AP_NavEKF3: Handle repeated FW flight without magnetometer

The EKF can build up large yaw errors on ground so it is safer to stop using GPS and re-align after launch as per first launch.

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -283,6 +283,15 @@ void NavEKF3_core::setAidingMode()
                                      (imuSampleTime_ms - lastPosPassTime_ms > maxLossTime_ms);
             }
 
+            // This is a special case for when we are a fixed wing aircraft vehicle that has landed and
+            // has no yaw measurement that works when static. Declare the yaw as unaligned (unknown)
+            // and declare attitude aiding loss so that we fall back into a non-aiding mode
+            if (assume_zero_sideslip() && onGround && (effectiveMagCal == MagCal::GSF_YAW) && !use_compass()){
+                yawAlignComplete = false;
+                finalInflightYawInit = false;
+                attAidLossCritical = true;
+            }
+
             if (attAidLossCritical) {
                 // if the loss of attitude data is critical, then put the filter into a constant position mode
                 PV_AidingMode = AID_NONE;

libraries/AP_NavEKF3/AP_NavEKF3_VehicleStatus.cpp

@@ -343,24 +343,18 @@ void NavEKF3_core::detectFlight()
             largeHgtChange = true;
         }
 
-        // Determine to a high certainty we are flying
-        if (motorsArmed && highGndSpd && (highAirSpd || largeHgtChange)) {
+        if (motorsArmed) {
             onGround = false;
+            if (highGndSpd && (highAirSpd || largeHgtChange)) {
+                // to a high certainty we are flying
                 inFlight = true;
             }
-
-        // if is possible we are in flight, set the time this condition was last detected
-        if (motorsArmed && (highGndSpd || highAirSpd || largeHgtChange)) {
-            airborneDetectTime_ms = imuSampleTime_ms;
-            onGround = false;
-        }
-
-        // Determine to a high certainty we are not flying
-        // after 5 seconds of not detecting a possible flight condition or we are disarmed, we transition to on-ground mode
-        if(!motorsArmed || ((imuSampleTime_ms - airborneDetectTime_ms) > 5000)) {
+        } else {
+            // to a high certainty we are not flying
             onGround = true;
             inFlight = false;
         }
+
     } else {
         // Non fly forward vehicle, so can only use height and motor arm status
 

libraries/AP_NavEKF3/AP_NavEKF3_core.h

@@ -1008,7 +1008,6 @@ private:
     bool inFlight;                  // true when the vehicle is definitely flying
     bool prevInFlight;              // value inFlight from previous frame - used to detect transition
     bool manoeuvring;               // boolean true when the flight vehicle is performing horizontal changes in velocity
-    uint32_t airborneDetectTime_ms; // last time flight movement was detected
     Vector6 innovVelPos;            // innovation output for a group of measurements
     Vector6 varInnovVelPos;         // innovation variance output for a group of measurements
     Vector6 velPosObs;              // observations for combined velocity and positon group of measurements (3x1 m , 3x1 m/s)