Thanks for the docs NCY.
To be clear, I'm not commenting on the merits or otherwise of the amendment, only describing my interpretation of the content of state letter AN 11/1.1-12/40. It's possible that I'm misinterpreting something, particularly as regards the minima. I'll check with contacts on the Ops panel.
At one particular airport I am thinking of, following the +V takes you 100 feet below a ridge line.
Some examples, including this one, would be very helpful please.
following the +V takes you 100 feet below a ridge line
Is that above or below the MDA?
Below the MDA, on a NP approach, all bets are off.
Unless I am missing something...
It is below the MDA and your understanding is correct. The problem is that pilots are trained to fly a GS with the understanding there will be no obstacles to the runway. Where ever there is official vertical guidance, this is a valid assumption. Extending this thinking to a NPA can be downright dangerous, particularly at night when obstacles can't be seen to be avoided.
Here is a presentation that I made to the ACF-IPG (FAA Aeronautical Charting Forum - Instrument Procedures Group) on the subject.
The presentation is helpful and thought provoking, thank you.
But doesn't Peter's point stand that even in the N23 case, the MDA is 2140 ft, and that must assure at least 246 ft obstacle clearance over anything between the FAF and the MAPt (in the absence of an SDF). If you treat the 2140 ft as a DA rather than an MDA, then, at 3.5 miles from the threshold, that's not a problem in itself, even if there is a 1550 ft obstacle on the glideslope at a point 1 mile from the threshold.
The problem would be assuming that the advisory glideslope continues to provide obstacle clearance below the 2140 ft level, when clearly it does not.
The problem would be assuming that the advisory glideslope continues to provide obstacle clearance below the 2140 ft level, when clearly it does not.
But if it is a DH then you are either visual or you are not. If not you couldn't continue. NCYankee, is your concern people following the advisory glideslope below the MDA/DH in IMC?
Bookworm,
Your analysis is all very true. In fact in this case if one were following the advisory GS to the MDA and treating it as a DA, you would be unable to see the runway threshold under any circumstance as the ridge line blocks the view from that point, and would always have to miss the approach. One might be able to see the far end of the runway and at night easily mistake it for being clear to the runway. Since the purpose of the VDA is to permit a constant descent to the runway, it is not appropriate for this or a significant number of runways. Note the Jeppesen chart shows a dotted line extending to the runway. This is only intended to show where the angle comes from and is not intended to indicate there are no obstacles, but IMHO it is very easily misinterpreted. There are many other examples where a TAWS would generate an alert if you followed the advisory path to the runway, but would not with a dive and drive. My major concern is that pilots do what they were trained to do and what they tend to do all the time, so a pilot that routinely uses the glidepath to the runway after the DA(H) is likely to do the same and make a terrible mistake when using a DH in lieu of a MDH. Not all approaches are suited for vertical guidance and in these cases the dive and drive method is appropriate.
Another point to consider is that with a DH, one may only have a single point in time to make the decision. Without approach lights, as is the case with most of these RNAV LNAV approaches, it takes some amount of time to recognize the runway is in fact the runway. If you are level at the MDA, you have much more time to spot the runway and distinguish it from the shopping mall parking lot or airport access road. The airlines won't fly into such an airport, but they are common for GA. A 3 degree glideslope is unnaturally flat for my Bonanza, in the pattern a 4 to 6 degree final is much more appropriate. To satisfactorily complete these approaches during low visibility, the dive and drive will get you safely down in many cases where using the MDA as a DA will require a missed approach. These approaches were designed for dive and drive and in many cases can't be fit into the constant angle method. Here in the US, if the approach qualifies for vertical guidance, it will get it.
But if it is a DH then you are either visual or you are not. If not you couldn't continue. NCYankee, is your concern people following the advisory glideslope below the MDA/DH in IMC?
Not so much if it is IMC at the MDA/DH, but if it is visual, low visibility, or night, when the runway may be mistaken to be insight or there are unseen obstacles between the DH and runway, then maintaining the same descent profile (for example following the advisory glidepath) it is possible to hit an obstacle. Most pilots are unaware of this possibility and the law of primacy says they do what they were trained to do under similar situations, even though it doesn't apply to the current situation. Here in the US, according to the Instrument Pilot Practical Test Standards on the Precision Approach Task : see the following criteria for pilots:
Maintains localizer and glideslope within ¾-scale deflection of the indicators during the visual descent from DA/DH to a point over the runway where glideslope must be abandoned to accomplish a normal landing.
