Emergencies & radio theory

090 CommunicationsChapter 7

International distress frequency

Key fact

121.5 MHz — international distress frequency. Make the first call on the frequency in use.

SSR emergency codes

Code	Meaning	Memory aid
7500	Hijack	75 taken alive
7600	Radio failure	76 radios…
7700	General emergency (distress)	77 going to heaven

Distress — MAYDAY

Key fact

Distress — aircraft threatened by serious/imminent danger and requiring immediate assistance. Transmit “MAYDAY MAYDAY MAYDAY”. Include: station addressed (unless circumstances permit omission), callsign, type of aircraft, nature of emergency, intentions, position, altitude, and heading.

Key fact

ATC should acknowledge the message. Relayed Maydays should be acknowledged, then the relay station takes control of comms and passes all necessary information to the ATS unit and aircraft operating agency, and warns other stations. Radio silence until the distress is over (unless giving assistance or given permission). Silence may be imposed by the aircraft in distress or ATC.

Distress cancellation phraseology

Phrase	From
STOP TRANSMITTING, MAYDAY	Imposes silence after a MAYDAY
CANCEL DISTRESS	Aircraft when emergency is over
DISTRESS TRAFFIC ENDED	ATC when emergency is over

Urgency — PAN PAN

Key fact

Urgency — safety of aircraft/person on board or nearby threatened but not requiring immediate assistance. Transmit “PAN PAN, PAN PAN, PAN PAN”. Include: station addressed, aircraft callsign, nature of urgency, intentions, position, level, and heading. When heard, pilots should monitor the frequency.

PAN PAN MEDICAL

Key fact

Callsign will be “PAN PAN MEDICAL”. Used for protected medical transport (established by 1949 Geneva Convention). Includes callsign, position, number and type of transports, ETE/ETD/ETA, and any other information. Does not apply for an onboard medical emergency.

Radio communication failure

VMC

Key fact

Land at the nearest airport and report ASAP to ATCU.

IMC

Key fact

Maintain last assigned speed and level for:

• No radar — 20 minutes following failure to report at CRP
• With radar — 7 minutes following either the last assigned level being reached, squawking 7600, or not reporting at a CRP (whichever is latest)

Then fly the current flight plan. Land within ±30 minutes of EAT/ETA.

Blind transmission

Key fact

Transmitting blind = transmit twice. Transmit reports at scheduled times. Advise time of next intended transmission. Use phrase: “TRANSMITTING BLIND DUE TO RECEIVER FAILURE”.

Technical failure

Key fact

Before departure — depart only to get repaired. After departure — notify ATC, who will try to get you to destination following the FPL.

VHF aeronautical communications

Key fact

Aeronautical comms use 117.955–137 MHz (amplitude modulated VHF). Available channels 118–136.975 MHz. Spacing is 8.33 kHz with 25 kHz as backup. Not equipped: “NEGATIVE EIGHT POINT THREE THREE”.

Frequency bands

Band	Frequencies
VLF (Very Low)	3–30 kHz
LF (Low)	30–300 kHz
MF (Medium)	300–3,000 kHz
HF (High)	3–30 MHz
VHF (Very High)	30–300 MHz
UHF (Ultra High)	300–3,000 MHz
SHF (Super High)	3–30 GHz

Key fact

HF uses 2.85 MHz – 22 MHz. Higher frequencies have: lower range, higher quality, lower wavelengths.

Radio wave propagation

Key fact

3 × 10⁸ = Frequency × Wavelength. Range = 1.23√(Transmitter height + Receiver height) — heights in feet, range in NM. Super refraction may extend range. Range is proportional to transmitter power squared.

Key fact

VHF/UHF waves travel basically straight (direct waves) and are affected by obstacles. Two aircraft at altitude will have the best reception. Attenuation is the weakening of radiated waves — increased with high frequencies and high densities. Fading occurs if the same signal arrives at different times.

Exam trap

7600 = radio failure (may still transmit blind). 7700 = emergency/distress. 7500 = hijack. Comm failure IMC with radar: 7 minutes at last level — not 20.