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...


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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

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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.

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