Progress Test Topic 2 - Page 2.22 - Question 3
Can you please let me know why the answer to this is [c] and not [a].
If I zero the hands on the altimiter the subscale would read the pressure height of the field.

With regards to density height and charts, if density height is provided, we enter the 'P' chart accordingly and move horizontally to the right till we reach the ISA temperature but for the Echo chart, when we enter with density height, we ignore temperature completely and move up vertically to the next entry argument box. Why is this?

Any help would be appreciated.

Thank you.

Two points to make here. Firstly, if you zero the main scale of the altimeter, the sub-scale will not read the pressure height. It will read the actual pressure at the field. The sub-scale is calibrated in hPa, so it can never read a height.

Also, when you enter the Echo chart at density height, you are not ignoring temperature. You have already accounted for temperature when you calculated the density height. The entry points on the density height scale assume you have already corrected the elevation for both non-standard QNH and non-standard temperature.

Bob

Thanks Bob.
Perhaps just confused with pressure height and QFE for a bit there.

With regards to the Echo chart and denisity height, I understand the reasoning.
Why then do we not do the same for the 'P' chart on page 3.3 and the example on page 3.4? Though we enter with density height, we still calculate the ISA temperature to intersect. Is it just the way the two charts are presented?

That's a good question and there is no answer really. CASA decided to use P charts that actually exist in the real world although they cannot use actual brand names for legal reasons. It just so happens that some companies use the Echo presentation, with an entry argument for density height and some use pressure height and temperature only. CASA didn't invent the P charts, they simply presented charts that do exist in the real world.

For some reason the Americans seem to think that 'Density Height' is a dirty word! Many American charts and tables talk about 'Pressure Height in the standard atmosphere', which is simply density height.

Thank you Bob. Appreciate your reply.