diff --git a/libraries/SITL/SIM_Aircraft.cpp b/libraries/SITL/SIM_Aircraft.cpp
index 0f6a622ab6..6cf84e1591 100644
--- a/libraries/SITL/SIM_Aircraft.cpp
+++ b/libraries/SITL/SIM_Aircraft.cpp
@@ -258,7 +258,6 @@ void Aircraft::fill_fdm(struct sitl_fdm &fdm) const
     fdm.pitchDeg = degrees(p);
     fdm.yawDeg   = degrees(y);
     fdm.airspeed = airspeed;
-    fdm.magic = 0x4c56414f;
 }
 
 uint64_t Aircraft::get_wall_time_us() const
diff --git a/libraries/SITL/SITL.h b/libraries/SITL/SITL.h
index 4cf492740f..dc4aa7113a 100644
--- a/libraries/SITL/SITL.h
+++ b/libraries/SITL/SITL.h
@@ -8,10 +8,8 @@ class DataFlash_Class;
 
 namespace SITL {
 
-struct PACKED sitl_fdm {
-    // this is the packet sent by the simulator
-    // to the APM executable to update the simulator state
-    // All values are little-endian
+struct sitl_fdm {
+    // this is the structure passed between FDM models and the main SITL code
     uint64_t timestamp_us;
     double latitude, longitude; // degrees
     double altitude;  // MSL
@@ -21,7 +19,6 @@ struct PACKED sitl_fdm {
     double rollRate, pitchRate, yawRate; // degrees/s/s in body frame
     double rollDeg, pitchDeg, yawDeg;    // euler angles, degrees
     double airspeed; // m/s
-    uint32_t magic; // 0x4c56414f
 };
 
 // number of rc output channels