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## PPL Worksheets

• JamesScot

### JamesScot replied the topic: PPL Worksheets

Ok, Thanks. I can appreciate that.

So in relation to the question I posed regarding the difference in answers from the worksheet.

QUESTION:
FE 4440ft / QNH 999hPa / OAT15 degrees

PH = 4840ft / ISA TEMP 5.32 / ISA DEV 9.68 / Add alt 1161.6 / =>>> DH=5602ft

DH= 6060ft.

A 460 ft difference in density height.
That's a significant difference. Would that be ok for the exam??

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 843

### John.Heddles replied the topic: PPL Worksheets

Perhaps, first, you might recheck your sums ?

Engineering specialist in aircraft performance and weight control.

• JamesScot

### JamesScot replied the topic: PPL Worksheets

Yes well, I am an idiot, I realised that I added the altitude correction to the field elevation rather than the pressure height.

Even so, the difference is still 34ft by using the decimal temperature values, rather than rounding them (or rounding the pressure height).

Is that an acceptable margin?
I've attached the working.
##### Attachments:

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 843

### John.Heddles replied the topic: PPL Worksheets

Don't be too hard on yourself - we all make mistakes. What the pilot needs is to see the mistake, acknowledge it, and fix it without the need for someone else to notice it or, even, remark on it.

34 ft ? Mere drop in the bucket.

2C/1000 is an approximation to the theoretical model, 30 ft/hPa is a ludicrous approximation other than for low levels, 120 ft/deg is an approximation, and so we could go on and on.

Bob's numbers are fine for the altimetry exam stuff but just keep in mind that they are very rough and ready in the real world.

You might run your temperatures to half a degree, and heights to the foot (only for fun) but any more than that is a bit like measuring the distance from Melbourne to Sydney with cigarette papers.....

Engineering specialist in aircraft performance and weight control.