I have searched long and hard for a user friendly explanation of PEC. In the end I worked it out and felt that I had to share it with anyone else in search of the same. Hope this helps
someone!!....
The
RATE at which pressure decreases, decreases with height. exponentially.
The
RATE at which pressure decreases, changes with a change in mean temperature of the air directly above the measuring point. That is to say if the temperature on the ground is + or - 1 deg C from ISA the
RATE that it decreases in pressure from that point is changed.
Altimeters are calibrated using ISA ~ +15 deg C and they show a decrease in pressure (or increase in altitude) at a fixed rate calibrated to that temperature. At least for a while! as it becomes increasingly less accurate at displaying a true altitude as it climbs. But because we are all using the same calibration we are all experiencing the same error.
The rate at which pressure decreases with height, INCREASES with a decrease in mean temperature.
That is, the pressure of a parcel of air will drop to a particular pressure at a lower true altitude than that of a parcel of warmer air. The pressure read by your altimeter on an ISA day at a TRUE altitude of 1,000ft will indicate 1,000ft but on a cold day that same amount of pressure required to indicate 1,000ft on your altimeter is actually found at a true altitude of say 900ft. So if you were to trust your altimeter you would be flying dangerously low. hence the pressure error correction.
Often a PEC table can be found in an A/C flight manual and used to determine height corrections for terrain clearance or DA adjustments. However i imagine most operators in Australia will just use a fixed buffer to save unnecessary calculations having to be made in flight, as it rarely gets
that cold here in sunny Aus!