QNH/True altitude checks

[chimichanga]

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!!!!!!

[Piltdown Man]

You are really making a correction for the error in layer of air between you (assuming you are where the altimeter is) and the ground. If you need to know true altitude, you have to make a correction whenever the temperature is non-ISA, both above and below. But the one that kills you (quickest) is when the temperature is below ISA and you are close to the ground. In our company we may correct for any non ISA temperature but we HAVE to correct for temperatures equal to or less than ISA-15˚. When making corrections for flying over high terrain, technically only the distance between the surface and the altimeter needs correcting however I think most people would correct the entire distance between MSL and the altimeter - if for no other reason than you don't know the elevation of the surface we are flying over.

PM

[chimichanga]

PD Man: Thanks for taking the time to explain.

So, let's assume I am checking the altimeter while over the OM while doing the ILS.
1. The airport sits at 2000 FT MSL.
2. According to my approach plate, it states that while on the ILS, I should cross the OM at 3500 MSL.
3. I listen to the ATIS and it tells me that the temperature is -25C at the airport.
4. QNH is 995 millibars.

I had a friend ask me what my altimeter would read in this scenario. My answer was that my altimeter is going to overread (thus my aircraft is actually physically lower to the ground than realised).

As for the exact numbers (how much will my altimeter overread), can you or anybody offer guidance?

[Piltdown Man]

Altimeter over-reads by (3,500 - 2,000)/1,000 × 4 x ( -25˚C - (ISA @ MSL 15˚C - 4 (altitude correction @ 2˚/1,000'))) = 216 ft.

PM

[chimichanga]

Cool, thanks a bunch!