The FAA has just issued a notice which amends procedures contained within Federal Aviation Administration (FAA) Order 7110.65W, Air Traffic Control, and introduces Unmanned Aircraft Systems (UAS) lost link procedures.
Unmanned Aircraft Systems (UAS) are unique as they are operated through commands sent via line of sight, relayed by satellite relay, or by responding to pre-set programming in the on-board computer.
There are two components to lost link: one is the uplink that transmits command and control (C2) instructions to the aircraft; the second is the downlink which relays the operation/status of onboard systems within the aircraft to the ground control station. If either link is disabled or malfunctions, the result is defined as “lost link,” and some aircraft transponders will automatically reset to code 7400, execute a pre-programmed flight profile and controllers will react accordingly.
NAS Automation changes have been made to all NAS platforms to recognize the Mode 3 7400 Code. Upon this introduction to the NAS, not all UAS platforms are adapted for this code, some will still Squawk Mode 3 7600, therefore ATC personnel should continue to treat each situation as a Lost Link and continue existing procedures.
The Notice can be viewed here.
Source: FAA
“Mode 3” code or “Mode C” code ?
Update : correction. I’ve been informed that Mode 3A is the Military equivalent of the civilian Mode C. My apologies for any confusion caused by my comment above.