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Questions and Answers
What type of instrument can be used to measure fuel level?
What is common to all instruments described in this module?
What is the purpose of the variable resistor in a moving coil instrument?
What is the principle of operation of the AC ratiometer?
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What happens when a coil carrying current is placed in the field of a permanent horseshoe magnet?
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What do the symbols ε and ⊕ indicate?
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What is the primary function of the power supply from the permanent magnet alternator in the FADEC system?
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What happens to the two channels in the ECU during engine operation?
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What is the purpose of the cross-channel data link between the two channels?
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When does the switch-over from one channel to the other typically occur?
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What is the purpose of the redundant power supply in each channel?
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What is the role of the channel in command?
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What does channel B use to calculate T25 when the two T25 signals are different?
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What happens if one channel is faulty and unable to control?
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What is the purpose of the ECU during automatic engine start on ground?
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What happens if both channels of electronic control are lost?
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What is the function of BITE in the FADEC system?
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What happens to fault data in flight?
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Study Notes
Engine Indicating Systems
- Engine indicating systems include instruments such as moving coil instruments, DC ratiometers, and AC ratiometers.
- These instruments can be used for various applications, including measuring fuel level, temperature, and pressure.
Moving Coil Instrument
- A moving coil instrument measures parameters such as fuel level, temperature, and pressure using transducers like floats, thermistors, and Bourdon tubes or bellows.
- The output from the transducer is a current that is changed by a variable resistor and drives the instrument windings against a spring.
- When a coil carrying current is placed in a permanent horseshoe magnet's field, the magnetic field due to the current interacts with the permanent magnet field, causing the coil to turn.
Current Flow and Magnetic Fields
- Conventional DC current flow direction is indicated by the symbols and ⊕.
- The symbol represents current flowing towards the reader (like the point of an arrow coming towards you), and the symbol ⊕ represents current flowing away from the reader (like the flights of a departing arrow).
- When current flows in a wire, a magnetic field is set up around the wire.
- The direction of the magnetic field is determined by the current flow direction: clockwise for current flowing away from the reader and anti-clockwise for current flowing towards the reader (the corkscrew rule).
Ratiometers
- DC ratiometers use DC to produce two opposing torques within the instrument, which balance when the instrument comes to rest.
- AC ratiometers work on a similar principle to DC ratiometers, but use AC to drive two separate (shaded pole) motors in the instrument, with opposing torques.
FADEC System Operation
- The heart of the FADEC system is the electronic control unit (ECU), which has two channels that operate independently from each other.
- Each channel has its own redundant power supply, one from the aircraft and one from the permanent magnet alternator, making the ECU almost fully independent from the aircraft.
ECU Channel Operation
- The two channels are operating simultaneously, but only one channel at a time has the authority to control the engine, known as the channel in command.
- The channel in command generates control signals, while the other channel is in standby.
- The channels alternate in command, with the switch-over usually occurring at the beginning of each engine start sequence.
Data Collection and Communication
- The two channels collect data from the engine and aircraft and communicate with each other via a cross-channel data link.
- The channel in command compares its input data with the respective data from the neighboring channel to ensure correctness.
Fault Detection and Recovery
- If one channel is faulty and unable to control, the command is taken over by the other channel automatically, based on the fault status of each channel.
- To completely lose a control function, more than one fault is necessary.
- The ECU can detect compressor stalls, flame-outs, and other abnormal conditions and take corrective action, such as restarting the combustion in the event of a flame-out.
Automatic Engine Start and Monitoring
- During automatic engine start on the ground, the ECU controls all necessary activities and continuously monitors the start sequence.
- If something goes wrong during engine start, the ECU can interrupt and repeat the start sequence.
Fail-Safe Mode
- In the unlikely event that both channels of electronic control are lost, the torque motors default to their fail-safe positions, resulting in minimum fuel flow, fully open stator vanes, and an open air-oil cooler.
Self-Test Routines and Fault Detection
- The FADEC includes extensive self-test routines that are continuously actuated.
- The built-in test equipment (BITE) can detect and isolate faults within the EEC and its input and output devices.
- Fault data is recorded in flight and can be recalled during shop repair.
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Description
Test your knowledge of engine indicating systems and moving coil instruments, including their applications and components.
