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pressure height calculations for performance charts.

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sandesh.bidari created the topic: pressure height calculations for performance charts.

Hi,

We usually need to calculate PH from given elevation and QNH before plotting the T/O or landing charts with the formula: Elevation+(1013-QNH)*30. My question is, do we sometimes need to change the base QNH i.e. 1013 depending on the OAT?

I came across this following questions while practicing:

-Refer to the cessna landing chart to find landing distance required.
Given,
ELE: 100 ft, QNH: 1030 hpa
OAT: +25
forecast 10kt headwind.
level field
landing weight: 1010 kg.

Now, when i calculate PH it comes as -410 ft. This doesn't seem right to me.
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  • John.Heddles
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John.Heddles replied the topic: pressure height calculations for performance charts.

.. with the formula: Elevation+(1013-QNH)*30.

One of the problems with formula dependency is that it is very easy to lose track of what the formula represents and, I suggest, this is what you have done here. It is always useful to draw a quick picture of what is going on so that the brain can follow the story. Formulae are only good when you have done the hard yards the long way and have an intimate understanding of what the formula is all about.

My question is, do we sometimes need to change the base QNH i.e. 1013 depending on the OAT?

No OAT related changes to QNH are required.

ELE: 100 ft, QNH: 1030 hpa. .. calculate PH it comes as -410 ft. This doesn't seem right to me.

Looks fine to me. How about you try drawing the basic diagram of where the pressure levels are ?

In this case, the 1013 surface is 510 ft above MSL which has QNH = 1030. The elevation is 100 ft above MSL, so the Hp is -510+1-00 = -410 ft

Engineering specialist in aircraft performance and weight control.
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sandesh.bidari replied the topic: pressure height calculations for performance charts.

Hi John,
Thank you so much for your reply. I really appreciate it. I do understand all that. My only question is how do i plot -410ft PH in a landing chart. I thought maybe I am missing some other calculations required for this particular type of question.

I can understand that this might be a silly question but I'm early into my theory studies.
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John.Heddles replied the topic: pressure height calculations for performance charts.

how do i plot -410ft PH in a landing chart.

I would need to see the particular chart to comment in detail. For landing data, it is OK to enter with density height instead of pressure height. Alternatively, round Hp up and (presuming the lowest charted Hp is SL) use 0 feet. This will be conservative

this might be a silly question

Absolutely not the case. As we often say, the only silly question is the one which is not asked.

Engineering specialist in aircraft performance and weight control.
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