Let's take them in order — Page 169, Lesson 149

I want to walk you through a set of answers from the navigation and instruments question bank. These cover some really important concepts — compass errors, Mach number calculation, magnetic terms, and altimeter positioning errors. Let's take them in order. Question 47 gives us a formula: C + D = M ± V = T. That stands for Compass heading plus Deviation equals Magnetic heading, then plus or minus Variation equals True heading. The worked example shows 244° plus 11° gives 255°, then minus 8° gives 247° True. So the answer is B. Question 48 is about using the CRP-5 flight computer to find Mach number. First you calculate True Airspeed from the given information — and don't forget compressibility! That gives a TAS of 481 knots. Then in the airspeed window, set the ambient temperature of -65°C against the Mach number index. Against the TAS of 481 on the outer scale, read off the Mach number on the inner scale — in this case Mach 0.855. Answer C. Question 49 deals with direct reading compass errors during acceleration and deceleration. During a deceleration, the compass will indicate an apparent turn towards the further pole. In the northern hemisphere, that means an apparent turn towards the south. In the southern hemisphere, it gives an apparent turn towards the north. Accelerations are exactly opposite — they indicate apparent turns towards the nearer pole. Answer D. Question 50 is just answer A — no explanation given in the excerpt. Question 51 defines three key magnetic terms. Variation is the angular difference between true north and magnetic north. Deviation is the angular difference between magnetic north and compass north. 'Dip' is the angle formed between the horizontal component — also called the H or directive force — of the earth's magnetic field, and the total magnetic force or intensity of the earth's magnetic field. An isoclinal is a line joining points on a chart of equal dip. The aclinic line is a line indicating zero dip, and it's also referred to as the magnetic equator. Answer C. Question 52 says the altitude is of no consequence here, and to use the same technique as shown in question 46. Answer B. Question 53 is about Mach number changes in the climb. Owing to density error, the True Airspeed will increase dramatically in the climb even at a constant Calibrated Airspeed. The Local Speed of Sound, being controlled by temperature, will also reduce. From the formula Mach Number equals TAS divided by LSS, an increase in TAS and a decrease in LSS must result in a marked increase in Mach number. A descent would mean a reversal of the speed changes seen in the climb. Answer D. Question 54 refers to the basic rules of magnetism: like poles repel, unlike poles attract, and the lines of flux run from the red north-seeking pole to the blue south-seeking pole. Answer C. Question 55: double integration means the second stage of integration — in this case, distance along the local meridian. Answer C. Question 56: a solid state gyro refers to the ring laser gyro as used in inertial reference or 'strapdown' systems. It's also referred to as a rate sensor. Answer B. Question 57 is just answer B. Question 58: the ISA temperature at 40,000 feet is -56.5°C. Using the airspeed window of the CRP-5, set the temperature of -56.5° against the Mach number index. Now against the TAS of 450 knots on the outer scale, read off the Mach number on the inner — 0.783. Answer B. Questions 59 and 60 are just answers A and C respectively. Question 61: in an EFIS system, wind information can be displayed in every mode except PLAN. Answer C. Questions 62 and 63 are answers C and B. Question 64: in the NAV mode, 1 dot equals 2 nautical miles. In the VOR mode, 1 dot equals 5°. In the ILS mode, 1 dot equals 1°. Answer C. Question 65 is answer B. Question 66 deals with an altimeter position error. The apron is 10 feet below the stated aerodrome elevation. So assuming the QFE is set for the aerodrome level, an altimeter on the apron should read minus 10 feet. However, the instrument is positioned in the aircraft 20 feet above the apron, so it should show minus 10 plus 20 equals plus 10 feet. Its actual reading is plus 40 feet, so it is over-reading by 30 feet — an instrument error of plus 30 feet. Answer B.