The power indicating system shows engine power data in the flight compartment, on the engine indicating and crew alerting system (EICAS) displays. Each engine has a related power indicating system that measures the speed (N2) of the high pressure (HP) rotor, and the speed (N1) of the low pressure (LP) rotor.

The power indicating system supplies the electrical signals related to the HP and LP rotor speeds and sends them to the engine control units (ECUs) through the engine wiring harness. From these signals, the related data processor calculates the engine power output. The power indicating system includes the speed sensing sub-system.

The speed sensing system has N1 and N2 sensors that measure engine speed. The speed sensors send the signals to the electronic control units (ECU) to monitor the engine speed. The ECUs control the engine on N1 sensor, and use N2 sensor for the engine overspeed protection. The ECUs supply limited authority N1 and N2 speed synchronization, by sharing data through the cross-channel data link (CCDL).

The N1 sensor is an engine speed sensor which has three coils wound on a magnetic pole (monopole). Each coil measures the turn of the teeth on the low turbine (LP) rotor shaft that pass through the magnetic field of the magnet. Each coil of the N1 sensor sends an output, through two connectors, to the ECU.

Two coils of the N1 sensor are connected to their related ECU. A third coil is shared by the two ECUs through the cross-channel data link.

The N1 sensor has a stainless steel housing and it is found at the 12 o’clock position on the engine. It is installed through the front frame and extends down to the LP rotor shaft. The N1 sensor is attached by two captive bolts. There is a gap control spring that supplies the correct gap and removes the need for measurements and shimming.

The N1 compensator is a small unit that is permanently attached on the engine. It has an internal resistor value that represents the N1 compensation value. This value equals the rpm, below rated rpm, that the applicable engine can turn and still supply a rated, or higher than rated, thrust.

The N1 compensator is connected to the two ECUs. There are two resistors in the N1 compensator, and each resistor is connected to one ECU. The N1 compensation value for the ECUs is shared through the CCDL.

The N2 sensor is an engine speed sensor with a two-coil monopole and has an interface with the two ECUs. The N2 speed signals for each ECU are measured from the applicable three-phase permanent magnetic alternator (PMA).

The N2 sensor has a stainless steel housing. It is attached to the accessory gearbox (AGB) and extends to the generator spur gear. A single captive bolt supplies attachment of the N2 sensor.

There is a cross-channel data link (CCDL) between two ECUs of an engine. The CCDL lets the two ECUs of an engine share data from the sensors with each other. And there are two cross-capable discretes between the engines. One cross-capable discrete connects between the channel A ECUs of the engines, the other cross-capable discrete connects between the channel B ECUs. The two cross-capable discretes let the channel A ECUs share data between engines, and let the channel B ECUs do the same. If an ECU fails on one engine, the other ECU can share data with the same channel ECU on the other engine through the cross-capable discrete.

The N1 sensor senses the turn speed of the LP rotor shaft. Each turn of the LP rotor shaft causes the magnetic field on the monopole of N1 sensor to change. The speed of this magnetic field changes in relation to the turn speed of the LP rotor shaft. The change of the magnetic field causes the current and voltage to increase or decrease across the wire coils on the monopole. The N1 sensor changes this current and voltage into an AC signal with frequency in proportion to the engine speed.

Three wire coils on the N1 sensor have an interface with their related ECU. Two coils go to their related ECU, the third coil is shared between the two ECUs through the CCDL. Each ECU has access to all three N1 signals. The ECUs supply these signals, through the ARINC 429 data bus, to the engine indicating and crew alerting system (EICAS) displays in the flight compartment.

The engine indicating system (EIS) area of the EICAS display shows the N1 speed in analog and in digital symbols. The analog symbol is a circular scale and a pointer, and the digital symbol is a value shown in percentage. The analog and digital symbols show N1 speed in percentage. The range for the N1 analog scale and pointer is from 0 to 110%. The 0% N1 RPM is at the 3 o’clock position of the circular scale, and 110% N1 RPM is at the 12 o’clock position. When the N1 speed increases in RPM, the pointer turns clockwise around the scale, and the digital RPM value increases.

A red tick mark on the N1 scale shows the N1 redline, and its position can change on the scale. If the N1 redline is not valid, the default redline position is at 80%. The pointer and the digital symbols are usually green. When the ECU senses N1 is at a warning speed, the pointer and the digital RPM value turn red and flash. If the N1 data is not correct, the digital RPM becomes dashes in magenta, and the pointer goes out of view.

There is a N1 overspeed shutoff detector (OSSD) which senses the LP turbine overspeed and sends a signal to the ECUs to automatically shutdown the engine

The N1 compensator adjusts the N1 actual speed value to get a N1 book value for the EICAS indication in percentage. An engine with an N1 book value of 10,000 rpm, but has an N1 actual speed of 9,800 rpm, the N1 compensator adjusts a 200 rpm value. The adjustment is added to the N1 actual speed to give a book value of 10,000 rpm.

The N1 compensator is connected to the two-engine ECUs. The engine ECU applies N1 adjustment in increments of 0.2% N1 on the range of 0.0 to 1.4% N1.

The N2 sensor’s two-coil monopole inside the AGB measures the speed of the HP turbine rotor from a pair of wire coils in the PMA assembly. The PMA assembly also has an isolated pair of windings, and its rotor is a permanent magnet. The permanent magnet is attached to a shaft in the AGB. Each of the PMA windings supplies a power source and an N2 speed signal to each ECU.

The gears and shafts in the engine turn the AGB which turns the rotor. While the rotor turns, each coil senses the turns of the teeth on the rotor shaft that pass through the permanent magnet’s magnetic field. The change in the magnetic field causes the current and voltage to increase or decrease across the coil. The N2 sensor changes this current and voltage into an AC signal and frequency in proportion to engine speed. The N2 sensor sends the frequency input to the ECUs through its electrical connector. The ECUs supply this signal for the EICAS display shown as N2 in rpm percentage.

Each ECU receives two N2 speed sensor inputs. One senses the N2 speed signal from the PMA. The other senses the N2 speed signal from the N2 sensor.

The EICAS display shows the N2 speed in percentage value, which can be more than 100%. In normal condition, the N2 is shown in green. It changes to yellow or red when the N2 rpm is more than its limit set by the ECU. When it changes to red, the N2 percentage value flashes. When N2 data is invalid, magenta dashes replace the percentage value.

The L(R) ENGINE EXCEEDANCE caution message shows when an overspeed of the related engines N1 or N2 has occurred.

The following EICAS messages are related to the speed sensing system: 

EICAS MESSAGE(S)	LEVEL (COLOR)
L ENGINE EXCEEDANCE	WARNING (red)
R ENGINE EXCEEDANCE	WARNING (red)

The speed sensing system interfaces with the following components and systems:

• Electronic Control Unit 
• Emergency Shutdown 

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