FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
2001
2001 - 2561.PDF
uenance Fan cowl door on an A320 showing overhead damage Hail damage on the upper skin of an elevator B737-300; V tion/removal in cloth and unidirectional reinforcements of carbon, glass or Kevlar. It involves drilling holes at appropriate locations and injecting resin to fill the delamination. Boeing says this method is superior to vacuum and syringe tech niques, and can be used for almost all delaminations regardless of size or location. Maintainers' views Lufthansa Technik supports both Boeing and Airbus aircraft. Michael Witt, manager customer support, airframe-related compo nents, says that sometimes airframers make structures and parts in advanced composites without considering the increase in part or repair costs. He compares, for example, the radome of the A320 which is made of aramid fibre reinforced plastic with that of the Boeing 737, which is glassfibre. They are similar in size. However, the A320 radome is 600 grams lighter and nearly five times more expensive than that of the 737. Lufthansa's support of its fleet of A320s, results in addi tional spares costs of $320,000 and repair costs of $20,000 per year - hardly worth the weight reduction. Witt thinks that some original equip ment manufacturers' keenness for weight saving at any price *:omes from a lack of understanding of costs, and the historic expectation that fuel prices would soar in real terms. Fuel prices were high until 1985, but have fallen to pre-1978 levels. Expensive fuel would make airlines very weight conscious. Boeing chose not to use composites extensively in the 747-400, which Witt believes was a wise decision. Another problem is the large number of different branded materials that have to be kept and their^hort storage lives. Some air craft may have more than 200 adhesives, prepregs and films. These have to be refrig erated and, even then, shelf life may only be three months. Much more standardisa tion of composite materials is needed. Damage tolerant designs are particularly desirable in short-haul aircraft which spend a much higher percentage of their time on the ramp where they might get hit by jetways, towing vehicles, etc. In addition, their under-wing and under-tail heights are less, which also increases the probability of collisions. An impact that might put a minor dent in a metal structure might crack a compos ite one, and require replacement. In addi tion, damage to composites can be less obvious on the surface than it would be with metals. So composite structures often need more frequent non-destructive test- ing/eValuation (NDT/E). In the air, hail can cause elevator com posite skins to be written off because it can penetrate the outer layer, rebound off an internal one and tear the first skin out wards, beyond economic repair. Fan cowl doors can suffer overheat dam age from engine hot air leaks and, again, composite ones may be worse affected than metal ones. Witt says that four metal doors were required as spares to support the 100 which Lufthansa had in service, whereas six composites ones were needed to support 52. In the mid-1980s, a metal door cost $30,000 whilst a composite one cost $100,000. Today, the composite doors cost over $200,000. „ Repairs to composites can also take much longer than to equivalent metal parts. Repairs to metal structures which take days can run to weeks for composite parts, and their repair costs can be up to seven times as high. It is ironic that composite parts are often found in high damage areas. A study by Witt showed which parts are most likely to be damaged and it found that composites are often used in areas such as wing tips, leading edges, trailing edges and wing-to- body fairings. Sandwich construction leads to different composite materials being joined. If they have different coefficients of thermal expansion, heating and cooling will inev itably cause microcracking and this absorbs water. If the structure were then to be struck by lightning, the water could vapourise and explode. Microcracks can also be induced in a composite structure when it is physically stripped of paint by sanding or blasting. Chemical strippers cannot be used, how ever, because they would react with and damage the composite. So stripping a com posite component can take up to five times as long as a metal one. Yet paint has a higher coefficient of thermal expansion than composites, leading to crazing and a more frequent need for repainting. The big advantage of composites is the major reduction in corrosion they offer and their virtual immunity to fatigue. Several composite task forces have been set up, and they resulted in SAE/AE-27:27, a document which provides guidance on producing durable, repairable and main tainable composite structures. Sometimes old materials are used if ser vice experience warrants it. For example, new Boeing 737s have aluminium alloy inboard flaps, replacing the composite ones of earlier models. These items are more likely to suffer foreign object and tyreburst damage and the metal ones survive better. Similarly, the A34O-5O0 and -600 have aluminium inboard flaps. However, these aircraft do have thermo- plastic/carbonfibre fixed wing leading edges (behind the slats). The Boeing 777 has an all composite empennage and passenger floor beams. In today's cost-con scious world, new materials must buy their way onto an aircraft. • www.fliqhtinternational.com FLIGHT INTERNATIONAL 10-16 JULY 2001 39
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
