We're starting on page 113, and the first topic is Navigation System… — Page 112, Lesson 181

I want to walk you through a new chapter now — Chapter 8, Long Range Flight and Polar Navigation. We're starting on page 113, and the first topic is Navigation System Degradation. Modern navigation systems on long-range aircraft are built around Inertial Navigation Systems, or INS, combined with radio navigation aids — and that includes GPS. These systems typically come in either a 2-set system or a 3-set system. Both configurations include comparator and/or warning devices. What that means is the equipment can compare the outputs of the individual units and alert you if something is wrong. A pure equipment failure — meaning the hardware itself has failed — would be indicated by the illumination of a warning light on your panel. Now, let's move to Course and INS Cross-checking. In an emergency, you can cross-check the course and the INS outputs to identify which unit might be giving bad data. In a 3-set system, you have what's called a voting system — you compare the output of each of the three systems. Since you have three independent sources, if one is inaccurate, the other two will agree, and the faulty one is quickly detected. In a 2-set system, it's more difficult. The failure of one system would not be readily detected unless the system captions malfunction codes. If it does display those codes, then interpreting the code should reveal which unit is faulty. If you don't have that, you need another way. If it's possible to obtain a fix — that's a known geographic position — you can compare it with the positions reported by your INS systems. That fix could come from the weather radar, from bearings taken using the ADF and an NDB, or from a VOR beacon. Comparing that known position with the system positions should reveal which system is inaccurate. If uncertainty still exists after all that, there are more basic methods. You can contact another aeroplane in the vicinity and cross-check spot winds, ground speed, and drift. As a last resort, you can compare the outputs from your navigation systems with the flight plan data for wind velocity at the DR position — that's the dead reckoning position — of your aeroplane. Next, we have Unable to Continue in Accordance with ATC Clearance. If your aeroplane is unable to continue the flight as per the ATC clearance you've been given, a revised clearance shall, whenever possible, be obtained prior to initiating any action. This also applies to aircraft that are unable to maintain the specified navigation accuracy. The revised clearance shall be obtained by RTF — that's radiotelephony — using distress or urgency traffic, as appropriate. The subsequent ATC action shall then be based on the intention of the pilot. That covers the first few pages of this chapter. We've looked at how navigation system degradation is detected, how to cross-check course and INS in both 2-set and 3-set systems, and what to do if you can't continue with your current ATC clearance.