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Old 24th Jul 2023, 04:34
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Im having some trouble with a few NAV questions, specifically these two.

1. An aircraft is approaching to land and is exactly on the ILS glide slope as it crosses the outer marker. The airport elevation is 2,600ft with a publish OM crossing altitude of 4,200ft. The ATIS gives a QNH of 1013 hPa, temperature +40 degrees Celsius. Which answer best describes the expected indicated altitude as the outer marker is crossed?

A. 4,400ft
B. 4,200ft
C. 3,850ft
D. 4,050ft

2. An aircraft is overflying an aerodrome of elevation 3000ft. The aircraft altitude is 5000ft on an area QNH of 1013 hPa. OAT at 5000ft is +25 degrees. The true height above the aerodrome is:

a. 2000ft
b. 2375ft
c. 1650ft
d. 5375ft

Answer for 1 = D

Answer for 2 = B

Any ideas how to attack these?

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Old 24th Jul 2023, 10:00
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So you know the answer but dont understand why they are correct? I'll have a go at explaining, hope this helps.

Lets say that your altimeter is reading 2000ft, and the temperture is greater than ISA. Your actual altitude will be greater than 2000ft. If the temeprature was below ISA, then your actual altitude is less than 2000ft. For details on how to calculate the correction you should refer to DAP INTRO 2-2 and 2-3. Note that the table also work in reverse (i.e for hot temps)

So to your first question. 40deg is about ISA +25 and you are 1600ft AGL. Using the table from DAP 2-2 you can enter the graph at an aerodrome temp of -10deg (becasue that is ISA-25, remember it also works in reverse). Go along until you hit the 2000ft line then go down until you reach the AGL line, and then go to the right where you should get a temp correction of about 150ft. For the cold temp scenario you would add this value but for the hot temp scenario you take it off. So your indicated height is 4200 - 150(from the correction graph) which gives you 4020ft. answer (d)

For the next question I couldnt get the answer listed. I suspect the answer is incorrect. However it is the only answer that is higher than 2000ft by about the right amount. I got 2200ft at a stretch. So yeah (b). But note that the method for the questions is basically the same...use the graphs!

Note, I do these calcs everyday for a living and just looking at this again (and punching it into my spreadsheets), I am defintely convinced that the answer given for the second question is inaccurate.

Hope this helps, Alpha

Sorry I might add that there is a rough rule of thumb of 4% of the altitude for every 10deg (+ or -) from ISA. So for question 2 its ISA+20. 8% of 5000ft is 400ft....this gets you much closer to the supplied answer...but its not terribly accurate.
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Old 25th Jul 2023, 00:30
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Great explanation above👍. Put simply, if its hot you indicate lower, if its cold you indicate higher (dangerous). Expect the variation to be around 100-200 feet at those sort of temperatures and you will find those answers👍. Question 2 is a good assessment of terminology, a classic RTFQ😉.
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Old 25th Jul 2023, 12:12
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Unfortunately in Oz its irrelevant unless its colder than Isa-15. That's the only time you are required to recalculate minima's due to temperature errors. Another example of a CASA gotcha question that bears no relationship to reality.
That said, a good understanding is nice to have to explain crossing height differences on approach, although this could also be for other reasons, Altimeter errors, etc.
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Old 25th Jul 2023, 22:32
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The question is asking you do you understand what Pressure Altitude, Density Altitude, Height (there is no untrue height) or Absolute Altitude is, what these terms mean and when these are used.
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Old 29th Jul 2023, 05:01
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Gday folks,

Thanks for all your replies - ending up passing with a healthy 92%, so pretty stoked.

Ended up using the CR3 for quick calculations (Above or below glide slope) and used the following formula for more precise true altitude calcs:

Find pressure height - - Elevation + (1013 + QNH) x 30 - - at temperature source (QNH @ Aerodrome OR OAT at aircraft)

Use pressure height to determine ISA DEV - - OAT - (15 x Pressure height x 2 / 1000)

Find true altitude with following formula - - 0.004 x ISA DEV x height above ground (parcel of air) - -

All the best,

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Old 29th Jul 2023, 22:57
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Well done, Excellent work🤙🏼
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Old 29th Jul 2023, 23:31
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Maybe it’s more relevant than ever. With RNP approaches down to DAs close to ILS minima but utilising VNAV, the view at the bottom of the approach can vary a lot based on density altitude on the day,
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