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It’s in the Details
The “NA”s can be lost in the fine print of a long note on an instrument procedure chart. The ILS RWY 5 approach at eastern Tennessee’s Tri-Cities Airport (KTRI), for example, has these limitations (Figure 1):
• Circling to Runway 9 not authorized at night. A quick look at the surrounding terrain (which, if you’ve ever flown to KTRI, you know is steep and devoid of lights) tells you why.
• Circling northwest of Runways 9 and 23 not authorized. Even in the daylight, the terrain is too close to the runway to permit circling in the low visibilities that would require you to
be flying a circle-to-land maneuver.
• Autopilot coupled approach
NA below 2200. Terrain or
other features make the localizer and/or glideslope signal unreliable below 2200 MSL, which is 300 feet above decision height and even higher than localizer-only and circling minimums. So if you need to fly all the way to minimums to make it in, you have to hand-fly at least the last part of the descent to the missed approach point.
• Procedure not authorized when the tower is closed. This approach requires radar, per the larger note on the approach chart. When no one is in the tower the entire ILS is NA.
The Amarillo/Tradewind Airport (KTDW) RNAV (GPS) RWY 35 approach chart (Figure 2) illustrates these approach limitations:
• Procedure not authorized at night. There are likely related to obstructions on the final approach, and/or limitations on the type of runway
lighting available.
• Helicopter visibility reduction below one nautical mile not authorized. In the flat terrain near Amarillo, Texas, this is most likely related to obstacles near the runway as well.
• Baro-VNAV approach not authorized. There are obstacles on the approach path that prevent use of a GPS-derived, non-WAAS glidepath below the published MDA for the nonprecision approach.
Both of these approaches are good examples of why we must review approach charts, including a close review of all the notes, well in advance of beginning a procedure.
Figure 1
Figure 2
Temperature-Limited GPS Approaches
This one is a little more esoteric but no less vital. Procedures that use GPS to generate an electronic glide- path through Baro-VNAV may not provide obstacle clearance below the procedure’s Minimum Descent Altitude (MDA). What is Baro-VNAV?
The FAA tells us:
Baro-VNAV is an RNAV [GPS] sys- tem which uses barometric altitude information from the aircraft’s altim- eter to compute vertical guidance for the pilot. The specified vertical path is typically computed between two waypoints or an angle from a single way point. When using baro-VNAV
guidance, the pilots should check for any temperature limitations which may result in approach restrictions.
This is why IFR approach-cer-tified GPSs prompt the pilot to enter the current altimeter setting into the box before loading an approach. The GPS uses its terrain database to calculate and display an electronic
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