A distress signal indicates that a person or group of people, ship, aircraft, or other vehicle is threatened by grave and imminent danger and requests immediate assistance. Use of distress signals in other circumstances may be against local or international law.
Maritime distress signals
Distress signals at sea are defined in the International Regulations for Preventing Collisions at Sea and in the International Code of Signals. They must only be used where there is grave and imminent danger to life. Other urgent signals should be sent using a pan-pan message.
Distress can be signalled by any of the following means:
- sending a Mayday message by radio on very high frequency (VHF) channel 16 (156.8 MHz) or high frequency (HF) on 2182 kHz
- sending the Morse code group SOS
- using red flares (either hand-held, foot-held or parachute flares)
- emitting orange smoke from a canister
- showing flames on the vessel (as from a burning tar barrel, oil barrel, etc)
- slowly and repeatedly raising and lowering arms outstretched to each side
- a continuous sounding with any fog-signalling apparatus
- a gun or other explosive signal fired at intervals of about a minute
- the international maritime signal flags NC 20px 20px
- a signal consisting of a square flag having above or below it a ball or anything resembling a ball
In addition, distress can be signalled using automated radio signals, such as from an Emergency Position-Indicating Radio Beacon (EPIRB).
If none of the above are available, attention can be drawn by anything unusual, such as a jib sail hoisted upside down.
Inverted national flags are no longer commonly used as distress signals. It is difficult or impossible to determine when the flags of many countries (such as the United Kingdom or 25px Argentina) or 25px Italy are inverted, or the viewer may not be familiar with the flag and may not realise it is not upright. If a national flag is all that is available, distress may be indicated by tying a knot in it, making it into a wheft.
Aviation distress signals
The civilian aircraft emergency frequency is 121.5 MHz. Military aircraft use 243 MHz (which is a harmonic of 121.5 MHz, and therefore civilian beacons transmit on this frequency as well). Aircraft can also signal an emergency by setting one of several special transponder codes, such as 7700.
A "triangular distress pattern" is a rarely-used flight pattern flown by aircraft in distress but without radio communications. The standard pattern is a series of 120° turns.
Mountain distress signals
The recognised mountain distress signals are based on groups of three (six in the UK). A distress signal can be 3 fires or piles of rocks in a triangle, three blasts on a whistle, or three flashes of a light, in succession followed by a one minute pause and repeated until a response is received. Three blasts or flashes is the appropriate response.
In the Alps, the recommended way to signal distress is the Alpine distress signal: give six signals within a minute, then pause for a minute, repeating this until rescue arrives. A signal may be anything visual (waving clothes or lights, use of a signal mirror) or audible (shouts, whistles, etc.). The rescuers acknowledge with three signals per minute. To communicate with a helicopter in sight, raise both arms (forming the letter Y) to indicate "Yes" or "I need help" and stretch one arm up and one down (imitating the letter N) for "No" or "I do not need help". If semaphore flags are available, they can possibly be used to communicate with rescuers.
As a plot device in science fiction
In works of science fiction dealing with starships, most notably the various incarnations of Star Trek, a distress call from another ship, or an outlying planet or space station, is a common plot hook. Also, false distress signals are sometimes used by space pirates and other unsavory types to lure would-be rescuers into a trap; in the Star Trek universe, since Starfleet vessels (among others) are required by law, and most (non-villainous) independent ship captains tend to feel morally obligated, to render any assistance they can to a vessel in distress, this can be a very effective tactic, though there is the possibility that the pirates will catch something bigger than they can handle, such as a capital warship.
- Vessel emergency codes
- Global Maritime Distress Safety System
- 500 kHz
- Emergency Alert System
- Mountain rescue
- Maritime mobile amateur radio
- What is the meaning of SOS?
- US Coast Guard: Visual Distress Signals for Recreational Boaters
- US Coast Guard: Flares and other Visual Distress Signals
- Transport Canada: Radio Distress Procedures Card
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