diff --git a/src/blackmisc/directoryutils.cpp b/src/blackmisc/directoryutils.cpp
index 2db4443cd..bd1e25df9 100644
--- a/src/blackmisc/directoryutils.cpp
+++ b/src/blackmisc/directoryutils.cpp
@@ -525,6 +525,17 @@ namespace BlackMisc
         return d1.absolutePath() == d2.absolutePath();
     }
 
+    bool CDirectoryUtils::isSubDirectoryOf(const QString &dir1, const QString &dir2)
+    {
+        QDir d1(dir1);
+        const QDir d2(dir2);
+        do
+        {
+            if (d1 == d2) { return true; }
+        } while (d1.cdUp());
+        return false;
+    }
+
     QSet<QString> CDirectoryUtils::fileNamesToQSet(const QFileInfoList &fileInfoList)
     {
         QSet<QString> sl;
diff --git a/src/blackmisc/directoryutils.h b/src/blackmisc/directoryutils.h
index dc9b864c8..3b43d930e 100644
--- a/src/blackmisc/directoryutils.h
+++ b/src/blackmisc/directoryutils.h
@@ -184,6 +184,9 @@ namespace BlackMisc
         //! Same existing directories
         static bool isSameExistingDirectory(const QString &dir1, const QString &dir2);
 
+        //! Is dir1 a subdirectory (possibly nested) of dir2 or the same directory
+        static bool isSubDirectoryOf(const QString &dir1, const QString &dir2);
+
         //! Result of directory comparison
         struct DirComparison
         {
diff --git a/src/blackmisc/simulation/settings/simulatorsettings.h b/src/blackmisc/simulation/settings/simulatorsettings.h
index a9ec7cf17..5195fbd51 100644
--- a/src/blackmisc/simulation/settings/simulatorsettings.h
+++ b/src/blackmisc/simulation/settings/simulatorsettings.h
@@ -20,6 +20,7 @@
 #include "blackmisc/statusmessage.h"
 #include "blackmisc/propertyindex.h"
 #include "blackmisc/blackmiscexport.h"
+#include "blackmisc/directoryutils.h"
 
 #include <QStringList>
 #include <QObject>
@@ -386,6 +387,22 @@ namespace BlackMisc
 
                 //! \copydoc BlackMisc::TSettingTrait::humanReadable
                 static const QString &humanReadable() { static const QString name("XPlane settings"); return name; }
+
+                //! \copydoc BlackMisc::TSettingTrait::isValid
+                static bool isValid(const CSimulatorSettings &value, QString &reason)
+                {
+                    const QString simDir = value.hasSimulatorDirectory() ? value.getSimulatorDirectory()
+                                         : CSpecializedSimulatorSettings::defaultSimulatorDirectory(CSimulatorInfo::XPLANE);
+                    for (const QString &modelDir : value.getModelDirectories())
+                    {
+                        if (!CDirectoryUtils::isSubDirectoryOf(modelDir, simDir))
+                        {
+                            reason = QStringLiteral("Model directory must be within the simulator directory structure");
+                            return false;
+                        }
+                    }
+                    return true;
+                }
             };
 
             //! Trait for simulator settings
diff --git a/src/plugins/simulator/xplane/simulatorxplane.cpp b/src/plugins/simulator/xplane/simulatorxplane.cpp
index adffcd59d..be1e18685 100644
--- a/src/plugins/simulator/xplane/simulatorxplane.cpp
+++ b/src/plugins/simulator/xplane/simulatorxplane.cpp
@@ -571,7 +571,14 @@ namespace BlackSimPlugin
             {
                 Q_ASSERT(package.s.endsWith('/'));
                 package.s.chop(1);
-                m_trafficProxy->loadPlanesPackage(package.s);
+                if (CDirectoryUtils::isSubDirectoryOf(package.s, getSimulatorSettings().getSimulatorDirectoryOrDefault()))
+                {
+                    m_trafficProxy->loadPlanesPackage(package.s);
+                }
+                else
+                {
+                    CLogMessage(this).validationError(u"CSL package '%1' can not be loaded as it is outside the X-Plane installation directory") << package.s;
+                }
             }
         }