Hi all. Just want to clarify please, question 1 on page 4.3 talks about an ISA+25 day and a climb to 5000'.

I calculated the DA correctly as 7400ft and then used that for all my climb cruise calcs but the answers were wrong because in the book the answer uses 5000' straight from the charts. Why would we not use DA as this is the 'performance' altitude of the a/c?

My answers were 4.9min, 9.1 miles, 2.6 gallons (10 litres) - all wrong

Thanks, Brenton

Hi Brenton,

On the climb through to 7400ft you will have better climb performance at sea level and relatively poorer performance as you approach 7400ft. So, which figures from this table should you use?

Sea Level figures will be too optimistic and those for 10000 will be too pessimistic.

So, we take an average figure for the climb that's about midway between the density altitude of 2400 ft and the 7400 ft mark. The midway point is 4900 ft which is close enough to the figures for 5000ft.

That's why the problem uses the 5000ft figures. It's not using the PA but rather the figures closest to a point half way up the climb. In this example it just happens to be the same as the PA.

Have another read of the worked answer on page 4.53 and see if that makes it any clearer.

Cheers,

Rich

Again Richard, thanks. I am going to have to really 'wade through' this and get my head around the interpolations ie: when to look at which altitude? I certainly can see your logic and with extra 'hours' hopefully the penny will drop.