Emergency is a serious event that needs immediate actions. The most common reasons for the crew to declare an emergency can be: mid-air explosion, serious fire in the cabin or engine, fire, door warning lights, loss of an engine, severe icing, severe turbulence, pressurization, hydraulic failure, bird strikes, lightning strikes and illness on board. It is totally impossible to define instructions for all cases and write such a document as phraseology for emergencies. Nevertheless, there are some standard procedures, which help to prevent chaos and make controller's work organized and a bit regulated. An aircraft under emergency gets priority over other aircraft.
Emergency situations can be classified as distress and urgency. Any distress or urgency message must be as short, full and clear as possible.
Distress is a dangerous situation requiring immediate assistance (imminent danger). A pilot begins his message with the word “MAYDAY” three times then says АТС unit call sign, identification of the aircraft, nature of the distress condition, intentions, position, level and heading of the aircraft and other useful information.
Urgency is a dangerous situation not requiring immediate assistance (potential danger). Urgency message starts with signal “PAN” three times and includes: АТС unit call sign, identification of the aircraft, the nature of urgency, intentions and other useful information.
Distress and urgency messages can be transmitted either via emergency frequency 121,5 or via frequency in use. Distress communications have absolute priority over all other communications. Urgency communications have priority over all but distress communications. PAN warns other stations not to interfere with urgency transmissions. In case of emergency controllers can observe special symbols and SSR codes of emergency (7700), communication failure (7600) and unlawful interference/hijacking (7500) near the blip.
Every station received any distress or urgency message should retransmit it to the АТС unit immediately and the controller must acknowledge it at once. All stations received the message should monitor the frequency and the controller should impose radio silence with the phrase: "STOP TRANSMITTING", addressed to all or one particular station. The language used in non-standard events demands the knowledge of common and Aviation English which helps to avoid ambiguity, to provide safety, to solve a problem before it becomes a disaster. When the aircraft is no longer in distress, it cancels MAYDAY status. The controller cancels silence with a phrase: "DISTRESS TRAFFIC ENDED". It means resuming normal communication operations.
If an aircraft is in distress phase, the rescue coordination centre has to be notified immediately. All the information notified to the rescue coordination centre by an area control or flight information centre should be reported, without delay, to the operator.