Question 16 — The answer is C — Page 632, Lesson 731

Let’s pick up with the answers to these questions, and I’ll walk you through each one so the reasoning sticks. Question 16 — The answer is C. This is based on radio altimeter height above terrain. So when we talk about this particular function, the primary input is the radio altimeter, which gives us the actual height above the ground, not pressure altitude. Question 17 — The answer is B. The inputs to the Ground Proximity Warning System, GPWS, are: radio altimeter, vertical speed, ILS glide slope deviation, undercarriage position, flap position, and Mach number. So six inputs total. Let me define each: radio altimeter gives height above terrain; vertical speed tells us rate of climb or descent; ILS glide slope deviation shows how far off the glide path we are; undercarriage position is whether the gear is down or up; flap position is the flap setting; and Mach number is the speed relative to the speed of sound. All of these feed into the GPWS to warn of unsafe terrain proximity. Question 18 — The answer is B. A system that can withstand at least one failure but leaves the system capable of completing the landing and roll is described as ‘fail-operational’. An alternative term is ‘fail-active’. So the key here is that after one failure, the system still functions well enough to finish the landing and the roll-out. Question 19 — The answer is C. The autopilot synchronization system prevents ‘snatching’ of the flying controls on engagement of the autopilot. The auto-trim system adjusts the trim of the aircraft during automatic flight to prevent ‘snatching’ of the controls on disengagement of the autopilot. So two different systems for two different moments: synchronization for when you engage, auto-trim for when you disengage. Question 20 — The answer is D. EGT is measured using a number of thermocouples connected in parallel to minimize the effect of failure of one of them. So Exhaust Gas Temperature is sensed by multiple thermocouples wired in parallel — that way, if one fails, the others still give a reading. Question 21 — The answer is B. This is Newton’s third law: ‘to every action there is an equal and opposite reaction’. In this case the forces cancel out and the compass will continue to indicate north. So we have a pivot, a centre of mass, and an acceleration — but because the forces balance, the compass stays pointing north. Questions 22 through 25 — The answers are B, C, B, B respectively. I’ll let those stand as the correct selections. Question 26 — The answer is A. A trihedron is defined as a figure having three sides. Additionally, the 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. So the Inertial Reference System gives true north, and we apply magnetic variation — stored in memory — to get magnetic headings, over that latitude range. Question 27 — The answer is A. Total Air Temperature, TAT, is warmer than Static Air Temperature due to the effect of compressibility. The faster the aeroplane flies, the greater the TAT. So as speed increases, the ram effect compresses the air, raising the temperature measured as TAT above the static value. That covers all the answers. Let me know if you want me to expand on any of these — for example, how the GPWS uses each input, or how the auto-trim actually moves the stabilizer.