Altimeter Correction - Temperature
The Chief Standards chappy hit me with this one today...and not as part of my yearly beasting session, but mainly because no one could agree how to apply the correction or interpret the tables.
Now assume the airfield in question lies at 2000 ft amsl, and the aerodrome OAT is -10 deg C, the ILS MDH is 200ft. Now according to the tables in the American equivalent of the ERS the pilot should add 20 ft to the approach minima, making the minima now 220 ft indicated. Now assume that we now have a platform 5000 ft above the airfield, at 7000 ft amsl. The temp up there (assuming ISA lapse rate 2 deg/1000 ft) is measured at -20 deg C. According to the tables the correction for a 200 ft ILS approach to the platform is also 20 ft, as the temp was measured on the platform. Is that correct..or have I misinterpreted it? In free air the the altimeter correction at 5000 ft above the airfield with an OAT of -10 deg C, assuming ISA lapse rates temp would be -20 deg C up there, would be 710 ft. How do you guys apply this correction flying into cold and high airfields (Kabul etc)?
The ICAO Air Ops Procedures doc Vol 1, states that corrections must be added to minima when the surface temp is lower than ISA. Approx 4% height increase per 10 deg variation from below standard temperature measured at the altimeter setting source. Now reading that makes me think that it's not the OAT that should be important, but the difference between OAT and ISA temp. Or have I just tied my self in knots?
Last edited by Hueymeister; 21st Feb 2005 at 14:05.