diff --git a/Src/Orbiter/Vessel.cpp b/Src/Orbiter/Vessel.cpp
index f1555c64d..71354e6f2 100644
--- a/Src/Orbiter/Vessel.cpp
+++ b/Src/Orbiter/Vessel.cpp
@@ -8297,6 +8297,7 @@ void VESSEL::IncThrusterLevel (THRUSTER_HANDLE th, double dlevel) const
 	if (vessel->bFRplayback) return;
 	ThrustSpec *ts = (ThrustSpec*)th;
 	ts->level_permanent += dlevel;
+	ts->level_permanent = max (0.0, min (1.0, ts->level_permanent));
 	if (ts->tank && ts->tank->mass)
 		ts->level = max (0.0, min (1.0, ts->level_permanent + ts->level_override));
 }
diff --git a/Src/Orbiter/Vessel.h b/Src/Orbiter/Vessel.h
index 01b61fef1..f8189a394 100644
--- a/Src/Orbiter/Vessel.h
+++ b/Src/Orbiter/Vessel.h
@@ -443,6 +443,7 @@ class Vessel: public VesselBase {
 	{
 		if (!bFRplayback) {
 			ts->level_permanent += dlevel;
+			ts->level_permanent = std::max(0.0, std::min(1.0, ts->level_permanent));
 			if (ts->tank && ts->tank->mass)
 				ts->level = std::max(0.0, std::min(1.0, ts->level+dlevel));
 		}