SELCAL

Question 1

High Frequency (HF) Radio

Answer 1

● Aircraft generally communicate on 3 frequency bands:
-Ultra High Frequency (UHF) 225–328.6 & 335.4-400 MHz
Line of sight; Military use
-Very High Frequency (VHF) 118.0 – 136.0 MHz
Line of sight; does not reflect off ionosphere
-High Frequency (HF) 2850KHz-24890KHz
Bounces off the ionosphere; over the horizon range
Used when line of sight coms not possible
-oceanic/sparsely settled areas

Question 2

HF

Answer 2

● Range depends on height of ionosphere.
● Height of ionosphere changes with setting of the sun.
● Higher frequencies have better range during day
○ Sun up/frequency up; Sun down/frequency down
- the higher the sun, the higher the Ionosphere, therefore the higher the frequency used. The lower the sun, the lower the Ionosphere, therefore the lower the frequency used
● HF radio has a high level of background noise.
○ It can be fatiguing to monitor such noise for long periods.
● This is why they invented….

Question 3

SELCAL

Answer 3

● SELCAL is an acronym for SELECTIVE CALL.
● The crew has the luxury of having the HF radio on, but the volume down.
● When calling, a ground station transmits the aircraft identifier through a series of tones.
● SELCAL equipment hears the identifier, and then alerts the crew.
● Crew turns up HF volume and establishes comms

● SELCALs identifiers are four letter codes: e.g. AH-GP
○ Most often set by maintenance person (no requirement for pilot to set)

● Listed in 18 Other Information of ICAO Flt Plan

Question 4

Aeronautical Radio Inc

Answer 4

● SELCALs are issued to the airlines by ARINC (Aeronautical Radio Inc).
○ With a total of only 10920 available codes, duplications are possible.
● This problem can be partly overcome by allocating duplicate codes to aircraft operating in different parts of the world.
○ In theory they should never be working on the same frequency!
● If a duplicate SELCAL does appear on the same frequency then the problem can also be resolved by moving one of the flights to another frequency

Question 5

SELCAL Checks

Answer 5

● The crew will conduct a SELCAL check with the controlling center prior to turning down HF volume.
● If SELCAL works crew can turn down HF and maintain “SELCAL watch” on HF.
● If SELCAL fails crew can not turn down HF and maintain “listening watch” on HF