**ON A/C ALL
HIGH PRESSURE COMPRESSOR ROTOR SPEED (NH) INDICATING SYSTEM −
DESCRIPTION AND OPERATION
1.
General
Refer to Figure 1.
A separate speed indicating system is provided for the high pressure compressor rotor of
each engine. Each system contains a rotor speed (NH) sensor, which is connected by
electrical wiring to a speed indicator in the flight compartment.
2.
Description and Operation
A.
High Pressure Compressor Rotor Speed (NH) Sensor
The high pressure rotor speed sensor is located on, and is supplied as part of, each
basic engine. The sensor (magnetic pickup) situated adjacent to a toothed wheel in the
accessory gearbox casing on the right side of each engine, provides an ac voltage signal
to 0 to 18990 Hz, at 0.5 to 6.0 Volts, peak to peak, directly proportional to rotor speeds of
0 to 100%. This signal is applied to the NH speed indicator, with an equivalent output to
the associated electronic control unit (ECU) (refer to Chapter 73). For location of the NH
speed sensor, see ENGINE INDICATING − GENERAL, Figure 1.
B.
NH Speed Indicator
The high pressure compressor rotor speed (NH) indicators are mounted on the engine
instrument panel in the flight compartment. The indicators are powered from the 28 V dc
essential buses, engine No. 1 indicator from the left bus and engine No. 2 from right bus,
protected by 5−ampere ENG 1 NH IND and ENG 2 NH IND circuit breakers. Lighting for
the indicators is white, powered from the 5 V dc lighting system (refer to Chapter 33).
Circuits in each indicator compute the ac signal from its associated speed sensor and
provide an equivalent readout of NH speed by means of a moving pointer against a fixed
dial and an equivalent digital display. A 0 to 5 V dc signal, proportional to NH speed, is
relayed to the flight data recorder from each indicator (refer to Chapter 31).
The indicator dial is marked NH % RPM and a scale on the dial is graduated with major
graduations in increments of 10% NH from 0 to 110% NH, with minor graduations of 5%
NH.
The scale is provided with green arcs 10 to 20% and 66 to 100% NH and a red radial at
100% NH.
The digital display is of the 4−digit liquid crystal display (LCD) type. Pre mod 8−1/1101
aircraft incorporate a heater associated with LCD operation in each indicator. The
heaters are powered from the 28 V dc secondary buses, engine No. 1 indicator from the
left bus and engine No. 2 indicator from the right bus (refer to ENGINE INDICATING
GENERAL).
A press−to−test pushbutton on the indicator can be used to verify correct operation of
the indicator. Pressing the test pushbutton, with power on the indicator causes the
indicator pointer to align with a blue dot at 105% NH reading on the dial with an
equivalent reading on the digital display. Releasing the pushbutton causes the pointer to
return to a zero reading and the digital display to show four digit zero reading.
In the event of indicator electrical power supply failure, the indicator pointer moves off
scale below zero reading and the digital display is blanked.
ModSum 8Q100813 introduces an Auto Relight function which monitors engine speed
D
SERIES 100
AIRCRAFT MAINTENANCE MANUAL
MASTER
EFFECTIVITY: See Pageblock 77−12−00 page 1
77−12−00
Page 1
Dec 20/2004
