Airframe or Engine?

Airframe or Engine? Today, maintaining this critical interface requires significant knowledge of avionics By Jim Sparks July 2001 It sure doesn’t seem like all that many years ago when I had to repair a leaking engine oil pressure...

Engine data made easier
Generally, a device such as an EICAS display will have a menu bar where several options may be found. For example, the Falcon 900EX has one function labeled "DEEC." When this mode is enabled by switch selection, the source of primary engine display information is shifted from data straight off the engine to data being observed by the respective engine DEEC. As you may recall, engine data was supplied to one DAU to become digitized while DEEC data was sent to the second DAU for the same purpose. Both DAU’s supply their coded information to all three IAC’s and, as the #3 IAC is normally responsible for engine information, it is that computer which will process the signal for display on the EID.
When the DEEC mode is initiated on the EID menu bar, the #3 IAC no longer passes on the signal from the DAU looking at engine data but instead sends through the information from the Data Acquisition Unit observing the DEEC. Of course, under normal circumstances, the displayed values should be the same. In some cases, while in flight, crews have reported an amber question mark appearing in one of the engine parameter displays. If this "?" should appear, say, on the number one engine N1 scale, it would be an indication that the #3 IAC Fault Warning Computer has detected a difference in data coming from the #1 engine as compared to data supplied by the specific engine DEEC. In a Falcon 900EX, troubleshooting would begin by finding out if the display went to dashed lines or if there was an actual difference when the crew went to the reversion mode to observe DEEC data. In this particular aircraft, the Data Acquisition Units contain a stand-by channel that can be activated by a simple flight deck switch selection. Likewise, another IAC can be substituted by a rotary knob selection. Should either of these actions result in disappearance of the question mark, it is almost certain that engine and DEEC data are not contaminated. If the display was represented by dashed lines and loss of the N1 vibration scale observed, it would confirm a faulty N1 monopole.

More accurate analysis
It used to be that a magnetic plug would have to be removed at random intervals to verify engine integrity. With current systems, a chip detector and even oil filter or fuel filter bypass sensor can be monitored by computer and a flight deck warning issued if this computer detects some abnormality. Sometimes, a word message is displayed for the pilots, while in other situations, a discretely located indicator will advise maintenance personnel that some action is needed such as inspection or the need to troubleshoot the chip detector for an electrical failure.
Bleed-air systems that in the past were often considered causes for engine performance issues also benefit from new technology. Some aircraft incorporate bleed-air system computers, which have internal programs based on pneumatic demand, aircraft altitude and engine operating status. This system will supply the airframe with a minimal amount of air to insure all demands are met, while preventing excess air extraction. It can also provide maintenance technicians with valuable tools to troubleshoot suspected problems. In some cases, a laptop computer interfaced with a bleed-air computer will enable the user to monitor bleed-valve operation, as well as the specific temperature of the air leaving any or all engines. Talk about taking all the fun and guesswork out of the troubleshooting process!
In short, the pylon or firewall is no longer the clear division of where the engine stops and airframe begins. In fact, in most current production aircraft, a great deal of the interface involves significant knowledge of the avionics systems. In all honesty, I can’t say I truly miss the days of capillary lines and flexible linkages; however, back then, the only mystery of the Airframe/Engine interface was who would clean up the oily mess on the floor.

Jim Sparks is Manager of Technical Information Support Services for Dassault Falcon Jet. He is an A&P and an Electrician, who began his aviation career as a technician in General and Business Aviation. Later, he became a Technical Instructor on Falcon aircraft and then a Field Representative.

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