Wednesday, July 27, 2005

American Visibility Limits

In response to my posting about minima, John commented in part:

In the US, if you can see the runway environment or the approach lights, you can descend down to 100' above the touchdown zone (lower under certain conditions). Visibility is what determines whether or not you can land, so visibility is much more than advisory - it's the controlling factor.

Maybe I misunderstood your comment or things are different in Canada?

I looked up the American regulation John cited (14 CFR 91.175 - albeit in my 1998 FAR/AIM) to discover that things are different in Canada.

Canada forbids descent below decision height before "the required visual reference has been established and maintained in order to complete a safe landing". That part isn't different. Both countries require "at least one of the following visual references for the intended runway [to be] distinctly visible and identifiable to the pilot" and follow that sentence with almost identical lists of ten things that constitute the runway environment.

Canada United States
(a) the runway or runway markings(ix) the runway or runway markings
(b) the runway threshold or threshold markings(ii) the threshold
(iii) the threshold markings
(c) the touchdown zone or touchdown zone markings(vii) the touchdown zone or touchdown zone markings
(d) the approach lights(i) the approach light system [...]*
(e) the approach slope indicator system(vi) the visual approach slope indicator
(f) the runway identification lights(v) the runway end identifier lights
(g) the threshold and runway end lights(iv) the threshold lights
(h) the touchdown zone light(viii) the touchdown zone lights
(i) the parallel runway edge lights(x) the runway lights
(j) the runway centre line lights

The order and grouping of the items differs between countries so in the table above I have rearranged the American list (but kept its original numbering) to make it easier to see that it matches the Canadian one. The differences like "runway identification lights" and "runway end identifier lights" are trivial. The major difference is in what I elided on the starred item.

The American reference adds "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." This makes the American regulation more restrictive in theory, because it adds a second decision height. What are the calls in a two crew environment in this situation? "Minima, approach lights only" then "100 feet, in sight" or "100 feet, no red bars"?

That was surprising, but not nearly as surprising as the rest of the section. Ther is a fundamental difference in our instrument approach rules.

In Canada, that's it. You're set up to land, you reach decision height, you see the runway, you are allowed to land. The approach plates give both a decision height and a visibility, but as the AIP says, "published landing visibilities are advisory only. These values are indicative of visibilities which, if prevailing at the time of approach, shouold result in required visual reference being established. They are not limiting and are intended to be used by pilots only to judge the probability of a successful landing when compared against available visibility reports at the aerodrome to which an instrument approach is being carried out." They are also used in determining a legal alternate, but that is at least one posting in itself. (The Americans won't believe our alternate rules.)

American pilots are restricted by visibility and ceiling, all the way to landing. They are not allowed to continue an approach if the flight visibility is less than the one written on the plate. They are, in a separate paragraph, forbidden to land if the visibility is below the prescribed one. I'm looking at the plate for the ILS/DME rwy 27 into Rock Springs, Wyoming. DH is 200' agl and the prescribed visibility is 1/2 mile. Lets say a pilot has completed a difficult approach through visibility of half a mile in snow showers, and is now over the runway approaching the flare. The snow shower intensifies and blowing snow drops his forward visibility such that he can see just four runways lights ahead on each side. He's perfectly aligned, the airplane is under control, but he knows that runway lights are 200' apart so that his visibility is now below a thousand feet, nowhere near the requirement printed on the plate. Is he seriously required to go around? Seems crazy to me. At that point in the flight the pilot should be able to decide what is the safest course of action.

By the way, American official publications actually use the word "minimums." It's not just an artifact of illiterate chief pilots, to go with "it's" being used as the possessive. Canadian pubs always use "minima" but I've just spotted the phrase "Category D circling minima applies." It appears that "minima" is becoming a singular noun for pilots.

I will save the radical difference between the Canadian approach ban and the American stipulations of CFR 135.225 for another post.


Anonymous said...

Q, we all know Americans are mad;-)

Avimentor said...
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Avimentor said...

he part about the red terminating or side row bars bears some explaining.

The only approach lighting systems in the US that have those bars are ASLF-1 or ASLF-2 systems. These lighting systems are pretty rare (usually installed where CAT-II or CAT-III approaches are supported). So in most cases, a pilot cannot descend more than 100' above the touchdown zone just the approach light system.

Approach lighting systems are so important that the visibility requirements increase if the approach lights are inoperative.

Again, under the US regulations visibility control whether or not you can land (legally). Ceiling is an important consideration for determining when an alternate airport is required and which airports can be used as alternates. Another long subject ...

Sorry for adding that stuff about CAT-II and CAT-III approaches. More grist for the mill, eh?

In your example, 14 CFR 91.3 allows the pilot as "the final authority" to make the decision to land. Of course, s/he might have some explanining to do ...

Aviatrix said...

Thank you, John. My CFS is still open to the lighting page as I was looking through all the lighting types going "red bars?" and in Canada too, found them only in the rare ALSF-2 system. I've landed at airports that have them, but haven't noticed the red bars. However the CFS notes that ALSF-2 "may be operated as SSALR during favourable weather conditions."

Will ATC tell someone on approach, inside the FAF, if the RVR drops below that published for the approach? What do they say?

Old Blind Dog said...
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Old Blind Dog said...
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Old Blind Dog said...

You said:
American pilots are restricted by visibility and ceiling, all the way to landing. They are not allowed to continue an approach if the flight visibility is less than the one written on the plate. They are, in a separate paragraph, forbidden to land if the visibility is below the prescribed one.

I think that you ought to point out that the restriction is applicable to part 135 or 121 operations but not to part 91 corporate or private operations. Also, the reported visibility or RVR is only controlling prior to commencement of approach. Once the approach commences (the aircraft has passed the FAF inbound), flight visibility is controlling and the approach can be completed. So, in your example, the pilot can continue as long as he/she has the visibility criteria spelled out by the regulations (the runway or runway "environment"), which you listed.

For operations under part 91 I can commence the approach no matter what the reported weather.


Sorry for the multiple post delete.

Anonymous said...

I have an posting on the approach ban here. I haven't tackled alternate minima yet (or takeoff minima, for that matter). Another poster already mentioned that most or all of the U.S. regs do not apply to part 91 operations -- they can take off (or attempt an approach) legally even in 0/0 conditions (imagine taking off when you cannot even see the edges of the runway!).

Avimentor said...

The tower controller may very will tell a pilot if the RVR drops. The pilot may also ask for updates.

I did just this going into a Central Valley airport in March in a Cirrus SR22. As we approached an initial approach fix, the ATIS reported a touchdown RVR of 800 feet and a roll-out RVR or 1000 feet and an indefinite ceiling of 100 feet.

We were operating under part 91, so we were legal to commence the approach. As we were vectored for a downwind leg to the ILS, we asked the approach controller for an RVR update. In just a few minutes, the touchdown and roll-out RVR had both increased and the controller said "touchdown RVR 1800 feet, roll-out RVR 1800 feet."

We did the approach with me flying and the other pilot looking for the approach lights. The tower controller cleared us to land. I called out "minimums" just as the other pilot said "approach lights." At that point, I continued the descent, saw the required visibilility and landed.

Doing this single pilot is more challenging.