once bitten twice shy
Hi Guys!
I have been blasted twice for not RTFQ but isn't this, well, pretty basic? As pilots it is thumped into us in groundschool that for each 10degC the OAT is warmer than ISA increase the indicated altitude by 4% to give true altitude and for each 10degC the OAT is cooler than ISA decrease the indicated altitude by 4%. So if the airfield is 3600 ft and the mountain is 8050ft and also +15 ISA the helo pilot has moved 4450 ft upwards but the QNH altimeter has moved only 4200ft and thus "should" read 7800ft. All assuming we can treat humid air as a dry ideal gas. (Which we don't in meteorology or propulsion but it'll do for now and I won't bother with the regs for what constitutes a serviceable altimeter and what it "could" as opposed to "should" read). So my answer is c) and I am pretty worried if someone disagrees with me because one of us is badly mistaken and I am pretty sure it isn't me. (What is being thumped into people at groundschool?) My next worry is more technical. I was taught at school (not Uni) that for a dry ideal gas V/T is a constant provided that pressure stays the same (Charles' Law)and PV is a constant provided that temperature stays the same (Boyle's Law). Thus for a dry ideal gas P1V1/T1 = P2V2/T2. That pretty well wraps up the whole she-bang. Is it just not fashionable to teach this anymore either?
Sorry if I sound a bit abrupt this evening.
PS Mrs E has asked me to apologise before tea so I do. Also Old Smokey, if it is say 30 feet with every hectopascal and +/- 2 hectopascals tidal variation from the mean in Oz that could be 120ft in six hours - twixt low and high tides. If I park my Piper near a beach in the afternoon and come back for it later that night do I have to dig the altimeter out of the sand?
Chiz from the chastened enicalyth
Last edited by enicalyth; 26th November 2004 at 05:48.