Frequencies to remember

Many times we tend to forget important frequencies like VOR frequency range, Squawk Codes, ELT operation frequencies etc… So we decided to put all of the important and ‘should know’ ones into one post so you can have a quick reference to them. Here it goes:

HF Communication: 2 MHz to 22 MHz (in practice upto 18 MHz). Amplitude Modulated with emission code J3E.

VHF Communication: 117.975 MHz to 137 MHz with spacing of 8.33 KHz (older radios use 25 KHz spacing). Amplitude Modulated with emission code A3E

Non-Directional Beacons (NDB): AM signals in the upper LF and lower MF bands between 190 KHz and 1750 KHz. Emission Code – N0N A1A or N0N A2A.

VHF Omni-Range (VOR): 108 – 111.95 MHz (using even first decimals – 108.20, 110.65) and 112 – 117.95 MHz. Emission code – A9W

Instrument Landing System (ILS):

Localizer - VHF band between 108 and 111.975 MHz (using odd first decimals – 108.30, 110.75). Emission code – A8W. 40 Channels available with 50 KHz spacing.

Glidepath - UHF band between 328.6 – 335.4 MHz (practically between 329.3 – 335 MHz). Emission code – A8W. Spacing – 150 KHz.

Marker Beacons - 75 MHz (Outer, Middle, Inner)

Airborne Weather Radar (AWR): SHF Band between 9 – 10 GHz

Distance Measuring Equipment (DME): UHF Band between 962 – 1213 MHz. Emission code – P0N.

Transponder: Interrogation signal at 1030 MHz, and transponded signal at 1090 MHz.

Squawk Codes:

  • 7700 – Emergency (Distress)
  • 7600 – Communication Failure
  • 7500 – Interference (Unlawful interference)
  • 2000 – No code assigned by ATC

Radio Altimeter: SHF Band at around 4.3 GHz (FM)

Emergency Frequencies:

  • 121.5 MHz – International Air Distress (IAD) or VHF Guard
  • 406 MHz – Military Air Distress (MAD) or UHF Guard

Reduced Vertical Separation Minima (RVSM)

Reduced vertical separation minima (RVSM) is the reduction of the standard vertical separation required between aircraft flying above FL290 and FL410 (both levels inclusive), from 2,000 feet to 1,000 feet.

In the mid 1970′s, the world fuel shortage and the resultant rapid increase in the fuel prices led to the growing demand for a more optimum and efficient utilization of the available airspace. This emphasized the need for appraisal of the proposal to reduce the vertical separation minimum (VSM) above FL 290 from 2000 ft (600 m) to 1000 ft (300 m). Various studies by several countries revealed that a separation of 1000 ft (300 m) VSM above FL 290 is technically feasible and does not compromise with the safety of the aircraft.

Indian Airspace and neighbouring airspace has become RVSM airspace between FL 290 and FL 410, inclusive of FL 290 and FL 410 on 27th November, 2003.

The principal benefits of implementation of RVSM are:

  • A theoretical doubling of the airspace capacity, between FL290 and FL410; and
  • The opportunity for aircraft to operate closer to the optimum flight levels resulting in better fuel economies.

Aircraft operators must receive specific approval from the aircraft’s state of registry (DGCA) in order to conduct operations in RVSM airspace. Both the individual aircraft and the specific aircraft type or types that the operator intends to use is to be approved by DGCA. Non RVSM approved aircraft must fly lower or higher than the airspace, or obtain special permission from the state’s aviation authority. They can transit through RVSM airspace  provided they are given continuous climb throughout the designated airspace, and 2,000 ft vertical separation is provided at all times between the non-RVSM flight, and all others for the duration of the climb/descent.

All departing flights requesting FL270 and above are required to indicate their RVSM approval status by inserting ‘W’ as the second letter in item 10-Equipment of Flight Plan or in item ‘Q’ equipment of RPL (Repetitive Flight Plan) for RVSM equipped aircraft.

Aircraft system requirements for RVSM approval

1.  Two independent altitude measurement systems, each system composed of the following:

  • Cross-coupled static source
  • Equipment for providing a digitally encoded signal corresponding to the displayed pressure altitude, for automatic altitude reporting purposes
  • Static source error correction / Position error correction, if needed to meet the performance critaria

2.  One Transponder that can be connected to the Altimeter for altitude reporting.

3.  An automatic altitude alerting system with aural and visual alerting if altitude deviates from the selected altitude by more than +/- 300 feet (for aircraft with type certification made before April 1997 ) or +/- 200 feet (for aircraft with type certification made after April 1997).

4.  ACAS II system

5.  An Autopilot with Altitude hold with tolerance band of +/- 50 feet (15 m).


RVSM Approved: Aircraft which have gained RVSM approval from the State of Registry (DGCA) or state of the Operator.

Non-RVSM Approved: Aircraft which do not have a RVSM approval, or was RVSM approved but has become non-approved due to equipment failure.

Approved Non-RVSM: Aircraft which have gained an approval to flight plan in exclusive RVSM airspace, as per special coordination procedures.

Procedures within RVSM airspace

1. Before entering RVSM airspace, the pilot should review the status of required equipment. The following equipment should be operating normally:

  • Two primary altimetry systems
  • One automatic altitude-keeping device
  • One altitude-alerting device, and
  • One operating transponder with operational Mode ‘C’

2. The pilot must notify ATC whenever the aircraft-

  • is no longer RVSM compliant due to equipment failure; or
  • loss of a altimetry system; or
  • encounters turbulence that affects the capability to maintain flight level.

3. During cleared transition between levels, the aircraft should not overshoot or undershoot the assigned flight level by more than 150 ft (45m).

4. Except in an ADS (Automatic Dependent Surveillance) or radar enviornment, pilots shall report reaching any altitude assigned within RVSM airspace.

Special procedures for in-flight contingencies in oceanic airspace

Although all possible contingencies cannot be covered, the follwing sections cover the more frequent cases such as:

  • Inability to maintain assigned flight level due to meteorological conditions, aircraft performance; pressurization failure
  • En-route diversion across the prevailing traffic flow, and
  • Loss of, or significant reduction in, the required navigation capability when operating in a airspace where the navigation performance accuracy is a prerequisite to the safe conduct of flight operations

General Procedure

    1. If an aircraft is unable to continue flight in accordance with its air traffic control clearance and/or an aircraft is unable to maintain the navigation performance accuracy specified for the airspace, a revised clearance shall be obtained, whenever possible, prior to initiating any action.
    2. The radio-telephony distress signal (MAYDAY) or urgency signal (PAN-PAN) preferably spoken three times shall be used as appropriate. Subsequent ATC action with respect to that aircraft shall be based on the intentions of the pilot and the overall air traffic situation.
    3. If prior clearance cannot be obtained, an ATC clearance shall be obtained at the earliest possible time and, until a revised clearance is received, the pilot shall -

(i) Leave the assigned route or track by initially turning 90° to the right or to the left. When possible the direction of the turn should be determined by the position of the aircraft relative to any organized route or track system. Other facts which may affect the direction of the turn are

    • The direction to an alternate airport, terrain clearance,
    • Any lateral offset be flown, and
    • The flight levels allocated on adjacent routes or tracks

(ii) Following the turn, the pilot should

    • If unable to maintain the assigned flight level initially minimize the rate of descent to the extent that is operationally feasible.
    • Take account of other aircraft be laterally offset from its track.
    • Acquire and maintain in either direction a track laterally separated by 28KM (15NM) from the assigned route, and
    • Once established on the offset track, climb or descend to select a flight level which differs from those normally used by 150M (500Ft).

 (iii) Alert nearby aircraft by broadcasting, at suitable intervals, aircraft identification, flight level, position (including the ATS route designator or the track code, as appropriate) and intentions on the frequencies in use and on 121.5MHz (or a backup, or the inter-pilot air to air frequency 123.45MHz)

(iv) Maintain a watch for conflicting traffic visually and by reference to ACAS (if equipped)

(v) Turn on all aircraft exterior lights (commensurate with appropriate operating limitations)

(vi) Keep the SSR transponder on at all times

(vii) Take action as necessary to ensure the safety of the aircraft.

4.  When leaving the assigned track to acquire and maintain the track laterally separated by 28KM (15NM), the flight crew should, where practicable avoid bank angles that would result in overshooting the track to be acquired, particularly in airspace where a 55.5KM (30NM) lateral separation minimum is applied.

5.  Extended range operations by aeroplane with two turbine power units (ETOPS).

6.  If the Contingency Procedures are employed by a twin engine aircraft as a result of an Engine shutdown or failure of an ETOPS critical system, the pilot should advise ATC as soon as practicable of the situation, reminding ATC of the type of aircraft involved and request expeditious handling.

Further Readings :

CAR – Section 8, Series ‘S’ Part II issued 17th January, 2013

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