Barometric instruments measure the change in barometric pressure to give altitude information. They are calibrated to ISA condition. Therefore, if there is any deviation from such environment, an error will be induced. If the temperature is higher than the ISA, the error will cause altitude information to be under-read, and will therefore not be of great safety concern. However, if the temperature is lower than ISA, the error will cause the altitude information to be over-read, causing a safety concern, especially regarding terrain clearance. ICAO PANS-OPS recommends cold altimetry correction to be applied when ambient temperature is ISA-15 or lower.
In practice, however, it is not so straightforward. When approaches were categorised by precision and non-precision only, correction application was relatively straightforward. The introduction of new approaches such as GPS, RNAV AR, GBAS, SBAS, and new FMS based approach has complicated matters.
