However, in PANS-OPS reference is still made to the software for ILS procedure design.
The current version is 3.3. For ILS, as far as I have seen, the software performs ok. For APV procedures not so much. We can ignore APV II as that concept does not exist anymore.
In APV I we have the following problems:
1. The C coefficient of the W' surface is wrong. It is calculated CW'=-50+RDH*tan(0.75VPA)/tanVPA. Whatever the VPA the result will always be the same at a given RDH. At an RDH of 15m this value is off by 1m.
2. As a result of the above the X coordinates of point C is also wrong. That coordinate is calculated X=|CW'|/AW', (C of W' as absolute value). The Y coordinate is also wrong of course. The Y of point C however needs to be calculated with the X surface coefficients. Y=(Z-C-AX)/B, where A, B, C are X surface coefficients and X is the X of point C that you calculated above.
Important note: Whoever doesn't get that math and is not familiar with the surface equation:a) leave the OAS surfaces alone!!!b) book the ILS Master Class or the advanced PBN course with us.
Then recalculate X/Y of point C as described above.
3. When the RDH is NOT 15m, there is a correction algorithm that moves all surfaces up and down by the delta of the RDH. That is the correct way for all surfaces except the W'. That one must be calculated in a different way:
AW'=tan(0.75VPA)
CW'=–50 + RDH tan(0.75θ)/tan(θ) for APV I
4. As a result of 3) above, the intersection of the W' and the W surface must be recalculated (point C""):
X=(CW-CW')/(AW'-AW).
Y=(Z-C-AX)/Y, where A,B,C are surface coefficients of the X surface, X is the X of point C"" as calculated just before.
Z=AX+C, where A,C are either the W or the W' surface coefficients.
