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the top of Camel’s Hump Mountain if tracking just slightly left of the highest point (4026’ MSL). Also, consider that the approach plate specifically forbids autopilot- coupled approaches, making precise adherence to vertical and lateral guidance just a little more challenging. Even on a somewhat warmer day, the buffer could be perilously thin. Scary!
RNAV approaches are much like ILS and LOC approaches, but it is important to understand when baro altimetry-related temperature correction applies. LPV approaches follow the same rules appropriate to ILS approaches because the glide slope is 100% determined by GPS WAAS satellite information. In contrast, LNAV approaches inherently dependent on barometric altimetry are subject to temperature correction, much like LOC or VOR approaches whenever the airport’s Final segment is called out within the NOTAM document and aerodrome temperatures apply. Temperature correction for baro-dependent approaches should not be confused with low temperature restrictions that apply to LNAV/VNAV approaches at certain airports. For example, in the case of Burlington’s RNAV Z (GPS) approach to 33, the baro- derived LNAV/VNAV approach is forbidden for airplanes not equipped with automatic temperature compensation (to adjust the glide
slope) when the airport temperature falls below -15C.
It is curious that there haven’t been more accidents related to low temperature altimeter error. Part of the reason may be that low temperature days in the USA tend to be clear days; mostly visual approaches. There is also a lot of buffer built into the design of instrument approaches to mitigate for other sources of mistakes, including less-than-precise navigation and instrument error. ATC also deserves some credit in the sense that, except for a very few sectors in mountainous areas where special procedures apply, the MVAs used by controllers already enjoy a buffer beyond any effect caused by the worst- case low temperature day. It is for this reason that the pilot is not required to perform low temperature corrections when receiving vectors from ATC.
This is a cautionary tale. While a temperature-related altimetry error by itself likely won’t result in an accident, the Bell 212 crash reminds us that it can certainly contribute. And special care must be taken in mountainous areas where approach fixes may be high above the airport , knowing that these errors can be very big indeed. And now that it’s a FAA requirement to perform temperature correction at designated airports when conditions demand, it’s the pilot’s legal obligatio•n-responsibility to do so. Remember, temperatures low, look out below. T&T
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