On an instrument approach to landing, pilots can adopt an LNAV procedure where only lateral navigation is considered. An APV (approach with vertical guidance) procedure can also be used where both lateral guidance and specific height data is given, so that the autopilot can fly the aircraft both laterally and vertically. In designing the procedure, care must be taken to ensure a safe obstacle clearance in both the approach to the runway and for a ‘go around’ after a missed approach.
In certain weather conditions e.g. unusually low temperatures, the standard atmosphere cannot be used for calculating the aircraft height above the aerodrome. Instead, a height correction needs to be calculated for the ‘off-standard’ atmosphere.
The Smith Institute helped gCAP by developing an iterative algorithm that can calculate height corrections given a ground temperature and pressure reading. The algorithm was straightforward to implement and was quickly ported to the gCAP system. The main challenge to be overcome was poor documentation of the off-standard atmosphere model. The Smith Institute overcame this by approaching the problem from first principles, in order to ensure the correct parameter values were applied to the model.
Using the algorithm, gCAP can quickly and accurately calculate the height corrections required in an off-standard atmosphere.
Much of the documentation behind this equation was not available so it was a great relief to find someone who was able to make sense of it and come up with a script which, as it turned out, gave more accurate results than the sample output provided. Many thanks.Captain Nick GribbleManaging Director & Chief Designer at gCAP Ltd