diff --git a/src/blackmisc/simulation/interpolator.cpp b/src/blackmisc/simulation/interpolator.cpp
index c6372ef4e..4c485b82c 100644
--- a/src/blackmisc/simulation/interpolator.cpp
+++ b/src/blackmisc/simulation/interpolator.cpp
@@ -260,7 +260,7 @@ namespace BlackMisc::Simulation
             const CInterpolatorPbh pbh = interpolant.pbh();
 
             // init interpolated situation
-            currentSituation = this->initInterpolatedSituation(pbh.getOldSituation(), pbh.getNewSituation());
+            currentSituation = this->initInterpolatedSituation(pbh.getStartSituation(), pbh.getEndSituation());
 
             // Pitch bank heading first, so follow up steps could use those values
             currentSituation.setHeading(pbh.getHeading());
@@ -269,7 +269,7 @@ namespace BlackMisc::Simulation
             currentSituation.setGroundSpeed(pbh.getGroundSpeed());
 
             // use derived interpolant function
-            const bool interpolateGndFlag = pbh.getNewSituation().hasGroundDetailsForGndInterpolation() && pbh.getOldSituation().hasGroundDetailsForGndInterpolation();
+            const bool interpolateGndFlag = pbh.getEndSituation().hasGroundDetailsForGndInterpolation() && pbh.getStartSituation().hasGroundDetailsForGndInterpolation();
             currentSituation = interpolant.interpolatePositionAndAltitude(currentSituation, interpolateGndFlag);
             if (currentSituation.isNull()) { break; }
 
@@ -284,7 +284,7 @@ namespace BlackMisc::Simulation
             if (!interpolateGndFlag) { CAircraftSituationChange::null().guessOnGround(currentSituation, m_model); }
 
             // as we now have the position and can interpolate elevation
-            currentSituation.interpolateElevation(pbh.getOldSituation(), pbh.getNewSituation());
+            currentSituation.interpolateElevation(pbh.getStartSituation(), pbh.getEndSituation());
             if (!currentSituation.hasGroundElevation())
             {
                 // we still have no elevation
diff --git a/src/blackmisc/simulation/interpolatorpbh.cpp b/src/blackmisc/simulation/interpolatorpbh.cpp
index c0cab60a0..0ec98866f 100644
--- a/src/blackmisc/simulation/interpolatorpbh.cpp
+++ b/src/blackmisc/simulation/interpolatorpbh.cpp
@@ -12,6 +12,16 @@ using namespace BlackMisc::PhysicalQuantities;
 
 namespace BlackMisc::Simulation
 {
+    CInterpolatorPbh::CInterpolatorPbh(double simulationTimeFraction, const Aviation::CAircraftSituation &start, const Aviation::CAircraftSituation &end) : m_simulationTimeFraction(simulationTimeFraction),
+                                                                                                                                                            m_startSituation(start),
+                                                                                                                                                            m_endSituation(end)
+    {
+        if (CBuildConfig::isLocalDeveloperDebugBuild())
+        {
+            BLACK_VERIFY_X(isValidTimeFraction(m_simulationTimeFraction), Q_FUNC_INFO, "Time fraction needs to be within [0;1]");
+        }
+    }
+
     CAngle CInterpolatorPbh::interpolateAngle(const CAngle &begin, const CAngle &end, double timeFraction0to1)
     {
         // determine the right direction (to left, to right) we interpolate towards to
@@ -38,37 +48,31 @@ namespace BlackMisc::Simulation
     {
         // HINT: VTOL aircraft can change pitch/bank without changing position, planes cannot
         // Interpolate heading: HDG = (HdgB - HdgA) * t + HdgA
-        const CHeading headingBegin = m_oldSituation.getHeading();
-        const CHeading headingEnd = m_newSituation.getHeading();
+        const CHeading headingStart = m_startSituation.getHeading();
+        const CHeading headingEnd = m_endSituation.getHeading();
 
         if (CBuildConfig::isLocalDeveloperDebugBuild())
         {
-            BLACK_VERIFY_X(headingBegin.getReferenceNorth() == headingEnd.getReferenceNorth(), Q_FUNC_INFO, "Need same reference");
+            BLACK_VERIFY_X(headingStart.getReferenceNorth() == headingEnd.getReferenceNorth(), Q_FUNC_INFO, "Need same reference");
         }
-        return CHeading(interpolateAngle(headingBegin, headingEnd, m_simulationTimeFraction), headingEnd.getReferenceNorth());
+        return CHeading(interpolateAngle(headingStart, headingEnd, m_simulationTimeFraction), headingEnd.getReferenceNorth());
     }
 
     CAngle CInterpolatorPbh::getPitch() const
     {
         // Interpolate Pitch: Pitch = (PitchB - PitchA) * t + PitchA
-        return interpolateAngle(m_oldSituation.getPitch(), m_newSituation.getPitch(), m_simulationTimeFraction);
+        return interpolateAngle(m_startSituation.getPitch(), m_endSituation.getPitch(), m_simulationTimeFraction);
     }
 
     CAngle CInterpolatorPbh::getBank() const
     {
         // Interpolate bank: Bank = (BankB - BankA) * t + BankA
-        return interpolateAngle(m_oldSituation.getBank(), m_newSituation.getBank(), m_simulationTimeFraction);
+        return interpolateAngle(m_startSituation.getBank(), m_endSituation.getBank(), m_simulationTimeFraction);
     }
 
     CSpeed CInterpolatorPbh::getGroundSpeed() const
     {
-        return (m_newSituation.getGroundSpeed() - m_oldSituation.getGroundSpeed()) * m_simulationTimeFraction + m_oldSituation.getGroundSpeed();
-    }
-
-    void CInterpolatorPbh::setSituations(const CAircraftSituation &older, const CAircraftSituation &newer)
-    {
-        m_oldSituation = older;
-        m_newSituation = newer;
+        return (m_endSituation.getGroundSpeed() - m_startSituation.getGroundSpeed()) * m_simulationTimeFraction + m_startSituation.getGroundSpeed();
     }
 
     void CInterpolatorPbh::setTimeFraction(double tf)
diff --git a/src/blackmisc/simulation/interpolatorpbh.h b/src/blackmisc/simulation/interpolatorpbh.h
index fe67954df..624761206 100644
--- a/src/blackmisc/simulation/interpolatorpbh.h
+++ b/src/blackmisc/simulation/interpolatorpbh.h
@@ -14,15 +14,15 @@
 
 namespace BlackMisc::Simulation
 {
-    //! Simple interpolator for pitch, bank, heading, groundspeed
+    //! Simple linear interpolator for pitch, bank, heading and groundspeed from start to end situation
     class BLACKMISC_EXPORT CInterpolatorPbh
     {
     public:
         //! @{
         //! Constructor
-        CInterpolatorPbh() {}
-        CInterpolatorPbh(const Aviation::CAircraftSituation &older, const Aviation::CAircraftSituation &newer) : m_oldSituation(older), m_newSituation(newer) {}
-        CInterpolatorPbh(double time, const Aviation::CAircraftSituation &older, const Aviation::CAircraftSituation &newer) : m_simulationTimeFraction(time), m_oldSituation(older), m_newSituation(newer) {}
+        CInterpolatorPbh() = default;
+        CInterpolatorPbh(const Aviation::CAircraftSituation &start, const Aviation::CAircraftSituation &end) : m_startSituation(start), m_endSituation(end) {}
+        CInterpolatorPbh(double simulationTimeFraction, const Aviation::CAircraftSituation &start, const Aviation::CAircraftSituation &end);
         //! @}
 
         //! @{
@@ -31,14 +31,10 @@ namespace BlackMisc::Simulation
         PhysicalQuantities::CAngle getPitch() const;
         PhysicalQuantities::CAngle getBank() const;
         PhysicalQuantities::CSpeed getGroundSpeed() const;
-        const Aviation::CAircraftSituation &getOldSituation() const { return m_oldSituation; }
-        const Aviation::CAircraftSituation &getNewSituation() const { return m_newSituation; }
+        const Aviation::CAircraftSituation &getStartSituation() const { return m_startSituation; }
+        const Aviation::CAircraftSituation &getEndSituation() const { return m_endSituation; }
         //! @}
 
-        //! Set situations
-        //! \remark mostly needed for UNIT tests
-        void setSituations(const Aviation::CAircraftSituation &older, const Aviation::CAircraftSituation &newer);
-
         //! Change time fraction
         void setTimeFraction(double tf);
 
@@ -46,9 +42,9 @@ namespace BlackMisc::Simulation
         //! Interpolate angle
         static PhysicalQuantities::CAngle interpolateAngle(const PhysicalQuantities::CAngle &begin, const PhysicalQuantities::CAngle &end, double timeFraction0to1);
 
-        double m_simulationTimeFraction = 0.0;
-        Aviation::CAircraftSituation m_oldSituation;
-        Aviation::CAircraftSituation m_newSituation;
+        double m_simulationTimeFraction = 0.0; //!< Value within [0;1] to blend between the situations
+        Aviation::CAircraftSituation m_startSituation;
+        Aviation::CAircraftSituation m_endSituation;
     };
 } // namespace
 #endif // guard
diff --git a/src/blackmisc/simulation/interpolatorspline.h b/src/blackmisc/simulation/interpolatorspline.h
index 22a93e27e..eab803b63 100644
--- a/src/blackmisc/simulation/interpolatorspline.h
+++ b/src/blackmisc/simulation/interpolatorspline.h
@@ -68,12 +68,6 @@ namespace BlackMisc::Simulation
             //! Perform the interpolation
             Aviation::CAircraftSituation interpolatePositionAndAltitude(const Aviation::CAircraftSituation &currentSituation, bool interpolateGndFactor) const;
 
-            //! Old situation
-            const Aviation::CAircraftSituation &getOldSituation() const { return pbh().getOldSituation(); }
-
-            //! New situation
-            const Aviation::CAircraftSituation &getNewSituation() const { return pbh().getNewSituation(); }
-
             //! Set the time values
             void setTimes(qint64 currentTimeMs, double timeFraction, qint64 interpolatedTimeMs);