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...


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.

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