PPRuNe Forums > Wannabes Forums > Professional Pilot Training (includes ground studies) > ATPL NAV question - True altitude calculation PDA View Full Version : ATPL NAV question - True altitude calculation anderchunk30th May 2012, 10:18E.g. On the Alice Spring ILS, the Glide Path intersects the Outer Marker at an altitude of 3120ft. An aircraft is making an ILS approach. The local ASP QNH 028 HPA is set correctly. The surface temperature is +3'C and the elevation of the airport is 1789ft. If the aircraft crosses the Outer Marker with its altimeter indicating the height shown on the instrument approach, its position in relation to the Glidepath is? My questions is: Where do I apply the altitude error (4% per 10'C ISA deviation in calculation? Many thanks if anyone can help. Capt Pit Bull30th May 2012, 14:37What options are you considering? Perhaps have a think about how QNH is defined. Maybe you're unsure about that. For example, with QNH set how will the altimeter reading vary, after landing, depending on whether it's an ISA, ISA+10 or ISA-10 day? (btw I presume the QNH is a typo and should be 1028) pb alphacentauri31st May 2012, 00:21If I remember my ATPL nav stuff correctly the answer is...the aircraft will be under the glide path. This answer is from a procedure design point of view but should explain it. All published alts on procedures are assuming ISA temp. The ISA temp at sea level is 15°C. Alice Springs is 1789ft AMSL and the ISA temp should be about 12°C. (ISA temp drops 2°/1000ft) The reported YBAS temp is 3°C so this makes it about 9° below ISA. Close enough to ISA-10. As the temperature drops below ISA the altimeter will over read. ie you will be lower than the altimeter is telling you. This is because as the temp drops the air becomes more dense, thus fooling the altimeter. The altimeter thinks you are at a certain height but you are actually lower. The altitude correction can be apllied to any altitude in the terminal area, and is indeed now a requirement in Oz. The temp corrn tables are in the front of DAP and should be used on all minimum alts in the terminal area. Hope this explanation helps Alpha:ok: Da-20 monkey31st May 2012, 00:38Can I refer you to this thread: http://www.pprune.org/tech-log/471780-temperature-corrections-high-temperature.html#post6923548 Checkboard's explanation. Basically make the temp correction on height only, not altitude, since it makes no sense to apply the correction ,,under the ground''. In your example the difference will be small ofcourse. :cool: alphacentauri31st May 2012, 06:31Da 20 is correct. So for the example above you need the isa-10 correction, for height above aerodrome, at the outer marker :)