ICAO Procedures for Mode C-Derived Level Information of Aircraft Operating in RVSM Airspace

[Reference: ICAO "Procedures for Air Navigation Services - Air Traffic Management" (Doc 4444, up to and including Amendment No. 5 which was applicable on 22 November 2007]

VERIFICATION OF ACCURACY OF MODE C-DERIVED LEVEL INFORMATION

1. The tolerance value used to determine that Mode C-derived level information displayed to the controller is accurate shall be ±60 m (±200 ft) in RVSM airspace.

2. Verification of the accuracy of Mode C-derived level information displayed to the controller shall be effected at least once by each suitably equipped ATC unit on initial contact with the aircraft concerned or, if this is not feasible, as soon as possible thereafter. The verification shall be effected by simultaneous comparison with altimeter-derived level information received from the same aircraft by radiotelephony. The pilot of the aircraft whose Mode C-derived level information is within the approved tolerance value need not be advised of such verification.

3. If the displayed level information is not within the approved tolerance value or when a discrepancy in excess of the approved tolerance value is detected subsequent to verification, the pilot shall be advised accordingly and requested to check the pressure setting and confirm the aircraft's level.

4. If, following confirmation of the correct pressure setting the discrepancy continues to exist, the following action should be taken according to circumstances:

a) request the pilot to stop Mode C transmission, provided this does not interrupt the operation of the transponder on Mode A and notify the next control positions or ATC unit concerned with the aircraft of the action taken; or

b) inform the pilot of the discrepancy and request that Mode C operation continue in order to prevent loss of position and identity information on the aircraft and notify the next control position or ATC unit concerned with the aircraft of the action taken.

DETERMINATION OF LEVEL OCCUPANCY

1. The criterion which shall be used to determine that a specific level is occupied by an aircraft shall be ±60 m (±200 ft) in RVSM airspace.

Note - For a brief explanation of the considerations underlying this value, see the ICAO Air Traffic Services Planning Manual (Doc 9426).

2. Aircraft maintaining a level. An aircraft is considered to be maintaining its assigned level as long as the SSR Mode C-derived level information indicates that it is within the appropriate tolerances of the assigned level, as specified in 1. above.

3. Aircraft vacating a level. An aircraft cleared to leave a level is considered to have commenced its manoeuvre and vacated the previously occupied level when the SSR Mode C-derived level information indicates a change of more than 90 m (300 ft) in the anticipated direction from its previously assigned level.

4. Aircraft passing a level in climb or descent. An aircraft in climb or descent is considered to have crossed a level when the SSR Mode C-derived level information indicates that it has passed this level in the required direction by more than 90 m (300 ft).

5. Aircraft reaching a level. An aircraft is considered to have reached the level to which it has been cleared when three consecutive renewals of Mode C-derived level information have indicated that it is within the appropriate tolerances of the assigned level, as specified in 1. above.

Note - In automated ATS systems, the cycles of renewals of Mode C data may not be evident to controllers. It may, therefore, be necessary to specify in instructions to controllers the number of display repetition cycles, or a time interval, corresponding to three consecutive renewals of Mode C data.

6. Intervention by a controller shall only be required if differences in level information between that displayed to the controller and that used for control purposes are in excess of the values stated above.

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