Hi,
My questions concern performing altitude checks while passing the OM inbound on the ILS, True altitude, and true alt calculations when not doing the ILS, yet rather at a high(er) Flight Level and there is an issue of terrain. I know some of this was on the ATPL theory, but it's been awhile and needed some fresh young minds to help me out!
Here is a little 'rule of thumb' formula which will tell you. The most important thing to remember is the temperature datum is the airfield that gave you the temp via ATIS or whatever and that temperature as ISA devn is what counts NOT the ISA devn at cruise alt.
+ or - 4' per degree of ISA devn(ht/1000).
So applying this formula to the above ILS...lets assume temperature 0 degree giving an ISA devn of 15.
- 4 x 15(1.5)
-60 x 1.5= -90'
OK, I understand this example.
My questions are these:
1. What if we are landing at an airport that is not S.L.? Say the elevation is 2000 ft. Do you still do your ISA calculations from 15C? Or do you do the calculation from 11C (2C per 1000 ft). So, you arrive at a deviation of 11C rather than 15C as in the example posted above.
2. What about if you are given a QNH in the ATIS of something other than 1013 millibars? Say, we have a QNH of 993 (20 millibar difference)? Do we also do a calculation with this in addition to the temperature calculation to get a true altitude of the aircraft?
3. Taking this further: What if I am not doing the ILS, but rather at a cruise altitude and crossing the Himalayan mountain range. Temperature at my flight level is ISA - 20C. I assume I would not use this 4 foot rule of thumb, but rather use 27 ft or 30 ft as my "rule of thumb" in the calculation. Example: I am at FL240 and temperature is -53C (which is ISA - 20C). How is this calculation performed and when is the cutoff from using the 4 ft vs. the 30 ft?
Thanks!!!!!!