diff --git a/src/blackmisc/simulation/interpolator.cpp b/src/blackmisc/simulation/interpolator.cpp
index a16024b0c..61ff04fd8 100644
--- a/src/blackmisc/simulation/interpolator.cpp
+++ b/src/blackmisc/simulation/interpolator.cpp
@@ -204,10 +204,14 @@ namespace BlackMisc
                 currentSituation = interpolant.interpolatePositionAndAltitude(currentSituation, interpolateGndFlag);
                 if (currentSituation.isNull()) { break; }
 
-                if (CBuildConfig::isLocalDeveloperDebugBuild())
+                // if we get here and the vector is invalid it means we haven't handled it correctly in one of the interpolators
+                if (!currentSituation.isValidVectorRange())
                 {
-                    Q_ASSERT_X(currentSituation.isValidVectorRange(), Q_FUNC_INFO, "Invalid interpolation situation");
+                    if (CBuildConfig::isLocalDeveloperDebugBuild()) { BLACK_VERIFY_X(false, Q_FUNC_INFO, "Invalid interpolation situation"); }
+                    return CAircraftSituation::null();
                 }
+
+                // GND flag.
                 if (!interpolateGndFlag) { currentSituation.guessOnGround(CAircraftSituationChange::null(), m_model); }
 
                 // as we now have the position and can interpolate elevation
diff --git a/src/blackmisc/simulation/interpolatorlinear.cpp b/src/blackmisc/simulation/interpolatorlinear.cpp
index 0af25b5c1..e2726b3c3 100644
--- a/src/blackmisc/simulation/interpolatorlinear.cpp
+++ b/src/blackmisc/simulation/interpolatorlinear.cpp
@@ -8,6 +8,7 @@
  */
 
 #include "interpolatorlinear.h"
+#include "interpolatorfunctions.h"
 #include "blackmisc/aviation/aircraftsituationlist.h"
 #include "blackmisc/aviation/altitude.h"
 #include "blackmisc/geo/coordinategeodetic.h"
@@ -62,12 +63,24 @@ namespace BlackMisc
             const std::array<double, 3> oldVec(m_oldSituation.getPosition().normalVectorDouble());
             const std::array<double, 3> newVec(m_newSituation.getPosition().normalVectorDouble());
 
+            if (CBuildConfig::isLocalDeveloperDebugBuild())
+            {
+                BLACK_VERIFY_X(CAircraftSituation::isValidVector(oldVec), Q_FUNC_INFO, "Invalid old vector");
+                BLACK_VERIFY_X(CAircraftSituation::isValidVector(newVec), Q_FUNC_INFO, "Invalid new vector");
+                BLACK_VERIFY_X(isValidTimeFraction(m_simulationTimeFraction), Q_FUNC_INFO, "Invalid fraction");
+            }
+
             // Interpolate position: pos = (posB - posA) * t + posA
             CCoordinateGeodetic newPosition;
             newPosition.setNormalVector((newVec[0] - oldVec[0]) * m_simulationTimeFraction + oldVec[0],
                                         (newVec[1] - oldVec[1]) * m_simulationTimeFraction + oldVec[1],
                                         (newVec[2] - oldVec[2]) * m_simulationTimeFraction + oldVec[2]);
 
+            if (CBuildConfig::isLocalDeveloperDebugBuild())
+            {
+                BLACK_VERIFY_X(newPosition.isValidVectorRange(), Q_FUNC_INFO, "Invalid vector");
+            }
+
             // Interpolate altitude: Alt = (AltB - AltA) * t + AltA
             // avoid underflow below ground elevation by using getCorrectedAltitude
             const CAltitude oldAlt(m_oldSituation.getCorrectedAltitude());
diff --git a/src/blackmisc/simulation/interpolatorspline.cpp b/src/blackmisc/simulation/interpolatorspline.cpp
index 3498bd377..6bc91c85a 100644
--- a/src/blackmisc/simulation/interpolatorspline.cpp
+++ b/src/blackmisc/simulation/interpolatorspline.cpp
@@ -7,7 +7,8 @@
  * contained in the LICENSE file.
  */
 
-#include "blackmisc/simulation/interpolatorspline.h"
+#include "interpolatorspline.h"
+#include "interpolatorfunctions.h"
 #include "blackmisc/network/fsdsetup.h"
 #include "blackmisc/logmessage.h"
 #include "blackmisc/verify.h"
@@ -221,7 +222,7 @@ namespace BlackMisc
                 m_nextSampleAdjustedTime = m_s[2].getAdjustedMSecsSinceEpoch(); // latest
                 m_prevSampleTime = m_s[1].getMSecsSinceEpoch();
                 m_nextSampleTime = m_s[2].getMSecsSinceEpoch(); // latest
-                m_interpolant = CInterpolant(pa, altUnit, CInterpolatorPbh(m_s[1], m_s[2]));
+                m_interpolant = CInterpolant(pa, altUnit, CInterpolatorPbh(m_s[1], m_s[2])); // older, newer
                 Q_ASSERT_X(m_prevSampleAdjustedTime < m_nextSampleAdjustedTime, Q_FUNC_INFO, "Wrong time order");
             }
 
@@ -245,7 +246,7 @@ namespace BlackMisc
             {
                 BLACK_VERIFY_X(dt1 >= 0, Q_FUNC_INFO, "Expect postive dt1");
                 BLACK_VERIFY_X(dt2 > 0, Q_FUNC_INFO, "Expect postive dt2");
-                BLACK_VERIFY_X(timeFraction < 1.01, Q_FUNC_INFO, "Expect fraction 0-1");
+                BLACK_VERIFY_X(isValidTimeFraction(timeFraction), Q_FUNC_INFO, "Expect fraction 0-1");
             }
 
             const qint64 interpolatedTime = m_prevSampleTime + qRound(timeFraction * dt2);