From df7988fb490260cd5ac72aa2319b98c1a693b239 Mon Sep 17 00:00:00 2001
From: James Turner <zakalawe@mac.com>
Date: Wed, 6 Feb 2019 20:48:18 +0000
Subject: [PATCH] Fix NaNs in the VSI when pressure reading is invalid

Detect zero/very low pressure reading and avoid generating NaNs in the
main update code. Thanks to Johnathan Redpath for the report.
---
 .../vertical_speed_indicator.cxx                | 17 +++++++++++++++--
 .../vertical_speed_indicator.hxx                | 12 ++++++------
 2 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/src/Instrumentation/vertical_speed_indicator.cxx b/src/Instrumentation/vertical_speed_indicator.cxx
index c364a700d..e4f3f908c 100644
--- a/src/Instrumentation/vertical_speed_indicator.cxx
+++ b/src/Instrumentation/vertical_speed_indicator.cxx
@@ -71,8 +71,21 @@ void
 VerticalSpeedIndicator::update (double dt)
 {
     if (_serviceable_node->getBoolValue()) {
-        double pressure_inHg = _pressure_node->getDoubleValue() ;
-        double pressure_Pa = pressure_inHg * SG_INHG_TO_PA;
+        const double pressure_inHg = _pressure_node->getDoubleValue() ;
+        const double pressure_Pa = pressure_inHg * SG_INHG_TO_PA;
+
+        // this occurs if the static pressure source didn't report valid data
+        // yet. Don't continue processing here since we will generate NaNs
+        // when dividing by zero
+        if (_casing_pressure_Pa < 1e-3) {
+            if (pressure_Pa > 1e3) {
+                // once the pressure becomes valid, reinit
+                reinit();
+            } else {
+                return; // no more processing
+            }
+        }
+
         double Fsign = 0.;
         double orifice_mach = 0.0;
 
diff --git a/src/Instrumentation/vertical_speed_indicator.hxx b/src/Instrumentation/vertical_speed_indicator.hxx
index e552e536c..e290f8064 100644
--- a/src/Instrumentation/vertical_speed_indicator.hxx
+++ b/src/Instrumentation/vertical_speed_indicator.hxx
@@ -45,13 +45,13 @@ public:
 
 private:
 
-    double _casing_pressure_Pa;
-    double _casing_airmass_kg;
-    double _casing_density_kgpm3;
-    double _orifice_massflow_kgps;
+    double _casing_pressure_Pa = 0.0;
+    double _casing_airmass_kg = 0.0;
+    double _casing_density_kgpm3 = 0.0;
+    double _orifice_massflow_kgps = 0.0;
 
-    std::string _name;
-    int _num;
+    const std::string _name;
+    const int _num;
     std::string _static_pressure;
     std::string _static_temperature;