The B-17 contains equipment for long and short-range two-way voice and code communication, intercommunication between crew members, emergency transmission, directional indication, and reception of marker beacon signals.
The interphone system provides for communication between crew members. Command radio, liaison radio, and radio compass signals are audible over the interphone system at all crew stations. Any crew station can talk over the command transmitters. Only the pilot, copilot, navigator, and radio operator can transmit over the liaison radio.
Interphone equipment includes a dynamotor and amplifier located under the radio operator's table, and 12 jackboxes located throughout the airplane: 3 in the nose (for the navigator, bombardier, and forward gunner), 3 on the flight deck (for the pilot, copilot, and top turret gunner) , 2 in the radio compartment, 3 in the waist compartment, and one in the tail compartment.
Remember: Crew members should wear headsets at all times during flight.
The "CALL" position on the jackbox enables the user to over-ride reception on all other jackbox stations for the purpose of calling any particular station. A spring returns the selector switch to "INTER" so that it cannot be left in the "CALL" position inadvertently. For obvious reasons, use of the "CALL" position should be held to a minimum.
The command radio is for short-range communication with aircraft and ground stations.
Voice transmission over the command set is available to all crew stations, but code transmission is limited to the pilot and copilot, who alone have a transmitting key. It is on the remote control box on the ceiling of the pilot's compartment.
The command radio consists of 3 receivers and 2 transmitters on the right forward bulkhead of the radio compartment. Remote controls are on ceiling of pilot's compartment.
Remote Control Units: The transmitter control box has an on-off toggle switch which turns on either transmitter, and a transmitter selector switch which selects either of the 2 transmitters. (Positions are provided for 4 transmitters, should the 2 extras be installed.) A wave selector switch turns on voice, CW (continuous wave) or tone as desired.
The receiver control is divided into 3 control units, one for each receiver. The low band receiver covers 190-550 Kc, the intermediate band from 3000 to 6000 Kc, and the high band from 6000-9100 Kc. Each receiver control unit has 2 switches to operate it.
The A-B switch selects either jackbox or control unit. Use "A" if plugged into jackbox; use "B" if plugged directly into control unit. A tone selector switch which can select "TONE," "CW," or "MCW" should be turned to modulated CW with "A" and "MCW" on. Then you can tune to desired frequency by means of a small handle which turns a calibrated dial.
The reliable transmitting range of the command set is 25 miles or less. Under good atmospheric conditions greater range may be obtained.
RADIO COMPASS (SCR 269-G)
The radio compass is a multi-purpose receiver designed primarily as a navigational instrument.
The power for this set comes from the airplane's batteries and inverters. The various relays and switches operate on the direct current supply, and the receiver and motors for rotation of the loop operate on the inverters.
This set has 2 antennas: a sensing (whip), or non-directional antenna, and a loop, or directional antenna.
The radio compass is a multi-band receiver and, as installed in B-17 aircraft, may be remotely controlled from either of 2 identical control boxes. One of these boxes is above and between the pilot and copilot; the other directly above and slightly to the left of the receiver itself in the navigator's compartment.
Each of the control boxes (BC-434-A) has (1) an antenna selector switch; (2) a band-change switch; (3) a control button; (4) a main tuning control; (5) a tuning indicator (meter); (6) an audio output control; (7) a loop L-R switch; (8) a control light; (9) a dial light, and (10) a dial light control.
This transmitter, on the aft bulkhead of the radio compartment, insures communication with aircraft in flight and ground stations over distances up to 3000 miles, depending on atmospheric conditions and method of transmission. The usual reliable distances are 250 miles on voice, 500 miles on tone and 750 miles on CW. Only 4 jackbox positions (radio operator, pilot, copilot and navigator) can transmit on the liaison set.
This set has 7 interchangeable turning units covering frequencies from 360-650 Kc and 1500-12,500 Kc, and including a low band from 200-500 Kc in some models. For tuning this set, see communication section of B-17 T.O.'s.
The liaison receiver on the radio operator's table covers a frequency range from 1500-18,000 Kc. It uses the same antenna as the transmitter: the skin of the airplane. This is connected to a throw switch on the left side wall. This switch can change over to the trailing antenna (also on left side wall). The trailing antenna is operated from a control box to the right of the change-over switch.
Antenna Selector Switch
This four-position ("OFF - COMP - ANT -LOOP") switch selects the type antenna or antennas to be used, AVC or MVC, and largely determines the indication and action of the loop. The 4 positions of this switch may be explained as follows:
"COMP": When in this position, the set is using both sensing (whip) and loop antennas. Automatic volume control is always present in this position, and the operation of the radio compass indicators and loop is automatic.
"ANT": This position utilizes only the whip or non-directional antenna; therefore the loop and indicators do not operate. Manual volume control is now present, and the volume is adjusted or regulated only by means of the audio control. This position should be used at all times for the initial tune-in of the station.
"LOOP": Now only the loop or directional antenna is in use. The operation of the loop and indicators are controlled by means of the Loop L-R switch. Again you have only manual volume control.
This electrically controlled switch selects the band or range of frequencies desired. There are 3 positions, or bands. One band covers frequencies from 200 to 410 Kc; another from 410 to 850 Kc; the third from 850 to 1750 Kc.
After selecting the desired band with the band change switch, use this control to select any desired frequency within this band.
This light on the control box indicates the control box actually controlling the compass receiver. When the light is on, the control box is in control of the radio compass.
This push button throws control of the radio compass from one control box to the other. If the light on the desired control box is un-lighted, press this control. When you release it, control is switched from the other control box to the desired box.
By varying this control, the operator may adjust the headset volume as desired.
This switch controls rotation of the loop when the antenna switch is in "LOOP" position. The loop can be rotated at two different speeds. When the Loop L-R switch is pressed in and switched to the desired position (L rotates loop to left, R to right) and held there, the loop rotates at a fairly rapid speed. When the switch is not pushed in, but only held in the desired position, the loop rotates slowly. When the loop is rotated by this switch the compass indicators rotate to show the position of the loop.
Tune for Maximum Indication
When tuning in any station, the main tuning control should be tuned for maximum swing of the needle on "Tune for Maximum" indicator.
Dial Light Control
This control regulates the brilliance of the dial light.
For instructions on how to use the radio compass, see Advanced Instrument Flying, T.O.-30-100.
RADIO SET SCR 522 A
The SCR 522 A VHF (very high frequency) transmitter-receiver radio set provides 2-way radio-telephone communication between aircraft in flight and between aircraft and ground stations. Provision is made for voice communication and continuous audio-tone modulation.
The pilot and co-pilot control the SCR 522 by means of the radio control box on the left side of the pilot's control pedestal in the B-17. The set operates on any one of 4 pre-set crystal-controlled frequency channels lying within the range of 100-156 Me. Line-of-sight communication is normally necessary for satisfactory operation of the radio set.
The following table lists the approximate range to be expected, assuming communication is taking place between the aircraft and a ground station over level country.
Radio Control Box
The radio control box to the left of the pilot's control pedestal provides the only complete remote control of communications functions. Five red push buttons are the means by which any one of the 4 channels (А, В, С, and D) is selected and the power turned off. When the "OFF" push button is pressed, the dynamotor is stopped. The push buttons are interconnected so that not more than one channel can be selected at a given time. A light opposite each push button indicates which channel is being used.
The "T-R-REM" switch (transmit-receive-remote) is normally in the "REM" position, permitting press-to-talk operation by means of the conventional push button microphone switch on the pilot's control wheel, which when depressed switches the equipment from receive to transmit. In the "T" position the transmitter is in continuous operation. In the "R" position the receiver is in continuous operation.
The lever tab, directly above the "T-R-REM" switch, when lowered, blocks the switch from "REM" position and spring-loads the switch lever so that unless it is held in the "T" position it will return to "R."
The small lever tab opposite the "OFF" push button is a dimmer mask to reduce the lamp glare. The lamp opposite the "T-R-REM" switch is on when receiving and off when transmitting.
The transmitter and receiver units are in a single case. The transmitter employs a crystal-controlled oscillator circuit and operates in the frequency range of 100-156 Me on one of the 4 pre-set channels А, В, С, and D. Average output power of the transmitter is 8 to 9 watts, using a total power input current of 11.5 amps at 28 volts.
The receiver is a sensitive superheterodyne unit employing a heterodyne oscillator whose frequency is controlled by any one of 4 quartz crystals. Thus the 4 crystal-controlled channel frequencies within the range 100-156 Me are available for instantaneous selection at the remote control position. For reception the total input current is 11.1 amps at 28 volts.
The dynamotor operates on the 28-volt power circuit and supplies 3 regulated voltage sources (300-volt DC, 150-volt DC, and 13-volt DC) required for operation of the transmitter-receiver assembly.
In addition to the equipment listed above, jackboxes, junction boxes, headsets, and microphones are used with the radio set.
Operation of the SCR 522 A
1. Transmission only
To start the equipment, press push button A, В, С, or D depending upon which channel is to be used.
Allow approximately one minute for the vacuum tubes to warm up.
Move the "T-R-REM" switch to the "T" position and speak into the microphone.
2. Reception only
Place the "T-R-REM" switch in the "R" position. It is held in the "R" position by lowering the small lever tab.
To start the equipment press push button A, В, С, or D for the desired channel.
3. Press-to-transmit (press-to-talk) operation
Place the "T-R-REM" switch in the "REM" position.
To start the equipment select a channel by pressing push button А, В, С, or D.
To receive: Under these conditions the receiver is normally in continuous operation.
To transmit: Depress the press-to-talk button and speak into the microphone.
To receive again: Release the press-to-talk microphone button.
4. To shut off the equipment, press the "OFF" button.
Common Uses of Channels
"A" channel is usually used for all normal plane-to-plane communication or for plane-to-ground communication with a Controller.
"B" channel is common to all VHF-equipped control towers. It is normally used to contact the control tower for takeoff and landing instructions.
"C" channel is frequently used in contacting homing stations.
"D" channel is normally used for plane-to-ground contact with D/F stations, and as a special frequency which is automatically selected at regular intervals by the action of a contactor unit.
Precautions During Operation
Avoid prolonged use of the radio on the ground to conserve the batteries and avoid overheating of the dynamotor.
If the transmitter and receiver fail to operate when a channel push button is pressed on the radio control box, press another channel push button, then again press the push button for the desired channel. Transmission and reception should now be possible.
A frequency meter is standard equipment on all B-17's and should be kept in the radio compartment. It is used to check and correct transmitters and receivers on frequency ranges from 125 Kc to 20,000 Kc. For use and operation, see Technical Orders.
The radio marker beacon receiver receives ultra-high frequency signals used in aircraft navigation and landing, and reproduces them visually by an amber light on the pilot's instrument panel. When the receiver is over a keyed transmitter, such as a C.A.A. marker, or certain types of Army transmitters, the indicator lamp flashes in accordance with the identifying signal of the transmitter.
EMERGENCY OPERATION OF RADIO EQUIPMENT
Interphone Equipment Failure
If the interphone equipment fails, the audio frequency section of the command transmitter may be substituted for the regular interphone amplifier. To make this connection, the pilot places his command transmitter control box channel selector switch in either channel No. 3 or 4 position (or whatever position is not being used with a transmitter). Set the interphone jackbox selector switch to "COMMAND" to place the interphone equipment in operation.
When the command transmitter control box channel selector switch is set in either the No. 3 or 4 position for emergency operation of the interphone equipment, it is not possible to establish communication with any ground station or any other airplane. It is possible at all times to resume normal command set operation by placing the channel selector switch of the command transmitter control box in either the No. 1 or 2 position.
Substitution of Radio Compass Receiver for Low-Frequency Command Set Receiver
If the low-frequency receiver of the command set fails, the radio compass receiver may be substituted, with the pilot having direct control over the compass receiver. To complete this emergency hookup, the pilot must set his interphone jackbox selector switch in the "COMP" position and then place the radio compass selector switch in the "ANT" position. The radio compass can then be tuned as desired.
Substitution of Liaison Receiver for Low, Medium, and/or High-Frequency Command Receiver
In case of the failure of the low, medium, and/or high-frequency receiver of the command radio equipment, the liaison receiver may be substituted, but the pilot will have only limited control over it. The pilot should first call the radio operator on the interphone system and tell him what frequency he desires to receive, that he is switching the interphone selector switch to the "LIAISON" position, and for him (the radio operator) to tune in this frequency and maintain the setting until further notice.
Command Set Transmitter Failure
If the command set transmitter fails, the liaison transmitter may be substituted. The pilot should first call the radio operator on the interphone and have him adjust the liaison transmitter to the frequency he desires to use. He should then set his interphone selector switch to the "LIAISON" position and operate his microphone button in the same manner that he did when the command set was in operation. When he is through using the liaison transmitter, the pilot should place the interphone selector switch in the "INTER" position and tell the radio operator to cut the liaison transmitter off, to reduce the load on the electrical system.
When substituting one receiver for another, such as the compass receiver for the command receiver, the pilot must move his interphone selector switch to the "COMMAND" or "LIAISON" position, as the case may be, in order to transmit. At the end of the transmission, he must switch back to the position of the receiver being used. He must do this every time he desires to hold a 2-way conversation.
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