Guidance on Identification Friend or Foe (IFF) Altitude Reporting (Mode C) Control

1. This MARADMIN provides critical guidance for all
platforms equipped with IFF or civil transponders to address
significant operational and safety concerns.
2. Background. A recurring issue has been identified where military
operators deselect the altitude reporting (Mode C) function on their
transponders. This action is often taken under the erroneous
assumption that it is a valid Emissions Control (EMCON) procedure.
Deselecting Mode C causes the transponder to report "unknown
altitude," which creates serious safety risks, degrades the tactical
picture, and negatively impacts air traffic control.
3. Guidance. Effective immediately, all units will adhere to the
following:
a. Do not deselect Mode C. Any military platform using an IFF
transponder must ensure the altitude reporting (Mode C) control
remains enabled at all times. Deselecting Mode C alone has no effect
on achieving EMCON.
b. EMCON procedures. For EMCON purposes, if a platform needs to
secure Mode C replies, the operator must disable Mode 3/A, which
disables replies to both Mode 3/A and Mode C. Mode C must remain
enabled, even during EMCON.
c. Exception. The only exception to this guidance is when explicitly
directed by an Air Traffic Control (ATC) authority due to verified
errors in the platform's pressure altitude reporting.
4. Impacts of incorrect procedures:
a. Military operations: Disabling altitude reporting negatively
impacts Combat Identification and the ability of C2 systems to
maintain a coherent and accurate tactical picture.
b. Civil aircraft safety: Nearby aircraft with Traffic Alert and
Collision Avoidance Systems (TCAS) will receive erroneous traffic
advisories, leading to unnecessary and potentially unsafe maneuvers.
5. Widespread dissemination of this guidance to all operational
aviation units is required.
6. Release authorized by Lieutenant General William H. Swan, 
Deputy Commandant for Aviation.