What Is Guard Frequency (121.5MHz) And Why Is It Important To Monitor It?

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  • Post last modified:July 23, 2020

Guard frequency is an international distress and emergency aviation frequency that pilots are expected to monitor on their second radio when possible. 

What exactly does it mean to be “On Guard”? 

Guard frequency 121.5 MHz (which you might also hear referred to as “twenty-one five”) and 243 MHz are international aviation distress frequencies with 243 MHz (think double 121.5) generally used by the military. ELT’s can also be head on guard and should be reported to ATC.

What is guard 121.5MHz used for?

121.5 MHz is generally reserved for civilian distress or emergency use, with 243MHz used for military operations (although you do hear military aircraft also on 121.5 MHz).

If ATC (Air Traffic Control) are unable to reach aircraft on their assigned frequency they will try to call them on guard frequency. Aircraft may not be able to respond to ATC on their assigned frequency because the frequency is blocked by a “stuck mic”, they’ve gone out of range, the crew have selected the wrong frequency, or because of full-blown “lost comms”.

Radio Management Panel (RMP) on Airbus A330 aircraft showing 121.5MHz guard selected on VHF radio 2.
Radio Management Panel (RMP) on Airbus A330

Lost Comms / NORDO

Although light aircraft that are operating without a radio can be considered to be “NORDO” (“no radio”), generally this is used to describe aircraft which have lost communication with Air Traffic Control. Often, if ATC lose contact with an aircraft on their assigned frequency they will try to raise the aircraft “on guard”. Losing comms with ATC can happen for a variety of reasons – changing to the wrong frequency, going out of range etc. 

General “lost comms” procedures depend if you’re VFR or IFR. VFR procedures state that you should remain in visual conditions (and visual with terrain) and land at the nearest airport.

If operating IFR then you should, if possible, land visually at an airport. If not visual with terrain then you should continue IFR (taking into account minimum altitudes) and comply with the last assigned IFR clearance:

Radio communication failure procedure.

Link: FAR AIM 6-4-1 Two-way Radio Communications Failure

For example, in Seattle a departure clearance will include a SID (Standard Instrument Departure), a squawk code, cleared level and “expect FL350 15 minutes after departure” due to the high terrain in the area – in case of lost comms, as above, you would climb to the level ATC has told you to expect.

Lost comms actions would include:

  • “Squawking 7600” (i.e. setting a squawk code of 7600 in the transponder which will signal to ATC you’re experiencing radio failure). 
  • You should also continue to “transmit blind” (i.e. continue transmitting in the hope that even though you can’t hear the reply, they can hear you).
  • Try all radios you have – so on airliners you would generally have 3 separate radios for VHF and 2 HF radios. 
  • CPDLC (more on Controller Pilot Data Link Communications here) may be available where you are – and you may be already be using it. You can send a message to the ATC center you’re logged on to advising them of your situation.
  • Many aircraft are equipped with SATCOM (Satellite Communication) – by selecting “SATCOM” on the RMP (Radio Management Panel) you can call any number as if you would with your phone. Pilots can access a list of useful numbers including ATC, maintenance, operations etc.
  • ACARS (Aircraft Communication Addressing and Reporting System) which is a text message-like system normally used for getting weather reports but it could be used to alert Dispatch/Ops Control of your situation.
  • Mobile phone – if you’re low and slow enough it might be an option. Also modern aircraft have the option of using your phone over the aircrafts SATCOM system (just don’t forget to send the bill to the company!).

As you can see, it’s almost impossible for a modern aircraft these days to lose communication completely. 

Learn more about CPDLC and ACARS here.

Noctilucent clouds visible from Airbus A330 aircraft flight deck
Noctilucent clouds visible over Northern Canada from A330 flight deck

What is a Stuck Microphone or “Stuck Mic”?

Another call you may hear on guard from ATC or other aircraft is reports of a “stuck mic” on a certain frequency. What is a stuck mic?

A stuck microphone, or a “stuck mic”, referes to the way that sometimes the transmit button or trigger that’s used to transmit radio calls to ATC gets stuck resulting in a continuous transmission that blocks the frequency in use.

(And it can be very dangerous if you don’t realise its happening to you and you’re chatting about how your latest date went!)

Modern aircraft will alert you about this embarrassing situation after about a minute of continuous embarrassment (transmission).

Link: Airbus Transient Loss of Communication due to Jammed Push-To-Talk

Emergency Locator Transmitters (ELTs)

ELTs (Emergency Locator Transmitters) are electronic transmitters which assist in the locating of downed aircraft and other vessels in distress. ELTs transmit on 121.5 MHz, 243 MHz and 406 MHz, with the 406 MHz signal often encoded with details such as the vessels identification, contact details and even location which can be received by satellite allowing much faster rescue. Here is what an ELT on guard sounds like.

In the U.S. 406, MHz ELTs must be registered with the National Oceanic and Atmospheric Administration (NOAA) as they are the department tasked with monitoring the satellite distress alerting system (SARSAT). The COSPAS-SARSAT system is a network of satellites that monitor signals from ELTs in aircraft (and EPIRBs in the maritime environment and PLBs for personal use) anywhere on earth and alerts ground stations when these signal have been detected.

Link: FAA Aeronautical Information Manual: 6-2-4 Emergency Locator Transmitter (ELT)

Emergency Location Transmitter ELT Panel on Airbus A330 Aircraft
ELT Panel on Airbus A330 Aircraft

ELT Testing

ELT’s should only be tested for the first 5 minutes of the house. The FAA AIM states that ELT’s should “be tested…preferably in a shielded or screened room…to prevent broadcast of signals which could trigger a false alert”. When this isn’t possible analog 121.5/243 MHz ELTs should only be tested for the first 5 minutes of the hour.. (Regardless, its good airmanship to report any observed ELT to Air Traffic Control).

In an older airliner I used to fly part of the “parking checklist” called for checking 121.5 MHz in case the ELT had activated accidentally. (Fortunately my landings, which weren’t always smooth, were never bad enough that the system thought we had crashed…). Modern ELTs provide a visual warning to the pilot they have activated – either with a warning light or on the EICAS/ECAM.

Link: FAR AIM ELT Testing

The newer model ELTs that transmit on 406 MHz and monitored by satellite must be able to transmit for a period of 24 hours from activation. ELT’s would normally be activated by impact but can be manually activated on its control panel (see above).

2 Eurofighter Typhoon fighter jets in flight

Life on the Line – a crash in the Alps and fighter jets on the port side

About 10 years ago we were routing over France when we started to pick up one side of a conversation on box 2 (which we had tuned to 121.5 MHz). It was from a light aircraft at low level over the Swiss Alps that was relaying a distress call on guard frequency. He was reporting another light aircraft he had been travelling with had crashed and was inverted and he was relaying the position. We monitored the conversation to make sure he was able to report the location to another aircraft that was closer to him, who in turn reported it to ATC. 

On another day, not too many years ago we started to pick up calls on VHF guard from UK ATC requesting a private jet check in. “ABC123 come up on guard, and squawk ident”. (“Squawk ident” mean to press the “ident” button on the transponder. This is used by ATC to identify particular aircraft and is also used in a lost comms situation so even if the aircraft is unable to reply on VHF, if they squawk ident ATC know they are at least able to receive them). This was repeated every minute or two for about 20 minutes when things got more serious: “This is the UK Ministry of Defence. I am instructed by her Majesty’s Government to order you to change course immediately”.

Yes, things were getting serious. This call was repeated a number of times but there was no answer from our silent friend. Usually calls on 121.5 don’t escalate this much – aircraft check in with ATC, apologise (or make an excuse) for being on the wrong frequency, and carry on their way (with a bit of paperwork to fill out at the end of the flight). In this case though, with no reply from ABC123, the inevitable call arrived: “ABC123 this is the RAF Eurofighter on your left hand side, come up on 121.5 MHz immediately. The sight of a fighter jet suddenly coming into view is going to wake anyone up!

All of a sudden ABC123, sounding just a little shocked, came up on the frequency. Apologising, he requested to carry on his flight to his destination – nice try, but no way that was going to happen! The fighter jets escorted him to Cardiff airport where I’m sure he had a lot of awkward explaining to do to his passengers.

Interceptions usually involve two fighters – the one you need to worry about is not the one you can see, but the one behind you!

Link to the story here.

Frequently asked questions about Squawking 7700

What is VFR flight?

VFR, or Visual Flight Rules, refers to a set of procedures and regulations that pilots operate aircraft to when operating in visual conditions. Pilots operating VFR must be able to navigate the aircraft with visual references to the ground, and be able to visually avoid obstacles and other aircraft.

What does VMC mean in aviation?

VMC (Visual Meteorological Conditions) refer to weather conditions good enough to allow pilots to operate VFR and navigate by reference to the ground and avoid other aircraft.

What is IFR flight?

Instrument Flight Rules (IFR) is a set of regulations governing flight where that flight may be operated solely by reference to the aircraft instruments.

What does IMC mean in aviation?

IMC (Instrument Meteorological Conditions) refers to specific weather conditions lower than VMC (Visual Meteorological Conditions).

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