APPENDIX Environmental System
APPENDIX: Environmental System
Bleed air is extracted from two places on the engine high-pressure compressor. The engine bleed air is ducted from the engine to a
manifold through combination regulator shutoff valves and check valves where it is distributed to the various aircraft systems. The bleed
air is directed through the aft fuselage to an air duct assembly. The bleed air is supplied by the engines at 150 psi and 600 F. This air is
regulated and cooled down for use in several environmental systems. Regulated 15-psi air is routed to the entrance door seal and the
hydraulic reservoir. Check valves in the system prevent bleed air from one engine to enter the other or escape from one engine through
the other during single engine operations. The systems that require bleed air are: Engine inlet anti-ice, Wing anti-ice and shutoff valve,
Hydraulic reservoir pressurization, Entrance door seal inflation, Cabin pressurization and control and Environmental system.
ENGINE ANTI ICE: The Engine anti ice is provided by allowing high-pressure bleed air to the engine inlet leading edge via an ejector and
then into a piccolo tube. It is also routed into the low compressor inner stators. Engine anti ice must be on for taxi and takeoff when in
visible moisture at on OAT/SAT of 5C or colder. Do not operate on ground if temperature exceeds 10C OAT/SAT except for preflight test. A
120F temperature sensor illuminates the R/L ENG ICE TEMP LO annunciator whenever the temperature at the inlet is less than 120F
indicating that the system has not reached its full temperature or that it failed.
ACM: The ACM is similar to air conditioners in other aircraft. Hot bleed air is extracted from the bleed air manifold, regulated from 150
psi to 30 psi and cooled through heat exchangers from 600oF down to 39F. This air enters a mixing chamber where it is combined with
hot bleed air to produce the temperature selected with the TEMP knob (60F-90F). If bleed air selector switch is in the OFF position, Ram
air is supplied to the cabin and bleed air is not available. In the L ENG position, bleed air is supplied by the left engine (9-10lb-min),
NORM, bleed air is supplied by both engines. HIGH same as Norm but airflow is increased for smoke evacuation, entrance door seal
failure etc (10-23 lb-min), in EMER position, unconditioned bleed air is supplied to the cabin for emergency pressurization. With the
system in emergency, the compressor unit and its associate refrigeration capability is rendered inoperative. With emergency
pressurization, the crew will be subjected to the inflow of the unregulated bleed air source with the potential for 600 degrees F and 150
PSI. VCCS can be used to mitigate thermal stress. The AIR COND FAIL annunciator indicates that the emergency pressure valve has
opened because of a high temperature (400F) or a high pressure (53 psi) condition in the ACU or because the bleed air selector has been
placed in the EMER position. Prolonged use of emergency pressurization can result in the melting of aircraft trim panels near the rear
bulkhead and can cause potential smoke fumes. Temperature sensors for the ACM can be selected from the cabin when the AUTO FLDK
is selected, duct temperature by selecting AUTO DUCT or manually by selecting MAN DUCT. In MAN DUCT the temperature is based on
solely on the position of the temperature rheostat, allowing for a temperature of 39 oF to 90oF.
WING ANTI ICE: The wings are anti-iced with bleed air taken from the engines and routed through the wing bleed air shutoff valve. Bleed
air is regulated to 38-44 psi and routed through the wings through a shutoff valve controlled by the WING AI ON or OFF sw and the delay
timers. An ejector located in each wings inboard area discharges bleed air into piccolo tubes for distribution along the wing leading edge.
Fresh air is also introduced into the wing by leading edge ram air scoops and mixed with the bleed air from the piccolo tubes. The wing
anti ice system has 3 over temperature switches in each wing. Two 212F switches are mounted on the front side of the forward spar and
one 350F switch is mounted near the center of the inboard wing leading edge skin. It only takes one over temperature switch in either
wing to trigger the respective WING OVHT annunciator. The WING ICE TEMP LOW annunciator will illuminate until the temperature in the
outside sensor reaches 140F. Do not operate wing anti ice on ground except for the preflight test and during landing rollout.
Cabin Air OVHT annunciator illuminates if cabin air supply exceeds 235F. Defog AIR OV HT illuminates if the defog duct temperature
exceeds 200F and the BL AIR DCT FAIL illuminates if any of the four sensors detects high temperature exceeding 350F due to a ruptured
bleed air duct or a 3.5 +/- 0.5 psi differential is sensed.
VCCS: When the engines are not operating, the Vapor Cycle Cooling System is the sole source of cockpit cooling air. The VCCS can also
supplement the ACU at low power settings on hot days on the ground or in flight. Because the VCCS uses so much current, it is
automatically shut down when starting either engine, generator drops off line, using windshield deice or horizontal stab de-ice and/or
anti-ice. In addition, it shuts down when the ambient temperature drops below 20F refrigerant pressure drops below 10 psi for more than
20 seconds or pressure exceeds 320 psi. The VCCS is similar to a car air conditioner.
EHS: The Electric Heat Subsystem provides heat through the AUX TEMP CONTROL switch. The aircraft must be on the ground to use the
EHS with external power connected. The EHS is selected using the ELEC HEAT1/ELEC HEAT 1&2 positions of the AUX TEMP control sw. It
can only be used when external power is connected. Temperature cannot be controlled which is about 120-150F. ELECT heat 1 powers
the #1 heater for window, windscreen defogging and flight deck floor outlets. the ELECT HEAT 1 & 2 power #1,2 and #3 heaters in the
supply duct for glare shield, observer floor outlets, observer eyeball outlets and forward evaporator blower motors.
PRESSURIZATION CONTROL SYSTEM: The cabin is pressurized by airflow from the cabin and flight deck air outlets. Cabin pressure is held
constant by two outflow valves that open and close to control the amount of air escaping the pressurized compartment. The amount of air
allowed to escape, is controlled by the CABIN CONTROLLER to the desired altitude and cabin rate of climb. Each outflow valve is
connected to a backup pressure regulator. In the event of failure of the controller to regulate cabin pressure, these backup regulators will
prevent the cabin altitude from exceeding 12500 +- 1500. The CABIN DUMP valve can rapidly and completely depressurize the cabin
bypassing the backup pressure regulators. CABIN PRESSURE LO annunciator illuminates when cabin altitude exceeds 9500 +- 500 ft.
CABIN PRESSURE HI illuminates when cabin pressure differential exceeds 11 +/-1 psi. (When operating properly, the outflow valve will
keep cabin pressure differential from exceeding 9.1 +- 0.1 psi. Do NOT depend on the CABIN PRESSURE HI annunciator to warn of a high
cabin pressure differential situation.) When throttles are advanced to the takeoff position on the ground, the cabin is pressurized to a
positive 80 to 120 foot differential. This prevents a pressure bump at lift off. Cabin pressure can be automatically controller by the cabin
pressure controller at a rate of change of approximately 50 to 2,000 fpm.
STAB ANTI/DE-ICE. 12 Electric heater mats provide anti ice and deice protection to the horizontal tail. The mats are installed on the left
and right horizontal tail leading edges and elevator horns. Each mat can perform anti ice and de-ice functions. In the event that either
horizontal stabilizer anti ice or deice system fails there is a backup system that uses the gap heaters located in the elevator horns and
stabilizer. Power is supplied to the gap elements for 30 seconds after activating the sw. The desired temperatures in the stab is between
260-280F, if the temperature reaches 310F the temperature controller removes power to the heating elements and the STB ANTI ICE
FAIL annunciator will illuminate. The horizontal stab anti ice system is prevented from operating on the ground through the landing gear
safety switch. During the test of the horizontal stab anti-ice, hold the switch in the test position for approximately 3 seconds for the STB
ANTI ICE FAIL annunciator to illuminate. The de-ice system when ON the stepper relay runs through 12 cycles (one in each zone mat) and
takes 3.5 minutes total. During the test of the stab de-ice system, all mats are energized for 0.6 sec, therefore the operation light will
illuminate 12 times.
AV BLO FAIL annunciator. A malfunction in or failure of the blower motor
Windshield Ice/Rain protection. The windshield is laminated. A heat conducting film is sandwiched between an inner acrylic layer and an
outer glass layer. The High setting of the windshield heat is not permitted for takeoff or landing because of the distortion it causes