Ground School Exam Questions & Question Banks

[PGT]

Not an easy task, but we try to please

I've been going on about pictures for ages when it comes to these questions, so I'm gonna make you draw a picture.

Draw a horizontal line and label it "QNH sea level 998hpa"
Draw another above it and label that, "height of obstacle". Label the distance between these two lines as 2000ft.
Draw another line above the "height of obstacle" line and label the distance from the "height of abstacle" line to this new line as 1000ft.
Your true distance must be 3000ft above the sea for you to clear the obstacle by 1000ft. This is obvious to see. The ditance from the bottom line, to the top line.
What we need to work out is the indicated distance (why they are different is gonna take an essay, or 2 secs with an instructor)
Use this formulae.
For every 1°C ISA deviation your altimeter will be in error from the true altitude by 4ft in every 1000ft of pressure altitude.
Lets work out pressure altitude (the height above 1013.2hpa) so that we can start to use this formulae.
Draw another line under the "QNH sea level 998" line and label this as "1013.2hpa". The distance between this line and the QNH line is 405ft (27ft x 15 hpa). So the pressure altitude of an aircraft that needs to clear the obstacle is 3405ft. Please don't look at the diagram again cos it'll start to mess with your head. We have all we need now from it.
OK, we have 19°C ISA deviation in the example and we have 3.405 thousands of feet pressure altitude. Now we can use that formulae.
So, 19°C deviation x 4ft gives an error per 1000ft of 76ft. But, we have 3.405 thousands of feet.
So, 76 x 3.405 gives a total error of 258ft.
When it's cold the true altitude (in our case 3000ft) is always less than the indicated altitude, so indicated altitude on QNH must be 3000ft plus 258ft.

Hope this helps.


I did try

Steve Francis