Garmin GNC 420W Pilots Guide - Page 202

SECTION 11 ADDITIONAL FEATURES Pilots should be aware of TAS/TCAS system limitations. If an intruder transponder does not respond to interrogations due to antenna shading or marginal transponder performance, it will not be displayed, or display may be intermittent. Pilots should remain vigilant for traffic at all times when using TAS/TCAS systems for non-transponder equipped airplanes or unresponsive airplanes. TCAS I Surveillance Volume Top and bottom mounted antennas allow an active surveillance range of up to 12 nm (GTS 800) or 40 nm (GTS 820/850) in the forward direction, and somewhat reduced ranges to the sides and aft of own aircraft due to the directional interrogation patterns. Interference limiting in GTS 820/850 units may automatically reduce range in high density traffic areas. TA Alerting Conditions The GTS 8XX automatically adjusts its TA sensitivity level (Table 11-5) to reduce the likelihood of nuisance TA alerting during flight phases likely to be near airports. Sensitivity In all other conditions, Level B (greater sensitivity) TA sensitivity is used to assess TA threats. Traffic Symbology Traffic information from the GTS 8XX is displayed on the GNC 420 unit using TAS/TCAS symbology (Table 11-4) on a dedicated Traffic page, and on the moving Map Page. The displayed traffic information generally includes the relative range, bearing, and altitude of intruder aircraft. The GTS 8XX also generates aural announcements heard on the cockpit audio system. Target altitude relative to own aircraft altitude (relative altitude) is displayed (in hundreds of feet) for each target symbol (Figure 11-20). If traffic is above own aircraft altitude the relative altitude is shown above the target next to a '+' symbol. If traffic is below own aircraft altitude the relative altitude is shown below the target next to a '-' symbol. Altitude trend (Figure 11-20) is displayed as an up arrow (≥ +500 fpm), down arrow (≤ -500 fpm), or no symbol if less than 500 fpm rate in either direction. Relative Altitude Level A (less sensitivity) TA is used when the aircraft's radar Traffic altimeter (if equipped) indicates own altitude is less than Type 2000 feet AGL. If no radar altimeter is present, Sensitivity Level A is active when the landing gear is extended. Level A Altitude Trend is also active when groundspeed is less than 120 knots with no radar altimeter present in a fixed gear aircraft. Figure 11-20 Traffic Symbol Components Sensitivity Level A A B B Intruder Altitude Available Yes No Yes No TA Alerting Conditions Intruder closing rate provides less than 20 seconds of vertical and horizontal separation. Or: Intruder range is within 0.2 nm and vertical separation is within 600 feet. Intruder closing rate is less than 15 seconds. Intruder closing rate provides less than 30 seconds of vertical and horizontal separation. Or: Intruder range is within 0.55 nm and vertical separation is within 800 feet. Intruder closing rate is less than 20 seconds. Table 11-5 TA Sensitivity Level and TA Alerting Criteria 11-12 GNC 420(A) Pilot's Guide and Reference 190-00140-20 Rev. K