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Coping with emergencies &
VHF radiocommunications

Safety and emergency communication procedures

Rev. 37a — page content was last changed 10 October 2013
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When a pilot is experiencing in-flight difficulties it is advisable to inform others as early as practical and to advise whether the pilot considers the situation to be an emergency or something less. The VHF frequency on which a distress call (a MAYDAY transmission) or an urgency message (a PAN-PAN transmission) is made, should be that which is most likely to provide a quick response from a responsible person. But the pilot's primary focus must be to continue to fly the aircraft.

In addition satellite radio distress beacons may be used, as a means of last resort, to alert the Australian Search and Rescue organisation that an aircraft's occupants are, or may be, in 'grave and imminent danger'. In some circumstances aircraft radar beacon transponders, particularly Mode S, can play a significant role in identifying and tracking an aircraft declaring an emergency.

The material in this module of the VHF radiocommunications guide covers part of the RA-Aus radio operator's endorsement syllabus listed in the RA-Aus Operations Manual section 3.08.

This module also forms part of the 'Coping with Emergencies' Guide.


6.1 Communications when in difficulties

Assess the probable outcomes of the available alternative actions
When a non-instrument rated recreational pilot realises that he/she is likely to be in difficulties (very low on fuel, lost or in failing light, encountering low cloud and rising terrain) or is already in difficulty (the engine or a control circuit has failed), the top priorities are: (a) fly the aircraft, (b) continue flying the aircraft whilst running through the pre-planned emergency drills and (c) decide the best landing area. During this period an assessment must be made of the probable outcome in terms of possible injury and/or survival following the landing.

  • If the aircraft is a low-momentum type, is normally controllable, pilot only onboard, visibility is okay and the area is clear terrain with a normal rural population density and road infrastructure, then the landing should not be life-threatening to a reasonably competent pilot. If unable to remedy the fault on the ground, the pilot won't have to walk far to find assistance. In this circumstance many recreational aircraft pilots, particularly those in single-seat taildraggers, would not consider communicating any form of alert except, perhaps, to advise an accompanying aircraft. This brings to mind the RA-Aus pilot who underwent three forced landings, due to engine stoppages, on one journey to NATFLY before he finally made it.

  • On the other hand if the pilot is experiencing control difficulty, or the terrain is rough and/or heavily treed, or in a more remote area, or the type of aircraft is such that it is likely that the landing cannot be carried out without some risk of occupant injury then the pilot would be well advised to initiate a distress broadcast — a MAYDAY call — even if there is little time available.

    Distress is defined as a situation where — in the opinion of the pilot in command — an aircraft (or vessel, vehicle or person) is in grave and imminent danger and requires immediate assistance. The word 'Mayday', an anglicised version of the French 'm'aider' [help me], was adopted in 1927 as the standard radiotelephony distress call.

The VHF frequency chosen, at the pilot's discretion, depends on circumstances and should be that which is most likely to provide a quick response or rapid assistance at the scene. The first choice response station will usually be Brisbane or Melbourne Centre on the flight information area frequency or a terminal area frequency. If aircraft height is such that Air Traffic Services are not contactable and the frequency already tuned is a CTAF and other aircraft or a Unicom operator are known to be listening out then use that frequency (but bear in mind CTAFs are not monitored by Air Traffic Services). In very remote areas another option is the international VHF voice distress frequency of 121.5 MHz, which, though also not monitored by Air Traffic Services, is continually monitored by RPT aircraft and others with a good citizen attitude and the communications equipment capability to monitor more than one frequency; see Boyd Munro's comments.

But the pilot's primary task is to fly the aircraft while selecting the best landing site and minimising risk to all persons; it is not productive to stall the aircraft while attempting to change frequencies (or just to find an appropriate frequency) or communicate, and you certainly don't want to risk dropping a hand-held transceiver.
Requesting assistance
There are circumstances that make some form of alert or urgency communication advisable, even if the pilot doesn't want to ask for help or feels a bit embarrassed about it. (But — in my book — better red than dead.) The pilot who is encountering difficulties might decide to request assistance from the ATC on-request flight information service Flightwatch — if contactable on the flight information area frequency — advising the difficulty, the aircraft's approximate location and the pilot's intentions: without the pilot initiating an emergency status. The Flightwatch operator may arrange to directly assist or may decide to treat the situation as an emergency and declare the appropriate emergency phase — uncertainty, alert or distress. See AIP GEN 3.6.

The call format might be:

*Note: ERSA-GEN-FIS 3.2 indicates it is not necessary to prefix the generic 'Flightwatch' callsign with the callsign of the ATC unit e.g. 'Brisbane Centre'.

If the pilot considers there is some uncertainty and/or urgency in the situation, and that assistance may be needed, then he/she may decide to advise of an urgency condition and initiate a PAN-PAN broadcast — stating the nature of the alert, pilot's intentions and assistance desired. Pan derives from the French 'panne' meaning 'breakdown'.

Declaring an emergency in an appropriate situation displays good airmanship — and people do like to help. Read the article 'Salvation from above' in the January–February 2001 issue of the Australian Civil Aviation Safety Authority's Flight Safety Australia magazine. A categorised index of articles of interest to recreational pilots contained in Flight Safety Australia since 1998 is available on this site.
The VHF urgency and distress calls
PAN-PAN and MAYDAY calls are internationally recognised emergency transmissions that initiate ICAO prescribed procedures and offer decided advantages to the pilot in difficulties.
  • Distress calls have absolute priority over all other communications on that frequency, and the word MAYDAY commands immediate radio silence. Radio silence should continue until listeners have determined that communication has been properly established between the station in distress and a responsible authority, and that assistance is being provided.

  • Similarly PAN-PAN urgency communications have priority over all other communications except distress calls.

  • The Flightwatch flight information service or the ATS alerting service will immediately acknowledge any distress or urgency message received, coordinate communications and alert the Australian Search and Rescue organisation [AusSAR] on receipt of a distress call.

  • If any station monitoring a distress or urgency message becomes aware that Flightwatch either has not received the message or, having received it, cannot establish contact with the originator, that station has a responsibility to contact Flightwatch and/or the aircraft, and offer assistance — possibly as a relay station — which may entail remaining in the area.

  • There is an understanding that "In an emergency requiring immediate action, the pilot in command may deviate from any rule ... to the extent required to meet the emergency." However, you would need to ensure that any such departure doesn't cause risk to someone else. Nothing in the CASRs acts to protect the pilot against civil liability in the case of damage to persons or property. Also declaration of an emergency while entering an active restricted area does not guarantee safe passage.

  • For transponder-equipped aircraft also see transponder emergency procedure.
MAYDAY call format
To remove any uncertainty whether a monitored call is an emergency call, it is most advisable to precede the call with the recognised priorities PAN-PAN or MAYDAY, then transmit as much of the following detail as circumstances allow — bearing in mind the pilot's first priority is to fly the aircraft. If experiencing controllability problems or an engine failure when close to the surface, there won't be much time to bother about formal communication formats.

If time is available, distress calls have the preferred format:
  1. Priority = MAYDAY (repeated three times)
  2. Calling station ID (repeated three times, if time permits) and aircraft type
  3. Nature of distress
  4. Calling station position, heading and altitude
  5. Intentions
  6. Other useful information
For example, with an engine failure over rough, hilly terrain:
Note the last line includes the information that there are two persons on board [POB] and repeats the call sign and the MAYDAY priority. It might help an Air Traffic Services operator, managing several frequencies, if the frequency in use was also transmitted.
PAN-PAN call format
Urgency calls have the preferred format:
  1. Priority = PAN-PAN (three times)
  2. Called station ID
  3. Calling station ID and aircraft type
  4. Nature of emergency
  5. Calling station estimated position, altitude and heading
  6. Request or intentions
Utilising GPS
If the pilot in distress is able to communicate, or has established contact, a functioning GPS is a great advantage to everyone concerned, because the pilot is then able to provide a latitude and longitude position probably accurate to 100 metres. Consequently any search only entails a direct flight to that position by one aircraft. Some distress beacons also include Global Positioning System input capability.
Other communication means
UHF citizen's band [CB]. In rural and outback areas, particularly in the vicinity of the arterial roads, there is widespread usage of UHF CB radios by truck drivers, four-wheel drive vehicles, road crews, mustering crews and fencers. There are 40 CB channels located between 476.425 and 477.400 MHz in 0.025 MHz steps. The road vehicles listen out on channel 40, and channels 5 and 35 are emergency frequencies. Some VHF handheld transceivers might include UHF CB capability and there is quite a good UHF repeater system (channels 1–8/31–38) established in Australia.

A cellular mobile communication device may be useful in advising your situation to others. An individual's ability to make radio frequency transmissions in the Australian cellular mobile communications 850, 900, 1800 and 2100 MHz bands is legitimised by the Radiocommunications (Cellular Mobile Telecommunications Devices) Class Licence 2002.

An activated mobile communication device in a high-speed aircraft may cause channel interference across cells, but in July 2010 the Australian Communications and Media Authority [ACMA] amended the class licence (which previously prohibited the airborne use of mobile communication devices) to allow operation of a mobile communication device in an airborne aircraft above an altitude of 10 000 feet or, perhaps, 20 000 feet, but only to communicate with a licensed public mobile telecommunications base transceiver station (a 'pico cell' such as those used in large buildings) onboard the aircraft with connection to telecommunications satellites. A control unit blocks onboard devices from terrestrial signals. Under these conditions the mobile devices in the aircraft operate at very low power.

So, the class licence authorises persons to use mobile communication devices in aircraft if they are in an airliner equipped with a 'pico cell' unit (and operating under a public telecommunications service licence). The class licence does not authorise the use of any other form of mobile communication device in any airborne aircraft at any altitude. However, in an emergency safety has priority so airborne pilots might contact the ATC centres by mobile 'phone. The telephone numbers of the state ATC centres and the SAR hotline (1800 215 257) are given in ERSA GEN-FIS 'Use of mobile 'phones in aircraft' — store the numbers in your 'phone.

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For recommended actions during and following an emergency please read all of the ERSA Emergency Procedures Section ERSA EMERG; particularly the 'Activation of ELT' and the survival sub-sections.

6.2 Distress beacons and AusSAR

In a life-threatening situation the pilot may activate a radio distress beacon when approaching, or on, the surface. The signal from the beacon will be detected by the specialised search and rescue system — Sarsat (Search And Rescue Satellite Aided Tracking system) and the Russian Cospas. The satellite-mounted Cospas-Sarsat receivers monitor only the global distress frequency, 406.025 MHz and are reputed to have been involved in more than 7000 rescues since the system was introduced in 1982. Analogue transmissions might also be picked up by nearby aircraft — regular passenger transport aircraft usually continually monitor the 121.5 MHz frequency and military aircraft monitor 243.0 MHz.

In the Australian aviation regulatory environment, the generic name for distress beacons is Emergency Locator Transmitters [ELTs]. ELTs are usually a fixed installation within larger aircraft, but may be demountable. When armed, ELTs are designed to be activated automatically (perhaps by a g-switch) under a high-impact deceleration; or they can be manually activated by the pilot. (Unfortunately ELTs may not survive a high impact landing or the antenna may be disconnected in a lesser accident.)

Similarly, the generic name for 406.025 MHz maritime environment beacons is Emergency Position Indicating Radio Beacons [EPIRBs]. The significant difference between EPIRBs and ELTs is that the former are buoyant and work at their best when floating freely and upright, while the ELTs work best on land — though they should be waterproof. The most expensive EPIRB is the 'float free' or 'float-to-the-surface', automatically activated maritime only type. Smaller, lanyard-equipped, manually operated, category 2 EPIRBs are designed to be placed in the water and allowed to float upright.

Personal Locator Beacons [PLBs] were originally designed for personal use by ground travellers in a rugged environment or by those recreational sailors who don't venture very far out to sea — they probably float but perhaps not upright. The manually activated, pocket-sized, analogue PLBs were extensively used by recreational pilots — among many other users.

In the Australian aviation scene PLBs and manually activated EPIRBs are classified as portable ELTs so, for aviation regulatory purposes, the ELT term encompasses fixed-installation ELTs and portable ELTs; the latter being the digital PLBs and the manually activated digital EPIRBs. Recreational aviation pilots carry PLBs or, if undertaking significant water crossings, should carry the personal EPIRBs that can be attached to a lifejacket or to clothing.

(The term ELB [Electronic Locator Beacon] is sometimes used but this term is no longer defined in aviation regulations or by the Australian Maritime Safety Authority — which has search and rescue responsibility in the Australian region — so the term has no valid usage and adds to the confusion between aviation and AMSA definitions. ELBs were once in use as a 121.5 MHz beacon but their transmission format was not satellite-compatible and production ceased in the early '90s.)

The now superseded analogue versions of PLBs/personal EPIRBs transmitted on the 121.5 MHz voice frequency and simultaneously on 243.0 MHz, but not 406.025 MHz. For aural recognition and homing that continuous wave transmission is modulated with a swept tone sounding like a two-tone siren and audible via a VHF transceiver. The 121.5 or 243.0 MHz transmission is used as a short range homing signal by search aircraft or surface vehicles. On 1 February 2010 the class licence for the 121.5/243.0 MHz distress beacons was finally withdrawn by the Australian Communications and Media Authority [ACMA], consequently it is now illegal to use those beacons for any purpose.

On land there might be a requirement that PLBs/EPIRBs, when activated, must be placed in the centre of a ground mat formed from a sheet of aluminium kitchen foil, about 120 cm square — which provides the 56 cm radius ground plane. Read 'Activation of ELT' within the emergency procedures section of ERSA.

Remember the requirement (AIP GEN 3.6 para 8.2) that pilots should monitor 121.5 MHz before engine-start and after engine-shutdown, to check for the 'two-tone siren' distress transmissions — and to ensure that your own beacon is not activated inadvertently.

Distress beacons have been used in Australian aviation for at least 45 years and are an essential item for pilots who fly in sparsely populated areas, and for vehicle drivers who operate in remote areas. The buyer of a distress beacon should be well aware of how to keep it secure and to use it correctly, effectively, and only when in a life-threatening situation; also how to finally dispose of it without possibly causing costly problems to AusSAR. For beacon disposal instructions see beacons.amsa.gov.au/batteries-disposal.html.
The 406.025 MHz ELTs
On 1 February 2009, the Cospas-Sarsat satellites ceased processing distress signals on 121.5 MHz and now only process signals from the 406.025 MHz digitally-encoded PLBs, ELTs or EPIRBs. So, search (and rescue) for persons using the 121.5 MHz only units is totally dependent on time-consuming, expensive and difficult — and possibly dangerous — air and ground searches.

GME's MT410G PLB The digitally-encoded PLBs, ELTs and EPIRBs that operate on 406.025 MHz, quickly provide position accuracy to within 5 kilometres or so using satellite trilateration. If the beacon has an integrated GPS input the location coordinate data are transmitted to the satellite, pinpointing the site to within 100 metres or less. This makes redundant the search portion of the rescue operation and greatly aids rapid recovery; and rapid recovery is vital when the aircraft occupants are injured or in difficult circumstances. The 406.025 MHz beacons generally also transmit a low power analogue 121.5 MHz final stage aircraft homing signal; for example, the Australian MT410G PLB at left.

When activated the 406.025 MHz beacons send a 0.4-second data packet every 50 seconds. The packet includes a 15 hexadecimal character beacon identity code plus the country/SAR authority code within a 30 hexadecimal character distress message. That message is retransmitted by the satellite to the two AMSA ground stations. The hexadecimal identity code, marked on the unit as purchased, must be known to AusSAR's database, and linked to your personal and aircraft details. Part of the functional working of the 406.025 MHz beacon search and rescue system is having the owner of the beacon register it with the Australian Maritime Safety Authority [AMSA].

Requirement to register and carry 406.025 MHz beacons
The requirement for an Australian aircraft to carry an approved distress beacon or emergency locator transmitter is stated in CAR 252A (as amended 1 February 2009). Every two-place recreational aircraft operating beyond 50 nm from their departure point is required to carry a 406 MHz beacon registered with AMSA. Single-place aircraft are amongst those exempted in CAR252A, so carriage of a beacon is not mandatory for CAO 95.10 aircraft — but it is certainly wise to do so.

So, recreational pilots should acquire a 406 MHz beacon with internal GPS input (for example, the MT410G costs about $650) and register that beacon. In order to make the process of registration and upkeep of details easier, AMSA have an online registration program. This system is available to all beacon owners to use and there is no charge for its use; go to beacons.amsa.gov.au to register your unit and to find more details regarding how to purchase PLBs. AMSA will provide a registration sticker to be placed on the unit, the stickers provide owners and Flight Operations Inspectors with proof of current registration.

The ELT registration must be renewed every two years and a new sticker attached to the device; see 'Renewing your registration'.

Note: if a beacon has been activated inadvertently, switch it off and notify the Rescue Coordination Centre Australia by calling 1800 641 792 to ensure a search and rescue operation is not commenced. There is no penalty for inadvertent activations.

According to their website — 'since its inception in 1982 the Cospas-Sarsat System has provided distress alert information which has assisted in the rescue of 26,779 persons in 7,268 distress situations [land, sea and air]. In 2008 only, the System provided information which was used to rescue 1,981 persons in 502 distress situations. The locations of these events are depicted on the map below.'

Cospas-Sarsat rescues in 2008

For further general information, the next page in this guide is a document Aviation Distress Beacons written some time ago by David McBrien of AusSAR.
Personal flight tracking systems
There are several flight tracking systems available which allow interested parties to follow the progress of a flight via the internet. For example, Spidertracks is a system developed in New Zealand that uses a small (12×6×3 cm) demountable transceiver in the aircraft (with its own GPS engine) to send location, heading, speed, altitude reports at nominated time intervals — via the Iridium satellite global communications network — to a host computer, which users can access via the internet. The display includes flight track, reporting times and locations overlaid on a Google Earth map. There is a facility available which will activate email or text notification — to a user-nominated person or group of persons — if three contiguous reports are missed. Cost may be a problem.
Australian Search and Rescue (AusSAR)
If a registered civil aircraft issues a MAYDAY call, or is seen to crash away from a controlled aerodrome or is reported missing, Australian Search and Rescue has national responsibility for coordinating the search and rescue. In addition, AusSAR monitors satellite-intercepted signals via two ground stations in Australia and one in New Zealand. AusSAR is responsible for delivering search and rescue coordination in response to an activated distress beacon within AusSAR's area of responsibility — which covers all the Earth's surface between 75° East and 163° East and roughly 10° South to 90° South.

Further information is contained in the document Understanding SAR services.

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6.3 Aircraft radar beacon transponders

Mode A/C transponders
Transponders are specialised radio devices that form the airborne part of the Air Traffic Control Radar Beacon System [ATCRBS "at-crabs"].

Transponders respond to a 1030 MHz interrogation pulse, from an air traffic control secondary surveillance radar [SSR], by returning a high-energy 1090 MHz pulse that strengthens the radar return signal. Lower power primary surveillance radar [PSR] exists only within about 50 nm of the major civilian and military airports but such radars don't interrogate airborne transponders. SSR range is at least 100 nm from the radar unit, depending on target height. The surveillance (i.e. computer-aided search and track) radars provide only bearing and distance from the radar site, target height is provided by the airborne transponder.

In addition, the response from transponders fitted to smaller civilian aircraft normally consists of a 12-bit ATC assigned identity/status code plus a 12-bit altitude reading (in units of 100 feet) which appear on the controller's SSR screen with the aircraft 'paint'. Civilian units with this identity (Mode A) plus altitude encoding (Mode C) interrogation response capability are known as Mode A/C transponders, sometimes they are referred to as 'Mode 3A/C'; the '3' just refers to a US military classification. The transponders receive the Mode C altitude data from altitude encoding devices.

The 12-bit Mode A identity code is separated into four three-bit numerals using octal rather than decimal notation. Thus each numeral will be in the range 0–7; i.e. the numerals 8 and 9 will not appear in any identity/status code. The standard four-digit non-discrete identity code 'squawked*' by VFR aircraft is '1200' (all non-discrete codes end in '00') until radio contact with Air Traffic Services, who might then instruct the pilot to squawk a particular discrete (i.e. individual) code; e.g. 4367. The maximum number of discrete identity codes available for assignment at any one time is about 4000 (in decimal notation).

*(Note: the 'squawk' term originated in Britain early in the second World War when the Chain Home early warning radar network was used for the first ground controlled fighter interception system against incoming air raids. The RAF fighters were equipped with a rudimentary 'identification friend or foe' (IFF) transponder, code-named 'Parrot'. When the ground controller required a flight or squadron to switch on their transponders the instruction was "Squawk your parrot". Conversely, "Strangle your parrot" to switch off.)

Mode A/C transponders have a very important 'identify' [IDENT] or 'special position identification' [SPI] facility which, when operated, momentarily adds an additional bit to the '1200' non-discrete identity code, or whatever discrete code is being used by the pilot. That causes the aircraft's 'paint' to brighten or change colour on the controller's display. So, for example, when the controller wishes to locate a particular aircraft on the display screen, among all those currently squawking '1200', the controller will request the pilot to "squawk ident"; i.e. operate the 'ident' button, knob or spring-loaded toggle switch. Pilots must not squawk 'IDENT' unless told to do so by ATC or when first squawking an emergency code.

The non-discrete transponder squawk codes (for emergency use only) are:
  • 7700 emergency
  • 7600 VHF communications failure
  • 7500 unlawful interference (i.e. hijacking).
See transponder emergency procedure below.
Mode S transponders
The Mode A/C radar surveillance system is rather limited. The Mode S transponders, carried by regular passenger transport aircraft, use their National Airworthiness Authority [CASA for Australian aircraft] assigned permanent 'ICAO 24-bit Aircraft Address'. The 24 binary digits allow a total of 16.8 million individual addresses; thus every aircraft can be permanently assigned a unique address, generally based on the aircraft's country of registration and issued by their National Airworthiness Authority. Consequently, those aircraft can be selectively addressed by ground stations or other aircraft for transfer of information as digital data. This message format is called Mode S (for 'selective address') but the transponders also have the normal Mode A/C functions.

*Note: Binary, octal, decimal and hexadecimal numerical notation. Our everyday decimal numbering system has a base of ten with 10 markers 0–9. Octal and hexadecimal notation refer to versions of computer numerical display that assist human perception of the binary digit representation used in computers. Binary numbering is base-2 with two states (on or off) per binary digit (bit) representing 0 and 1. Octal notation is base-8 with eight markers 0–7 and uses one group of three bits to represent any of the eight numerals 0–7. The hexadecimal (or hex) numbering system is base-16 with 16 markers 0–9 plus A–F, the latter representing the decimal numerics 10 through 15. A decimal number of '255' is represented by the hex number 'FF'. Hexadecimal uses one group of four bits to represent any of the sixteen numerals 0–15 rather than the 8-bit byte normally used for alphanumeric coding.

For Australian aircraft the ICAO 24-bit Aircraft Address code, also known as the 'Mode S Transponder Code is usually stated in 6-digit hexadecimal notation format. All Australian civil aircraft with a Mode S transponder installed are required to have a registered permanent ICAO 24-bit Aircraft Address assigned; this is accomplished by emailing CASA at [email protected] who will assign a permanent ICAO 24-bit Aircraft Address code for that aircraft in the range '7C0000' to '7F0000'. For RA-Aus registered aircraft the code may be entered into a Mode S transponder by the aircraft owner; for aircraft with national registration (i.e. VH) the code must be entered into a Mode S transponder by an appropriately trained and rated licensed aircraft maintenance engineer (LAME), or CASA authorised person, at the time of transponder installation and re-tested at 2-year intervals.

Note: only the CASA assigned aircraft address should be entered into the 24-bit hexadecimal field otherwise there is the possibility of duplication of aircraft addresses. If a CASA-assigned aircraft address has not been entered and verified in a Mode S transponder then the unit may only be operated in A/C mode.

Also, as with the Mode A/C transponders, the Mode S transponders have an identification function that may be known as 'Aircraft Identification', 'Flight Identification' or 'FLIGHTID'. This Aircraft Identification may be no more than seven alphanumeric characters but, for RA-Aus registered aircraft, CASA require the Aircraft Identification to be five alphanumeric characters consisting of the four numeric digits of the aircraft's registration mark preceded by the letter 'R' (for RA-Aus) without hyphens or included spaces, e.g. Jabiru 24-7147's identification is 'R7147'. For RA-Aus aircraft the Aircraft Identification is a permanent code, for other aircraft it may be entered/changed by the pilot as required.

In Australia, prior to 2010, there was no Mode S secondary surveillance radar network so the main Mode S transponder function was to allow aircraft equipped with Traffic Alert and Collision Avoidance Systems [TCAS] to communicate directly with each other, thereby enabling mutual resolution of potential traffic conflicts. The transponders – in combination with a GNSS receiver – periodically 'squitter' a burst of data containing tracking information such as the aircraft's position, altitude, vector and velocity. (Squitter means a rapid R/F emission.) Such transponders also act as the aircraft's digital modem terminal for data upload/download and distribution.

Mode S can also provide faster, more accurate ATC surveillance, provided the ground SSRs are of the fast, single-pulse interrogation Mode S type. The non-Mode S Australian SSRs are now in the process of replacement, both in the main city hubs and en route.

When interrogated by a Mode S SSR a Mode S transponder replies with its Flight Identification plus its ICAO Aircraft Address, plus other relevant data.

From February 2014 an aircraft that is newly registered (or that is modified by having its transponder installation replaced) and that is operated in Class A, B, C or E airspace, or above 10 000 feet amsl in Class G airspace, must carry a serviceable Mode S transponder, but that Mode S transponder is not required to have the 'extended squitter' hardware and software (known as '1090ES') to transmit Automatic Dependent Surveillance–Broadcast [ADS-B] data. The term 'extended squitter' refers to an additional [112-bit] ADS-B data packet, which is part of the enhanced Mode S transponder data link standards for ADS-B. The 1090ES satellite-based surveillance and traffic management system is currently implemented for Australian airspace above 29 000 feet. See the Australian ADS-B implementation program.

The Traffic Alert and Collision Avoidance Systems [TCAS II], fitted to all Australian RPT aircraft exceeding 30-passenger capability, also send out Mode C interrogation pulses in the same manner as an SSR, and use the interrogation responses broadcast from aircraft Mode A/C transponders (within a range of 14 nm) to determine collision risk. (TCAS computers determine the velocity vector of an aircraft within range — ascertaining distance by the response time, bearing by a directional antenna and altitude from the 12-bit reading encoded in the response.) If there is no altitude given then the computer can only provide a traffic alert rather than a 'resolution advisory' recommending a particular action to the pilot. TCAS II won't detect an aircraft fitted with an operating Mode A-only transponder.

TCAS systems also utilise their Mode S-capable transponders to transfer data between aircraft TCAS systems for mutual resolution of traffic conflicts, or to provide a data upload/download link with a ground station. For a description of TCAS read the article 'Collision Avoidance' in the April 1999 issue of the Australian Civil Aviation Safety Authority's Flight Safety Australia magazine.
Transponder operating regulations
For traffic separation purposes all aircraft — including recreational aircraft — operating in Class A, C and E Australian airspace, or in any airspace above 10 000 feet, must be fitted with an operating Mode A/C transponder. If an aircraft is transponder-equipped the unit must be operated constantly, whether in controlled or non-controlled airspace. There are some exemptions in Class E if the aircraft's electrical system is not capable of continuously powering a transponder. No aircraft may operate in Class E within 40 nm of a Class D tower without a functioning transponder. For further information see controlled airspace. A recreational aircraft operating in Class E should check with Air Traffic Control to confirm that the transponder is functioning correctly.

Normal operating procedure:

1. After engine-start turn the transponder mode switch from 'OFF' to 'STBY' (standby) to warm up the unit — which may take a couple of minutes. When the transponder is in 'STBY' it will not respond to an SSR interrogation. Set the identity code '1200' unless advised otherwise by ATC.

2. Before take-off turn the mode switch to 'ALT' (altitude) rather than the 'ON' position. Unless ATC instructs you to do so there is really no need ever to use the 'ON' position. The 'ON' position directs the transponder to respond only to a Mode A interrogation. When 'ALT' is selected, even if there is no altitude encoder fitted, the transponder will still return a response pulse to a Mode C interrogation coming from a ground radar or from a TCAS aircraft, but without any altitude data of course. Leave the switch in the 'ALT' position until turning off the runway at the destination, unless the identity code is to be changed during flight; in which case place the unit in 'STBY' mode while the change is being effected.

3. For further information on operation of transponders see AIP ENR 1.6 subsection 7.

A user's manual for the Australian Microair T2000 transponder may be downloaded from the Microair website.
Transponder emergency procedure
For any transponder-equipped aircraft within radar coverage — say, up to 100 nm from the SSR site for lower altitudes — and whether outside (or underneath) controlled airspace, the ATC radar emergency service will provide navigation assistance if the aircraft is in distress or experiencing navigational difficulties.

In an emergency situation the pilot should select the emergency status code 7700, operate the 'IDENT' function and, if possible, contact the service on the overlying en-route area control frequency shown on the ERC-L, call-sign CENTRE; e.g. BRISBANE CENTRE.
  • SQUAWKING 7700
Deviation into an active restricted zone
Should an aircraft be forced to deviate into an active restricted zone due to the weather — without an ATC clearance — then the pilot must declare a PAN-PAN, squawk 7700 and broadcast on 121.5 MHz and on the appropriate ATC frequency. ATC will declare an 'Alert Phase'.

The declaration of an emergency will not guarantee safe passage in a hazardous restricted zone.
Mode C transponder maintenance
RA-Aus aircraft owners should note that transponders with an active altitude reporting facility (altitude encoding altimeter or a blind encoder) must be maintained in accordance with CASA regulations not RA-Aus regulations. CAO 100.5 appendix 1 requires that the system is tested by a CASA-licensed maintenance engineer at intervals not exceeding 24 months or after any change/modification to the altitude reporting system component(s) or interwiring. Code 2100 is used by maintenance personnel for testing purposes.

[ The next section in the airmanship and safety sequence describes Microscale meteorology and atmospheric hazards ]

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6.4 Can it ever be appropriate to monitor 121.5 MHz en route?

The following was written by Boyd Munro of Air Safety Australia

121.5 is the International Distress Frequency. A recent survey by Air Safety Australia has revealed that few Australian pilots monitor 121.5, apart from those who work or have worked for an airline, and those with significant overseas experience. I got a big surprise from this, because I always monitor 121.5 en route without even stopping to think why. It’s just something I do, like getting dressed before I leave the house in the morning.

Remember that “monitor” in this context means “listen without talking”. The survey also showed that the term “monitor” is quite widely misunderstood.

For the most part we Australian pilots are not trained to monitor 121.5 when flying en route, but there are powerful reasons why we should.

1. We are instantly available to another pilot who experiences an emergency in the air, or crashes but still has a working radio and calls on the International Distress Frequency. This is not merely good airmanship, it is responsible citizenship.

2. We can pick up ELT signals, so if another pilot crashes we can bring help to him. ELT signals are also picked up by satellites [this capability ceased 1 February 2009 ... JB] but hours can elapse before one of those satellites passes over the accident site, and if the ELT’s antenna was damaged in the crash the high-flying satellite may not be able to pick up the signal at all. Airmanship/citizenship again.

3. We can be contacted at any time. For example “Aircraft at position X, you are entering restricted area R123 and will be intercepted unless you make a 180 turn and leave the area forthwith.”

4. All airlines monitor 121.5 en route.

5. ICAO requires that all aircraft monitor 121.5 at all times in areas where ELTs must be carried (which includes the whole of Australia).

6. ICAO recommends that all aircraft monitor 121.5 at all times to the extent possible.

7. If you crash and survive but are injured, 121.5 is, overall, the best frequency to use to summon assistance. A call on 121.5 is almost always answered anywhere in the world except in the polar regions. That’s because of the large number of good airmen and good citizens who monitor 121.5 when flying en route.

8. An intercepting aircraft is required by ICAO Annex 2 to call us on 121.5 before shooting us down.

Until 27th November 2003, the Australian recommendation (it was never a requirement) was that we should monitor the “Area Frequency” whilst en route VFR. The Australian recommendation now is that we monitor an appropriate frequency.

One practical benefit of monitoring 121.5 as opposed to the old “Area Frequency” is that 121.5 is almost silent. The only transmissions ever heard on 121.5 are those relating to distress or an aircraft which ATC has “lost” or transmissions made unintentionally (when the pilot intended to transmit on a different frequency). There is not the noise and distraction that occurs on an area frequency, leaving the pilot better able to fly the aircraft and maintain a good lookout.

Air Safety Australia urges all members to become familiar with monitoring 121.5 when flying en route, and then to always consider 121.5 when choosing which frequency to monitor when flying en route.

When you monitor 121.5 for the first time, remember that it is a silent frequency. Don’t make any transmissions on it unless you experience an emergency or you are responding to another aircraft which is experiencing an emergency and has transmitted on 121.5

Boyd Munro, 19th March 2004

The next section of the Coping with Emergencies Guide is a further discussion of aviation distress beacons written by David McBrien of AusSAR.

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Groundschool – VHF Radiocommunications Guide

| Guide content | Abbreviations and acronyms |

| 1. Transmitter licensing | 2. R/T phrasing | 3. VHF characteristics and radio operation |

| 4. Microair 760 transceiver | 5. R/T procedures | [6. Safety and emergency procedures] |

| 7. Aviation Distress Beacons | 8. Understanding SAR services |

Next – Aviation distress beacons The next section of the VHF radiocommunications guide is a further discussion of aviation distress beacons written by David McBrien of AusSAR.

Copyright © 2000-2013 John Brandon     [contact information]