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.
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
i realised that i had to work backwards just didn't know the right procedure for the right calculation.
thank you for your help 🙂
