Originally Posted by
Fly Boy Knight
I agree with this above with the exception of RVR for air carriers. Most air carriers have ops specs that specifically state "TDZ RVR is controlling (if available)" meaning that even though the "flight visibility" may very well be met when you arrive at DA/DH, because the RVR is below what the FAA determines to be safe, you may not continue (or even start the approach if not yet beyond the FAF/FA Point).
GA/Part 91 world, flight visibility = visibility out the window and nobody knows that except the pilots. As someone stated before, if SFC Vis and/or RVR are both WAY below the min flight vis required, you may have a hard time defending your decision to continue during an enforcement action but according to the letter or the law, it is legal IF you actually had the min flight vis. Unfortunately, when it comes to this type of "gray area", it's not 100% what was legal to the T, but more about what you can prove / defend / justify when brought in to a hearing during a possible enforcement action. A good general rule for pilots... C.Y.A. and don't be stupid.
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.