The function of the analyzers is to continuously monitor the engine for forces that are not balanced. An internally defective engine can cause too much vibration. Engine vibration, caused by out of balance forces, is changed to a numerical value (inches per second) which shows on the EICAS primary page (N1 VIB) (digital).

Each engine has a vibration transducer which is on the intermediate case. It connects electrically to the engine vibration-monitoring unit (EVMU) which is in the main avionics compartment. Speed sensors, one for each compressor, also transmit signals to the EVMU. The EVMU transmits an indication of the vibration level to the engine indicating and crew alerting system (EICAS) in the flight compartment.

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The function of the engine vibration indicating system is to give a continuous flight compartment display of engine vibration levels.

The operation of the high pressure (HP) and low pressure (LP) compressors is continuously monitored by a vibration transducer. This is installed at approximately the 11 o’clock position on the intermediate case. The transducer connects with an electrical harness to the engine vibration-monitoring unit (EVMU) which is in the main avionics compartment. A connector kit attached to the tray has all the electrical connections between the EVMU and outputs for flight compartment indication, fault isolation and system test. The vibration level felt by the transducer is changed into an electrical signal. This signal goes to the EVMU which also gets inputs from N1 and N2 speed sensors. The EVMU compares the signals and transmits an N1 value to the engine indicating and crew alerting system (EICAS) in the flight compartment.

The EICAS primary page shows a digital value for N1 only. If the vibration level of the HP compressor becomes more than a specified limit, VIB shows on the EICAS primary page. There is also an amber warning light if N1 or N2 vibration levels are too high.

The engine vibration indicating system has the components that follow:

• Engine Vibration Monitoring System
• Engine Vibration Transducer

Engine Vibration Monitoring System

The primary function of the Engine Vibration Monitoring (EVM) System is to provide the crew with the means of continuously monitoring any imbalance of the rotating assemblies in the engine in order to detect possible internal failure.

In addition, the EVM system has a secondary maintenance function. This is to record, both on-ground and in-flight, engine balance data that may be used to compute a fan trim balance solution.

The system comprises one airframe mounted Engine Vibration Monitoring Unit (EVMU) processing signals from dedicated N1 and N2 speed probes and vibration transducers, (one per engine). The EVMU provides an ARINC output for flight deck displays of engine vibration. The EVM unit is located in the lower Avionics Bay on the left-hand side of the aircraft.

The signal from each transducer on each engine is input into two tracking filters which use analog signals of N1 and N2 speeds to track vibration for each rotor. The narrow band filter for the N1 once-per-rev signal is compared to the measured engine vibration input. This gives a continuous indication of N1 vibration in inches/second. A narrow band filter for the N2 speed signal monitors N2 vibration. In addition to N1 and N2 vibrations, the EVMU will also process the transducer signals to provide a broadband signal, used for cabin noise assessment and to provide a backup indication in case of a speed probe failure.

Engine Vibration Indication

N2 Vibrations

If either engine N2 rotating assembly experience high vibrations, an amber "VIB" icon is posted above that engine N2 RPM indication on the primary EICAS page. The "VIB" icon is displayed when the exceedance trigger level of 1.0 inches per second (in/s) is reached. When the N2 vibration level falls below 1.0 in/s, the "VIB" icon is removed.

N1 Vibrations

If either engines N1 rotating assembly experience high vibrations, at 0.5 IPS, a white N1 VIB label is posted under the OIL PRESS icon on the primary EICAS page. The actual N1 vibration levels for both engines will be posted under the oil pressure digital display as follows:

• Between 0.0 and 0.9 IPS the digits will be green
• If the N1 vibration exceeds 1.0 IPS the digits will be amber, indicating an exceedance
• The N1 vibration indicating range is 0.0 to 9.9 IPS

When both N1 vibration levels fall below 0.5 IPS the N1 vibration icons are removed. Also, any time a N2 VIB icon is displayed, the N1 vibration levels for both engines is displayed.

Engine Vibration Transducer

The vibration transducer is a piezo-electric accelerometer type, which has no parts that move. It includes crystal rings which make an electrical charge when they are shaken. The Vibration transducer (accelerometer) transmits a signal to EVM system to enable N1 and N2 vibrations to be monitored. The unit is located on the intermediate casing slightly left of Top Dead Center (TDC).

The transducer sends out an analog signal that is directly proportional to the acceleration produced by the vibration of the engine. The unit is hard-wired to the EVMU using low noise cable.

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The compressor discharge temperature T30 is continuously monitored by the EEC as a means of detecting high moisture content due to inclement weather. Under such circumstances, the EEC is able to take appropriate action to avoid engine flameouts by selection of continuous ignition, opening the handling bleed valves, and increasing the engine idle speed.

There are two T30 thermocouple units per engine, located on the combustion casing, in line with the fuel spray nozzles at the 9 o’clock and 3 o’clock positions. The thermocouple leads are connected into the main electrical harness by special connectors with different diameter holes.

Inclement weather detection is inhibited when:

• N2 > 84%
• IAS < 50 knots

T30 Thermocouple

The T30 thermocouple is a probe, mounted to the engine via a three hole flange, and a takeoff lead terminating in three flying leads with terminal tags: a positive thermocouple connection, a negative thermocouple connection, and a connection to ground. The ground probe contains a Mineral Insulated (MI) type K (alumel/chromel) thermocouple element.

The flange of the probe incorporates a groove to accommodate a metal C seal to seal against engine pressure. The thermocouples operate on the same principle as the ITT thermocouples. Each probe is wired to a separate EEC channel.

Access to the thermocouple units is gained through the bypass case lower left and lower right access panels respectively. Care must be taken to ensure that the lower body of the probe is fully withdrawn before attempting to remove the probe.

The validation of the probe is as follows:

• Range-check of compensated T30
  
  
• Cross-check with other channel
  
  
• If the cross-check of the compensated T30 fails, a cross-check of raw T30 is made with the highest being accepted as valid, if a difference exists

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There are two different pressures that are sensed around the engine and are passed to the EEC. The EEC uses them to monitor and control the various functions that collectively, along with the temperature sensors, ensure fully automatic control of all engine-operating parameters. There are two pressures that are measured are P0 and P30.

Although P0 and P30 are not strictly engine sourced signals, which provide direct engine indications, they are dealt with here because they are used by the EEC to provide engine control that effect all indicating parameters.

P0 Pressure Signal

The purpose of the P0 pressure signal is to provide the EEC with an ambient pressure value for system scheduling and signal validation. The P0 signal pick-up is part of the EEC assembly and consists of a protective mesh screen over the pressure inlet port on the EEC pressure transducer module.

P0 is actually undercowl pressure that is relative to ambient pressure. The P0 pressure is validated by a range check. The range check is 1.0 to 18.0 psia. There are no separately removable parts on the P0 pickup.

P30 Pressure Signal

The P30 pressure signal is used to provide indication of surge and recovery by the engine, as controlled by the EEC, and for fuel scheduling.

The P30 pressure pick-up is bolted to the outer combustion case and is a flanged hole in the casing to which an elbow and pipe are attached. The pressure (pipe) is then routed to two (channel A and B) strain gauge transducers in the EEC. The P30 line incorporates a moisture trap at the exit from the interservices panel.

Validation of the P30 pressure is as follows:

• Range-check to detect and identify measurements which are outside the range of possible values
• A cross-check to detect in range failures
• A model-check to determine which signal path is correct, following a cross-check failure
• The range-check is 2 to 200 psi when N2 <70% and 2 to 550 psi when N2 >70%
• If P30 input is invalid, the engine will auto shut down

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