Tuesday, August 1, 2006
B787 Promises To Be Easier to Maintain
Boeing engineers have looked at every component with an eye to accessibility, ease of maintenance, and extended intervals between scheduled maintenance. Here is our look at what they accomplished.
You've got to love us for more than our body" might be the operative phrase for the ease of maintenance being built into the Boeing 787. This is the first Boeing commercial jet to make really extensive use of composite materials in the wings and fuselage, shaving an estimated 20-40 percent of weight off a similar-sized airplane built of aluminum (see AM. May 2005, p. 13).
But the use of composites does not address the overall maintenance burden of the airplane, and how Boeing plans to make the airplane easier to troubleshoot and to keep flying.
Herewith, a brief tour across the airplane and the maintainability features being incorporated. We begin with the lavatories, which are used at least once in flight by everybody aboard, and which traditionally have been a headache to maintain, not least because of leakage that contaminates and corrodes aluminum structure.
For the B787 lavatories, overhead tie-in's to structure have been eliminated, which simplifies removal and installation. The floor fittings are made of titanium, which saves weight and, from a maintenance perspective, eliminates the aluminum corrosion issue. The lavatory interface with the floor is not achieved with the application of a liquid sealant; rather, Boeing has opted for physical seals made of silicone rubber, which provide a better blockage for fluid and are easier to replace than hard-cured sealant.
Gray waste (the drainage from lavatory and galley sinks) is no longer drained overboard. Rather, gray water sources are coupled with the airplane's "black water" vacuum waste system, which feeds to a storage tank at the back of the aircraft. While this arrangement is similar to that found on the B767 and B777 for the toilet waste, the vacuum waste system on the B787 has been expanded to include sinks in the lavatories and galleys as well. The tank is periodically emptied during ground servicing.
For both lavatories and galleys, decorative appliqu�s are no longer used. These films can be as difficult and messy to repair as wallpaper. On the B787, decorative panels are used in lavatories and galleys. To change d�cor, or to overhaul worn or unsightly decoration, the entire panel is simply removed and replaced.
Lavatories and galleys have been sized to fit through the doors. While this has been true for lavatory units, it is a new feature on the B787 for galleys. By splitting the galley into multiple components, it can fit through the door, and being able to do so after the airplane is assembled is significant.
The Seating Area
In the B787 cabin, seat power and entertainment system wires are not run on the top of the floor, which has been the case on some aircraft, particularly those with upgraded passenger services like in-flight entertainment networks (IFEN). For example, a B767 undergoing heavy maintenance at a major carrier a few years ago had the wires from seat row to row running on top of the floor, protected by a rubber dam under the carpet. This arrangement got the wires into the airplane, but the circuits faced abuse as passengers stepped on them, even with the rubber dam for protection.
On the B787, a recessed groove in the floor accommodates wires to the seats. Seat tracks also fit in a recessed groove, providing better protection for attachment fittings.
The IFEN itself, which features on-demand video to every seat, is wireless. The only wires running to the seats are the power wires, explained Justin Hale, deputy chief mechanic on the B787 project. "This means a huge reduction in maintenance, as there won't be any soft drinks spilled down onto electrical connectors," he said.
The wireless IFEN offers a number of potential advantages from a maintenance standpoint. "Only power wires run to the seats. That means a huge reduction in maintenance," said Hale. An industry standard has been established for the head end of the system, in terms of power and data requirements. With this standardization, Hale said different producers of IFEN simply "drop their box in a slot and it'll work."
By this means, he said, "We hope to avoid massive changes for IFEN providers when the airplane is in for service."
The wireless system has another advantage: caged boxes containing components are no longer necessary under the seats. "There won't be huge boxes on the floor; we've reduced the size and the heat signature of the components," Hale said. Testing of the new wireless IFEN is now under way, and DVD-quality video seems eminently attainable.
The Triumph of Electricity
Overall, the B787's electrical system will be markedly different than on previous Boeing aircraft. For one thing, electric power replaces traditional pneumatic and hydraulically activated systems. "We're looking at reduced mechanical complexity," said Hale. For example, leading edge slats are electrically rather than pneumatically anti-iced, and the landing gear brakes also are now electrically activated.
In addition, the B787 will feature variable speed generators to produce electrical power at the speed of the engine. Generators traditionally have been of the 115 volt AC, 400 Hertz variety. On the B787 they will be 235 volts, 300-800 Hertz. "With variable frequency power converted to the necessary power inside the pressurized fuselage, we believe there is a huge improvement in reliability," Hale said. He estimates the new variable speed generators are about 300 percent more reliable than their constant-speed cousins on the B767. "Today, B767 generators stay on the wing for about 10,000 hours; on the B787, the generators will be guaranteed for 30,000 hours between unscheduled removal," he said.
The generators supply power to a distributed network, so all circuitry needn't be routed through the electronics and equipment (E & E) bay.
This design feature translates into less wiring. A significant weight saving is envisioned, compared to routing all circuits through the avionics bay. With less wiring, it needn't run though the passenger floor beams; rather, a scalloped recess accommodates the wiring and other system routings underneath the floor, where they are more maintainable. In addition, the wiring is touted as having a more flexible insulation. While this new insulation may be able to "handle" tighter bend radii, the real benefit is seen in its resistance to wear in high vibration areas.
All circuit protection devices (CPD's) are of the new fast-acting arc-fault type. They protect against thermal overload of a circuit and should offer greater protection to electrical components in the face of arcing. Not only does this technology protect the wiring, it should help prevent egregious damage to black boxes and other devices to which the wiring is connected.
No More Wet Blankets
Wiring often is placed under thermal/acoustic insulation blankets, which makes access difficult. On the B787, the blankets have been eliminated from the lower lobe of the fuselage. "We've seen blankets in this area saturated with water, corrosion inhibiting compound, and cleaning solutions, as well as debris collected on blankets in the bilge area; without blankets, that problem goes away," said Hale.
Since the blankets provide sound insulation, the question is whether their removal doesn't make for a louder cabin. Hale says no, because the airplane design is "cleaner," so there'll be less noise generated by the airplane's passage through the air, and the engines are quieter. In addition, a thin layer of rigid insulation has been added to the cargo liners on the underside of the passenger floor, helping to isolate passengers from any sound coming from below.
Overall, Boeing has reduced the square feet covered by blankets some 25 percent compared to the B767. The blankets in the B787 also feature a new covering film that meets the latest FAA standard for flammability (the so-called radiant panel and vertical flame test).
Recall that potable water lines are routed in the bilge area, and to keep them from freezing, heater tape has been used. The line has been wrapped with an electrical tape containing a heating element. Over the years, numerous problems with the heater tape have occurred, most notably on an Air Canada B767-300 on May 13, 2002 as the airplane was just 10 miles from landing at Toronto. The heater tape arced and burned, the fire spread (aided by flammable thermal/acoustic blanketing) and burned through a floor beam.
On the B787, the heater tapes around water and drain lines have been eliminated, have been replaced by a heating element integral to the water line. At the least, this feature will prevent external damage to the heater tape - damage that can lead to dangerous electrical arcing and fire.
The cargo handling system in the lower bay features the kind of independent monitoring that is pervasive on the B787. Instead of system health monitoring by zone, the monitoring is done at the component level. On the cargo handling system, this means, for example, that an electric motor on the power drive will be monitored. Mechanics will thus know where and what to fix.
The air-conditioning system will not take bleed air from the engines. Rather, four large electrically driven air compressors will feed ambient air to the air-conditioning packs. Boeing engineers believe this system has a number of advantages to siphoning inlet air off the engines.
First, the engines will be more efficient, as all air sucked into them will contribute to thrust. Also, oil and other contaminants will not be sucked into a bleed air system, thus eliminating engine-related fumes in the cabin. By foregoing bleed air, leaks in hot duct work are avoided. And finally, Boeing engineers regard the electrically-driven air compressors as more reliable.
The compressors are independently monitored, however; and Hale said they "swap out as a unit." With four compressor motors, one can be off-line until repaired or replaced, and the airplane can continue flying, relying on the remaining three for necessary redundancy.
Also located below deck is an inerting system that will provide a flow of nitrogen-enriched air to all fuel tanks. By this means, the ullage in the tanks - that is, the vapors between the fuel and the top of the tank - will be prevented from entering an explosive condition. This is done by displacing oxygen - which can sustain an explosion if an ignition source is present - with a flow of inert nitrogen enriched air. The system works by passing ambient air through air-separation modules that lead to two streams, one of nitrogen-enriched air and the other of oxygen-enriched air. The latter is pumped overboard, while the former is routed to the fuel tanks.
Boeing positioned the inerting system in the wing-to-body fairing on the right side. The air separation modules have a 27,000-hour on-wing life, which essentially means six years of operation before replacement (at 4,500 flight hours per year). An air filter is also part of the inerting system, and it has a change interval of 6,000 flight hours.
Should the inerting system malfunction and require repair, a 10-day Minimum Equipment List (MEL) proviso is envisioned. That is, the airplane can be kept flying for up to 10 days with the system on the blink.
In the fuel tanks, the fuel quantity indication system (FQIS) is one of the unproven suspects in the fatal explosion of TWA Flight 800, a B747, in 1996. Investigators believe a surge of electrical energy may have found its way into that capacitance system, leading to arcing, which exploded the flammable vapors in the empty center wing tank.
"We went to a lot of component improvements on the FQIS system," said Hale. "We don't daisy chain anything." Which is to say that two probes can be failed in any one tank, but there is sufficient redundancy to get a good fuel-level reading. "We have two completely redundant and separate FQIS systems in the airplane," declared Hale.
For the B787 landing gear, Boeing eschewed hydraulic brakes in favor of electrical brake actuation. One advantage, according to Boeing officials, is the absence of hydraulic fluid leak and their potential to contribute to a fire hazard. On the main landing gear, which features four-wheel trucks, there are 16 individual brake actuators. They are completely independent.
As Hale explained, "Now you lose 25 percent of braking on one wheel in the event of actuator failure, versus 25 percent of the entire truck," Hale explained.
The airplane features sensors to monitor possible adverse aerodynamic loads on structure, which may occur during overspeed conditions and from hard and heavy landings. Without quantitative ways to measure these events, "no fault found" inspections are commonplace. "Structural health monitoring will take the guesswork out of assessing these events," Hale said. "It will help reduce lost revenue and costs by telling the airlines precisely what does or does not need to be inspected before they fly the airplane again," he explained.
Less Maintenance for Lights
Lights are traditionally a source of maintenance. Hale announced proudly, "On the 787 we've made incandescent and fluorescent lights obsolete."
The flight deck and cabin are illuminated by light emitting diodes (LEDs). These have at least 20 times the life of incandescent bulbs. Recall that on the old TWA 800 accident aircraft the investigation by the National Transportation Safety Board indicated that some 200 reading lights and such in the cabin were burned out; these were of the old incandescent variety.
Outside the airplane, the strobe and position lights are also of the LED type. Logo illumination lights and landing lights are of the high intensity discharge type. Rather than using a heated element, the approach used in traditional lights, these lights use an excited gas to create the illumination. (This type of light is finding use on headlights in some high-end automobiles.) For the B787, the 200-300 hour life expectancy for Halogen incandescent bulbs has been superseded by 2,000-3,000-hour high intensity discharge lights.
In sum, the lights are more reliable and the replacement intervals are extended by an order of magnitude.
Less Maintenance Overall
There is another, more subtle but nonetheless important, aspect that makes B787 maintenance easier. Boeing has dedicated an entire B787 team solely to the issue of maintainability and inspectability. It evaluated every duct and wire-bundle routing, everything that might be difficult to access. The team examined some 4,000 areas of maintenance on the airplane and provided guidance to the designers for maximizing maintainability. In addition, not everything was done with an eye to weight savings. For instance, the airplane will carry the weight of a built-in maintenance platform in its tail section. "It was done to make the airplane more operable, because certain things have to be done on the ground. That `operability' criterion justified the extra weight of the platform," Hale explained.
How do all of these improvements translate to the maintenance program? Consider the A-checks for routine maintenance, the C-checks for systems and for structure, and the D-checks, which involve overhaul. Boeing is promising a doubling of the interval from the B767:
Check B767 B787
A 500 hours 1,000 hours
C (for systems) 18 months 36 months
C (for structure) 3 years 6 years
D 6 years 12 years
These extended intervals mean an estimated 60 percent fewer hours for scheduled maintenance for the B787 compared to the B767. While composites are a major contributor to the overall economical improvements being promised to the operators, the advantages are more than skin deep. Features, technologies and innovations are being incorporated throughout the B787 to make it more reliable, more maintainable, and more available for making money.