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One requirement of stand-alone IFR GPS installations is an external/separate CDI for each GPS unit. These external CDIs are usually incorporated into a Horizontal Situation Indicator (HSI), Electronic HSI (EHSI), or traditional Nav-Heads (combo OBS/CDI instruments). They must be installed within the scope of a “normal instrument scan.” However, in the event of an external CDI failure, most stand-alone GPS units incorporate an internal digital CDI. While this CDI is not approved for primary navigation purposes, it can be used in lieu of the external CDI in emergency situations. For emergency and abnormal situations that might call for it, GPS users should know how to make their system display its backup CDI, how to determine its lateral limits (needle sensitivity), and how that CDI may or may not be coupled to installed Flight Director (F/D) and/or Autopilot (A/P) systems. Modern integrated flight deck systems need not have external CDIs, as those systems have built-in redundancies that traditional panel-mounted GPS installations do not. Such systems generally have multiple GPS units, with each unit capable of displaying its navigation signal on multiple pilot- selectable CDIs and bearing pointers. While this lessens the chance of single-point failures in such advanced systems, pilot proficiency in system programming is vital when dealing with abnormalities that require using secondary navigation sources and/or non-standard arrangements of navigation instruments/indicators.
Figure 3: A portion of FAA Flight Advisory CHLK 16-08 advising of GPS testing causing unreliable or unavailable GPS signals over a huge area and altitude spectrum across the southwestern United States.
Complete GPS Loss
GPS is also subject to inference, rendering signals unreliable or unavailable. Such was the case throughout most of June 2016 in a multi-state area in the southwestern United States, due to GPS testing periods. In those instances, the FAA issued a Flight Advisory (Figure 3), as well as multiple NOTAMs related to the times, areas, and altitudes of the GPS outages.
AUGUST 2016
KING AIR MAGAZINE • 13