Looks like the RDS has an updated % for this. 3.78%

Its been changed again in the new ERSA. Its now 4.14%

Threw me off there for a bit. Had to do a google to see where I was going wrong.

Keep in mind that the airport authority will commission resurveys at regular intervals. The surveyor generally does the usual x,y,z point survey of all interesting obstacles and then runs a calculation to convert that discrete data to obstacle gradient slopes.

It follows that the ERSA data will change at each survey (and for problems which arise between routine surveys). For the EOS surface, you are looking at all the close-in obstructions - fences, trees, power lines and so forth. Tree growth and the like is the principal cause of increasing slope gradients. This is why there are powers for some airports to require landowners to maintain growth within regulated limits.

The ERSA slopes are a very simplistic operational form of data which presents a variety of difficulties for pilot use, especially with larger aircraft above 5700 kg, for which the AFM data is geared to discrete obstacles. Should you want to use discrete data, you can obtain that from the airport or the airport's surveying folk or, for a little loss of accuracy and a bit of conservatism, you can run a simple trigonometric set of calculations to determine a quasi-discrete obstacle set from the declared ERSA gradients.

Generally, for light aircraft, the simple gradient approach works satisfactorily and it was for this purpose that the ERSA data was set up as gradients rather than the sort of discrete data you get with Type A charts for the larger airports which serve heavy aircraft.

Hi, I have a question on Question 38 of CPL Final Test (p.241) (see attachments below):

1. Why is the answer 3.52%?
Looking at the ERSA, I would have thought that the obstacle free gradient is 4.14%...
2. Also, I am not very sure what is meant by "RDS has an updated % for this. 3.78%" in this post. RDS for which aerodrome?

Sorry if the questions are silly, I am struggling a little with this subject (and with air law...)

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Link to ERSA:
www.airservicesaustralia.com/aip/aip.asp...date=30NOV2023&ver=2

The answer was correct until Air Services changed the entry in the ERSA, something they do quite often and is difficult to keep updated, we do the best we can but sometimes things get through. I have updated the Question to reflect the change, I don't know how it was missed for so long.

"RDS has an updated % for this. 3.78%" That means the entry has been updated in the ERSA.

The answer is 4.14%


Cheers

Thanks so much, Stuart!
I think it would help if the answer key shows how the figure is extracted from the ERSA (maybe like a screen shot of ERSA with the answer circled).
Then even if the ERSA is updated, we would still know how to arrive to the answer.