ENSTROM F-28F/280F SERIES MAINTENANCE MANUAL SUPPLEMENT
Rev.
4
MM-21-4
Jun
21/12
The system is armed by the same switch which activates the clutch disengagement light.
Thus, the low rotor rpm light will be on only when the clutch disengaged light is out, and the
main rotor rpm is below 334. In addition, the warning horn is wired through a position switch
on the collective torque tube such that the horn will not operate with the collective fully
down. The horn has a pulsing tone of 2900 Hz at 80-95 decibels.
After the engine is started, the following sequence should occur in the annunciator panel.
The red clutch disengagement light will be on until the clutch is engaged. When the clutch
snaps into the engaged position, the red clutch disengagement light will go out and the
amber low rotor rpm light will turn on. The low rotor rpm light will remain on until the rotor
rpm exceeds 334. The horn will sound if the rotor rpm is below 334 and collective is raised
off the lower stop.
This system is set to trigger the light and horn at 334 rpm ±1 rpm. The system can be
adjusted with a potentiometer on the top of the signal conditioning unit. The position of the
magnetic sensor was set at the factory and will need no readjustment. Contact Enstrom
Product Support for service on the magnetic sensor. For electrical schematic see Diagram
21-4, for RPM adjustment on low rotor indicator see Figure 21-2. Press to test warning
indicator on F-28F.
E.
Hi/Lo Rotor RPM Circuit
F-28F S/N 831 and subsequent and 280FX S/N 2136 and subsequent are equipped with a
Hi/Lo Rotor RPM Circuit. Refer to paragraph 21-7 for maintenance procedures.
F. Overboost
Circuit
The overboost circuit consists of a pressure switch in the intake manifold which activates an
amber warning light in the annunciator panel. The light will turn on between 36 and 40 in-Hg
manifold pressure. The light may be triggered by short pressure pulses which will not
appear on the manifold pressure gauge. The manifold pressure gauge is the primary
manifold pressure indicator, while the annunciator panel is a secondary indicator. The
manifold pressure gauge should be used to determine severity of any overboost. Press to
test warning indicator on F-28F.
G.
Main and Tail Rotor Gearbox Chip Detectors
The main and tail rotor gearboxes are equipped with magnetic chip detectors which operate
amber lights in the annunciator panel. There is a separate circuit and light for each gearbox.
When a chip attaches to the chip detector, it completes a circuit to ground, turning on the
light. Further information on the chip detectors is included in Sections 19 and 25, which
describe the main and tail rotor transmissions, respectively.
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