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One more question to the experts:
When doing 1-in-60 exercises I am having a problem to calculate the New HDG after Intercept (M).
For example:
Given FPT= 273 M, Heading held= 274 M, Distance along Track= 80 nm, Distance off Track = 13 nm left, Distance to re-join Track= 50 nm.
I have found Track Error= 10 , TMG = 263 , Drift = 11 left, Closing angle = 16, TTI = 289, and HDG to Intercept = 300 M.
I just missing the step of how to calculate the new HDG after Intercept.
Hi Nikita,
Now that you have calculated the drift, 11° in this example, you can now apply it to your FPT. This will give you a new HDG which should keep you on track much better than what you originally planned. So we end up with planned track of 273° + 11° to the right = New HDG 284°.
Always found DR nav to be the most fun to do and it has saved my neck more than once when things like GPS and navaids failed.
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
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.
I thought that we weren't supposed to round until the final answer?
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.
I didn't think that level of accuracy/inaccuracy was acceptable.
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 ....
Engineering specialist in aircraft performance and weight control.
