FLUG REVUE August 2003: Aircraft windows are high-tech

LOOKING AHEAD THROUGH HIGH-TECH WINDOWS

By Matthias Gründer

On 26 May 2003, a British Midland Airbus A321 from Larnaca flew into a severe hailstorm over Germany. Despite enormous damage, the crew managed to get the aircraft safely to its destination of Manchester, along with its 213 passengers and eight crew members on board.

Exactly why the captain of G-MIDJ decided to continue on a high-risk flight rather than divert immediately to the nearest German airport for an emergency landing is neither here nor there. What is of interest here is the aircraft cockpit windows, which within the space of a few seconds were blinded by hail the size of golf balls but nevertheless withstood the might of the icy barrage. The dramatic odyssey of the A321 shows only too clearly that we are not dealing here with ordinary windows but with complex and complicated high-tech subassemblies.

The windows of an aircraft are exposed to extreme stress both on the ground and also in the air. They have to withstand tropical temperatures in the holiday resort and also the icy cold of the cruise altitude, as well as surviving unharmed the impact of weather of all kinds – right through to hailstorms like the one mentioned above –, tough ultraviolet solar radiation and even birdstrike.

Normal window glass, such as was used in the pioneering days of flying, simply cannot satisfy the demands made on it in modern aircraft construction. Today cabin windows are normally constructed from single-layer acrylic. Cockpit windows on the other hand are multi-layered and ever better hardened against the rising demands of frequent, fast flying at high altitudes. A comparison between the cockpit windows on the Boeing 707 and 737, both manufactured by PPG Industries of Huntsville, Alabama, demonstrates just what enormous advances in window technology have occurred in recent decades. Whereas their basic design has remained the same, thanks to over a dozen technical improvements their service life has risen from about a year or 1,500 flying hours to being virtually without limit, yet at the same time the amount of maintenance they require, with its associated costs, has declined dramatically.

Meanwhile, at the few producers of such cockpit windows, which include Saint-Gobain Sully of France, design data and planned performance parameters provided by the aircraft manufacturers are transformed into three-dimensional computer models, which in turn form the basis for subsequent machine processing. Depending on the purpose for which the aircraft will be used, the materials used will be specially hardened polycarbonate, acrylic or laminated glass with low weight and high visual quality that can endure rapid changes in material tensions and mechanical effects and can be heated to counteract icing.

At PPG, two kinds of glass are used for civil aviation: Herculite, a thermally reinforced glass that was developed back in 1938, and Herculite II, which is chemically processed. Here, lithium is added to the sheet glass during the manufacturing process, and after the glass has been moulded into shape it is placed in an immersion bath that contains liquid sodium nitrate. The sodium ions detach the lithium ions from the material and take their place, resulting in compaction of the surface. A third type of glass, Airphire, which possesses special optical characteristics, is produced only for military aircraft.

The synthetic glass acrylic is also used in multi-layer windows. This is very light and hardy and is bonded with glass panes into windows. Special, wafer thin, polyurethane-based coatings in turn raise the chemical and mechanical resistance of the material and are vapour deposited. Depending on composition, they can also prevent ultraviolet light from penetrating.

To make the cockpit glass high-impact resistant, it needs plastic interlayers, which nowadays are generally made of polyvinyl butyral (PVB). This material can absorb high mechanical energies and is not sensitive to changes of temperature, which is particularly important as regards the installation of windshield heating. Sheets of laminated glass, thus comprising several panes of glass and interlayers and manufactured under clean room conditions, are permanently bonded together in the autoclave without any air bubbles or inclusions.

The windshield heating in turn is as simple as it is ingenious: in aircraft with an AC power supply it consists of a wafer thin film of metal oxide generally based on indium that is vapour deposited on the inside of the outer pane and is heated with a supply of approx. five watts at a resistance of 30-50 Ohm per square inch (6.452cm2). The heat thus generated provides even protection, right into the corners, against misting or icing.

Fine heating wires, such as will be familiar from car manufacture, are generally installed only on business jets that have an AC power supply. This product is known at Saint-Gobain Sully as Airplex, and at PPG as Aircon. Finally, the panes of high flying commercial aircraft are often vapour-coated with a last layer of gold or silver to reflect the solar radiation and protect the cockpit from heating up.

All that is need now to turn the composite panes into a window is the frame. This too is a complicated matter as the frame must fit snugly and tension-free with the fuselage, with total seal proofing. If it was up to the aircraft designers, their products would not have any cutouts, as these perforate the structure and impair its strength. But aircraft cannot manage without windows, doors, loading and maintenance hatches etc., and these cutouts have to be laboriously sealed up again. The biggest problem with the windows is how to protect the side panels from penetrating moisture. One tried and tested solution is to use a Z-shaped seal made out of glass fibres or stainless steel, which in turn is bonded with polysulfide, a synthetic rubber, and padded out with various silicons.

The panels are always inserted from the inside towards the frame, so that they cannot be pushed outwards by the enormous internal pressure of the cabin. Two technologies are employed here: hoop tension, whereby an internal holding frame is screwed on, and plug-loading with multi-functional gasket frame made of vulcanised silicon. The now complete window is bolted tightly to the fuselage in such a way that it can be changed rapidly in the event of damage. In the case of the British Midland A321, the damage sustained during the hailstorm necessitated replacement. The most important thing was, however, that the window remained in place under the weight of the hail. For the safety of the aircraft, all this effort was definitely worthwhile.

From page 74 of FLUG REVUE 8/2003

Last updated 7 July 2003

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