diff --git a/python/setup.py.in b/python/setup.py.in
index 516a19dca..c78991db6 100644
--- a/python/setup.py.in
+++ b/python/setup.py.in
@@ -214,10 +214,11 @@ for d in os.scandir('data/aircraft'):
         data_files.append(('share/${PROJECT_NAME}/'+dir_name,
                             list(XML_files(dir_name))))
 
-        # Some aircraft folders include a `Systems` and/or an `Engines`
-        # sub-directory so make sure it is copied in the wheel archive
-        # (see GH issue #687)
-        for sub_dir in ('Systems', 'Engines'):
+        # Some aircraft folders include a "Systems" and/or an "Engines"
+        # sub-directory (with several alternative spelling) so make sure they
+        # are copied in the wheel archive (see GH issue #687)
+        for sub_dir in ('Systems', 'systems', 'Engines', 'engines', 'Engine',
+                        'engine'):
             sub_dir_name = dir_name + '/' + sub_dir
             subdir_dir_fullname = os.path.join('data', sub_dir_name)
             if os.path.exists(subdir_dir_fullname) and os.path.isdir(subdir_dir_fullname):
diff --git a/src/models/FGFCS.cpp b/src/models/FGFCS.cpp
index 9409af82c..a5ffadfaa 100644
--- a/src/models/FGFCS.cpp
+++ b/src/models/FGFCS.cpp
@@ -38,6 +38,7 @@ INCLUDES
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
 
 #include <iomanip>
+#include <array>
 
 #include "FGFCS.h"
 #include "input_output/FGModelLoader.h"
@@ -613,8 +614,17 @@ SGPath FGFCS::FindFullPathName(const SGPath& path) const
   SGPath name = FGModel::FindFullPathName(path);
   if (systype != stSystem || !name.isNull()) return name;
 
-  name = CheckPathName(FDMExec->GetFullAircraftPath()/string("Systems"), path);
-  if (!name.isNull()) return name;
+#ifdef _WIN32
+  const array<string, 1> dir_names = {"Systems"};
+#else
+  // Check alternative capitalization for case sensitive OSes.
+  const array<string, 2> dir_names = {"Systems", "systems"};
+#endif
+
+  for(const string& dir_name: dir_names) {
+    name = CheckPathName(FDMExec->GetFullAircraftPath()/dir_name, path);
+    if (!name.isNull()) return name;
+  }
 
   return CheckPathName(FDMExec->GetSystemsPath(), path);
 }
diff --git a/src/models/FGPropulsion.cpp b/src/models/FGPropulsion.cpp
index 685498b0a..319203546 100644
--- a/src/models/FGPropulsion.cpp
+++ b/src/models/FGPropulsion.cpp
@@ -45,6 +45,7 @@ INCLUDES
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
 
 #include <iomanip>
+#include <array>
 
 #include "FGFDMExec.h"
 #include "FGPropulsion.h"
@@ -457,11 +458,23 @@ bool FGPropulsion::Load(Element* el)
 
 SGPath FGPropulsion::FindFullPathName(const SGPath& path) const
 {
-  if (!ReadingEngine) return FGModel::FindFullPathName(path);
-
-  SGPath name = CheckPathName(FDMExec->GetFullAircraftPath()/string("Engines"),
-                              path);
-  if (!name.isNull()) return name;
+  SGPath name = FGModel::FindFullPathName(path);
+  if (!ReadingEngine && !name.isNull()) return name;
+
+#ifdef _WIN32
+  // Singular and plural are allowed for the folder names for consistency with
+  // the default engine folder name "engine" and for backward compatibility
+  // regarding the folder name "Engines".
+  const array<string, 2> dir_names = {"Engines", "engine"};
+#else
+  // Allow alternative capitalization for case sensitive OSes.
+  const array<string, 4> dir_names = {"Engines", "engines", "Engine", "engine"};
+#endif
+
+  for(const string& dir_name: dir_names) {
+    name = CheckPathName(FDMExec->GetFullAircraftPath()/dir_name, path);
+    if (!name.isNull()) return name;
+  }
 
   return CheckPathName(FDMExec->GetEnginePath(), path);
 }