What does each control in an aircraft radio stack do?

An aircraft radio stack groups the radios and related avionics used for voice communication, navigation, audio routing and surveillance. In aviation and real-world flying, its controls typically select a radio, tune and swap frequencies, adjust volume or squelch, identify navigation stations, choose what the crew hears, and set the transponder code and mode.

There is no universal radio-stack layout. A basic training aircraft may have separate boxes for each function, while newer aircraft combine COM, NAV and GPS functions in one unit. Autopilots and engine instruments are sometimes mounted in the same vertical stack, but they are not radio equipment.

What units are found in a typical aircraft radio stack?

A conventional general-aviation radio stack contains some combination of the following units.

This FSX panel placing COM, NAV, ADF and transponder controls together is a useful visual example of the traditional arrangement.

What do the common radio knobs and buttons mean?

The same labels recur across many radio models, although a push function may be hidden inside a rotary knob.

• PWR/VOL: Turns the unit on and adjusts received-audio volume. On some installations, power comes entirely from the avionics master and the knob controls volume only.
• SQ or squelch: Suppresses background hiss when no usable COM signal is present. Pulling or pressing the knob may open the squelch so the crew can check receiver audio.
• Large and small tuning knobs: The outer knob normally changes the larger frequency step; the inner knob changes the smaller digits or channel. Exact behaviour depends on the radio and its channel-spacing configuration.
• Active and standby displays: The active frequency is the one being used. The standby field is where the next frequency is prepared.
• XFR, swap or a double-arrow button: Exchanges the active and standby frequencies. A mistake we see constantly in simulators is tuning the standby field but never transferring it.
• MON: Monitors a second receiver while retaining the selected transmitter. It does not necessarily change the radio used for transmission.
• ID: On a NAV receiver, this enables or emphasises station-identification audio. On a transponder, IDENT has a completely different purpose.
• ENT and CLR: Confirm or cancel data entered into a digital radio or integrated navigator.
• TEST: Tests lamps, displays or annunciators. Its effect is equipment-specific and should not be confused with transmitting a test signal.

How do you tune and use a COM radio?

A COM radio is used by tuning the required frequency, making it active, selecting that radio as the microphone source and transmitting with push-to-talk.

1. Check power and volume. Confirm that the avionics master and radio are on, then set a usable receive volume.
2. Tune the standby frequency. Use the outer and inner knobs, or enter the frequency directly on equipment that supports keypad entry.
3. Transfer it to active. Press the swap button and verify the active display rather than relying on memory.
4. Select the transmitter. On the audio panel, choose COM 1 MIC, COM 2 MIC or the equivalent label.
5. Listen before transmitting. This avoids transmitting over another call already in progress.
6. Hold push-to-talk while speaking. Release it to receive. A stuck or incorrectly mapped push-to-talk control can block normal communication.

COM frequencies may connect to ground, tower, approach, departure, area control or advisory services. Our explanation of how ATC services use those radio communications covers the operational distinction.

How do NAV, ADF and DME controls work?

NAV, ADF and DME units receive navigation signals rather than providing normal two-way voice communication.

What does a NAV radio control?

A NAV radio tunes a VOR or localiser frequency and sends guidance to a CDI, HSI or integrated display. After tuning, identify the station, select VLOC rather than GPS when required, and set the desired course on the separate OBS or course selector.

For an ILS, tuning the localiser frequency also pairs the associated glideslope where the installation supports it; there is no separate glideslope frequency knob. The NAV volume control changes identifier or voice audio, not needle sensitivity.

What does the GPS/VLOC or CDI button do?

The GPS/VLOC or CDI control chooses which navigation source drives the course indicator and, where connected, the autopilot. GPS mode follows the programmed route; VLOC mode follows a VOR, localiser or ILS receiver.

Switching source does not tune the NAV radio. If an ILS frequency is correct but the course needle remains tied to the magenta GPS route, the source is still set to GPS.

What do ADF controls do?

An ADF is tuned to an NDB frequency and placed in ADF mode to provide a bearing. ANT mode is mainly for reception and identification and normally does not provide a usable bearing indication. BFO is needed only for certain unmodulated station identifiers.

What does DME hold mean?

DME normally follows the station paired with NAV 1 or NAV 2, while HOLD keeps the previous DME channel when the NAV frequency changes. Forgetting that HOLD is engaged can leave a believable but unrelated distance on the display.

DME indicates slant-range distance, not map distance along a route. Its computed ground speed and time-to-station are useful only when tracking substantially towards or away from the station.

What does the aircraft audio panel control?

The audio panel independently controls which radio transmits and which receivers the crew can hear. Selecting COM 1 for reception does not always select COM 1 as the microphone source.

Only one transmitter is normally selected at a time, but several receivers can be monitored together. NAV and ADF audio selections let the crew hear station identifiers, while intercom controls may isolate the pilot, crew or passengers. Legacy marker controls select sensitivity, mute marker tones and test the blue, amber and white lamps.

What do the transponder controls mean?

The transponder code and mode determine how the aircraft replies to surveillance systems and whether pressure altitude is included.

• Code or squawk: A four-digit code using digits 0 through 7. Use the code assigned by ATC or the published code for the operation and jurisdiction.
• STBY: Powers the unit without providing normal airborne replies.
• ON: Provides identity replies but, on conventional controls, may omit altitude reporting.
• ALT: Provides identity and pressure-altitude reporting. The altitude comes from an encoder or air-data system; the pilot does not type in the displayed altitude.
• IDENT: Temporarily highlights the aircraft's return for a controller. Press it only when requested or when procedures require it.
• VFR: Loads a preset visual-flight-rules code. That code differs between jurisdictions, so the button should not be assumed to mean the same number everywhere.

Digital Mode S and ADS-B control heads may also request a flight identification and select ground or airborne modes automatically. Codes 7500, 7600 and 7700 are reserved for unlawful interference, radio failure and general emergency respectively; they are not test codes.

Why is the radio stack not working?

Most apparent radio-stack failures are caused by power, audio routing, source selection or an unswapped standby frequency rather than by the tuned number itself.

In FSX, keyboard input can help separate a control-binding problem from a panel or gauge problem; the FSX commands for selecting COM, NAV, ADF, DME and transponder controls show the relevant assignments.

Are airline radio panels different?

Modern airliners split the traditional stack functions between radio-management panels, audio-control panels, flight-management systems and transponder controls. The jobs are broadly the same, but the controls are distributed around the centre pedestal rather than arranged as separate general-aviation boxes.

For example, our guide to where radio and transponder controls sit in an A320 cockpit shows how those functions are divided. In an actual aircraft, the approved flight manual and avionics supplements remain the authority because button labels, automatic modes and backup tuning arrangements vary by installation.