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Airliner doors are high-tech items
By Michael Lagemann
When Hollywood directors turn to the subject of aircraft, often they allow themselves to get carried away by their imaginations, resulting in quite spectacular scenes. Thus we get people sucked out of the aircraft because the bad guys opened a door during the flight. Yet, when it comes to the effectiveness of door seals, nowhere is the discrepancy between truth and fiction more glaring than in a film.
As Karl Specht, Fleet Engineer for Airframe and Systems Engineering at LTU Technik in Düsseldorf, with responsibility for the passenger doors of large commercial aircraft, points out, “Since the introduction of commercial aircraft with a pressurised cabin, it has been virtually impossible to open the doors in-flight.” If they are not with Karl Specht in the hangar, then most of these doors will be in the air at an altitude somewhere in the region of 10,000 to 11,000 metres. Up there, bleed air from the engines is used to keep the cabin pressure raised to a level that is tolerable for people, since above 2,500 metres the air gradually becomes too thin to breathe. “It is precisely that pressure differential between the cabin and the thin external air that is harnessed in keeping the doors locked shut,” Specht explains.
Boeing, for example, relied for many years on rotatable doors referred to as “plug-type doors”. These are larger on the inside than the opening in the fuselage. This means that they can only be opened inwards into the cabin. But when the raised cabin pressure exercises force on the door, it is pressed into its frame like a wedge and sealed.
This design offers a high degree of safety against loss of pressure at high altitudes. But on the ground, doors opening inwards tend to get in the way of passengers embarking and disembarking. To ensure that the exit area, which in any case is on the tight side, is not also blocked by the door plus its in-built emergency chute, rotatable “plug-type” doors can also be swung outwards at the side. But how does the door pass through an opening for which, as previously mentioned, it is too big?
“The trick lies in a special mechanism,” Specht explains. When one activates the door handle, four cam followers are released from their catches in the frame. At the same time “gates” swing back at the top and bottom of the door. These metal flaps only a few centimetres high open the cabin to release the last minimal pressure differential. As a result the door can now be swung inwards by 90 degrees on two hinges. A cam on one of the hinges forces it into a slightly inclined position. Thus inclined and effectively shortened, the door fits through the opening.
The principle of swinging out the enormous “plug-type doors” was used in many of the well-known Boeing types up to the beginning of the 1980s. It was not until the introduction of the 767 in 1981 that a different solution was adopted. This aircraft with its wider fuselage was fitted with vertically opening doors which could be moved at the press of a button. These “slide-up doors” spring inwards a few centimetres when the door lever is actuated and are then drawn upwards into the fuselage by electrical winches. This design creates a lot of space as the area available for the passengers is not constrained by the radius of swing or by jutting hinge pivots.
“A light touch on the button and up it goes!” This was especially welcome to the more slightly built air hostesses, who had sometimes had quite a struggle manoeuvring the unwieldy and often heavy swinging doors of previous Boeing models. For safety reasons, this feature was there right from the design phase, as the slide-up doors were also bigger than the opening in the fuselage which was to be locked. Once lowered from their hiding place in the ceiling area, the cabin pressure pushed them firmly into the smaller frame. Nevertheless, this concept was not pursued further. “The complex actuators and associated extra weight quickly cancelled out the benefits in operation,” Karl Specht believes.
As a result Boeing's next widebody jet, the 777, which made its debut in 1994, featured a door concept much more similar to the cabin doors Airbus employs. These “translating doors” open directly to the outside. Unlike the swinging doors of the older Boeing models, which were rotated by 180 degree, only the outside of the door is ever seen. However, the doors are no longer protected from opening in flight by their larger dimensions. “Nevertheless, they are still plug-type doors,” explains Karl Specht. “They may not be bigger than the fuselage cutout, but they ensure through a special mechanism that the cabin pressure makes it impossible for them to open outwards.”
In the closed state, all the door stops are latched in behind their counterparts on the fuselage. When the opening lever is actuated, the door is then initially opened inwards by only a few millimetres and raised by a few centimetres. This is accomplished with the aid of slide rail guides in the frame in which shank rolls move on a fixed track. In this way the door stops are unlatched from their supports on the fuselage and the door can be manoeuvred outwards on two swinging arms.
To close the entrance so that it is airtight, additional rubber seals are used which are specially encased for longer life and against freezing on the fuselage. “These seals have a design similar to that of a car door. But they are also perforated and expand under the impact of the cabin pressure so that the smallest gap is effectively sealed,” says Specht. In his experience, the process of fitting the door to the fuselage is quite a bit more complicated at Boeing than at Airbus. “At Boeing, the doors are always built larger than needed and then fitted on-site to the exact dimensions of the door opening with a milling machine, so that every opening has its own, special door.” Moving a door to a different fuselage is very difficult even with the same models and the same installation locations. At Airbus on the other hand, it is easy to move a door from one fuselage to another, as long as it is in the same position on the aircraft.
In Karl Specht's view, Airbus doors are easier to handle and also simpler to maintain. “The old plug-type swingable doors became more and more difficult to operate over time as, in particular, the bearings of the complex swing mechanism were allowed more and more slack,” he recalls. “The mechanics of the Airbus doors are significantly more wear-resistant.” Nevertheless, all doors are checked over once a year and every five years they are completely taken apart and examined.
To ensure that the passengers embark and disembark in the right places, LTU's A330's have door monitors in the cockpit and the cabin. These enable one to see at a glance which door is currently open or closed and whether the emergency chutes have been activated. With this system, the crew would notice immediately if a door had been opened either unintentionally or deliberately, something which, however, is difficult to imagine in the case of a pressureless cabin on the ground. The seal on a pressure cabin is only really 100% guaranteed once it is at cruise altitude. Perhaps the Hollywood directors will one day make a concerted effort to ensure that even in films the seal on the door reflects the true situation.
From FLUG REVUE 8/2007
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