Airctaft maintenance goes high-tech

Aviation International News » September 2008
September 3, 2008, 9:55 AM

To see where business jet maintenance is headed, just look at mechanics’ toolboxes. Those who turn wrenches on the most modern airplanes still need the standard-issue screwdrivers, wrenches, sockets and ratchets, but they also need a powerful computer to analyze what ails their charges. The computer–usually a laptop PC–replaces bulky and expensive test equipment for previous-generation aircraft and plugs directly into the aircraft’s own maintenance computer to download normal and abnormal maintenance information, help mechanics record events surrounding intermittent problems and troubleshoot problems before they cause more expensive damage.

This development is an inevitable result of the increasing complexity of modern jets and the growing integration of aircraft systems. Engines, via fadec, talk to avionics, which interact with air-data and navigation systems and know the status of flight controls and environmental systems and even the temperature of the jet’s skin, as well as a lot more data delivered by sensors mounted all over the airplane.

At the heart of all this inter-aircraft communication is a maintenance computer. Honeywell’s Central Maintenance Computer (CMC) is installed on Hawker Beechcraft’s Hawker 4000 and Gulfstream’s large-cabin G350 through G550 as well as other aircraft equipped with Honeywell avionics and systems. The Rockwell Collins Maintenance Diagnostic Computer (MDC) is part of the Pro Line 4 and Pro Line 21 avionics systems installed in aircraft ranging from Beechcraft King Airs to Bombardier Challenger 605s and CRJs.

The tight integration of avionics, aircraft systems and engines means that today’s technicians face maintenance tasks different from those they confronted on previous-generation jets. When the first business jets were developed, nothing was integrated. The engine sat by itself, all systems worked independently and avionics were separate from the rest of the airplane.

As microprocessors found more application in aircraft design, individual systems became more powerful and began talking to each other via databuses. A new tool in the aircraft technician’s arsenal was an oscilloscope, used to troubleshoot bus problems, but this was the realm of the avionics expert. Modern technicians need to be more than just airframe or engine or avionics specialists because they will be troubleshooting an integrated system–the entire aircraft–and dealing with more complicated problems that can affect multiple parts of the aircraft.

Aircraft as an Organism
Douglas Davidson, manager of customer support for Hawker Beechcraft’s Hawker 4000 program, sees an airplane “as more of an organism than a piece of hardware.” A former molecular biologist who is also an avionics technician, Davidson, and his team, will be on the receiving end when Hawker 4000 operators call for help. The support team was heavily involved in the design of the Hawker 4000 and has a big stake in making sure the jet is reliable and maintainable.

The Hawker 4000 is equipped with Honeywell’s Epic avionics platform, which Davidson considers the airplane’s central nervous system or, to use a computer analogy, the hardware and operating system software that runs the airplane. Any system supplier that wants to participate in the Hawker 4000 program has to learn how to work within the Honeywell architecture.

Smiths, for example, manufactures the Hawker 4000’s fuel system, including fuel probes, measurement system and indication components. The microprocessor guts of the Smiths system live in a Honeywell avionics rack, not as a separate system, and the Smiths fuel system software runs on the Honeywell host (operating system). All Hawker 4000 systems communicate with each other via the Honeywell central nervous system, which consists of high-speed bidirectional databuses. “Everything on the ship is integrated,” he said, “flight controls, cabin management system, even skin temperature sensors that can tell all sorts of information about the aircraft at any given time.”

The Hawker 4000 resembles a modern computer in yet another way: it is an open-source system. Personal computers are baseline systems to which third-party designers can add new software and hardware features to make the computer more useful. “On the 4000 program, that’s exactly how we approached the supplier partners,” Davidson said. “We told them, ‘You’re going to design the system,’ and we gave them the parameters and the space in which they could design.” Normally, aircraft designers will stipulate the exact specifications for any vendor-supplied products, but this method turned conventional design practices upside down, allowing vendors to design optimal systems and work closely with the Hawker Beechcraft (formerly Raytheon Aircraft) engineers.

The development process did encounter some bumps in the road. “Early on we had some issues,” said Davidson. “We had to go back to [some] suppliers and say, ‘Your system is not performing.’” And the supplier had to redesign the system to make it work properly, not just replace one suspect component. This even affected Honeywell, because it also provided the pneumatic, air-conditioning and pressurization systems, but from parts of the company other than the avionics division.

Another feature that is becoming increasingly common at jet manufacturers is the modern version of the “iron bird,” a replica of the airplane’s systems built independent of the airframe, so engineers can make sure that all systems work together properly before hard-wiring them into the airframe.

For Hawker Beechcraft, the modern iron bird is the Integrated Systems Design Facility (ISDF). Gulfstream Aerospace calls its bird the Integrated Test Facility (ITF).
“It’s a sophisticated flying aircraft in a lab,” said Jim Gallagher, Gulfstream program director for aircraft health and trend monitoring systems. The ITF has a cockpit with avionics, full aircraft systems, wiring harnesses, hydraulic components and engine simulators.

The biggest advantage of the ITF, he said, is that engineers can ensure that all systems work properly together during the design and certification process. Another advantage is that when some component needs an upgrade, engineers can plug it into the ITF to see how it affects the rest of the aircraft’s systems before they test it on a live jet. This also works for software upgrades. Finally, when technicians isolate a component that is causing an intermittent problem, they can yank it off the airplane and install the possibly defective unit on the ITF for more extensive and less costly troubleshooting.

There is a training benefit to these airplane simulacrums. Davidson’s team at Hawker Beechcraft is also involved with training program development, and he found that it made sense to bring the Hawker 4000 training provider, FlightSafety International, into the development team early in the program.

“The instructors who teach the maintenance class and the instructors who teach the pilot training courses have all been involved in the development of the aircraft,” he said. “They know not only how things work but why they work the way they do. Obviously in the development of an airplane you’ve got to make some tradeoffs between design and engineering, so they know why we went down one path or another. And I think that helps a lot in the classroom.”

The Hawker 4000 ISDF is located across the street from FlightSafety’s maintenance training facility in the Hawker Beechcraft customer support facility in Wichita, so when students need to see how the airplane’s systems work, they can visit the ISDF. “There’s no substitute for a picture, or even better yet, a chance to get your hands on something,” Davidson said.

He continued, “Today’s maintenance technician, yours truly included, is a little intimidated by technology. The best way to get him comfortable with it is to let him play with it, let him explore. And that’s what we’re doing, letting them get their hands dirty on a molecular level, letting them play with the software.”

Sensors Support Maintenance
The source of much of the information about the aircraft and its systems is data delivered by components as well as by many sensors all over the aircraft. There is a risk that adding more sensors could introduce potential system faults.

In the Hawker 4000, however, engineers used sensors that seldom wear out or cause problems. Landing-gear position is determined not by unreliable microswitches but by using proximity sensors with no moving parts. Thermistors record temperature in many areas, and the only time that Davidson was able to get a thermistor to fail was during a test in which the team purposely exposed the device to an extreme environment that the airplane will never experience.

“As the level of integration has gotten higher,” said Gallagher, “we’ve got to make sure that sensors are not providing bad information.” Sensors have been widespread since development of the Gulfstream IV, he added. “We’ve been dealing with that for 20 years. The level of sensors on the G550 is not much more than on the GIV, and we’ve been well through that learning curve.”

Having so many sensors throughout the aircraft means that problems can be isolated much faster than they would if the airplane did not have the sensors. “The system will take you directly to the root cause of something,” said Davidson.

For example, when a “check engine” light in a car illuminates, that doesn’t tell the driver anything about the problem, and an auto mechanic will have to plug in a test box to read the trouble code, which might pinpoint the faulty system but not the exact component that has failed. “It could be any number, maybe two dozen sensors that cause that [light] to go off,” he said. In an airplane equipped with a CMC, when a CAS [crew alerting system] message lights up on the cockpit display, clicking on that message will show the exact cause of the problem, according to Davidson, “even if it’s a right-hand or left-hand installation.”

When he caused that thermistor to fail during the extreme-condition test, he added, the system said exactly what failed and where it is located. “Just go out,” he said, “open up the access panel, find the little [component], pull it out, snap a new one in. And with the system still on, just have it run its own self test. If the test passes, the CAS message extinguishes, you’re done.”

For those who prefer a more traditional troubleshooting approach, he said, “it’s not so far advanced that you can’t actually get in there with a meter and check things if you want.”

Most flight operations prefer to get the airplane back in the air, however, and this system helps make that possible, Davidson explained. “You don’t have to pull your test equipment out; you don’t have to access anything. You could have the crew performing some of the checks as they’re rolling in before the airplane gets chocked and it will tell you exactly where to go. Assuming that you’ve got the part on hand, you can be flying again within a couple of hours.”

Troubleshooting the component that failed has always been challenging, but the maintenance computer helps prevent the possibility of replacing an expensive part that isn’t the cause of the problem.

A worst case scenario might be a broken wire. “When something like that happens,” Davidson said, “the CMC not only knows that it can no longer communicate with the component, but it also knows that that component must be functioning because other systems aren’t affected.” Say a wire to one of the air data computers is severed. The CMC will notice that it has lost communication with ADC 1, he explained.

In older systems, the technician would be led to think that there was a problem with that ADC. But the CMC knows that the flight guidance system, the weight-on-wheels and the airspeed indicator are all working. “So,” Davidson said, “it has now deduced, ‘Hmm, air data must be OK and it must have power and ground, so chances are it’s just a wire.’ It’ll tell you that. It kind of takes some of the thinking out of it for you.

“It’s easy to just say ‘Go change the box,’” he continued. “Nine times out of 10, that’s probably what it’s going to be. But that one time out of 10, I don’t want to have to change that component if that’s not the issue. And I certainly don’t want to change something that doesn’t fix the problem. So, developing the smarts behind the CMC was the hardest part. We’re not done with that, and that will continue to evolve over time.”

Delivering Dispatch Reliability
The goal for Gulfstream is more dispatch reliability and availability, according to Gallagher. “It’s supporting the customer and reducing the number of missed trips.” Dispatch reliability for Gulfstream’s large airplanes is at 99.7 percent, but tackling that last little bit is proving the most challenging.

“We spend a lot of time looking at that [small] percentage of trips that are missed and analyzing everything to do with that. We track both dispatch reliability and availability. If you can save a missed trip or reduce the downtime on an aircraft, you help both of those.”

Gulfstream believes that the datalinking of CMC information to the ground is a key element of chipping away at that last bit of dispatch reliability. The airplane is a sophisticated piece of equipment and Gulfstream’s product support system is also a sophisticated system, Gallagher explained, but the link between them is not. “Using voice and fax and e-mail is fine, but the aircraft is smart enough to know what’s going on with it and should be able to plug into our system and get supported a lot faster and with a lot more accuracy,” he said.

Gulfstream now offers a top-of-descent data-link called PlaneConnect, which automatically sends trouble messages to Gulfstream and the owner/operator before the jet’s arrival. The messages arrive at Gulfstream’s technical operations group as e-mails and are reviewed to see if any repairs need to be planned. The game plan, according to Gallagher, is to check the messages for any problems that could ground the airplane, then check the availability and location of parts and the airplane so maintenance can be arranged.

Another benefit of the CMC system is that if operators are willing to share the data with their manufacturer or service provider, it becomes much easier to spot fleet-wide problems. “There’s no downside to sharing,” said Gallagher.

Gulfstream also uses the CMC to track down intermittent problems. Engineers create a condition-monitoring file and load it into the system. The file will tell the CMC to record an event when certain parameters occur, or the pilot can trigger the system to make a recording when the intermittent fault occurs. After the aircraft lands, Gallagher said, “we can get that data and look at it and understand what’s going on in the system.” This avoids having to test fly with extra instrumentation or special equipment to track down the fault.

Gulfstream is now looking at how to interface the CMC system with an airplane’s master minimum equipment list (MMEL), so that the system would automatically look up MEL items without pilots or mechanics having to take so many steps to fly with allowed inoperative equipment. “We’re looking at automating that,” Gallagher said, “maybe with a separate system.”

Davidson’s team took the MMEL-writing task upon themselves; typically, engineers write the document, but it’s the product support team that has to deal with consequences of MELs. “In our case,” he said, “probably nobody knows the whole ship as well as my team does.”

The team consulted with the FAA’s aircraft evaluation group and flight standardization board about what they would like to see in a MMEL. Then they met with the maintenance and operator advisory teams and engineering and flight test personnel to fine-tune the MMEL. “We created an MEL,” he said, “that basically will allow you to dispatch safely in almost any type of issue, other than fire, smoke, those kinds of things. The MEL was designed to work with the CMC. As the CMC evolved, we refined our MEL.”

Now that airplanes can announce their ills to pilots in flight and even broadcast their condition to support teams on the ground, how does this affect airworthiness? In other words, if something happens to an airplane in flight that renders the airplane unairworthy, aren’t pilots supposed to land and take care of the problem? With maintenance-computer-equipped airplanes, the dynamics of airworthiness are changing, and the integrated MEL, like that of the Hawker 4000, has a lot to do with that.

Davidson used a hypothetical pressurization failure in the Hawker 4000 as an example of how the system works. “Big issue, right? Let’s say you’re flying along and you get an auto-pressurize fail CAS message. And so the crew sees this in flight, and they say, ‘Do we need to land at the nearest available airport or can we continue?’ They determined through their AFM that they can continue on to their destination as long as they can maintain cabin pressure. And they land, deplane their passengers, and then they don’t have to call out maintenance; they can use the CMC to interrogate the system and find out why that CAS message came up.

“Let’s say the CAS message tells you that the number-two main air valve isn’t reporting correctly. The valve is open, and it’s closing properly, but it doesn’t tell the computer that it’s open or closed properly. So the computer knows I’m getting air, and the air, it’s good, so the system is working OK, but I’m not seeing what I’m expecting to see. So here’s a case where you might have a broken wire. If they look at the MEL, they can see, if I can manually check the position of the valve, which is very easy to do, then I’m good to go, I can go fly. As long as the valve opens and closes on request, I’m good to fly. Here’s a case where instead of an airplane being grounded, you can  fly, and we’ll give you up to 10 days on that one.”

The Hawker 4000 MMEL writers incorporated maintenance and operations procedures for dealing with MEL issues inside the MEL, not in separate documents. Davidson instructed the team to try to find operational procedures for handling MEL items instead of maintenance procedures, so pilots could handle MEL disposition.

A New Role for Aircraft Mechanics
The transition to learning the new technology on airplanes with maintenance computers  underscores something business aviation has known for a long time: mechanics are more than wrench-turners who only know how to change parts that are obviously defective. Modern aircraft mechanics play a key role in aviation safety, and now they have the tools that will help operators keep costs under control and raise dispatch reliability and availability to unheard-of levels and also improve safety.

Rockwell Collins’ plans for its MDC include enhanced aircraft systems integration,
including LRU self-test and reporting, fleet maintenance operations quality assessment, enhanced diagnostic, and prognostic capabilities and integration with operators’ back-office computer systems.

The next step is adapting trend-monitoring techniques that have long been used in engine maintenance. “If I can predict when it will fail,” said Gallagher, “I can fix it before it fails.” That means that expensive unscheduled maintenance gets shifted to lower-cost scheduled maintenance.

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