If a pilot is already speaking to ATC (or monitoring a frequency)
they should continue to use that frequency for any emergency or abnormal situation.
If they are not currently receiving ATC services or on an ATC frequency, attempt to use 121.5.
Many ATC providers and enroute airliners monitor this frequency and can relay or provide assistance.
The FAA's AIM (6-3-1) describes emergency use of 121.5 in the United States: h.
Although the frequency in use or other frequencies assigned by ATC are preferable, the following emergency frequencies can be used for distress or urgency communications, if necessary or desirable: 1. 121.5 MHz and 243.0 MHz.
Both have a range generally limited to line of sight.
121.5 MHz is guarded by direction finding stations and some military and civil aircraft.
243.0 MHz is guarded by military aircraft.
Both 121.5 MHz and 243.0 MHz are guarded by military towers, most civil towers, FSSs, and radar facilities.
Normally ARTCC
emergency frequency capability does not extend to radar coverage limits.
[emphasis mine]
If an ARTCC does not respond when called on 121.5 MHz or 243.0 MHz, call the nearest tower or FSS.
aviation.stackexchange.com/users/39/egid
Yes, the aircraft emergency frequency (which we often call "guard") is 121.5 MHz.
This is monitored by many control towers and flight service stations.
It is also typically monitored by airplanes on cross-country trips (especially airliners).
You will occasionally hear ATC ask an airplane to try to contact another airplane "on guard" in an attempt to relay a message to an aircraft which they have lost communications with.
However, if you are currently in communication with ATC on another frequency, or there is a frequency where you know you can communicate your distress and it will be heard, you should use that frequency instead, because there is no guarantee your message will be heard on 121.5 in all parts of the country/world.
Older ELT's (emergency locator transmitter) also transmitted a siren type sound on this frequency when activated which sounds like this .
In addition, it is used as an intercept frequency.
In other words, if you get intercepted by a fighter jet, you better be listening in on 121.5 for any commands they give you.
aviation.stackexchange.com/users/6/Bret Copeland
As a general rule, declaring an emergency should be done on the frequency you are currently communicating on.
However, there is a dedicated emergency frequency as Bret said (121.5 MHz), which is the same all over the world.
It's generally used if there is no response from the ATC facility the pilot was communicating with.
On many glass cockpits there's even a shortcut to tune to the emergency frequency.
For example, on Garmin equipment like the G1000
and G530/430
you have to press and hold the "flip-flop" key for 2 seconds and it will automatically tune 121.5 for you.
aviation.stackexchange.com/users/4/Philippe Leybaert
Canada has similar procedures to the US.
From the Transport Canada Aeronautical Information Manual , in the COM section: 5.11 Emergency Communications
[...]
The first transmission of the distress call and message by an aircraft should be on the air-to-ground frequency in use at the time.
If the aircraft is unable to establish communication on the frequency in use, the distress call and message should be repeated on the HF general calling or distress frequency
3 023.5
kHz, 5 680 kHz, 121.5 MHz, 406.1 MHz, or other distress frequency available, such as 2 182 kHz, in an effort to establish communications with any ground station or the maritime service.
aviation.stackexchange.com/users/487/200_success
If you have two radios it is a good idea to monitor 121.5 on your second radio.
You can set the volume slightly lower than the primary radio you are listening to.
Doing this allows you to hear ELT's (remember the old ELT's are not monitored by satellites anymore - just the 406 elt's)
So if you hear an ELT you can notify ATC and they can investigate it.
This has helped more than one downed pilot.
Also, if you are flying VFR and nearing a TFR, or other restricted airspace, you may get a call in the blind on 121.5 giving your position, direction of flight, and altitude readout along with a warning that you are about to enter, or have entered restricted airspace.
This call may keep you from having a violation.
If you are in the restricted airspace, the call will include a direction to exit the airspace and usually a frequency to contact for directions.
We all need to help and watch out for each other.
If you do this long enough you will make a call on 121.5 in error.
At that time all of the people who never make mistakes will point it out to you.
Just remember, by monitoring 121.5 you can keep yourself stay out of trouble and help someone else who has experienced trouble.
aviation.stackexchange.com/users/3965/Mark
121.5 MHz is the international civilian distress frequency.
It has been designated for emergency communication by an international agreement.
ICAO Annex 10, Volume V, § 4.1.3.1.1 states that frequency 121.5 MHz “shall be used only for genuine emergency purposes” broadly covering the following activities: The handling of an emergency situations; air-ground communication with aircraft with airborne equipment failure; search and rescue operations and the operation of emergency locator transmitters (ELTs); and air policing/interception action.
aviation.stackexchange.com/users/3775/usernumber
Coast patrol has a dedicated frequency for emergency that must be watched at almost all hours, and even if I recall, it had a "dead silent period" to prioritise whatever emergency might be reported. Is there a dedicated frequency for aviation? Or shall MAYDAYs just be sent to the standard ATC frequency?
Is there a dedicated emergency frequency?
aviation.stackexchange.com
<air-traffic-control><emergency><radio-communications>
aviation.stackexchange.com/questions/809
aviation.stackexchange.com/users/327/ppp
Attempt to use the aircraft emergency frequency, 121.5 MHz. It is a good idea to monitor it on your second radio.  It is generally  agreed that in the US it is 121.5 and in Canada it is 5.11.
It is generally  agreed that in the US it is 121.5  In Canada it is 5.11