+void
+AirspeedIndicator::computeMach(double ias)
+{
+ if (!_environmentManager) {
+ return;
+ }
+
+ FGEnvironment env(_environmentManager->getEnvironment());
+
+ // derived from http://williams.best.vwh.net/avform.htm#Mach
+ // names here are picked to be consistent with those formulae!
+
+ double oatK = env.get_temperature_degc() + 273.15; // OAT in Kelvin
+ double CS = 38.967854 * sqrt(oatK); // speed-of-sound in knots at altitude
+ double CS_0 = 661.4786; // speed-of-sound in knots at sea-level
+ double P_0 = env.get_pressure_sea_level_inhg();
+ double P = _static_pressure_node->getDoubleValue();
+
+ double DP = P_0 * (pow(1 + 0.2*pow(ias/CS_0, 2), 3.5) - 1);
+ double mach = pow(5 * ( pow(DP/P + 1, 2.0/7.0) -1) , 0.5);
+
+ // publish Mach and TAS
+ _mach_node->setDoubleValue(mach);
+ _tas_node->setDoubleValue(CS * mach);
+}
+