Possible to shut off ACARS and SATCOM modem? Why can pilots turn the transponder off?

Right. I am going to answer this question and link it to the wiki. For the record I am a type rated Senior First Officer on the 777.
My answer will most likely get technical with jargon. To address this please use this list of abbreviations.

ABS - absolute

ABV - above

ADIRS - air data inertial reference system

AIMS - airplane information management system

ALT - altitude

ant - antenna

ATC - air traffic control

ATCRBS - air traffic control radio beacon system

BITE - built-in test equipment

BLW - below

CMCF - central maintenance computing function

CMCS - central maintenance computing system

coax - coaxial

CPU - central processing unit

gnd - ground

I/O - input/output

ident - identification

INS - instrument

LED - light emitting diode

MAT - maintenance access terminal

MAX - maximum

PSR - primary surveillance radar

RA - resolution advisory

REL - relative

RF - radio frequency

RPTG - reporting

SDI - source destination identifier

SLS - side lobe suppression

SSR - secondary surveillance radar

SPI - special purpose identification

STBY - standby

sw - switch

TA - traffic advisory

TAS - true air speed

TCAS - traffic alert and collision avoidance system

tpr - transponder

XFR - transfer

xpdr - transponder

xpndr - transponder

My aim here is to firstly demonstrate that an aircraft's transponder is nothing like the simplistic boxes attached to bank vans and automobiles that help lower insurance premiums. They are infact rather complex bits of kit. I will then explain the SATCOM and ACARS in depth and explain that they can indeed be "switched off".

Firstly. As has already been mentioned previously an aircraft needs to be seen on radar to receive a radar control service in the relevant airspace categories mandating this level of ATC service. In increasingly busier skies it is important that all the blips (primary radar echoes) can be positively identified. To aid this we have TRANSPONDERS with altitude encoding abilities. (MODE C) and surveillance capabilities (MODE S). Even little Cessna 150s are fitted with mode C transponders in certain states that require mode C capability as a minimum.

Both inside controlled airspace and uncontrolled airspace it is important for aircraft to avoid each other. As such TCAS or ACAS computers are fitted with their own antenna and interface with the TRANSPONDER.

To help ATC identify which aircraft is which, each control authority has a set of 4 digit codes to issue to each flight using their airspace at a certain time. Codes 7500/7600/7700 are reserved. Many states also reserve 7000 and 200. So baring those 4 codes when an aircraft receives it's airways or departure clearance it is issued with this number. The pilots enter this number and when the transponder is selected to a mode that enables ATC to see then (either with or without alt encoding at this point) the surface movement radar stars to label the blip on the apron as flight MH370. From this point on ATC can positively identify the flight. More depth in just a minute but why do you need to switch it off? Well flight MH370 flew in from somewhere then sat on the ground before departing. ATC shan't o close the flight plan and re issue the code to the next Flt plan requiring it. As such the transponder has to be placed in standby so that they no longer take up a code and add to ATC 'clutter'.

Now for an indepth technical bit. I am using a copy of the AMM and FCOM. That way, when reading this you will know everything you need to know plus more.

In General -

The air traffic control (ATC) ground stations interrogate the airborne ATC/Mode S transponder system. The ATC/Mode S transponder replies to the interrogations in the form of coded information that the ground station uses.

The ground station uses a primary surveillance radar (PSR) to get radar returns from airplanes within the radar range. To make a communication link with the airplanes in the radar range, the ground station uses a secondary surveillance radar (SSR) to interrogate the ATC/Mode S transponder. The ground station transmits a side lobe suppression signal to inhibit close ATC replies that come from a SSR side lobe transmission.

On the ground radar display, the ATC operator sees the radar returns, altitude, and a four digit airplane identifier.

The ATC/Mode S transponder also replies to mode S interrogations from the traffic alert and collision avoidance systems (TCAS) of other airplanes.

When a ground station or a TCAS computer from another airplane interrogates the ATC/Mode S transponder system, the transponder transmits a pulse-coded reply signal. The reply signal identifies, and shows the altitude of the airplane.

General Description

The components of the ATC system are:

- Top antenna - Bottom antenna - ATC coaxial relay top - ATC coaxial relay bottom - Transponder panel - Two ATC/Mode S transponders - Two program switch modules.

The two antennas supply transmit and receive signals to the ATC/Mode S transponder through the ATC coax relays.

The transponder panel sends identification and control data to the transponders and control to the ATC coax relays.

The AIMS cabinets supply altitude and central maintenance computing function (CMCF) data to the ATC/ Mode S transponders.

The ATC/Mode S transponders have interface with the TCAS computer.

Flight Deck

The transponder panel is on the P8 aft aisle stand.

Main Equipment Center

The ATC/Mode S transponder system components in the main equipment center are:

- Left ATC/Mode S transponder - Right ATC/Mode S transponder - Program switch modules - Top ATC coax relay - Bottom ATC coax relay.

Power

The left ATC/Mode S transponder gets 115v ac power from the ac standby bus. The right ATC/Mode S transponder gets 115v ac power from the right ac transfer bus. The dual transponder panel gets 115v ac power from the ac standby bus.

Standby/On Discrete

The transponder panel sends a (ground) standby discrete to the transponder that is not in use.

Identity Code

The flight crew sets the four digit identity code and it shows on the transponder panel. Both transponders get the identity code from the transponder panel.

Control Data

The transponder panel also sends control data to the transponders. This control data does these functions:

- Permit mode C altitude reporting - Permit the transponder to send the special position identification (SPI) pulse or airplane identification code.

Air Data

The air data inertial reference unit (ADIRU) and the secondary attitude air data reference unit (SAARU) send

air data to the AIMS cabinets. ATC/Mode S transponders receive the air data from both AIMS cabinets.

The left transponder gets air data at ADC input 1 from the ADIRU general purpose (GP) bus 3 from left AIMS cabinet. Air data at ADC input 2 comes from the SAARU GP bus 3 from the right AIMS cabinet.

The right transponder gets air data at ADC input 1 from the SAARU GP bus 3 of the right AIMS cabinet. Air data at ADC input 2 comes from the ADIRU GP bus 3 from the left AIMS cabinet.

THE TRANSPONDER PANEL

General

The dual transponder panel controls the ATC/Mode S transponder and the TCAS computer.

The ATC controls and displays on the transponder panel are:

- IDENT switch - Four digit identification code - Active system display - Transponder mode selector - Transponder code selectors - Altitude source selector - Transponder selector.

Altitude Source Selector

The altitude source selector has two positions, NORM and ALTN. Each transponder has two ADC inputs. With NORM selected, the active transponder uses the altitude data that comes from ADC input 1. With ALTN selected, the active transponder uses the altitude data that comes from ADC input 2.

Transponder Selector and Active System Display

The transponder selector has a left and a right transponder (XPNDR) position. With the left position selected, the control panel sends a ground standby/on discrete to the right transponder only. Then the right transponder is in standby. The left transponder is active.

With the right position selected, the control panel sends an open standby/on discrete to the right transponder to make it active. A ground standby/on discrete goes to the left transponder and an antenna transfer discrete goes to the ATC coax relays. This puts the left transponder in standby, and makes the relays connect the ATC antennas to the right transponder.

The selected ATC system shows on the LCD active system display.

Identification Code Display and Selection

The flight crew uses the code selectors to set the four digits of the identification code. The four digits show on the liquid crystal display (LCD). Codes are from 0000 to 7777, with 4096 different selections.

CAUTION: DO NOT SELECT CODES 7500, 7600 OR 7700. THESE CODES ARE USED ONLY FOR EMERGENCIES.

Transponder Mode Selector

The transponder mode selector has several positions. These are the functions that the ATC system uses:

- STBY (standby) - a ground discrete goes to both transponders. This ground discrete prevents operation of the transponder, but does not prevent built-in-test-equipment (BITE) functions.

- ALT RPTG OFF (altitude reporting off) - the active transponder responds to ATC interrogations with mode A replies only. - XPNDR (transponder) - the active transponder responds to ATC interrogations with mode A and mode C (altitude reporting) replies.

IDENT Switch

When the ATC controller requests the airplane identifier, the pilot pushes the momentary IDENT switch. The transponder adds a special position identification (SPI) pulse to the interrogation reply for the next 18 seconds.

TCAS

The traffic alert and collision avoidance system (TCAS) helps the flight crew and air traffic control maintain safe air traffic separation. TCAS is an airborne system. TCAS uses an air traffic control radar beacon system (ATCRBS) transponder or an ATC/Mode S transponder to track other airplanes. TCAS also coordinates maneuvers with other airplanes that have TCAS.

TCAS supplies a traffic display and visual and aural vertical commands to the flight crew. All commercial airlines and some general aviation airplanes have TCAS.

The traffic alert and collision avoidance system (TCAS) interrogates other airplanes to get altitude, range, and bearing data. TCAS uses the replies and inputs from other onboard airplanes systems to calculate traffic avoidance data.

General Description

These are the TCAS components:

- ATC antennas - Transponder panel - TCAS directional antennas - TCAS computer.

TCAS interfaces with these other system components:

- Left and right airplane information management system (AIMS) cabinets - Left and right ATC/Mode S transponders - Left and right warning electronic units (WEUs) - Left and right radio altimeters - Ground proximity warning computer (GPWC) - Left and right weather radar receiver-transmitters (R/Ts) - Left and right proximity switch electronic units (PSEUs).

Suppression

The TCAS computer connects to the DME interrogators and the ATC transponders through a suppression splitter.

The TCAS computer has interfaces with these components:

- Overhead circuit breaker panel - Top and bottom TCAS directional antennas - Left AIMS cabinet - Right AIMS cabinet - Air/ground relay - Ground proximity warning computer (GPWC) - Proximity sensor electronics unit (PSEU) 1 - PSEU 2 - DME/ATC/TCAS suppression splitter - Left warning electronic unit (WEU) - Left WXR RT - Right WXR RT - Right WEU.

There are program pins to supply control of the TCAS computer.

Power

The TCAS computer gets 115v ac from the left ac secondary 2 bus, through the overhead circuit breaker panel.

Antennas

There are two TCAS directional antennas, one on the top and one on the bottom of the airplane. The TCAS directional antennas receive traffic airplane reply

signals. They also transmit the TCAS interrogation signals.

AIMS - Display Status

An analog discrete from either AIMS cabinet goes to the TCAS computer when the display function of AIMS fails. When the TCAS computer gets this discrete, the TCAS computer does not do these functions:

- Send TCAS display outputs to the AIMS - Send TCAS aurals to the warning electronic system (WES) - Transmit coordination data to traffic airplanes with TCAS.

Air/Ground Relay

This discrete from the air/ground relay supplies in-air or on-ground status to the TCAS computer. The air/ ground discrete inhibits resolution advisories (RAs) when the airplane is on the ground and inhibits tests when in the air. Also, the air/ground discrete increments flight legs in the TCAS nonvolatile memory when the central maintenance computing system (CMCS) fails.

GPWC - Advisory Delay Discretes

The GPWC sends three analog discretes to the TCAS computer. These discretes inhibit advisories during some ground proximity warning and alert conditions.

PSEU - Landing Gear Position

This analog discrete from the PSEUs supply the TCAS computer with the position of the landing gear. When the TCAS computer gets this discrete, the TCAS computer makes the bottom directional antenna become an omnidirectional antenna.

Suppression Input/Output

The TCAS computer gets a suppression pulse when an ATC transponder or DME interrogator transmits. When the TCAS computer transmits, it sends a suppression pulse to the ATC transponders and the DME interrogators.

WEU - TCAS Aurals - Voice Outputs

The TCAS computer sends RA and TA aural signals to the warning electronic units (WEUs). The WEUs amplify the RA and TA aurals and send them to the aural warning speakers to alert the flight crew.

WXR - TCAS Alert Inhibit

The TCAS computer gets an inhibit discrete from the predictive windshear (PWS) function of WXR. When there is a PWS alert, this discrete does these functions:

- Changes TCAS resolution advisories (RAs) to traffic advisories (TAs) - Inhibits TCAS aural alerts.

Program Pins

These are the functions that the program pins on the TCAS computer enable:

- A self-test inhibit to prevent self-test in the air - The audio level of the voice outputs - The airplane altitude limit of 48,000 feet so TCAS does not command a climb or increase climb above this altitude.

TCAS - DIGITAL INTERFACES

General

The TCAS computer has digital interfaces with the:

- Left ATC/Mode S transponder - Right ATC/Mode S transponder - Left radio altimeter transceiver - Right radio altimeter transceiver - Left AIMS cabinet - Right AIMS cabinet.

ATC/Mode S Transponder - Control and Coordination Data

The dual transponder panel sends this control data through the selected ATC/Mode S transponder to the TCAS computer:

- The TCAS mode selection (TA only or TA/RA) - Control of the altitude limits for the TCAS display that shows on the ND.

For the TCAS computer to calculate coordination data, ATC transponder sends this to the TCAS computer:

- 24-bit airplane address - Barometric altitude - Maximum true airspeed.

The TCAS computer sends this to the transponder:

- General TCAS operational status - Mode S coordination data.

Radio Altimeter Inputs

The TCAS computer gets radio altitude from the left and right radio altimeter transceivers.

TCAS Inputs from AIMS

The left AIMS cabinet supplies these inputs to the TCAS computer:

- Inertial reference unit (IRU) data from the air data inertial reference system (ADIRS). - Data loader data - Data loader enable analog discrete - Central maintenance computing system (CMCS) data.

The right AIMS cabinet does not supply any inputs to the TCAS computer.

TCAS Outputs to AIMS

The TCAS computer sends TCAS data load status to the left AIMS cabinet.

The TCAS computer supplies these outputs to both AIMS cabinets:

- Traffic advisory (TA) and resolution advisory (RA) data, which includes all traffic data for TCAS displays - TCAS maintenance data.

PILOTS VIEWPOINT

TCAS alerts the crew to possible conflicting traffic. TCAS interrogates operating transponders in other airplanes, tracks the other airplanes by analyzing the transponder replies, and predicts the flight paths and positions. TCAS provides TCAS ND messages, voice annunciations, PFD vertical flight path guidance, and traffic displays of the other airplanes to the flight crew. Neither ND messages, voice annunciations, PFD vertical guidance, nor traffic display is provided for other airplanes that do not have operating transponders. TCAS operation is independent of ground–based air traffic control. TCAS identifies a three–dimensional airspace around the airplane where a high likelihood of traffic conflict exists. The dimensions of this airspace are contingent upon the closure rate with conflicting traffic. TCAS provides:
• resolution advisory (RA) and display • traffic advisory (TA) and display

• proximate traffic display • other traffic display.


TCAS messages and TCAS traffic symbols can be displayed on the ND in the map, map centered, VOR, and approach modes. TCAS messages and TCAS traffic symbols cannot be displayed on the ND in the VOR–centered, approach–centered, or plan modes. TCAS messages TRAFFIC, TA ONLY, and TCAS TEST may be displayed in all ND modes. TCAS processing priorities may reduce display of certain air traffic on the ND. Reduced display of air traffic does not affect system collision avoidance alerting capability.

Resolution Advisories (RA) and Display An RA is a prediction that another airplane will enter the TCAS conflict airspace within approximately 20 to 30 seconds. If altitude data from the other airplane is not available, no RA can be provided. When TCAS predicts an RA: • the TCAS red message TRAFFIC is displayed on the ND • a TCAS voice annunciation sounds • TCAS PFD vertical guidance is displayed When the TCAS cyan message TFC is displayed on the ND, and the RA is within the display range of the ND, the TCAS RA Traffic aircraft symbol and its accompanying data tag is displayed on the ND. The TCAS RA Traffic aircraft symbol is a filled red square. The RA data tag contains the altitude and the vertical motion arrow. For no–bearing RAs, the red RA label is displayed below the red message, TRAFFIC, and the RA data tag information is displayed to the right of the label. The RA red data tag contains the distance, altitude, and the vertical motion arrow. When the RA is further from the airplane than the ND range currently displayed, the TCAS red message OFFSCALE is displayed on the ND.

Traffic Advisories (TA) and Display A TA is a prediction another aircraft will enter the conflict airspace in 25 to 45 seconds. TAs assist the flight crew in establishing visual contact with the other aircraft. When TCAS predicts a TA: • the TCAS amber message TRAFFIC is displayed on the ND • the TCAS voice annunciation TRAFFIC, TRAFFIC sounds once When the TCAS cyan message TFC is displayed on the ND and the TA is within the display range of the ND, the TCAS TA Traffic aircraft symbol and its accompanying data tag are displayed on the ND. The TA Traffic aircraft symbol is a filled amber circle. The TA data tag contains the altitude and the vertical motion arrow. For no–bearing TAs, the amber TA label is displayed below the TRAFFIC message, and the TA data tag information is displayed to the right of the label. The TA labels are displayed below the RA labels. The TA data tag contains the distance, altitude, and vertical motion arrow. When the TA is further from the airplane than the ND range currently displayed, the TCAS amber message OFFSCALE is displayed on the ND. I

Proximate Traffic Display Proximate traffic is another airplane that is neither an RA or a TA but is within: • six miles • 1,200 feet vertically. When the TCAS cyan message TFC is displayed on the ND, and the proximate traffic aircraft is within the ND display range, the TCAS proximate traffic aircraft symbol is displayed on the ND. The TCAS proximate traffic aircraft symbol is a filled white diamond. When TCAS is receiving and processing altitude data, the Proximate Traffic data tag is displayed on the ND. The proximate traffic data tag contains the altitude and vertical motion arrow.

Other Traffic Display Other Traffic aircraft is an aircraft that is within the ND display limits but is neither a RA, a TA, or proximate traffic aircraft. When TCAS is not receiving and processing altitude data from the Other Traffic aircraft, the Other Traffic aircraft becomes Proximate Traffic aircraft automatically when within six miles. When the TCAS cyan message TFC is displayed on the ND and the Other Traffic aircraft is within the ND display range, the TCAS Other Traffic symbol is displayed on the ND. The TCAS Other Traffic symbol is a hollow white diamond. When TCAS is receiving and processing altitude data from the Other Traffic aircraft, a data tag like that described in Proximate Traffic Display is displayed.

TCAS PFD Vertical Guidance When TCAS predicts an RA, TCAS vertical guidance is displayed on the PFD for a maneuver to ensure vertical separation. Traffic avoidance is ensured by adjusting or maintaining a pitch attitude and vertical speed outside the red RA regions. If the traffic airplane also has TCAS and a mode S transponder, TCAS vertical guidance is coordinated with the traffic aircraft TCAS.

TCAS Normal Operation TCAS is controlled from the transponder panel. TA/RA is normally selected. However, it is sometimes necessary to select TA ONLY to prevent nuisance RAs. TA ONLY is selected during engine out operations to prevent RAs when adequate thrust is not available to follow the RA commands.

TCAS Non–Normal Operation The EICAS advisory message TCAS OFF is displayed if TCAS is not operating. The message is inhibited below 400 feet radio altitude. No TCAS RA guidance is displayed on the PFDs, no TCAS traffic symbols are displayed on the NDs, and no TCAS voice alerts sound. An amber TCAS OFF message is displayed on both NDs. The EICAS advisory message TCAS RA (CAPTAIN or F/O) is displayed if TCAS cannot display RA guidance on the respective PFD. The ND traffic displays and voice alerts are unaffected. The EICAS advisory message TCAS is displayed if TCAS cannot display TCAS RA guidance on either PFD, and cannot display TCAS traffic symbols on either ND. TCAS voice alerts will not occur. An amber TCAS FAIL message is displayed on both NDs.


So as you can see the TRANSPONDER is a complex bit of kit that requires manipulation by the flight crew for a range of reasons, most of which are normal day to day operational requirements.

When looking at the FCOM Boeing state which mode is to be selected and when depending on phase of flight.


ACARS AND SATCOM.

ACARS and SATCOM can be disabled.

General

The SATCOM system provides a total of five voice channels and one data channel.

The control display units (CDUs) connect with the satellite data unit (SDU). The flight crew use the CDUs to control the SATCOM system.

Voice Operation

Audio control panels (ACPs) connect to the audio management unit (AMU) to control the voice signals from the flight deck to the SATCOM system.

The AMU connects with the SDU to send and receive audio signals for the flight crew.

The cabin telecommunication unit (CTU) connects with the SDU to send and receive voice signals.

Data Operation

The airplane information management system (AIMS) connects with the SDU to send and receive data signals.

The CTU connects with the SDU to receive and transmit data signals.

Signal Flow - Receive

The high gain antenna (HGA) receives radio frequency (RF) signals. It sends them to the LNA/Diplexer (LNA/ DIP).

The LNA/DIP increases the power level of the received signals. The LNA/DIP sends the amplified RF signals to the radio frequency unit (RFU).

The RFU receives the amplified signals and changes the RF signals to intermediate frequency (IF) signals for the satellite data unit (SDU).

The SDU demodulates the received IF signals and sends them to the AMU, the AIMS cabinets, or the CTU.

Signal Flow - Transmit

The AMU, AIMS, or the CTU sends transmit signals to the SDU. The SDU modulates the signals and sends them to the RFU.

The RFU sends the RF signals through the radio frequency attenuator and the radio frequency splitter (RFS) to the high power amplifier (HPA).

The HPA increases signal strength and sends the signal to the low noise amplifier/diplexer (LNA/DIP).

The LNA/DIP sends the RF signal to the top mounted high gain antenna (HGA). The HGA receives beam steering information from the beam steering unit (BSU). The HGAsplits the RF into thirty nine identical signals. The HGA electronically steers the RF signals to the satellite by sending them through the antenna arrays in an aligned beam.

Low Gain Antenna

The low gain antenna system is for data only. It operates if a high gain antenna system fails or the class A high power amplifier fails.

The satellite data unit (SDU) supplies control and failure data for the SATCOM system.

These components have an interface with the SDU for control and status data:

- High power amplifiers (HPAs) - Low noise amplifier/diplexers (LNA/DIPs) - Beam steering unit (BSU) - Airplane information management system (AIMS) - Control display units (CDUs) - Radio frequency unit (RFU).

High Power Amplifiers

The high power amplifiers (HPAs) receive control signals from the SDU. The HPAs receive the control signals to adjust the output of their transmit signals. The HPAs send BITE data to the SDU. The class A HPA receives a mute signal from the BSUs to shut down the HPA when the system switches outputs from one antenna to the other.

Low Noise Amplifier/Diplexer

The low noise amplifier/diplexer (LNA/DIP) for the low gain antenna (LGA) receives on/off signals from the SDU. The LNA/DIP sends BITE data to the SDU. The LNA/ DIPs for the high gain antennas send BITE data to the beam steering units (BSUs).

The LNA/DIP for the high gain antenna (HGA) receives on/off control from the beam steering unit. The LNA/DIP for the HGA sends BITE data to the beam steering unit (BSU).

Beam Steering Unit

The BSU receives direction signals from the SDU. The BSU sends BITE data to the SDU. The BSU sends on/off signals to the LNA/DIP. The BSU sends beam steering data to the high gain antenna (HGA). The BSU receives BITE data from the HGA.

Airplane Information Management System (AIMS)

The central maintenance computing function (CMCF) in the airplane information management system (AIMS) receives SATCOM system BITE data from the SDU.

AIMS sends this data to the SDU:

- International Civil Aviation Organization (ICAO) address - Inertial reference data - Software for system configuration - Data load enable - BITE commands.

Radio Frequency Unit

The radio frequency unit (RFU) receives frequency selection data from the SDU. Each satellite operates on a different frequency to decrease interference from other satellite signals.

Audio Management Unit

The audio management unit (AMU) sends PTT signals to the SDU. The SDU sends call light signals to the AMU.

Control Display Units

The flight crew uses the control display units (CDUs) to send control signals to the SDU for SATCOM system operation.

The flight crew can instigate a manual log-off of the SATCOM system.

The only remaining element for data transmitting is via the DATA channel of the VHF Centre and Right radios. This is simply done by selecting a voice frequency on both radio tuning panels.