5-2-15

Validation of Mode C Altitude Readout

JO 7110.65BB, Change 3, effective July 9, 2026 · Official text on faa.gov

Ensure that Mode C altitude readouts are valid after:
1. Initial track start.
2. Track start from coast/frozen status.
3. During and after an unreliable Mode C readout.
4. Accepting an interfacility handoff, except:
  1. CTRD‐equipped tower cabs are not required to validate Mode C altitude readouts after accepting interfacility handoffs from TRACONs according to the procedures in paragraph 5-4-3, Methods, subparagraph a4.
  2. ERAM facilities are not required to validate Mode C altitude readouts after accepting interfacility handoffs from other ERAM facilities, except:
    1. After initial track start or track start from coast is required, or
    2. During and after the display of a missing, unreasonable, exceptional, or otherwise unreliable Mode C readout indicator.
      Note
      
      Consider a Mode C readout unreliable when any condition exists that indicates the Mode C may be in error, not just those that display an indicator in the Data Block.
Consider an altitude readout valid when:
1. It varies less than 300 feet from the pilot reported altitude, or
  Phraseology
  
  (If aircraft is known to be operating below the lowest useable flight level),
  SAY ALTITUDE.
  or
  (If aircraft is known to be operating at or above the lowest useable flight level),
  SAY FLIGHT LEVEL.
2. You receive a continuous readout from an aircraft on the airport and the readout varies by less than 300 feet from the field elevation, or
  Note
  
  A continuous readout exists only when the altitude filter limits are set to include the field elevation.
  
  Reference
  
  FAA Order JO 7110.65, Para 5-2-21, Altitude Filters.
  FAA Order JO 7110.65, Para 5-13-5, Selected Altitude Limits.
3. You have correlated the altitude information in your data block with the validated information in a data block generated in another facility (by verbally coordinating with the other controller) and your readout is exactly the same as the readout in the other data block.
When unable to validate the readout, do not use the Mode C altitude information for separation.
Whenever you observe an aircraft below FL 180 with an invalid Mode C readout:
1. Issue the correct altimeter setting and confirm the pilot has accurately reported the altitude.
  Phraseology
  
  (Location) ALTIMETER (appropriate altimeter), VERIFY ALTITUDE.
2. If the altitude readout continues to be invalid:
  1. Instruct the pilot to turn off the altitude‐ reporting part of his/her transponder and include the reason; and
  2. Notify the operations supervisor‐in‐charge of the aircraft call sign.
    Phraseology
    
    STOP ALTITUDE SQUAWK. ALTITUDE DIFFERS BY (number of feet) FEET.
Whenever you observe an aircraft at or above FL 180 with an invalid Mode C readout, unless the aircraft is descending below Class A airspace:
1. Verify that the pilot is using 29.92 inches of mercury as the altimeter setting and has accurately reported the altitude.
  Phraseology
  
  VERIFY USING TWO NINER NINER TWO AS YOUR ALTIMETER SETTING.
  (If aircraft is known to be operating at or above the lowest useable flight level),VERIFY FLIGHT LEVEL.
2. If the Mode C readout continues to be invalid:
  1. Instruct the pilot to turn off the altitude‐ reporting part of his/her transponder and include the reason; and
  2. Notify the operations supervisor‐in‐charge of the aircraft call sign.
    Phraseology
    
    STOP ALTITUDE SQUAWK. ALTITUDE DIFFERS BY (number of feet) FEET.
Whenever possible, inhibit altitude readouts on all consoles when a malfunction of the ground equipment causes repeated invalid readouts.

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