A fail-safe is a pre-programmed behavior designed to prevent a crash in the event of an unsafe situation. All ArduCopter vehicles include a GPS fail-safe activated by default. 3DR RTF copters are also preset with radio control (RC) and low-battery fail-safe behaviors.
If the drone loses GPS signal in a flight mode that requires GPS (loiter, auto, return-to-launch, guided), it lands automatically, as indicated by a blinking blue-and-yellow status LED and a high-high-high-low tone. For best GPS-signal strength, always fly outside in an open area.
If the drone loses contact with the RC transmitter, it lands automatically, as indicated by a blinking yellow status LED. If it loses RC signal during a mission, it returns to the launch point before landing.
If the battery reaches 25% charge, the drone lands automatically, as indicated by a blinking yellow status LED and a quick repeating tone. If the battery runs low during a mission, the drone returns to the launch point before landing.