diff --git a/src/Instrumentation/airspeed_indicator.cxx b/src/Instrumentation/airspeed_indicator.cxx
index 27f2f0f17..480d6dab3 100644
--- a/src/Instrumentation/airspeed_indicator.cxx
+++ b/src/Instrumentation/airspeed_indicator.cxx
@@ -54,13 +54,17 @@ AirspeedIndicator::unbind ()
 # define FPSTOKTS 0.592484
 #endif
 
+#ifndef INHGTOPSF
+# define INHGTOPSF (2116.217/29.9212)
+#endif
+
 void
 AirspeedIndicator::update (double dt)
 {
     if (_serviceable_node->getBoolValue()) {
         double pt = _total_pressure_node->getDoubleValue();
         double p = _static_pressure_node->getDoubleValue();
-        double q = pt - p;      // dynamic pressure
+        double q = ( pt - p ) * INHGTOPSF;      // dynamic pressure
 
                                 // Now, reverse the equation
         double v_fps = sqrt((2 * q) / SEA_LEVEL_DENSITY_SLUGFT3);
diff --git a/src/Systems/pitot.cxx b/src/Systems/pitot.cxx
index 0a58805b5..3cd8f9485 100644
--- a/src/Systems/pitot.cxx
+++ b/src/Systems/pitot.cxx
@@ -37,16 +37,20 @@ PitotSystem::unbind ()
 {
 }
 
+#ifndef INHGTOPSF
+# define INHGTOPSF (2116.217/29.9212)
+#endif
+
 void
 PitotSystem::update (double dt)
 {
     if (_serviceable_node->getBoolValue()) {
                                 // The pitot tube sees the forward
                                 // velocity in the body axis.
-        double p = _pressure_node->getDoubleValue();
+        double p = _pressure_node->getDoubleValue(); // static
         double r = _density_node->getDoubleValue();
         double v = _velocity_node->getDoubleValue();
-        double q = 0.5 * r * v * v; // dynamic pressure
+        double q = 0.5 * r * v * v / INHGTOPSF; // dynamic
         _total_pressure_node->setDoubleValue(p + q);
     }
 }