Ok, there is that Appendix 1 to OPS 1.430 which explains how to get minimum RVR/CMV for CAT I and non precision approaches provided you have DH/MDH, slope angle and approach lights system length:
Required RVR/visibility (m) = [(DH/MDH (ft) × 0,3048)/tanα] – length of approach lights (m)
And there is that table containing lowest allowable values of RVR/CMV for instrument approaches with vertical profiles
up to 3.77° for Cat C/D aircraft and various approach lights systems.
Everything goes perfect when you fly your standard 3° GS / DH 200' / RVR 550 m approach to at least CAT I runway.
But here's the question. Let's assume we're flying RNAV(GNSS) approach to LKPR RW24 using LNAV managed guidance only, and reported
RVR is 1200 m.
At DH we will be
412 × 0,3048 / tan(3°) = 2396 m from touchdown,
or 2396 - 1020 =
1376 m from the beginning of the ALS...
MINIMUM? GO AROUND, FLAPS!
Ok, second approach, RVR became
2000 m, but approach lights have failed.
At DH we will be 2396 - 300 =
2096 m from threshold.
MINIMUM? GO AROUND, FLAPS!
Where is the trick?
These minimum RVRs are extracted from that table, which was calculated using slope angle 3.77°. You can check it yourself:
For NALS: RVR = (412 × 0,3048)/tan(3.77°) =
1905 m.
For FALS reduce it by ALS length: 1905 - 720 =
1185 m.
And note that these values include approx. 300 m margin, because ALS starts from RW threshold.
But if you fly 3° approach instead of 3.77°, at the same altitude you will be tan(3.77°)/tan(3°) = 1.26 times further
away from the runway!
So, three questions:
1. Why is that table applicable for approaches with vertical profile
up to 3.77°, and not
"down to"?
2. Why is distance from threshold to touchdown not used in minimum RVR calculation?
3. What am I doing wrong?
