cougar

This statement isn't correct. From AC 120-29A:

6.2.7. Continuing Category I or Category II Approaches in Deteriorating Weather Conditions.
The following procedures are considered acceptable in the event that weather conditions are reported to drop below the applicable
Category I or II minima after an aircraft has passed the final approach point or final approach fix, as applicable
(reference section 121.651).

a. Operations based on a DA(H) may continue to the DA(H) and then land, if the specified visual reference is
subsequently established by the pilot no later than the DA(H).

b. Operations based on an MDA(H) may continue to the MDA(H), and then to the point of intercept of the
VNAV path to the runway, to the VDP, or equivalent, or to the MAP, as applicable, then land, if the specified visual
reference is established by the pilot no later than point at which descent below the MDA(H) commences.

NOTE: For wind constraint applicability on final approach see paragraph 6.2.11.

For approaches involving DA(H)/MDA, where a decision is made(i.e. you have to see the some part of the approach lighting at minimums), once established on the final segment, the approach can continue to minimums even if the WX drops to 0/0. This applies to both Cat I and Cat II approaches. Cat III is carrier specific as defined in their Ops Spec C060. If at the DA(H) and see the approach lighting system, then you have the required visibility, regardless of what is reported.

Back to 91.175 (c) (3) (i)i) "The approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable."

The statement is two-part, which means you can't descend below 100' with the approach lights as the sole reference. But you can descend TO 100' with only the approach lights.

Assume a Cat I approach with 2400' RVR required and a 200' DA. Using Denver's logic, if the WX was actually at 2400 RVR, you would never be able to land. Looking at the geometry of the aircraft when it is at the 200' minimums the distance from the runway can be determined. Assuming a 3 degree glide path, at 200', the aircraft would be 3800' from touchdown, and considering a 50' TCH, the aircraft would be approximately 2800' from the end of the runway. Using a Cat I with 1800 RVR mins would magnify the issue even more. With reported vis at minimums, your slant range at 200' would always be greater than the minimum published RVR in these two examples.

But at a 200' DA, the above 91.175 reference allows a descent to 100' if the approach lights are in sight.

At 100' the approximate slant range to the runway would be 2800/2=1400'. At that point the crew needs to have something from the "runway environment" in sight to continue.

Denver

The confusion here I imagine is because we are assuming a standard 200 and a half ILS where you are going to have visual reference if you are at or above published minimum flight visibility at DH. The geometry works out as such and they designed it this way on purpose.

So now imagine the Reno ILS when your flight visibility you have determined at DH is 6 miles and the minimums are 7 miles but you barely have the approach lights in sight and lets say this distance is 6 miles from the aircraft. The way I read 91.175 is that you cannot descend below this very high 2200 or so foot DH because your flight visibility is below that stated on the approach plate. The other side of the argument is that you can descend down to 100 feet above but just cannot continue further if it is still under 7 miles.

In other words 3(i) is simply one of the other 10 items of required visual reference but has it's separate clause about a limitation on using the approach lights. I do not read anywhere that it says you can ignore the visibility if you can see them. In other words 3(i) is not an exemption to (c)(2) but I think many read it as such.

Correct me if I am wrong but 6.2.7 just talks about the requirements to continue an approach once beyond the FAF with new weather that is now below minimums. 91.175 C2 still governs once at DH for the visual portion.


Note that between (c) (2) and (3) the word "and" exists and not the word "or"


(c) Operation below DA/ DH or MDA. Except as provided in paragraph (l) of this section, where a DA/DH or MDA is applicable, no pilot may operate an aircraft, except a military aircraft of the United States, below the authorized MDA or continue an approach below the authorized DA/DH unless—

(1) The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and for operations conducted under part 121 or part 135 unless that descent rate will allow touchdown to occur within the touchdown zone of the runway of intended landing;

(2) The flight visibility is not less than the visibility prescribed in the standard instrument approach being used; and

(3) Except for a Category II or Category III approach where any necessary visual reference requirements are specified by the Administrator, at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

(i) The approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable.

(ii) The threshold.

(iii) The threshold markings.

(iv) The threshold lights.

(v) The runway end identifier lights.

(vi) The visual approach slope indicator.

(vii) The touchdown zone or touchdown zone markings.

(viii) The touchdown zone lights.

(ix) The runway or runway markings.

(x) The runway lights.

buckeye88

Yes it is legal. The approach light system is one of the visual references that allows you to descend below DH.

Upon reaching DH the approach lights are in sight. At this point you are in position to land (hopefully) and you have the required visibility for the approach to identify a visual reference which allows you to descend below DH.

Since the approach lights are not a runway visual there's the restriction of the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable.

Now note the visual references must be distinctly visible and identifiable to the pilot. Only seeing one bar of an approach light system in my opinion wouldn't be distinctly visible and would necessitate a missed approach.

Ominous

When have you ever been at 2200' AGL with 6 miles vis (which I would consider 'good'), and ONLY seen the rabbit or approach lights? An unrealistic (I didn't say impossible) situation.

When have you done a CAT I appch with 1/4 mile vis and VV100, only to see the rabbit at mins? Probably quite a few times!

Denver, I appreciate your wanting to dissect the regs and have everything as legal as possible, but i am not sure you will find a black and white answer to your very specific question. Using good judgement in your scenario is a better approach than worrying about finite details of the regs, IMO. I can't wait to see you try and pick through the regs concerning climb gradient requirements for part 91 turbojets!

Denver

No I know. I am over thinking again. The structure of an IAP is set up so that if you have the minimum visibility, than in all likely hood you will also have required visual reference. So the point is kind of ridiculous. I was using this more from the point of an interview question in which the Reno ILS is a popular one but I forgot we are working in statute miles here and in this case you would have it.

So we can say that at DH or MDA if you see the approach lights than that means you must have by default the required visibility as well so you may continue. Inside of 100 above TDZ you must be able to see according to foreward flight visibility as well as the new references ahead of you (side rows, etc.). After all when flaring to land you can have the required visual references but only able to see 500 feet in front of you which would be illegal to land with.

So I get why it is worded in this way now. 91.175 does not explicitly say you can descend below DH/MDA with lower than minimums visibility because it does not have to. This would only apply when you are close enough or over these references like for the landing.

Anytime TERPS deems it necessary to raise or to have a higher than 200' DH on an ILS due to see and avoid obstructions in the visual segment the visibility also needs to go up accordingly so the pilot can see the approach lights at DH and for a NP approach w/o lights the runway environment.

It makes sense now.

Now what was that about part 91 turbojets now?

501D22G

200 feet, lights in sight

The ILS 16R at Reno is not an ILS.

This is an ILS.

USMCFLYR

I take it that you are just making fun of the more simple nature of the Cat I Reno ILS than the Cat III DFW?
But on the internet you just never know.

USMCFLYR

501D22G

No,

I thought the one I linked to was simply a category I ILS 35R @ DFW. But the minima are 1800 RVR and 200 feet.

I also put the link to that article for the OP.

Think about 91.175 in regards to a 2031 ft /7sm ILS and then more pertinently a 200 /18 approach.

USMCFLYR

You linked the Cat I approach plate at DFW, but the ILS system in place based off AJQ, is a Cat III/E performance capable system. The RNO ILS is a Cat I system with only a performance capability of I/D (not nearly as good).

I liked the article.
Explains the concept in a pretty straight forward manner.
Hope that helps Denver (and the rest of us too).

USMCFLYR

Denver

Yes it does, that is a great article. It becomes apparent that if you do not have the minimum required forward flight visibility you will also not have required visual reference. I think they could leave out (c)(2) since they already have the landing visibility minimums part further down. Makes for a confusing read the way it is written but it is probably just legal CYA and such.