I think QNH 1015 was for the question below the one we're discussing. You had me thinking though!

Cheers,
Mister W.

I'm not sure M W... I saw 1015 in the very first post, so now I'm really confused :-S

Hi Abargri,

The actual question in the workbook is Alt: 13800 ft, QNH 1022 and OAT 22 degrees C. This gives the Pressure altitude of 13530 ft. There was a misprint in the first post which mentioned the QNH being 1015. I have edited Nikita's original post to make the QNH 1022.

If the QNH was indeed 1015, your calculation would be correct 🙂

Cheers,

Rich

I thought that we weren't supposed to round until the final answer?
i.e., ISA = 15- (2*13530/1000) = -12.06
ISA Dev = 22- -12.06 = 34.06 degrees
Add elevation 34.06 *120 = 4087.2
Density Altitude = 17617ft
This particular one isn't massively out. But the first one is
My answer
FE 4440ft / QNH 999hPa / OAT15 degrees => PH = 4840ft / ISA TEMP 5.32 / ISA DEV 9.68 / Add alt 1161.6 / =>>> DH=5602ft
But the answer on that sheet is 6060ft. A massive 460 ft difference in density height.

This is entirely due to the rounding of the pressure height before calculations start.

Is that an acceptable error difference? How does one know when to round the PH? Does it go up in 500ft or 250ft increments or 100ft increments?

What about the temperature deviation? Bear in mind, that a 0.5 degree dev from ISA results in a 60ft difference. If you round to the nearest 100ft for pressure height and then to the nearest whole degree for ISA temp deviation (that's 100ft + 0.5*120ft = 110ft) is a 110ft deviation from actual DH. If you round pressure height to the nearest 500ft and the nearest whole degree for ISA temp deviation (500ft + 0.5*120)=560ft deviation from actual DH.

I didn't think that level of accuracy/inaccuracy was acceptable. I thought we weren't supposed to round until the very end.

Help!

[color=blue][i][b]I thought that we weren't supposed to round until the final answer?[/b][/i][/color]

As a general rule approach to things, that's a good idea, noting that, for exam questions, the examiner may use words which vary any generally expected technique.

[color=blue][i][b]I didn't think that level of accuracy/inaccuracy was acceptable.[/b][/i][/color]

Things get a tad messy with atmospheric hydrostatics. The equations are not linear (generally exponential - and these, themselves, are only theoretical approximations to the real world) and the various approximations we use in aviation are just that, and pretty rough and ready at best. Although we use approximations such as 30 ft/hPa, 120 ft/C°, for example, such numbers are very rubbery, especially for pressure and density variations.

The end result is that the answers are fine for the exam as the aim is to check that the candidate has an idea of the story but, in reality, they don't represent anything particularly accurate in the real atmosphere - they are, at best, useful indications of what's going on. You want the correct answer, you need to use the correct equation - which is rather somewhat beyond what would be reasonable for pilot training.

The takeaway is that one ought not to get too fussed about super accuracy in altimetry exam question matters ....

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

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

BOBTAIT ANSWER:
DH= 6060ft.

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

Perhaps, first, you might recheck your sums ?

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.

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.....