Weather, Landing, Other Factors At Issue
The Aug. 2 landing of Air France Flight 358, an Airbus A340 from Paris with 309 passengers and crew aboard, at Toronto Pearson International Airport was to have been the last before the airport was closed due to inclement weather, forcing other inbound aircraft to divert to other airfields, according to preliminary accounts.
The skies over the airport were dark with clouds, a blackness punctuated by the bright flashes of lightning from a heavy thunderstorm.
The aircraft touched down, but its speed was not arrested sufficiently, and at 4:03 p.m. it skidded off Runway 24L, running through a fence and ending up in the Etobicoke Creek ravine, the aircraft’s fuselage tipped down and its tail in the air.
An emergency evacuation was ordered and all 297 passengers and 12 crewmembers were able to evacuate before the airplane was consumed by fire. As of this writing, 14 were still in the hospital, but all survived the crash.
The investigation by the Transportation Safety Board (TSB) of Canada will take a year or more to complete. Flight data and cockpit voice recorders have been recovered, and if they are readable should provide a wealth of data. The fact that the flight and cabin crews lived through the experience will add additional detail and insights into what happened. At this point, the accident sequence is speculative, but scenarios can be pieced together.
It sounds possible, from multiple witness reports, that the aircraft was hit by lightning very shortly before or after touchdown. Passengers reported that the interior lights cycled off and on. If that cycling was due to an electrical bus change-over induced by the lightning strike, and that affected the ground-air sensing circuit, the anti-skid and auto-brake systems may have been deactivated or cancelled. There’s also the question of spoiler arming becoming deactivated (that circuit also plumbing through the weight-on- wheels circuitry). If a lightning strike cancelled the auto-armed spoilers due to bus switching, then this accident might parallel the American Airlines Flight 1420 scenario at Little Rock, Ark., in 1999. In that event, the crew landing in a severe thunderstorm forgot to arm the spoilers.
If the auto-throttle was affected, that also may have caused a fast touchdown.
Then, too, the captain may have added speed during the final approach as a hedge against wind shear, which can literally push an airplane into the ground. R�al Levasseur, the investigator-in-charge for the TSB, said at a briefing that the airplane’s straight track prior to touchdown ruled out a strong crosswind from the thunderstorm.
If the pilot realized that systems were affected by a lightning strike, he may have applied maximum reverse thrust and started manual braking. The rocking and rolling of the airplane described by a witness may have resulted from the differential braking coefficients between the downwind and upwind (of the runway) mainwheels. “Differential” is here meant to infer that, courtesy of runway cross-camber, the upwind main-gear would be swimming in water and aquaplaning, and the downwind gear would be firmly in contact with a relatively dry runway. The end result would be a swerving rock and roll that would result in less than efficient manual braking – even on a benign surface.
But a lightning strike aside, even though the runway is grooved, the water depth on the runway may have been sufficient to cause the airplane to aquaplane off the end. And rubber buildup in the grooves, particularly around the touchdown zone of the reciprocal runway, would have compromised their function of increasing runway friction.
Data from Pearson airport’s ground radar suggests that Flight 358 was nearly halfway down the 9,504 foot runway before it touched down. In thunderstorm conditions, there might have been a micro-burst and the aircraft may have picked up a strong tailwind after touchdown. It’s not known if Toronto has a windshear alerting system. A runway awash with water and hail would afford very little braking action, particularly if the rubber deposits were heavy.
The ground radar indicates that the airplane increased speed after touchdown, possibly indicative of a go-around, which was then abandoned.
In any event, the investigation seems likely to focus on the following areas:
Closing the airport. The airport’s decision to close for landings after Flight 358. Should this decision have been made earlier? The crew’s decision to land will also be examined, no doubt.
The dynamics of landing. Was extra speed added to account for microbursts and/or wind shear? Where was touchdown? What systems were fully functional to retard speed on the airplane, to include lift dumping devices, auto-brakes, anti-skid and thrust reversers? What systems were not, and why not? The role played by lightning, if any, will be assessed, and the implications for aircraft design, including fly-by-wire airplanes like the A340.
Was there standing water on the runway, which would have impeded the airplane’s stopping capability?
Did the captain brief the first officer on a possible go-around? Was a go-around attempted, and then abandoned? One scenario that comes to mind is that the aircraft landed normally in a window between fairly fierce but localized storm cells, and that immediately after touchdown the window closed, presenting the flight crew with a split-second choice between a go-around straight into the cell, or to complete the arrival. In a few seconds, they would have passed from a wet, grooved section of runway to one with rubber deposits, little braking adhesion, “hailstones the size of marbles” and nil forward vision.
The essential question remains: why did the airplane depart the end of the runway at around 90 mph? The fact that all occupants survived the experience suggests that speed was abated somewhat by the aircraft’s ability to soak up the impact forces, and that the subsequent fire was slow to take hold. But full credit must be given to the cabin crew for overseeing a rapid and complete evacuation, by any standards.
Runway safety areas. The adequacy of Runway 24L’s safety area will be examined. The safety area is that space beyond the runway pavement available to an airplane to come to a stop; in the U.S., that area is to extend 1,000 feet beyond the macadam. The runway at Toronto is roughly parallel to Highway 401, one of the city’s busiest roads, and it is only a few hundred feet distant from the end of the runway. The presence of a ravine in close proximity will also be examined. This is not the first time an airplane has broken its back in that ravine. Air Canada Flight 189, a DC-9, overran a rejected takeoff there on June 26, 1978, and two passengers were killed and a hundred injured. The classic case of this kind of thing was the Swissair DC-8 crash at Athens (the old Athens airport) in 1979, in which a score of people were killed.
More recently, recall the crash of American Airlines Flight 1420 at Little Rock, Ark. The airplane landed in the teeth of a severe thunderstorm, touched down long and roared off the end of the runway at an estimated 70 mph, then plunged down an embankment, coming to rest in the Arkansas River floodplain (see ASW, Feb. 14, 2000, and May 22, 2000). As a result of this tragedy, the airport authority elected to install an arresting system for aircraft overruns. Whether a similar installation would be appropriate at Toronto Pearson will most likely be examined in the course of the investigation.
In terms of highways abutting runways, recall the March 5, 2000, runway overrun of a Southwest Airlines B737 at Burbank-Glendale-Pasadena Airport (see ASW, Aug. 26, 2002). Because of the airport’s location in a built-up area and the presence of a major thoroughfare off the end of the runway, the safety area was only 32 feet instead of 1,000 feet. The airplane careened off the end of the runway, blew through a blast barrier and came to rest on the busy road next to a gas station. As in the case of the crash at Little Rock, the Federal Aviation Administration (FAA) provided $1.9 million to install an arresting system at Burbank.
The robustness of the cabin. Passengers on Flight 358 reported overhead bin doors popping open, bags and other contents spilling out, and ceiling panels coming off. The forces at touchdown and when the airplane came to a stop will probably be reconstructed, and the strength of overhead bin doors and other cabin fittings likely will be assessed.
The robustness of the structure. Holes or fractures in the fuselage ahead of the wing may have facilitated passenger egress, although such fracturing, mentioned in initial news reports, is not clear from photos of the evacuation. Clearly, why and exactly when the airplane caught fire will be looked at. If the fire was caused or fed by igniting fuel from a ruptured tank or system, the subject of fuel system crashworthiness will probably come under scrutiny once again.
The overall flammability of structure and components will be examined; after all, the airplane was reduced to a charred shadow of its former self and burned out all the way forward to the cockpit bulkhead. And if passengers availed themselves of fractures in the fuselage to escape, then the 90-second time permitted by regulation for escape via emergency exits and slides will bear reexamination.
The emergency evacuation. The cabin-cockpit coordination of the crew, and announcements/commands to the passengers will be reviewed. Some passengers reportedly were getting out of their seats before the plane came to a halt. The evacuation, initially orderly, reportedly devolved into chaos when fire was seen outside some windows, and in their haste to get to emergency exits some passengers were climbing over seats. At least one inflatable slide failed to function, maybe two, according to passengers. Some degree of smoke wafted through the cabin. Passengers tried to take their carry-on bags or other personal effects during the evacuation, which is a definite no-no because it slows evacuation. Other passengers took photographs with their mobile phones during the evacuation, instead of heading immediately to safe ground away from the airplane.
Whether or not some passengers opened doors will also be assessed; one report has the cabin crew restricting exit to the right side, because of fire on the left rear side, yet some passengers apparently opened the forward left side doors. This will all have to be sorted out.
The failure of some slides to deploy will certainly be examined. Recall that the FAA recently issued a supplemental notice of proposed rulemaking on A319, A320 and A321 slides that may not deploy properly (see ASW, Aug. 1, July 21 entry).
Cockpit voice and flight data recorders (CVR/FDR). It appears that the CVR/FDR in the aft portion of the aircraft were recovered and may be readable. But what if they were destroyed? Photographs of the wreckage show that the aft portion of the airplane was badly burned, while the fuselage forward of the cabin/cockpit bulkhead was fairly intact. Recall that the TSB and the NTSB have called for a second set of CVRs/FDRs to be installed in the front of the airplane to ensure that cockpit voice and data is recovered (see ASW, Aug. 1). Note also that the reduced wiring runs to such front-mounted CVRs/FDRs also increase the likelihood that they’ll receive data and voice to be recorded. This issue of front-mounted recorders will doubtless receive attention by the TSB.
The emergency response. The airport rescue and firefighting (ARFF) units were on the scene within 52 seconds of being alerted. When they arrived at the wreckage, three- quarters of the passengers had already evacuated the aircraft. The adequacy of the ARFF response will be assessed, including first aid, firefighting and communications.
The security of the perimeter. Some passengers ran from the burning wreckage to Highway A401. If people can get out of the airport so swiftly, the adequacy of perimeter fencing for security against unauthorized entry to the airport is in question.
The TSB has much to investigate, and the fact that no one was killed is fortuitous.