OAS constants - anomaly

14.06.2006

Submission:

As part of ILS work that I did recently I had to determine the Obstacle Assessment Surfaces. I used the ICAO Pans-Ops software. I then, for some reason decided to check the constants obtained from the software against the values in the old Pans-Ops green pages and I noticed a slight difference.

Parameters used:

LLZ to threshold: 2000
Glide path angle: 2.5 degrees
Missed approach: 3 degrees
ILS category: II

The value obtained:

By constant value:
Pans-Ops software value: 0.242328
Green pages value: 0.242329

I know the difference is small but I was just wondering why?
Also will this affect the ILS results that I calculate and which is correct?

Ian Whitworth

Answer or Commentary:

There are small and non-significant differences between the OAS constants in PANS-OPS Volume II (pre-Amendment 12) and those obtained from the OAS calculator now included on the CD ROM in Amendment 12. These differences (+/- 0.000001) affect about 2% of the A and B constants.

They arise because the original FORTRAN program used to generate the Table I values made the calculations in a different order to the order needed for printing the results. Because of memory limitations in early computers, the results were written to an external file and then read back for printing. Unfortunately this resulted in the results being rounded twice, first to 7 or 8 decimal places (depending on the constant) and then to the 6 decimal places for the printout. Any mathematician will tell you that this is a mathematical no-no and will produce exactly the anomalies observed. The PC version of the program also had the same fault - with the additional complication that the errors are also dependent on the FORTRAN compiler used.

The PANS-OPS OAS program contained on the ICAO CD-ROM stores the results to the full precision of the computer, and only rounds the results for printing. Thus, the ICAO CD-ROM is correct, and both the PC version and Table I suffer from these occasional small errors.

Ralph Sexton - Procedure design instructor - USA