A downdraft on the otherwise wonderful winglet?

Washington DC
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Reports from at least one major airline indicate that blended winglets installed on large commercial aircraft are being damaged or destroyed by lightning strikes as well as by ground incidents, but the overall experience appears to be positive.

With thousands of winglets in the field, operators are only just getting a clear picture of the true costs associated with maintenance and repair of the energy-saving composite devices, although the data is being closely held.

Southwest Airlines has a great deal of experience with the devices to date, with 370 aircraft out of a total of 410 Boeing 737-300s Classics and Next Generation 737-700s in the fleet already fitted with blended winglets provided by Aviation Partners Boeing, also the maker of winglets for other 737 models as well as for the 757-200 and 767-300.

From a cost-benefit analysis, the airlines have not questioned their decisions to invest roughly $1 million for each 737 and more for larger aircraft, given the rapidly escalating cost of fuel. APB's website carries a running tally of its estimate for how much fuel its winglets have saved the industry - as of 23 June, the number was greater than 3.8 billion litres (1 billion USgal).

According to a 2007 report prepared by the National Academy of Sciences, 737s and 757s fitted with winglets can see fuel burn reduced by as much as 4% and 3.3%, respectively, despite the addition of hundreds of kilogrammes in extra weight for the devices and the necessary structural modifications to the wing. Accompanying the weight - 155kg (340lb) for a 737-700 retrofit and 615kg for a 757-200 - the modified aircraft have a longer wingspan (an extra 1.9m (76in) for the 737-700 and 1.4m for the 757-200) that pilots must consider when taxiing in close proximity to other aircraft. Also to be considered is the substantial vertical element of the new wingtips, 2.5m in the case of the 737-700.

Close behind Southwest is American Airlines, which now has winglets on all 77 of its 737-800s. The airline has retrofitted 83 of its 124 757-200s so far, and is testing a leased 767-200 with winglets at APB, says the carrier.

'Core' damage repair

Neither the carrier nor APB were available to comment about maintenance and repair experience with the winglets. Composite winglet suppliers to APB include Ducommun, Fischer Advanced Composites, Kawasaki Heavy Industries, and most recently, GKN, which delivered its first 737 Classic winglets to APB in February.

Winglet leader Southwest did provide a glimpse of its maintenance and repair experience with the devices. "We have no issue with the winglet itself," says Craig Roberts, structural project engineer for the carrier, "but we do see lightning damage." Roberts says Southwest removes damaged winglets on the line, repairing external blemishes in its own shops and sending the units back to APB when there is "core" damage.

He says Southwest stocks two sets (left and right) of spare winglets for its 737-300 fleet and "at least" one set of spares at each of the four major repair stations in Chicago Midway, Dallas, Houston and Phoenix for the much larger 737-700 fleet.

Texas Air Composites, which provides core repair services to APB, revealed several years ago that it had repaired a dozen 737NG winglets, mostly due to lightning and ramp damage. One employee of the company said in April that repairs of lightning-damaged winglets are commonplace, with the composite structures sometimes split in half.

 

Discharging static electricity

Southwest's Roberts says Boeing incorporated more static wicks to the outboard portion of the wings and empennage of the 737 Classic when it developed the supplemental type certificate for the winglets, although he does not know if extra wicks were put there for lighting protection based on the 737NG experience.

The wicks help to discharge static electricity in the airframe as well as to help dissipate a lightning strike should it occur. Roberts says the 737-700 has the same number of wicks with or without the winglets.

As for planned maintenance, Roberts says the 737-300 winglet structures must be inspected externally every six months and internally on an annual basis using a borescope snaked into three access ports on the winglet.

The 737-700 winglets, by contrast, must be inspected every five years, says Roberts. During the five-year check, mechanics must check the internal reinforcement changes to stringers and extra ribs in the wing, a task with minimum impact given that "you'll be in that area anyway" as part of the five-year maintenance check, Roberts adds.

One type of unplanned maintenance introduced by the new vertical dimension of Southwest's winglets is detailed in the National Transportation Safety Board's account of a 26 December 2006 taxiing collision between two Southwest 737s in San Diego. According to investigators, pilots of a 737-700 with winglets taxied too close to a 737-300, the top of its left winglet clipping the right horizontal stabiliser of the -300, causing "substantial damage" to the -700, but no injuries.