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Aviation History
1953
1953 - 0556.PDF
552 FLIGHT, i May 1953 COMET ENGINEERING . . . quences of windows blowing out at 40,000ft. The de Havilland company have approached the undisputed problem by treating windows as though they were part of the basic airframe, and stressing them accordingly. In fact, not only have Comet win dows been tested to 100 lb/sq in, but another has been pressur ized to 8J lb/sq in daily for several years and cleaned regularly with scratch-remover, as are windows in service. The Air Regis tration Board is quoted as saying that explosive decompression at 40,000ft "shouldn't happen any oftener than a wing falling off." A related subject which is also causing transatlantic concern is the provision of doors in pressure cabins. The Comet has two main doors, one aft on the port side for passengers and one on the starboard side of the forward fuselage for the crew. The passenger door hinges inwards in a straightforward manner, while the crew door lifts away from the wall and slides upwards around the roof in runners. Both, therefore, open inwards. The American Civil Aeronautics Administration, on the other hand, are currently favouring outward-opening doors—the assumption being that an inward-opening door constitutes a barrier to rapid passenger- escape after a crash. This proposal seems to suggest a lack of experience on the part of the organization concerned, for quite apart from considerations of in-flight safety, it appears very unlikely that any economic jet airliner could have such doors—such is the weight-penalty involved. Underneath the Comet fuselage are other doors: two, opening outwards, cover the unpressurized nosewheel bay, while inward- opening hatches seal the equipment bay and the rear freight hold. The two latter are opened from the outside only; like the main doors above floor level they are pressure-sealed, and lock without slamming. No leakage has been noticed with any door, neither has any locking trouble occurred. De Havillands decided, early in the design, to use single panes for the pilots' direct-vision panels and for the small, circular win dows in the entrance doors. Although the C.A.A. disagrees with such practice, D.H. feeling is that—provided the windows are properly designed and stressed—two panes are no better than one. The rest of the Comet windows are double-thickness units, the layers being (from the inside): a thin, unstressed sheet of Perspex capable of accommodating the great temperature-difference be tween the interior and ambient air without great internal stress; a gap supplied with dry air from a desiccator; and an outer, thick sheet of Perspex carrying the pressure-difference. An exception to this rule is made in the case of the windscreen panels directly in front of the pilots; these have the thick, pressure-resisting pane on the inside and a splinterproof sheet of laminated glass on the outside. Four of the passenger windows form emergency exits. They can be released from either inside or outside the aircraft and, since they open inwards, inadvertent release in the air is impos sible. These exits are above the wing. No modifications have been necessary to the airframe as a result of severe treatment in service. The structure is immensely strong and has shown no signs of tiredness, in spite of continued small- amplitude, high-frequency vibration and, sometimes, "cobble stone" acceleration-loading in clear-air gusts. Another point is that there has been no measurable performance-deterioration arising from skin wrinkling. Power Plant.—There is no doubt that it was a wise move when the decision was taken—jointly by de Havilland and B.O.A.C.— to put Ghost-Comets into service at the earliest possible date, and not to wait for the civil Avon. By so doing, at least 18 valuable months of experience are being accumulated, and the more efficient axial-engined Comets will thereby benefit. This is not to suggest that the Ghost is proving an inefficient transport power-plant. Far from it; the de Havilland Engine Company have shown that the turbojet can be developed for air line service until it requires fewer man-hours in maintenance than the average equivalent piston engine. Only one major fault has occurred: during the first few months of 1952 B.O.A.C. experienced a run of cracked compressor impellers, which necessitated immediate rectification. The trouble was quickly traced to high-frequency fatigue and a cure was effected by slightly cropping the impellers which did not affect the engine performance. A failed engine was then subjected to a 17-hour simulated Johannesburg-London flight, without trouble. Incidentally, none of these compressor failures was noticed by the crew at the time of occurrence—which meant that the aircraft was, in most cases, at the London maintenance base when the trouble was discovered. It is particularly important to phase-in inspections of the com bustion chambers and flame tubes with the main overhauls of the engines. "Can" inspections are, although of minor consequence in themselves, annoying in that they demand removal of the engine from the airframe. The initial engine-overhaul experience was obtained with G-ALZK, the Ministry of Supply Comet 1. At first, main over hauls were fixed at 250-hr intervals, but B.O.A.C. started sched uled operations with a 375-hr period. This was not a particularly good schedule, as the can inspections were set at 150-hr intervals —a figure which does not divide neatly into 375. As a consequence, A.R.B. approval was obtained for a 450-hour cycle on the main overhauls, and this has been the arrangement used during most of the Comet's first year of work. At the present time, B.O.A.C. have eight Ghosts running to a 600-hr main-overhaul cycle, which allows three can-inspections between each overhaul. Additionally, Joseph Lucas have developed a new flame-tube—the Mk V—which B.O.A.C. expect to adopt, initially on a 200-hr basis and later with inspections every 300 hr. Very soon, therefore, the engine cycle will be: 300 hr, can inspec tion; 600 hr, overhaul; 900 hr, can inspection; 1,200 hr, overhaul; and so on. Admittedly this may appear to entail more work than does the upkeep of well-established piston engines, with overhaul periods of the order of 1,000 hr, but it should be appreciated that the actual work involved in carrying out a Ghost overhaul is consider ably less. Some figures will bear out this contention. With the Stratocruiser, some 40 man-hours are needed to change an engine, and even when removed from the airframe the Wasp Major is a complex power plant requiring much breaking- down and building-up. But Comet engine changes at base run The power plants of the Comet are well and truly buried in the wing. This drawing, which shows the port inner Ghost, illus trates well the manner in which the various related equipment is installed. The entire engine space is divided into three bays (by steel firewalls stippled in the small sketch) and each bay has its own extinguishing nozzles or spray rings. Cabin-air heat-exchanger intake. Cabin-air heat exchanger intake duct. Hot air to heat exchanger. Shutter control. Detachable bottom panel. Hydraulic pump. Air supply duct. Low-pressure fuel cock. Fuel flowmeter trans mitter. Fuel pipe to outboard engine. Engine trunnion mounting location. Access panel in engine rib. De-icing air valves. Methyl bromide extinguisher bottle. Inner (split) flap centre hinge. Flap-operating cable wheel and linkage. Tailpipesupporting roller. De-icing air duet. Detachable lagging. Tailcone sling. Access panels. Alternator. Starter. De-icing duct. Cooling air from alterna tors and rectifiers. De-icing mixing chamber spray. Wing equipment-bay spray. Fire zone No. 1. Fire zone No. 2. Engine spray rings. Methyl bromide extinguisher bottles.
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