How to find altimeter settings for higher altitudes?

Question

I am trying to calculate cruise performance, but how would I find the altimeter setting for a given altitude?

I’ve looked at charts but they only show altimeter settings for the surface.

Would I use the alt settings at the surface and subtract an inch per thousand feet, or does this only apply in std conditions and if it does how would I find that altimeter setting so that I can calculate cruise performance..

Answer 1

There's no such thing as an "altimeter setting for a given altitude." You use the same altimeter setting all the way up to the transition altitude (18000 feet in the US, but it depends where in the world you are).

Performance charts are usually given in terms of pressure or density altitudes. You can use the altimeter setting and your intended cruise altitude to compute a pressure altitude and temperatures aloft to compute a density altitude from there if needed. Then use this to compute performance.

Your cruise altitude is measured in terms of indicated altitude, so if you are cruising at 4500 feet with an altimeter setting of 29.92 inches of mercury, your pressure altitude will always be 4500 feet (ignoring instrumentation error). On a standard day, your true altitude will also be 4500 feet, while on a non-standard day your true altitude may be considerably higher or lower than this.

Answer 2

Use the current altimeter setting at a surface airport within 100 NM of your location and that will be accurate enough for cruise calculations.

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Here is the reasoning behind this. The airplane is flying a pressure altitude of (for example) 8,500 FT. It is not going to be the true altitude above mean sea level as the pressure can change based on many factors. The airplane senses it is at a pressure altitude of 8,500 FT.

Since we are properly using the Kollsman window correctly and setting the current altimeter setting within 100 NM of your location, the difference between a pressure altitude of 8,500 FT and your altitude is the difference in the calculation for the altimeter setting. At altimeter setting of 30.02 indicates you are flying a pressure altitude of 8,400 FT when the altimeter shows 8,500 FT.

The pressure altitude and the temperature at that altitude is what is required for the cruise calculations.