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Density Height

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rhysmor00 created the topic: Density Height

Hi All,

During my first CFPA exam attempt I failed this question and I am having trouble understanding why, seeing as it is such a basic question to fail.

QNH = 1028hPa
Shade Temperature = 35c
Strip Elevation = 1500ft

After calculation, the answer is 3810ft. I chose the closest, as it asked for the closest,at 3800ft.
Why am I wrong?

Thanks in advance.
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  • John.Heddles
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John.Heddles replied the topic: Density Height

Perhaps you might post your solution to the forum for comment ?

Engineering specialist in aircraft performance and weight control.
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rhysmor00 replied the topic: Density Height

Density Height = (1500 + (1013-1028) x 30) + (35 - (15-(2x1.5)) x 120)
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John.Heddles replied the topic: Density Height

Boy, but my head is still reeling from the equation .....

Side comment - it doesn't matter all that much, I guess, but you have more brackets, there, than you can poke a stick at. If you want to play with equations, consider making them as simple as reasonably feasible ? Brackets can foul up arithmetic calculations really quickly if you don't nit pick along the way when you set the equation up.

Most folks who know me, know that I detest long equations if I can run the sums other (simpler) ways -

(a) long equations make it very easy to make mistakes and even harder to spot them most of the time. Those of us with any computer programming background know just what a curse coding equations and debugging the inevitable errors is ..

(b) if you can break the calculation up into stages, especially with a picture or two along the way ... makes things much easier for dumbos like me.

So, when I have a look at your equation, a couple of things jump out at me -

(a) if I run your equation, as it is, I get an answer which, probably, is not what you intended - indeed, a long way from what you intended ? Perhaps you might revisit your equation and see if you have made an error with brackets along the way ? When I make the pertinent correction, the answer is far more reasonable and agrees with your answer - still wrong, though.

(b) if I have a second look at the temperature deviation correction component I am wondering if you might have another look at the numbers and consider what the pressure height (from which you are figuring the temperature correction) might be ? A good point for why the odd picture and running calculation elements sequentially helps the brain figure out what's going on. Again, when I make the pertinent correction, the answer is even a bit more reasonable.

Once you have done both the above suggested corrections, you probably might find yourself closer to the question's preferred answer ?

Keep in mind that -

(a) 120 ft/deg is only an approximation anyway

(b) the usual calculations run the atmospherics as being dry which imposes yet another error - the usual pilot calculation being run here ignores humidity.

(c) if you use the ISA calculation equation, things get more complicated along the way - fortunately, we don't go to those sort of extremes so we don't get too much brain strain.

Engineering specialist in aircraft performance and weight control.
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Qfly replied the topic: Density Height

Your working looks sound except for an extra 120ft in the calculation.

The PH is (1013 - 1028 = - 15 x 30 = - 450ft). (1500 - 450 = 1050ft)
PH is 1050ft (round down to (1). Seems you put this in as 1.5 (1500ft).

15 - (2 x 1) = 13. 35 - 13 = 22 x 120 = 2640ft. (2640 + 1050) = 3690. DA = 3690ft.

Hope this helps, see diagram using Bob's method.
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Qfly replied the topic: Density Height

Your working looks sound except for an extra 120ft in the calculation.

The PH is (1013 - 1028 = - 15 x 30 = - 450ft). (1500 - 450 = 1050ft)
PH is 1050ft (round down to (1). Seems you put this in as 1.5 (1500ft).

15 - (2 x 1) = 13. 35 - 13 = 22 x 120 = 2640ft. (2640 + 1050) = 3690. DA = 3690ft.

Hope this helps, see diagram using Bob's method.

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Carello replied the topic: Density Height

When I first started learning how to calculate PH and DH we would start with a simple diagram.


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John.Heddles replied the topic: Density Height

we would start with a simple diagram

Absolutely the way to go (even if the yacht cartoonist has no idea about wind and sail trimming).

A sketch and a think about what is going on usually is a winner ...

Engineering specialist in aircraft performance and weight control.
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