diff --git a/ArduCopter/ArduCopter.pde b/ArduCopter/ArduCopter.pde
index cb79a6fe7c..48c2f289de 100644
--- a/ArduCopter/ArduCopter.pde
+++ b/ArduCopter/ArduCopter.pde
@@ -353,7 +353,7 @@ static union {
         uint8_t motor_test          : 1; // 24      // true if we are currently performing the motors test
         uint8_t initialised         : 1; // 25      // true once the init_ardupilot function has completed.  Extended status to GCS is not sent until this completes
         uint8_t land_complete_maybe : 1; // 26      // true if we may have landed (less strict version of land_complete)
-        uint8_t throttle_zero       : 1; // 27      // true if the throttle stick is at zero, debounced
+        uint8_t throttle_zero       : 1; // 27      // true if the throttle stick is at zero, debounced, determines if pilot intends shut-down when not using motor interlock
         uint8_t system_time_set     : 1; // 28      // true if the system time has been set from the GPS
         uint8_t gps_base_pos_set    : 1; // 29      // true when the gps base position has been set (used for RTK gps only)
         enum HomeState home_state   : 2; // 30,31   // home status (unset, set, locked)
diff --git a/ArduCopter/control_acro.pde b/ArduCopter/control_acro.pde
index 3b08a0cc59..64bcb5b2fb 100644
--- a/ArduCopter/control_acro.pde
+++ b/ArduCopter/control_acro.pde
@@ -19,7 +19,7 @@ static void acro_run()
     int16_t pilot_throttle_scaled;
 
     // if motors not running reset angle targets
-    if(!motors.armed() || g.rc_3.control_in <= 0) {
+    if(!motors.armed() || ap.throttle_zero) {
         attitude_control.set_throttle_out_unstabilized(0,true,g.throttle_filt);
         return;
     }
diff --git a/ArduCopter/control_autotune.pde b/ArduCopter/control_autotune.pde
index 2396b444d8..46e7067562 100644
--- a/ArduCopter/control_autotune.pde
+++ b/ArduCopter/control_autotune.pde
@@ -233,7 +233,7 @@ static bool autotune_start(bool ignore_checks)
     }
 
     // ensure throttle is above zero
-    if (g.rc_3.control_in <= 0) {
+    if (ap.throttle_zero) {
         return false;
     }
 
diff --git a/ArduCopter/control_flip.pde b/ArduCopter/control_flip.pde
index c82dd87bf0..1b048f3a6f 100644
--- a/ArduCopter/control_flip.pde
+++ b/ArduCopter/control_flip.pde
@@ -45,7 +45,7 @@ static bool flip_init(bool ignore_checks)
     }
 
     // if in acro or stabilize ensure throttle is above zero
-    if ((g.rc_3.control_in <= 0) && (control_mode == ACRO || control_mode == STABILIZE)) {
+    if (ap.throttle_zero && (control_mode == ACRO || control_mode == STABILIZE)) {
         return false;
     }
 
diff --git a/ArduCopter/control_sport.pde b/ArduCopter/control_sport.pde
index 64ccb1dc77..08841ee307 100644
--- a/ArduCopter/control_sport.pde
+++ b/ArduCopter/control_sport.pde
@@ -26,7 +26,7 @@ static void sport_run()
     float takeoff_climb_rate = 0.0f;
 
     // if not armed or throttle at zero, set throttle to zero and exit immediately
-    if(!motors.armed() || g.rc_3.control_in <= 0) {
+    if(!motors.armed() || ap.throttle_zero) {
         attitude_control.set_throttle_out_unstabilized(0,true,g.throttle_filt);
         pos_control.relax_alt_hold_controllers(get_throttle_pre_takeoff(g.rc_3.control_in)-throttle_average);
         return;
diff --git a/ArduCopter/control_stabilize.pde b/ArduCopter/control_stabilize.pde
index 4640ce15a0..722a16904b 100644
--- a/ArduCopter/control_stabilize.pde
+++ b/ArduCopter/control_stabilize.pde
@@ -24,7 +24,7 @@ static void stabilize_run()
     int16_t pilot_throttle_scaled;
 
     // if not armed or throttle at zero, set throttle to zero and exit immediately
-    if(!motors.armed() || g.rc_3.control_in <= 0) {
+    if(!motors.armed() || ap.throttle_zero) {
         attitude_control.set_throttle_out_unstabilized(0,true,g.throttle_filt);
         return;
     }
diff --git a/ArduCopter/radio.pde b/ArduCopter/radio.pde
index b6a5ad7e74..64f3cefec3 100644
--- a/ArduCopter/radio.pde
+++ b/ArduCopter/radio.pde
@@ -172,14 +172,19 @@ static void set_throttle_and_failsafe(uint16_t throttle_pwm)
 }
 
 #define THROTTLE_ZERO_DEBOUNCE_TIME_MS 400
-// set_throttle_zero_flag - set throttle_zero flag from debounced throttle control_in
+// set_throttle_zero_flag - set throttle_zero flag from debounced throttle control
+// throttle_zero is used to determine if the pilot intends to shut down the motors
+// Basically, this signals when we are not flying.  We are either on the ground
+// or the pilot has shut down the copter in the air and it is free-falling
 static void set_throttle_zero_flag(int16_t throttle_control)
 {
     static uint32_t last_nonzero_throttle_ms = 0;
     uint32_t tnow_ms = millis();
 
-    // if non-zero throttle immediately set as non-zero
-    if (throttle_control > 0) {
+    // if not using throttle interlock and non-zero throttle and not E-stopped,
+    // or using motor interlock and it's enabled, then motors are running, 
+    // and we are flying. Immediately set as non-zero
+    if ((!ap.using_interlock && (throttle_control > 0) && !ap.motor_estop) || (ap.using_interlock && ap.motor_interlock)) {
         last_nonzero_throttle_ms = tnow_ms;
         ap.throttle_zero = false;
     } else if (tnow_ms - last_nonzero_throttle_ms > THROTTLE_ZERO_DEBOUNCE_TIME_MS) {