Test the unusable fuel feature (see issue #759).

aircraft/c172x/c172x.xml

@@ -283,6 +283,25 @@
       </channel>
     </system>
 
+    <system name="Fuel content in volume">
+        <fcs_function name="propulsion/tank[0]/contents-volume-gal">
+            <function>
+                <quotient>
+                    <p> propulsion/tank[0]/contents-lbs </p>
+                    <p> propulsion/tank[0]/density-lbs_per_gal </p>
+                </quotient>
+            </function>
+        </fcs_function>
+        <fcs_function name="propulsion/tank[1]/contents-volume-gal">
+            <function>
+                <quotient>
+                    <p> propulsion/tank[1]/contents-lbs </p>
+                    <p> propulsion/tank[1]/density-lbs_per_gal </p>
+                </quotient>
+            </function>
+        </fcs_function>
+    </system>
+
     <system name="Mixture control">
       <!--
       E = 1.3 * Mixture * P_std / P_amb

tests/CMakeLists.txt

@@ -49,7 +49,8 @@ set(PYTHON_TESTS ResetOutputFiles
                  TestLinearization
                  TestLinearActuator
                  TestPlanet
-                 TestLighterThanAir)
+                 TestLighterThanAir
+                 TestUnusableFuel)
 
 foreach(test ${PYTHON_TESTS})
   add_test(NAME ${test}

tests/TestUnusableFuel.py

@@ -0,0 +1,60 @@
+# TestUnusableFuel.py
+#
+# Checks that the unusable fuel feature work (see issue GH#759)
+#
+# Copyright (c) 2022 Bertrand Coconnier
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free Software
+# Foundation; either version 3 of the License, or (at your option) any later
+# version.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+# details.
+#
+# You should have received a copy of the GNU General Public License along with
+# this program; if not, see <http://www.gnu.org/licenses/>
+#
+
+from JSBSim_utils import JSBSimTestCase
+
+
+class TestUnusableFuel(JSBSimTestCase):
+    def test_unusable_fuel(self):
+        fdm = self.create_fdm()
+        fdm.load_model("c172x")
+        fdm.load_ic("reset01", True)
+
+        fdm["propulsion/tank[0]/unusable-volume-gal"] = 1.5
+        fdm["propulsion/tank[1]/unusable-volume-gal"] = 1.5
+        fdm["propulsion/tank[0]/contents-lbs"] = 10
+        fdm["propulsion/tank[1]/contents-lbs"] = 10
+        fdm.run_ic()
+
+        # Check the engine is running
+        self.assertNotEqual(fdm["propulsion/engine/set-running"], 0.0)
+        # Check that tanks are not yet emptied.
+        self.assertLess(
+            fdm["propulsion/tank[0]/unusable-volume-gal"],
+            fdm["propulsion/tank[0]/contents-volume-gal"],
+        )
+        self.assertLess(
+            fdm["propulsion/tank[1]/unusable-volume-gal"],
+            fdm["propulsion/tank[1]/contents-volume-gal"],
+        )
+
+        # Run until the engine starves.
+        while fdm["propulsion/engine/set-running"] != 0.0:
+            fdm.run()
+
+        # Check that the fuel content in gallons is lower or equal to the unusable volume.
+        self.assertGreaterEqual(
+            fdm["propulsion/tank[0]/unusable-volume-gal"],
+            fdm["propulsion/tank[0]/contents-volume-gal"],
+        )
+        self.assertGreaterEqual(
+            fdm["propulsion/tank[1]/unusable-volume-gal"],
+            fdm["propulsion/tank[1]/contents-volume-gal"],
+        )