A trihedron is defined as a figure having three sides — Page 160, Lesson 147

I want to walk you through a set of answers that cover some really important navigation and instrument principles. Let's start with question 26. A trihedron is defined as a figure having three sides. Additionally, the Inertial Reference System, or IRS, establishes true north. Magnetic indications are obtained by the application of variation, which, in the case of the Boeing 737-400, is stored in each IRS memory. The range covered for variation is from 73° North to 60° South latitude. Now, question 27. Total Air Temperature, abbreviated TAT, is warmer than Static Air Temperature due to the effect of compressibility. The faster the aeroplane flies, the greater the TAT becomes. Moving to question 28. Isoclinal lines refer to magnetic dip. Isobars refer to atmospheric pressure lines on a meteorological chart. And a 'deviation card' is a compass correction card positioned alongside the compass in the aircraft. Question 29 deals with temperature effects on altitude. If the actual temperature of the column of air in which the aircraft is flying is COLDER than ISA, then the True Altitude of the aircraft above mean sea level will be LOWER than the Indicated Altitude. If the actual temperature is WARMER than ISA, the True Altitude will be higher. Question 30 tells us that the air driven artificial horizon is also known as the 'classic attitude indicator'. For question 31, angular momentum means speed of rotation, measured in rpm. Question 32: the error detector is also known as a signal selsyn. Question 34 refers to a ring laser gyro. Now, question 42 is about gyroscopic axes. Vertical axis means horizontal plane. The two are at 90° to each other. It follows, therefore, that heading information only requires one degree of freedom in the horizontal plane about the vertical axis of the gyroscope. Question 45 gives us the formula for Local Speed of Sound, or LSS. LSS equals 38.94 multiplied by the square root of temperature in Kelvin. Or, you can use the CRP-5 navigation computer. Finally, question 46. The first thing to remember is that the local speed of sound is dependent on AMBIENT temperature, not ISA. In this case, applying the temperature deviation to the ISA temperature at mean sea level gives an ambient temperature of +5°C. There are two methods of calculating the local speed of sound. The first is to use the formula from question 45: LSS equals 38.94 times the square root of 278 Kelvin, which gives 649.26 knots. The second method is to use the CRP-5 navigation computer: in the airspeed window, set the ambient temperature of +5°C against the Mach number index. Then, against 10 on the inner CAS or Mach number scale, read off the LSS on the outer TAS scale, which gives 650 knots.