VARIABLE INLET DUCT RAMP
is on or the APU is operating in the ground
SYSTEM
maintenance mode and FCCs off, the bleed air
doors will open. When FCCs are turned on, the
bleed air doors will close. The FLT CONTR GND
Learning Objective: Recognize operating
PWR RELAYS No. 11 and No. 12 prevent the
principles and features of variable inlet
cycling of the bleed air doors during ground
duct ramp systems.
operations.
The Air Data Sensor, DT-600/ASW-44,
High-speed aircraft usually operate in pre-
computes the free air Mach number and sends the
determined Mach numbers instead of specific
signal to FCCA and FCCB. When the Mach
airspeeds. The Mach number is the ratio of the
number is below 1.23, the FCCs supply a ground,
speed of an object to the speed of sound in the
energizing the retract circuits. Each actuator
same medium and at the same temperature. Sonic
velocity and Mach number vary with air tempera-
torque motor energizes, and the actuator brakes
release. The torque motors retract the bleed air
ture; therefore, at standard day conditions, the
doors (if doors are not retracted) until the actuator
airspeed that corresponds to a given Mach number
varies with changes in altitude.
retract limit switches close. The retract limit
In aircraft that fly at speeds of Mach 2.0 and
switches de-energize the retract circuits, and the
greater, the air velocities at the inlet duct are much
actuator brakes are applied by spring pressure.
When the Mach number is above 1.33, the
higher than the engine can efficiently use. The
FCCs remove the ground, de-energizing the
velocity of the air at the inlet duct entrance must
be decreased before entering the engine to prevent
control relays and energizing the extend circuits.
Each actuator's torque motor energizes and the
engine stall. Some aircraft accomplish this by
actuator brakes release. The torque motors extend
using a variable inlet duct ramp. The duct ramp
the bleed air doors (if doors are not extended) until
is a moving part of the surface attached to the
the actuators extend limit switches close. The
leading edge of the engine inlet duct. It positions
extend limit switches de-energize the extend
the supersonic shock waves to allow only subsonic
air to enter the duct. Through wind tunnel tests,
circuits, and the actuator brakes are applied by
the exact ramp position necessary at any particular
spring pressure.
The extend limit and retract limit switches in
speed is determined.
each bleed air door actuator makes a ground to
signal the signal data converter when the inlet
ENGINE INLET BLEED AIR
bleed door fully extends or retracts. When ground
SYSTEM
is removed, the signal data converter measures the
time in transient until the other limit switch closes.
Learning Objective: Recognize operating
If the time in transient exceeds 8 seconds, the
principles and characteristics of aircraft
signal data converter signals the digital computer.
The digital computer commands the cockpit DDI
engine inlet bleed air systems.
to display a caution.
The engine inlet bleed air system of the F/A-18
provides the best inlet airflow for subsonic and
supersonic flight. The inlet bleed air doors control
REVIEW SUBSET NUMBER 13
inlet airflow by opening at 1.33 Mach to bleed
boundary layer air off the compression ramp and
Q1. The ratio of the speed of an object
close at 1.23 Mach. This makes sure the doors will
not oscillate when the aircraft travels at 1.33
to the speed of sound in the same medium
Mach. A bleed air channel controls the fuselage
and at the same temperature is known as
boundary layer airflow near the wing roots.
The bleed air doors open when three-phase,
115-Vat power is applied, and the controlling
FLIGHT CONTROL COMPUTER (FCC) has a
loss of power. After normal engine shutdown, the
bleed air doors normally close. In some models
Q2. The F-18 engine bleed air system duct doors
of the F/A-18 aircraft when external power
are controlled by
.