Aviation Distress Signals
The civilian aircraft emergency frequency for voice distress alerting 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.
The COSPAS/SARSAT signal can be transmitted by an Electronic Locator Transmitter or ELT, which is similar to a marine EPIRB on the 406 MHz radio frequency. (Marine EPIRBS are constructed so as to float while aviation an ELT is constructed so as to be activated by a sharp deceleration and is sometimes referred to as a Crash Position Indicator/CPI).
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.
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