diff --git a/libraries/SITL/SIM_JSBSim.cpp b/libraries/SITL/SIM_JSBSim.cpp
index 22010c333e..ad4cdfe68f 100644
--- a/libraries/SITL/SIM_JSBSim.cpp
+++ b/libraries/SITL/SIM_JSBSim.cpp
@@ -423,6 +423,9 @@ void JSBSim::recv_fdm(const struct sitl_input &input)
     airspeed = fdm.vcas * FEET_TO_METERS;
     airspeed_pitot = airspeed;
 
+    // update magnetic field
+    update_mag_field_bf();
+    
     rpm1 = fdm.rpm[0];
     rpm2 = fdm.rpm[1];
     
diff --git a/libraries/SITL/SIM_last_letter.cpp b/libraries/SITL/SIM_last_letter.cpp
index 965bb7ec3d..ffb198f1f8 100644
--- a/libraries/SITL/SIM_last_letter.cpp
+++ b/libraries/SITL/SIM_last_letter.cpp
@@ -125,6 +125,9 @@ void last_letter::recv_fdm(const struct sitl_input &input)
     airspeed = pkt.airspeed;
     airspeed_pitot = pkt.airspeed;
 
+    // update magnetic field
+    update_mag_field_bf();
+    
     // auto-adjust to last_letter frame rate
     uint64_t deltat_us = pkt.timestamp_us - last_timestamp_us;
     time_now_us += deltat_us;