Airframe or Engine?

July 1, 2001

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 indicator in the instrument panel of a radial-powered aircraft. Back then, it was not uncommon to find flight deck instruments connected to the engine via mechanical means. Pressure gauges used capillary lines connected to the induction manifold or the outlet of the oil pump. Some tachometers even used flexible drive cables. In fact, the only wiring involved the use of Alumel and Chromel conductors and provided information like cylinder head or exhaust temperatures.
Even turbine engines of the day used classical means of supplying operating information to the cockpit displays. RPM systems employed tachometer generators producing power outputs proportional to engine speed, and temperature reporting systems used the ageless thermocouple. The rest of the stuff up front, like the navigation displays, were always the responsibility of the guys upstairs in the avionics shop. Back then, everyone had specific responsibilities; the engine people would take care of the powerplant inside and out, and the airframe folks would help out when the engine group decided that the problem was beyond their expertise.

Out of the dark ages
Of course, airframe technicians did have certain limitations regarding how far they could go with instrument repair. In those days, when you went through an airframe school and engine school, you came out well qualified to troubleshoot the airframe/engine interface.
Turbojet or even turbofan technology of yesteryear used hydromechanical fuel controls and electromechanical indicators. The majority of fault recognition came from flight crews reporting differences in parameters of multi-engine aircraft. If it was a case of an engine indicator dropping off, it was often the job of someone like an A&P with an electrical background to make the determination where the actual failure occurred. In most cases, troubleshooting involved connecting a multimeter at various points in the system and performing a variety of tests. Usually, the cause of the fault was accurately identified in short order. Frequently, specialized equipment such as a Jetcal Engine Analyzer was required to verify that flight deck indications were in fact reporting true conditions.

New skills required
Aircraft manufactured in recent years have powerplants that utilize electronics as the means of control as well as indication. In today’s aircraft maintenance world, even though a technician goes through both an Airframe Maintenance Class and the Engine manufacturer’s school, this still may not be enough to understand the complexity of the marriage of a computerized engine to a digital airframe. In many cases, powerplant data is displayed in the same fashion and even on the same screens as navigation information.
Even though electronic controls and indicators in basic appearance seem very complex (and they really are), from a line maintenance perspective, they can be a technician’s best friend, once an understanding of the communication link is achieved.
Most of the gas turbines currently being produced will use some type of electronic control. Two of the more common devices are the
1. FADEC (Full Authority Digital Engine Control)
2. DEEC (Digital Electronic Engine Control)
For a computerized device to regulate an engine using electronic means, it needs the same information available as the person operating an engine with a hydromechanical control system.
First, the power requested is supplied to the electronic control by a variable output transducer, such as a potentiometer or variable core transformer connected to the throttle assembly. Engine computers still use monopoles or tachometer generators to monitor engine spool speeds, and internal temperatures continue to use the ageless thermocouple harness. In most cases, these devices produce two outputs; one signal feeds to the flight deck while the other feeds the computer. Other circumstances dictate that data for cockpit display is extracted from the device controlling the engine, as it is with many FADEC’s. A significant benefit of this technology is redundant data paths for engine displays. If the flight crew notes a discrepancy, chances are, it is also noted by one of the computers controlling or monitoring engine operation. Captured data often can be retrieved using internal or external maintenance diagnostic devices.

Engine Vibration Monitor Computer (EVMC) alongside Maintenance Diagnostic connectors permitting laptop interface for fault interrogation on a Falcon 900EX.

Complex equipment
Understanding the union of today’s engine and airframe still requires a fundamental knowledge of both the aircraft and the powerplant. The first objective is to understand the data path for engine parameters. As with the new generation Honeywell TFE 731 engines, each sensor has dual outputs. This includes N1 (Fan Speed), N2 (Core Speed) and ITT (Interstage Turbine Temperature). One output is supplied to the DEEC, while the second is destined for cockpit display. This analog signal from monopole or thermocouple has to be converted into a digital format to be used by any modern flight display system. This transformation takes place in a device called a Data Acquisition Unit (DAU) or an Engine Data Computer (EDC).
In addition to being analog to digital converters, these devices may also blend information. In other instances, they may need to convert a standardized digital signal into a manufacturer’s specific language, which means a DAU also may be a digital to digital converter. Multiple DAU’s are often fitted to take into account most any possible failure. A flight deck installed reversionary system enables the cockpit crew to switch data source in an effort to alleviate problems. In multiple engine aircraft, it is not uncommon for normal engine data feed into one DAU, while information used by the DEEC is converted into a digital form and may supply a second DAU.
Once the DAU works its magic and digitizes the engine data, it is then passed on to a device that will interpret the signal and determine how to display it.

Electronic Flight Instrument Systems

Falcon 2000 Maintenance Panel. Red Minelco (mechanical indicator) advises of a fault. FUEL computer is actually Airframe/Engine Interface computer.

In Electronic Flight Instrument Systems (EFIS), a symbol generator is used to receive data, then develop a format to paint a real-time picture with the information provided. The Honeywell Primus 2000 system, for example, has an Integrated Avionic Computer (IAC), which can perform several functions. First, it will provide the brains for the automatic flight control system or an auto-throttle computer for aircraft so equipped. In addition, the IAC contains the symbol generator circuitry along with a fault-warning computer. The Falcon 900EX is an example of one airframe incorporating this system, and uses three IAC’s. The #1 and #2 IAC Symbol Generators supply Pilot and Copilot Primary Flight Displays (PFD), as well as Pilot and Copilot Multi Function Displays (MFD). Also, these two IAC’s supply needed data for the two-channel Automatic Flight Control System (AFCS). Often, these two computers are interchangeable.
The third IAC symbol generator is, under normal circumstances, dedicated to supplying the Engine Indicating Display (EID), or as it sometimes is called Engine Indicating and Crew Alert System (EICAS), and may include an Auto Throttle Computer instead of an Autopilot. This EID presentation includes one section dedicated to basic engine parameters, including engine spool speeds, engine temperature and fuel flow. A second section of the display can include some optional selections such as aircraft fuel system information or engine system data, including oil-pressure, temperature and even engine vibration levels. A third section is dedicated to messages. That is, if an annunciator warning light should illuminate — and there are several things that may influence the warning — the message section will highlight the specific area of concern.

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.