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## Density height

• Posts: 161

### MissSoph created the topic: Density height

Just working through ex 1.4 p 15….
The second line has
Ele 1800
QNH 1027
Oat 26
Working out the isa temp…. 15-2x thousand of feet….
So I used 15-2x1.8
But the working out show using 15-2x1.5
Did I make it too technical for myself ?

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 697

### John.Heddles replied the topic: Density height

You're getting a little ahead of yourself.

(a) work out the pressure height. You can find yourself in strife if you work on the elevation when QNH is well away from standard pressure. Be aware that, when we use the 30 ft/hPa thing, it is wrong for most situations and the further you get away from the height for which it is correct, the larger the error you end up with. But, like many other things, we use 30 ft/hPa and don't fuss too much about the engineering.

(b) then work out the standard temperature. Be aware that the 2 deg/1000ft thing is not quite correct but is not too bad for government business.

(c) then work out the temperature difference between standard and actual, ie ISA deviation.

(d) then work out the density height. Be aware that the 120 ft/deg thing is wrong for most situations but it's not too bad, I guess. Some discussion in this thread (and another hyperlinked from the first)

bobtait.com.au/forum/general-enquiries/6...o-500-feet-wtf#13501

A useful takeaway from this is that we use a lot of rough and ready approximations. There is not much point worrying about figuring sums to the nearer 100 foot level when the techniques just don't justify that sort of accuracy. Hence, we usually just round the height off to the nearer 500 ft which fits in nicely with 2 deg/1000.

Engineering specialist in aircraft performance and weight control.

• Posts: 15

### Qfly replied the topic: Density height

All you have done is miscalculated the ISA temp, you deduced it right when you thought (1.5). In your calculatoin call 1380ft (1.5). The math now would be 15 - (2 x 1.5) = 12.

Always round to the nearest 500ft, if the pressure height is say 1780ft, call this (2000ft) which becomes (2), now to work the ISA temp it would be; 15 - (2 x 2) = 11. Conversely if the pressure height was 1240ft, round this down to 1000ft so it then becomes (1) and so; 15 - (2 x1) = 13.

I use the basic rule that if within 500ft if the ALT is less than 250ft, round down to nearest 1000ft, if it is above 250ft round up to nearest 500ft. It is not exact science but neither are performance calcs, I have added a rough depiction using BT's method. There are a few ways of working out the DA but use what is easiest for you.

Hope this helps