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Density Height question

  • bosh-777
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bosh-777 created the topic: Density Height question

Advanced questions on density height.

5. A certain multi - engine aeroplane has a maximum single - engine ceiling of 5000' density height. What altitude would you expect it to be able to maintain on a day when QNH is 1005 hPa and the ISA DEV is plus 15 ?

- I've figured out how to get the pressure height of 3200 by 15 X 120 = 1800, 5000 - 1800. But i cant seem to figure out how to get the Elevation o 2960 in the answer section.
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Richard replied the topic: Re: Density Height question

Hi Bosh,

The QNH is 1005 hPa which is 8 hPa LOWER pressure than ISA. Since pressure decreases with height at an accepted rate of 1 hPa per 30 ft, a drop in pressure of 8 hPa (1013 - 1005) is the same as an altitude increase of (8 x 30) or 240 ft.

So, the actual altitude that gives the pressure height you calculated must be 240 ft lower than 3200ft.

3200 - 240 = 2960 ft altitude

So, under the conditions of the day, 2960 ft altitude is the equivalent of 5000ft density altitude.

Remember, we are working backwards up the flow chart you'll find in the textbook for working Density Altitude problems.

Cheers,

Rich
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  • bosh-777
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bosh-777 replied the topic: Re: Density Height question

i realised that i had to work backwards just didn't know the right procedure for the right calculation.

thank you for your help :)
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