Frequency 121.5: Why Is It Important to Monitor?

Guides · 4 min read · Aug 10, 2022
Frequency 121.5

Frequency 121.5 MHz, also known as the guard frequency, is a frequency reserved for civil aviation emergencies by the FAA. On the other hand, emergencies in military aviation are more often channeled through the 243 MHz frequency even though the 121.5 MHz can sometimes be used for similar purposes. As such, both frequencies are international aviation distress frequencies particularly in the United States National Airspace System, NAS, which pilots can rely upon when they lose communication with the ATC because of many reasons.

So, let’s take a look at some of the major reasons to use Frequency 121.5 MHz.

Stuck microphone, “stuck mic”

Even heard of a hot mic and an open microphone? These are similar to a stuck mic, with the terminology used in aviation. If the toggle switch on the microphone remains undesirably on, then there will be an unintended continuous flow of signals that then jams or blocks the assigned frequency.

Modern aircraft have however come to the rescue of most pilots who could experience this apparent error. For instance, the Airbus Company reports that the transmission time is limited to 35 seconds for each VHF radio, and with just over 30 seconds of inactivity, the pilot is warned via 5 beeps, which signals that there would be an automatic cut-off of communication via the microphone. Well, as a pilot on duty who has little influence on how modern the aircraft is, as long as it is airworthy, the emergency frequency got you covered.

An aircraft pilot communicating with ATC on a radio frequency.

Selection of the wrong frequency

It is common to have different frequencies assigned to the aircraft depending on the flight mission among other considerations. Regardless, the traffic controllers at the airport must agree with the flight crew on whether to change the frequency or continue the flight on the same frequency as assigned in the previous flight.

However, if the aircraft is out of range with the previous controlling station, for example, where many agencies are involved or the assigned frequency is overcrowded, then a selection of the wrong frequency can occur, causing a loss of communication.

Out-of-range frequency

Each line of communication between the crew, or with the controlling station at the airport, or when airborne must be within a frequency band. A selection of an out-of-range frequency would thus hinder two-way communication.

What is unique or special about 121.5 MHz?

This frequency is monitored by every ATC and once an aircraft loses communication with the ATC, it can be relied upon to restore the lines of communication. Probably you could be wondering how the air traffic controllers know each frequency that every airborne civil airplane is on. Well, while they would normally know, at least as they are expected to, they would still be able to maintain contact with the aircraft, in this case, a “no radio” aircraft if they do not know, for whatever reasons, the frequency an aircraft is on.

No radio

A “no radio” airplane is termed as a NORDO aircraft, which has lost communication with the ATC. That is, the ATC can neither hear nor be heard by the pilots and vice versa.  Sometimes, NORDO refers to those aircraft that do not have a radio, especially small aircraft. However, in this context, we have relied on the former definition.

Search and rescue

Equally, the frequency is reserved for emergencies such as search and rescue operations. Therefore, it is nearly impossible for it to be jammed or have interference with other frequency bands. Considerably, both the traffic controllers and the pilots can maintain contact until a new frequency is assigned to the aircraft.

Airplane cockpit and control column with a pilot controlling it.

Should you always monitor the guard frequency in flight?

Considerably, as a disclaimer, the Aeronautical Information Manual, AIM, requires that airplanes maintain a listening watch on the emergency frequency to the maximum possible range. However, the answer to this question is a relative one, which means, it depends on you, the pilots, and to a larger extent, the aeronautical requirements.

While there is absolutely no harm to always monitor the guard frequency in flight, failure to monitor the frequency when it would have been necessary can compromise essential communication with the ATC and consequently, be a blow to safety. The bigger picture is for pilots to ask themselves the question, why not monitor the emergency frequency?

Benefits of regularly monitoring the frequency

First, as a pilot, you’ll never know that you have lost communication with the ATC via the guard frequency if you have the radios set on other frequencies other than the 121.5 MHz. This is because there are no system warnings about this. On the contrary, yes. You would immediately tell where the signal transmission from one end of the channel, the ATC, goes silent. In other words, for easier support to the flight crew, it is important to remain vigilant by having a stronger sense of situational awareness as recommended by the FAA.

Secondly, most modern airplanes are equipped with at least 2 VHF radios. This additional communication channel can be used to the advantage of the pilots by setting the guard frequency on one of the radios, usually the second radio. It also ensures that you have one dedicated channel for the initial flight rules and still maintain another for an emergency.

However, because of the cognizance that flight conditions are dynamic and the different flight profiles present a range of difficulties, then it is imperative for pilots to focus on the controllability and maneuverability of the airplane in those flight conditions at the expense of monitoring the VHF guard.

A pilot controlling a plane and communicating with ATC on radio.

Further, some flight crew members abuse the VHF guard through immaterial blathering on the frequency, often creating a false alert. For example, AOPA acknowledges that some pilots have gone overboard to the extent of imitating the ATC’s commands. This is after an incident when one pilot erroneously requested for a pushback clearance via the VHF guard and another pilot, on another aircraft, responded, “Cleared to push.”

Notably, it is least expected that pilots, who have undergone a series of technical, safety, and safety management system training, can sabotage the careers and safety, whether indirectly or directly, of other pilots and passengers. As a reminder, safety is essential both in the air and on the ground, anywhere in the world!

Final thoughts

Unless we remain responsible airmen, our abuse of the guard frequency may defeat its purpose. As brainstorming homework, is 121.5 MHz still used? Yes, but perhaps not as broadly as before. Want to know why? Don’t miss this next conversation, here on your favorite blog.

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Jaap Brian profile picture
Jaap Brian
A highly passionate aviator, with a solid background in aeronautical engineering. His journey to writing about aviation topics is founded on sharing insights into aviation safety and technical aircraft performance – a journey that is 6+ years and counting.

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