Guy Norris/LOS ANGELES
Boeing plans to complete work by the end of the year on improvements to tackle long-standing operational problems on the MD-11. They include new coatings to prevent excessive erosion to the winglets, modifications to stop skin cracks and changes to system fittings and valves to prevent uncommanded shutdowns.
Worse than expected erosion has been a problem on the MD-11 winglet for more than two years. Boeing is evaluating the use of a tungsten carbide coating to the composite surface as a long-term solution. The company says the coating has "performed well for erosion protection when applied to the metal surface of the leading edge slats on DC-9s but current technology of the coating does not permit installation on composite surfaces".
Until that issue can be resolved, Boeing is evaluating a urethane erosion boot, a corrosion-resistant steel doubler on the leading edges and possibly a new topcoat. Two different types of topcoat have been applied experimentally by one operator, thought to be FedEx, and the erosion qualities of both will be monitored for one "C" check interval (about 4,500 flight hours). One is a Teflon paint and the other a rain-erosion resistant topcoat, used on DC-10 andMD-11 radomes.
Boeing also plans to issue a service bulletin by the end of the fourth quarter of this year, instructing MD-11 operators on the removal of pneumatic system sense lines in the wing and tail.
The sense lines, in the pylons, collect data and help control the system, but have been prone to problems because of the failure of brazed fittings which connect them. The solution is to replace them with sense lines that have been orbitally welded. Boeing is working with BFGoodrich to study the option of providing a kit enabling operators to replace the lines during scheduled maintenance.
Orbital weld type sense lines are also being installed in the wing leading edge and empennage for the last few production MD-11s (aircraft 621 onwards). In a related area, operators also complained that bleed systems would shut down uncommanded. This was traced to freezing of the pilot valves and has been cured by incorporating integral heaters on all high-stage valves and a heated pilot pressure regulator valve.
With General Electric, Boeing has also completed work on plans to provide modified tail pylon lower aft fairings for all GE-powered MD-11s after several operators reported skin cracks.
Source: Flight International