The sheer size and capacity of the A380 brings with it greater than ordinary safety problems
DAVID LEARMOUNT / LONDON
Making a leap in passenger capacity is what the Airbus A380 is about. It means the manufacturer's primary new safety challenges are the aircraft's huge dimensions and managing large numbers of people.
The early versions will carry about 550 people in a three-class cabin configuration, but the company is now designing the aircraft with even larger versions in mind.
As a result, safe emergency evacuation is uniquely challenging for Airbus and the A380. It must meet the same conditions for certification as its smaller forbears: to be able to evacuate the cabin within 90s using half the available exits. Airbus says it does not see any insuperable problem in meeting this demand. It claims that the A380's passenger density is lower than that of the Boeing 747, as it has 49% more floor space but only carries 35% more passengers. That, of course, depends on whether airlines decide to go for a higher density seating arrangement than Airbus's projected standard fit.
The most dramatic difference between the 747 and the A380 accommodation, however, is that the Airbus has a full-length upper deck which will seat 199 passengers. Accommodation on the main deck is planned at 356 seats. Airbus points out that its upper deck floor is only slightly higher than the 747's - 8.1m (27ft) from the ground compared with 7.8m. But anyone who has looked down - let alone exited via - a 747's deployed upper deck slide admits that this is frightening enough without adding another 0.3m to the drop.
In fact, unless there is a need for a particularly rapid exit, the standard 747 upper deck evacuation drill is for passengers to use the stairs to the main deck and exit via the doors there. This will not be the A380 drill, says Jean-Michel Govaere of Airbus' Large Aircraft Division. The aim is to have slides capable of evacuating 70 passengers per minute "per lane", with each slide capable of handling two lanes of passengers (two exiting side-by-side).
Airbus, aware that passenger hesitation can dramatically slow an evacuation, is working with US-based slide manufacturers BF Goodrich Evacuation Systems and AirCruiser to meet several difficult certification challenges. Not least of these is to make the descent as passenger-friendly as possible.
So far, claims Govaere, trials have shown that people are scarcely more likely to hesitate at the threshold of the upper deck slide than at the main deck emergency exit which, at 5.2m, is about the height of an A340 main deck. Professor Edward Galea of the UK's Greenwich University, a world authority on passenger behaviour during emergency evacuation, confirms that trials he has observed suggest "exit hesitation time" is only slightly more from the upper deck, but admits that the upper-deck study sample has been far smaller than that for main deck evacuation.
One aspect of the 747 upper-deck slide which makes it more intimidating is that the emergency doors are small, and the slides are narrow and prone to movement in quite low winds. Airbus says that all exits on the A380 - at least five planned for the main deck and three on the upper - "will meet or exceed the size of type A" (full size) doors, and the slides will be reassuringly wide. They will also be shaped to look less steep. In addition, they will have larger side barriers, making them feel safer as slides and perform better as ditching rafts. Govaere says eight full-size doors for the entire aircraft could allow 880 passengers to evacuate within 90s.
Airbus commissioned Galea to use the Air Exodus computer tool - which he has been refining for more than a decade - to test A380 exit positions for evacuation effectiveness. He confirms that any of the exit and cabin layouts proposed by Airbus easily meet the 90s-criterion for evacuating 550 passengers, but says that if the seating densities were rearranged in non-traditional ways, evacuation could be completed even more quickly. He also remarks that, in tests not specifically associated with any aircraft type, he has discovered that an apparently sacrosanct rule requiring that the maximum distance between pairs of Type A exits should be 60ft (183m) is not relevant. Only the number of people between the exits has relevance until the distance becomes far greater, says Galea. He explains that the determinant of time to exit is not the distance alone - because queues form in the aisles - it is the rate at which people can get out of the exits.
Overcoming passenger reluctance to jump is not the only challenge for the slide designers. Fully inflating such massive slides within the required 6s is not going to be easy. In addition, the slides have to remain stable in a 25kt (45km/h) wind "from the worst direction", with all engines running at idle; and if the aircraft comes to rest nose-up or nose-down, the exit highest from the ground will still have a usable slide.
Appropriate sill height
For example, Govaere says, the forward main deck door sill is normally 5.2m above ground, but if the nose gear collapses the sill height is reduced to 2.7m. If the aircraft rocks backward onto its tail because of an aft centre of gravity, the forward main deck door sill will be 9.5m above the ground. The intention, says Govaere, is to provide for "all forseeable aircraft attitudes except the aircraft turned on its back". He says they are looking at developing smart systems so that the slide deploys to a length appropriate to the height of the door sill above the ground. One slide manufacturer will be chosen as the A380 supplier.
Slide storage for the upper deck exits will not be in the door, but "in the fuselage as it is in the A321". This makes it less likely, Govaere explains, that a passenger could inadvertently trigger inflation, and since the slide (if armed) begins to inflate the moment the door starts to open, it is also less likely that a passenger would jump before inflation is completed.
A crucial component in preparing for and co-ordinating a safe evacuation in a very large twin-deck aircraft, because of the distances involved, is first-class communication between cabin crew. France's Direction Générale de l'Aviation Civile is currently researching cabin crew communication needs. It may be necessary, for example, for crew to carry mobile intercoms rather than using only the fixed systems available on today's large aircraft.
Professor Helen Muir of the UK's Cranfield University, who devised and ran the live evacuation tests from which Galea derived much of the data for his computer models, maintains that one important lesson learned from the trials has been ignored in regulation. To achieve the most rapid safe passenger flow towards and through various exit types, Muir's studies showed that the aisle width between galley or lavatory bulkheads - often either side of the aisle just before exits - should be increased to the optimum of about 760mm (30in). The current required minimum is 510mm, which airlines normally use in practice. Airbus will use a 30in aisle space, tallying with Muir's recommendations and in harmony with a notice of proposed amendment by the Joint Aviation Authorities. Although airlines would be free to adopt the minimum unless the rules are tightened, Airbus says none of the A380 customers has rejected the wider passage concept.
Evacuation rig
An advanced aircraft evacuation research rig, commissioned by the UK Civil Aviation Authority (CAA) and partially funded by the European Union, is being built at Cranfield, UK. The modular rig can be set up to imitate various seat layouts and cabin widths for widebody aircraft, including aircraft with two decks. Robin Ablett, head of the CAA's Research Management unit, says it was constructed to include twin-aisle widebody passenger evacuation tests because most work had been done with single-aisle models, although the A380 was not a factor in the decision to construct the rig. When it goes operational in July the rig will be fully capable of simulating A380 evacuation procedures.
Galea wants to use the rig early to carry out more testing of passenger behaviour when using high exits, to increase the sample size and therefore the reliability of estimates of the average passenger hesitation time. He also wants to check the effect of the passenger use of the A380 staircase in evacuation, arguing that this may be necessary - or may happen anyway - depending on the combination of accident circumstances, whatever Airbus recommended drills are.
The policy of including a number of technology and safety improvements well above basic regulatory specification is the philosophy behind the A380's design and manufacture, says Govaere. Apart from saving customers the need to upgrade or retrofit, the purpose is to meet the as yet undefined target for long-range operations (LROPS). LROPS builds on the established extended range twin engine operations (ETOPS) standards, but is independent of the number of engines. The idea is to produce an aircraft in which the risk of having to divert is virtually eliminated, an objective that is becoming more desirable with the opening up of polar routes or services with very long oceanic sectors (Flight International, 10-16 April).
Every A380 will be designed and built with the LROPS philosophy in mind, Airbus says. The only exception to delivering total LROPS capability is that Airbus would not, as standard, fit navigational equipment which is above the customer's specification.
The need to raise standards in fire prevention, protection, detection and suppression has been highlighted by accidents like the 1996 Trans World Airlines 747 accident near Long Island, the ValuJet McDonnell Douglas DC-9 crash in Florida the same year, the 1998 Swissair Boeing MD-11 crash following an electrical fire near the cockpit, and this year's suspected fuel tank explosion on a Thai Airways International 737-400. As these events revealed the extent of world fleet vulnerability to in-fuselage fires, the US Federal Aviation Administration has released a raft of airworthiness directives requiring systems checks and insulating material upgrades.
The CAA's Ablett says the phase-out of halon as a lone extinguishant will mean that change is inevitable, and points out that the Airbus plant at Hamburg, Germany is carrying out tests with a nitrogen/watermist combination for freight-bays. Watermist systems are highly effective, he points out, providing they can be "zoned" properly, meaning directed only to where the fire is, not operating throughout the entire aircraft regardless of the fire's location. The problem with the latter method - that aircraft would have to carry prohibitively large quantities of water - is what killed off the early hopes for watermist as a viable system, following cost-benefit analysis. Now, says Ablett, halon's demise and the possibilities for zoning change the cost/benefit equation for watermist for the cabin.
Anticipating standards
Airbus, meanwhile, says it is not waiting for new rules and intends to use fuselage insulation material with greater "burn-through" fire resistance and lower toxicity, and higher specification electrical wiring coating. Govaere says: "We are making a voluntary application of future rules that are not [currently] applicable or published."
Fire detection and suppression well above traditional standards is a prerequisite for achieving the LROPS standard in reality. For that reason, says Airbus, it plans more extensive fire detection in "hidden areas" - those to which there is no access during flight - to enable crews to deal with fires more rapidly and accurately.
Freight bay fire specifications, already much improved across the world fleet since ValuJet, are to be improved further in the A380, Govaere says. One aim is to eliminate a frequent cause of diversion: false fire alarms from freight bay detection systems. These can be triggered by live cargo or fresh produce which generate unusual humidity or other atmospheric conditions.
Both real trials and software tools are being used to determine how smoke propagates in the cabin of such a large aircraft, particularly from the lower deck to the upper. The indications so far, says Govaere, are that propagation via the stairs is unexpectedly low.
Years ago, while an aircraft like the A380 was just a dream for Airbus, former chief engineer Bernard Ziegler mused that pilots and cabin crew alone should not be left in sole charge of any future large aircraft. He maintained that real-time datalinking of systems and engine performance information to the ground must enable the crew, during flight, to draw upon a bank of expertise in airline engineering and operations departments, or at Airbus itself, as soon as they needed any help.
Sufficiently good man-machine interfaces for such datalinking on the ground, however, have not become a commercial product yet. The technology is there, however, so Ziegler's prediction may ultimately come true.
Source: Flight International
