OK, this one's a little complicated to explain so please bear with me. Any text book I've ever read has said that every degree of temperature deviation off a standard ISA day (15C at sealevel, temperature lapse rate 1.98C for every 1000ft) is equivalent to 120 feet. So on an ISA +10 day (ignoring humidity) a pressure altitude of 0ft should equate to 1200ft of DA. This is easily confirmed by checking a PA/DA conversion chart such as
this one from the R22 manual. Pretty standard stuff and up to here no problem (I hope!
) .
Now, if you want to calculate the IGE ceiling for an R22 Beta (at say 1300lbs for the sake of argument) using
this IGE Ceiling chart (from the same manual) you will see that your ceiling is 8,500ft of density altitude (point of intersection of ISA line 'Standard Day' with vertical to 1300 lbs). Now at this stage we could stop using the graph and work out the rest in our heads: DA = 8,500ft, ISA +10 day, PA=(8,500-(10x120))=7,300ft also confirmed by aforementioned PA/DA conversion chart.
If however you decide to use the IGE Ceiling chart to calculate your PA, it will give you an answer of 8,000ft PA (intersection of interpolated ISA +10 line (not OAT +10 line!!) with vertical to 1300lbs). So for our 10 degrees of deviation from ISA we have paid a price of only 500ft which equates to 50ft for every degree and not 120ft.
Can anyone explain this discrepancy??? Or have I lost the plot and am incapable of reading a perfomance chart?
If at this stage you are totally lost and wish to brush up on DA and PA calculations, I recommend going to my
previous post 'Want something to read?' If you are not totally lost, please enlighten me! Now, where did I leave those Tylenol??
Thanks!
Irlandés