diff --git a/src/blackmisc/directoryutils.cpp b/src/blackmisc/directoryutils.cpp
index bd1e25df9..90605396e 100644
--- a/src/blackmisc/directoryutils.cpp
+++ b/src/blackmisc/directoryutils.cpp
@@ -527,12 +527,21 @@ namespace BlackMisc
 
     bool CDirectoryUtils::isSubDirectoryOf(const QString &dir1, const QString &dir2)
     {
-        QDir d1(dir1);
+        if (dir1.isEmpty() || dir2.isEmpty()) { return false; }
         const QDir d2(dir2);
+        return CDirectoryUtils::isSubDirectoryOf(dir1, d2);
+    }
+
+    bool CDirectoryUtils::isSubDirectoryOf(const QString &dir1, const QDir &dir2)
+    {
+        QDir d1(dir1);
         do
         {
-            if (d1 == d2) { return true; }
-        } while (d1.cdUp());
+            if (d1 == dir2) { return true; }
+        }
+        while (d1.cdUp());
+
+        // not found
         return false;
     }
 
diff --git a/src/blackmisc/directoryutils.h b/src/blackmisc/directoryutils.h
index 3b43d930e..f95df2714 100644
--- a/src/blackmisc/directoryutils.h
+++ b/src/blackmisc/directoryutils.h
@@ -185,8 +185,13 @@ namespace BlackMisc
         static bool isSameExistingDirectory(const QString &dir1, const QString &dir2);
 
         //! Is dir1 a subdirectory (possibly nested) of dir2 or the same directory
+        //! \remark testing on real directory structure
         static bool isSubDirectoryOf(const QString &dir1, const QString &dir2);
 
+        //! Is dir1 a subdirectory (possibly nested) of dir2 or the same directory
+        //! \remark testing on real directory structure
+        static bool isSubDirectoryOf(const QString &dir1, const QDir &dir2);
+
         //! Result of directory comparison
         struct DirComparison
         {
diff --git a/src/plugins/simulator/xplane/simulatorxplane.cpp b/src/plugins/simulator/xplane/simulatorxplane.cpp
index be1e18685..c28e9fe3e 100644
--- a/src/plugins/simulator/xplane/simulatorxplane.cpp
+++ b/src/plugins/simulator/xplane/simulatorxplane.cpp
@@ -567,11 +567,16 @@ namespace BlackSimPlugin
                 if (package.isEmpty()) { continue; }
                 packages.insert(it, { package.append('/') });
             }
+
+            // comment KB 2019-06
+            // a package is one xsb_aircraft.txt file BB has 9, X-CSL has 76
+            // the reason for the append("/")/chop "/" is explained here: https://discordapp.com/channels/539048679160676382/539925070550794240/594891288751505418
+            const QDir simDir = getSimulatorSettings().getSimulatorDirectoryOrDefault();
             for (auto &package : packages)
             {
                 Q_ASSERT(package.s.endsWith('/'));
                 package.s.chop(1);
-                if (CDirectoryUtils::isSubDirectoryOf(package.s, getSimulatorSettings().getSimulatorDirectoryOrDefault()))
+                if (CDirectoryUtils::isSubDirectoryOf(package.s, simDir))
                 {
                     m_trafficProxy->loadPlanesPackage(package.s);
                 }