EU-OPS new RVR minima

[bookworm]

I'm puzzled. Here's EU-OPS (http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2008:254:0001:0238:EN:PDF), Appendix 1 (New) to OPS 1.430 starts at page 70.

(d) Determination of RVR/CMV/Visibility minima for Category I, APV and non-precision approach operations

1. The minimum RVR/CMV/Visibility shall be the highest of the values derived from Table 5 or Table 6 but not greater than the maximum values shown in Table 6 where applicable

2. The values in Table 5 are derived from the formula below.

Required RVR/visibility (m) = [(DH/MDH (ft) × 0,3048)/tanα] – length of approach lights (m)

Note 1: α is the calculation angle, being a default value of 3,00 degrees increasing in steps

It all looks very logical. The required RVR is effectively the distance of the nearest approach light at the DH on the glideslope (or nominal glideslope for a CDFA).

But there's a catch. Table 5 does not appear to use that formula with a consistent value of α = 3,00 degrees. Using α = 3,00 degrees,

Required RVR/visibility (m) = [DH/MDH (ft) × 5.816] – length of approach lights (m)

At a DH of 200 ft, the formula gives RVR = 1163 m – length of approach lights. Looking at the table, no approach light system (NALS) gives 1200 m, which is believable, and a FALS gives 550m, which assumes 720 m of approach light. OK, so far so good.

But now try some higher DHs. Let's try 500 ft. The formula gives RVR = 2908 m – length of approach lights. But Table 5 gives 2300 m for NALS, and 1500 m for FALS. Those are much lower than expected, by 600 m or so. At 1000 ft the formula gives RVR = 5816 m – length of approach lights, but Table 5 gives 4500 for NALS and 3800 for FALS.

What's going on? Why are the numbers in the table not consistent with the formula? Something to do with "being a default value of 3,00 degrees increasing in steps"? If so, what on earth does that mean?

[bookworm]

No replies?

I think I've tracked down the discrepancy. EASA OPS Part-CAT (http://www.easa.europa.eu/rulemaking/docs/crd/part-ops/CRD%20b.3%20-%20Resulting%20text%20of%20Part-CAT%20(A,H)-corrigendum-1.pdf) (status is a CRD) says at AMC1-CAT.OP.AH.110 Aerodrome operating minima paragraph 4:

"The values in Table 5 should be derived from the formula below,

Required RVR/VIS (m) = [(DH/MDH (ft) x 0.3048) / tanα] - length of
approach lights (m)

where α is the calculation angle, being a default value of 3.00° increasing in
steps of 0.10° for each line in Table 5 up to 3.77° and then remains constant."

In other words it looks like a line was omitted from the EU-OPS text. And indeed table 5 is consistent with the use of:

DH.........α
200-210 3.0
211-220 3.1
221-230 3.2
231-240 3.3
241-250 3.4
251-260 3.5
261-280 3.6
281-300 3.7
301+.....3.77

But that simply demands the question "why is α chosen like that?" Surely the nominal glideslope is 3 degrees??

[keithl]

Bookworm, perhaps the difficulty is in the meaning of words, rather than the calculation technicalities. In other words, in EU-OPS "english", they use "default" when they mean "nominal".

I haven't read the original, just replying to your quoted text - but I would ask, "Why 3 deg "increasing in steps"? Specific approaches, especially NPAs, can be much less than 3 deg (e.g. Wick VOR 2.5 deg).

[bookworm]

Thanks for the reply Keith. The odd thing is that α doesn't seem to be related to the angle of the vertical profile. Rather it depends on the DH.

Para 3 does say "With the approval of the Authority, the formula may be used with the actual approach slope and/or the actual length of the approach lights for a particular runway."

But why, on a 3 degree vertical profile, would you use 3.77 degrees in this calculation for a DH of 300 ft, but 3.00 degrees for a DH of 200 ft?

[keithl]

Ugh! I see what you mean.
Beyond me - I'm out!

[RAT 5]

I suggest you address this to your in-house technical pilot, copy C.P & HOT and come back to us with the answer. I suspect it will take quite a while after much head scratching to try and understand the question.

[hvogt]

Interesting thread! Unfortunately, I don't fully understand the relationship between DH or MDH and alpha either, so I'm afraid I can't contribute too much to the discussion.

For what it's worth the angle of 3.77 degrees is equivalent to a gradient of 6.5 per cent which would be the maximum final approach gradient according to PANS-OPS.

[bookworm]

I emailed the original question to my contact at (UK CAA) Flight Ops. I'll let you know if anyone anywhere owns up to understanding this. ;)

[mgTF]

Anyone has come to any conclusion regarding this topic?

I'll make you an example and let's see who gets the right answer..

Airport in Russia, no euops/standard minima written on jeppesen chart.

Vor/dme, mdh 374, descent angle 2.7*, HIRL=720mt therefore FALS.

What's the required RVR/CMV?

Approach flown using CDFA technique, but not APV being on QFE operation and therefore snowflake being unreliable...