By Avionics Magazine Staff | September 1, 2001
At Northwest Airlines’ maintenance facilities in Minneapolis, Minn., engine maintenance crews save the airline millions of dollars, thanks to a state-of-the-art engine instrument display system (EIDS). Developed by Spirent Systems Wichita Inc., the AvVance EIDS is installed on Northwest’s fleet of 45 Boeing 747s and 45 DC-10s.
EIDS proved its cost-saving performance in helping Northwest corral a major problem: exhaust gas temperature (EGT) exceedances.
"Before the new EIDS was installed in the DC-10 fleet, EGT incidents were the No. 1 ATA [Air Transport Association] code responsible for delays and cancellations. As [EGT is] one of the parameters for flight, if there is an EGT failure, the aircraft can’t take off. With the new EIDS, EGT has moved from No. 1 down to No. 8," states Tim Blaney, Northwest’s DC-10 reliability manager.
Noting that delays and cancellations in the DC-10 fleet due to EGT failure dropped 30% between July 1999 and June 2000, Blaney adds, "That is a significant reduction, considering all the associated costs that accompany every no-go decision."
Jon Cox, lead avionics project engineer for Northwest’s DC-10s, explains further, "The EIDS has reduced the number of unnecessary engine removals because it gives us exact data on EGTs, how high the temperature was, its duration, what band it fell into, and other valuable data.
"In the past, when an exceedance indicator light went on, pilots had to estimate how long it had been on," Cox adds. "If unsure, they would give a conservative answer, one that favored safety. This meant you might take an engine out for repair only to find out that there was nothing wrong with it…EIDS has eliminated the guesswork and cut down on unnecessary repairs."
Buying into EIDS
Northwest launched its EIDS program in 1995, teaming with Spirent (then Penny & Giles Aerospace) to retrofit Northwest’s Boeing 747-200s with the EIDS. After receiving a Supplemental Type Certificate (STC) for the EIDS on the 747-200 in 1997, Northwest and Spirent began installing and certifying the EIDS on DC-10-30s and DC-10-40s.
AvVance has several intriguing features. For one, it is said to be the first commercially certified, two-display EIDS. Spirent received the certification after proving AvVance had the requisite reliability and redundancy to eliminate the need for a third, backup display. It also meets the rigorous, level A software certification standard.
Available in 5.0-inch, 5.5-inch or 6.4-inch diagonal sizes, the color active matrix, liquid crystal displays (AMLCD) on AvVance present data at a crisp resolution of 72 color groups per inch (standard) or 128 color groups per inch (optional). Each unit weighs 7.5 pounds (3.4 kg) maximum, and consumes between 40 to 70 watts, running on 28 volts DC or 115 volts AC power.
AvVance presents engine parameters in a vertical-tape format familiar to pilots. The parameters displayed include EGT, engine pressure ratio (EPR), fan speed (N1), turbine speed (N2), internal turbine temperature (ITT) and fuel flow.
More important, AvVance records engine data in real time and automatically records every exceedance and its duration. Thus, it is a valuable resource for engine condition monitoring and usage trending.
The EIDS also can be programmed to simultaneously record numerous engine parameters, such as during takeoffs. And it has a "snapshot" function, which simultaneously records numerous engine parameters during a 30-second period before and after an exceedance. Snapshots can be programmed to be automatic, or they can be taken manually by pressing a button on the EIDS. Armed with the snapshot’s data, maintenance personnel can more readily analyze the correlation between parameters during an event and better determine corrective action.
"That additional volume of data and its accuracy have allowed Northwest Airlines to significantly extend the engine on-wing-time," says Bob Trimmer, marketing manager, Spirent Systems Wichita.
Besides recording data, the EIDS can expeditiously deliver information to maintenance crews in several ways. The crews can download engine data after each flight. Or the system can transmit real-time engine data during flight, using an interface to a data link provided by such services as the Aircraft Communication Addressing and Reporting System (ACARS). If it is interfaced to a wireless data link that communicates with the maintenance facility at the specific airport, the EIDS also can transmit recorded data while taxiing to the gate. And it can deliver data to an on-board aircraft condition monitoring system (ACMS).
Taking a snapshot
Technical Standard Order (TSO) and STC approvals for AvVance have be granted on Boeing 747-100, -200, -300, DC-10-30 and -40 aircraft. It is a plug-and-play system that uses the aircraft’s existing engine sensors, transducers, wiring, etc. On Northwest’s B747s, AvVance’s two-display suite fits in the original display’s panel space. On the DC-10s, AvVance needs a slightly larger hole. Northwest personnel created a new panel that accommodated the EIDS without relocating other cockpit instruments.
The AvVance system used at Northwest can simultaneously record numerous engine parameters. Northwest’s EIDS is programmed to automatically record the first three minutes after liftoff, when the engines are working hardest. And the snapshot function is regularly used manually, during certain phases of flight and when the pilots notice any engine exceedances or anomalies.
Scouting Out Exceedances
"The automatic and snapshot recording capability of the EIDS has made a significant improvement in the quality of the data," remarks Rhonda D. Walthall, a Northwest propulsion-performance and testing project engineer. "Before, we wouldn’t know how much time had elapsed between when the crew recorded the first engine parameter and the last one, the time between readings or what occurred in the cockpit between readings. Now the snapshot records all engine parameters simultaneously."
An EIDS snapshot helps Northwest technicians determine the cause of an exceedance. According to Jeffrey Marcolina, Northwest’s JT90 powerplant engineering manager, "if an engine needs to be pulled, we know it for sure, without guessing. You might actually pull an engine for performance degradation rather than waiting for more obvious symptoms."
When EIDS generates exceedance snapshot data during a flight, Northwest’s pilots input the reported information into their ACARS terminals in the cockpit. This data is then transmitted to the airline’s maintenance center.
Northwest has not interfaced its EIDS to ACARS–though it could. The airline instead is installing into its aircraft a Honeywell aircraft condition monitoring system (ACMS) that will collect the EIDS data digitally, so pilots won’t have to input it, and forward it to a data link-equipped system for transmitting. Northwest began receiving the Honeywell ACMS earlier this year and has installed the system on one aircraft so far.
All Data Reported
Securing comprehensive and accurate data from the EIDs is a major improvement over Northwest’s previous method of data reporting, which depended on crew interpretation. Frequently, not all pertinent data was reported, such as exceedances of very short duration. Or data was reported incompletely if a pilot, for example, did not remember how long an indicator light had been on.
Because of these inherent inaccuracies, Northwest employed conservative maintenance policies. Engines were pulled if any doubts existed about the data’s accuracy. Now problems are better diagnosed and repairs are made only when necessary.
In reference to better diagnostics, John Alabach, Northwest’s maintenance operations technical representative, refers to a collection of EIDS snapshots of engine malfunctions and anomalies that were translated into graphs. As a troubleshooter for chronic maintenance problems, Alabach studies the graphs intensely.
Pointing to one graph, he says, "You can see where fuel flow was increased during throttle advancement, and where it decreases during throttle retard. When the throttle was pushed up, we lost our engine pressure ratio, which is essentially an indication of air flow through the engine. It dropped in less than a second."
Showing on the chart where the throttle was pulled back, Alabach adds, "The EGT continued to rise because the engine wasn’t getting enough air passing through it. That’s a data graph picture of what a compressor stall looks like. The next time we see a graphic depiction like it from EIDS data, it won’t take us long to recognize the anomaly."
Pointing to a graph of an over-temping engine, Alabach says, "I want to know if the flight crew is causing it or if it is an engine problem. I can see the engine pressure ratio and other data is all on target, which indicates that the crew didn’t over-boost it."
Indicating another point on the chart, he adds, "You can see the EGT creep over the red line and the fuel flow is right about there, and here’s the N1 and N2 speeds. So from that, I know I’ve got a problem with the engine."
Alabach explains that each EIDS snapshot provides a unique pattern that, when compared with future anomalies, would help Northwest identify similar situations more efficiently. "It is a positive dimension of the EIDS that we hadn’t anticipated but are very happy to have discovered," he concludes.
Curing Bad Habits
The EIDS is proving equally useful in helping Northwest identify practices that negatively affect engine performance. One example is EGT exceedances during takeoffs.
When the EIDS was first installed, recordings of EGT exceedances during takeoff were so prevalent that Northwest mechanics and engineers thought the EIDS units were inaccurate. But with both EIDS displays agreeing that exceedances had occurred, Northwest looked at cockpit procedures and discovered that rapid application of the throttles was causing the EGT exceedances.
"Because there are a lot of gears and pulleys, the old indicator lagged the engine as the engine RPM spooled up," Cox explains. "By the time the pilot noted the proper settings and backed off the throttles, it was too late."
Alabach provides another example. Pointing to a chart of EIDS data, he explains, "This indicates several exceedances have occurred because all three engines were pushed over the limit. That information helps us identify that the EGT was caused by operational procedures and was not an engine problem."
Summing up the significance of Northwest’s switch to the EIDS on its B747s and DC10s, Walter Branz, B747 project engineer, says, "The knowledge provided by EIDS data translates into very big savings for Northwest Airlines. It helps us identify engines that need to be pulled and, just as importantly, it helps us identify engines that can stay on the wing. At a million dollars or more per removal, there is no question that the EIDS is making Northwest Airlines more competitive on the bottom line and in the minds of our customers."