help with a tech question

[Checkboard]

The question is asking you to make a temperature correction to the indicated altitude to find the true altitude.

If you have set the local QNH, then the altimeter will read correctly at the field elevation, in this case 2500'. As you are indicating 10,000' the correction applies to an indicated height of 7500'.

At 10,000' ISA temperature is -5°C, your temperature is -20°C, so today is ISA - 15°.

The correction factor is: 4' per °C per 1000 feet.

In your case this is 4 x 15 x 7.5 = 450 feet.

As the air is colder, it is more dense than standard, so the pressure will drop off faster with altitude than standard. This means that the altimeter's 10.000 feet will be lower in the atmospere than standard, so the correciton is negative. i.e the altimeter will overread - temperature low, look out below!

True altitude: 7,500 feet - 450 feet = 7,050 feet above the station. This is your true height above the airport, but the quesiton is asking for a true altitude, so you must now add the airport elevation you removed before.

7,050 feet + 2,500 feet = 9,550 feet true altitude.

So today your altimeter says that you're flying at 10,000 feet, above a 2,500' airport, which would tend to make you think that you are 7,500 feet above the airport when you are really only 7,050 feet above that airport. If the chart showed a 9,000 foot hill between you and the airport, you won't have 1000 foot clearance (as you might expect) but only 550 feet.

This is one of the reasons that Grid MORA's shown on charts give 2000, clearance above terrain when the terrain is higher than 5,001 feet, but only 1,000 feet when the terrain is less than 5,000 feet (on Jeppesen supplied charts at any rate).