 a.m. There was no distress call from the pilots. Brazilian air force planes searching the area found a five kilometer strip of floating debris including cables and fuel slicks. Brazilian and French ships should arrive early Wednesday to begin the accident investigation and the recovery of bodies. (See early news bulletins, 1 and 2, and later bulletin 3)
Determining what actually happened will require recovering and examining the remnants of the airplane, in particular the major fragments of the airframe, the engines, and most importantly the flight recorders (flight data recorder and cockpit voice recorder). Without the benefit of a pilot’s radio report, investigators would only have the flight histories of the selected components and parameters that are monitored by the flight recorders. One worry is that the flight recorders are submerged at between 9,000 ft (2744 m) and 14,000 ft (4268 m) and may be difficult to find; their casings are designed to withstand the pressure of 6000 m (19,680 ft) depth for up to 30 days.
Speculation about what may have happened centers on lightning causing a massive electrical surge which caused the failure of (fused, short-circuited, overloaded) the fly-by-wire flight controls; and severe buffeting in a thunderstorm, which disrupted the flight-path.
Pilots of fly-by-wire airplanes do not normally use muscles or hydraulics to move the many flaps and the rudder; electric motors controlled by computers do this to an extent set by the pilots’ manipulation of their control levers, pedals and wheels. The lightning-blackout scenario is unlikely because lightning strikes happen regularly in commercial and military aviation, and airplanes are designed to withstand them (by keeping the electrical charges outside the plane’s interior, because of the all-metal hull and wing surfaces). Also, pilots are trained (in simulators) to compensate for loss of the fly-by-wire system by using the mechanical control system of the trim tabs to push the bigger flaps and the rudder into place. Trim tabs the smallest of the many types of movable flaps, which act like the tails of weathervanes, pushing the larger flaps or rudders they are embedded in into new angles; in normal operation the trim tabs are an assist and a fine adjustment.
The lack of a distress call from the pilots suggests two possibilities: their radios were inoperative or had lost power (which would be odd since the automated data transmission system was functioning), or a very sudden breakup of the airframe in flight. Airframes of modern commercial airliners like the Airbus A330-200 are designed to withstand the buffeting of air turbulence and the stresses of the severe turns and dives that may occur in emergencies. If the airframe broke up prior to impact with the water, what was its cause? Obvious guesses are: missile, bomb and fuel tank explosion.
There is no evidence of a missile attack, so we eliminate that guess. Sabotage by bomb is also discounted, because that guess requires too many elaborate assumptions. Actually, any speculation is entirely unjustified at this point, since recovering evidence and systematic analysis have yet to begin. However, events like this inspire fear and cause minds to race, speculating on causes and meanings.
So, we are led to the question of a fuel tank explosion, could it have happened in AF 447 as either a natural event (electrostatic discharge into fuel-air vapors above liquid fuel in an agitated tank) or an unintended electro-mechanical failure (spark from wire exposed by damaged insulation, into fuel-air vapors) as in the TWA Flight 800 disaster of 1996.
The fuel tanks of airliners are fitted into the wings and the section of the hull below the passenger deck and along the length of the wing roots. The central fuel tank between the wings is usually the single largest volume fuel container. As a plane climbs to higher elevation, the atmospheric pressure drops and so the air originally contained in a fuel tank at the airport seeks to expand; if the tank were sealed this would cause the internal pressure to increasing exceed the external pressure and put great stress on the tank walls. Similarly, as fuel is pumped out of a tank to the engines, the space evacuated must be filled with ambient air to avoid creating a vacuum that would resist subsequent pumping. So, fuel tanks are designed with vents that allow air to flow in, and fuel-air vapors to flow out as needed to equalize pressures at any altitude. The vents are in the form of pipes that run from the central fuel tank through the wing tanks and out to the wing tips where an orifice, at each wing tip, allows for the exchange of air.
Because hydrocarbon liquids and vapors are very insulating electrically, and metals are excellent conductors, there is always a build-up of electrostatic charge between fuel flows and metal containers. This is why sparks can be generated when fuels are pumped into rapid flows or sprays near metal surfaces. There have been many accidents caused by electrostatic discharges into fuel-air mixtures, which were initiated by improperly grounded or excessively turbulent pumping procedures. The petroleum industry has long known about this phenomenon, and developed many standards for the design and operation of fuel pumping and storage equipment. Also, many fuels have chemical additives that enhance their electrical conductivity, to significantly reduce their ability to hold electrostatic charges (reducing the electrostatic build-up relative to the metal piping and containers during pumping and/or sloshing).
The fuel vent pipes of airliners are one type of fuel pumping system. These must be designed to minimize the electrostatic build-up between the flowing fuel-air mixture and the pipe walls. Larger diameter vent pipes will keep flow velocities low (electrostatic build-up and the possibility of sparking increase with flow velocity). Plenum chambers and baffles along the flow path can help prevent bursts of rapid flow in reaction to some mechanical jolt to the wing structure or some sudden drop in external air pressure (which would impulsively draw out fuel-air vapors). Using additives to significantly increase the fuel’s conductivity is also extremely helpful.
Because the airplane manufacturing, airline transport and fuel industries routinely do a good job of managing the fuel and fueling risks, it is rare that we hear about fuel fires and explosions on the tarmac or in flight. However, fuel vapor explosion accidents still do happen; a Boeing 737-400 parked at the gate in 2001 had its center tank explode, killing one person (See 4).
Despite the many airliners that experience lightning strikes without harm, it may be that the destruction of AF 447 was the rare instance of lightning igniting the fuel-air vent flow, which subsequently caused a major fuel tank explosion.
Whatever the cause of the loss of AF 447, the loss of 228 lives demands that it be found, and the lesson applied to improve air transport safety.
MANUEL GARCIA, Jr., a former physicist at Lawrence Livermore Nuclear Laboratory, can be reached at mango@idiom.com
Notes:
[1] AFP news bulletin,
[2] CNN news bulletin,