I received an outstanding question this afternoon concerning instrument approach visibility minimums from Michael. His question went something like this:
I’m looking at the RNAV (GPS) Z Runway 31 at McNary Field in Salem, Oregon (SLE) and I can’t make sense of the minimum visibility requirements of the approach. Why is it that the visibility requirements for the LNAV only are so low? On this particular approach the required visibility for the LNAV only is 2400 RVR or about a 1/2 mile. That doesn’t make any sense! I wouldn’t be able to descend from a MDA of 940 with only 1/2 mile of visibility! Can you help shed some light on it?
Like I said mentioned before Michael, great question. Let’s take a look at the RNAV Z RWY 31 approach into SLE. For those who are interested, you can view a copy of the approach plate from EchoPlate:
Maybe at some point we’ll go over the terms and definitions of this approach plate a little more throughly, but for now, I’m going to try and just answer your question about the confusing visibility requirements.
Why are the visibility requirements so low for the MDA on the LNAV only approach?
The reason is the differences in the physical location of the Missed Approach Point (MAP) between the different approach types authorized for this approach. Take a look at the MAP for the LNAV only approach and then compare it to the MAP for the LNAV/VNAV approach. I have highlighted in red the locations of these two different MAPs in the illustration above. For the LNAV only, the MAP is basically the start of the runway. If you reach this MAP without having 2400 RVR AND you don’t meet the requirements of 14 CFR 91.175 then you must begin the missed approach procedure. For the LPV approach and the LNAV/VNAV approach, the MAP corresponds with the DA which is most likely going to be physically further from the runway thus the higher visibility requirements.
Now you bring up an interesting point about the practicality of this approach. Let’s say you are at a MDA of 940 and suddenly you have the airport environment and the required min. visibility, so you should “chop and drop” in right? Well, let’s read 91.175 again. 14 CFR 91.175 says that in order to operate below MDA you have to be:
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
So it may very well be that you have the required visibility before the MAP and you still have to execute a missed approach simply because you are not in a condition that you’ll be able to make it to the runway surface using safe and normal maneuvers.
I hope this clarifies things for you Michael. If you have any other questions, feel free to comment on the this post and I’ll do my best to find an answer for you.
If anyone else has questions about the terms used in this explanation, feel free to ask me a question about which term you are confused about.
Thanks again for your question and ….