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.
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.
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?
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.
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.
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...
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.
Bookworm,
With all due respect, this is dangerous thinking IMHO and would not be permitted in the US. The air carriers fly into a minority of the airports which have much tighter criteria for the approach path obstacle clearance. GA airports don't necessarily meet these requirements. At one particular airport I am thinking of, following the +V takes you 100 feet below a ridge line. The airlines will never fly into this airport. Although the airlines have an Opspec that permits them from using their FMS vertical guidance on a NPA approach and use the MDA as a DA, their are criteria that assure there is a clear path on those runways. In other cases there is a derived DA where a value typpically 50 feet is added to the MDA to obtain the derived DA. Turbo Jets have a strong need to be able to fly a constant angle stabilized approach to the runway and they simply avoid flying those runways where this is not possible. They are willing to have the higher minimums if necessary. Part 91 operators are not permitted to use vertical navigation +V or otherwise to treat a NPA MDA as a DA.
There is a lot more scrutiny being given to NPA here in the US. When the NPA is flight tested, it is flown with a FMS VNAV glidepath, one dot fly up. If the test pilot gets a TAWS warning, this is noted on the flight test report and a VDA and TCH are not published on the AeroNav Charts. Garmin and Jeppesen will not code the +V into the database at such runways. That doesn't prevent some FMS systems from generating an advisory glidepath anyway, so a warning note is added to the AeroNav approach charts indicating "Descent Angle NA". Airlines that use Jeppesen Charts have a note added if the NPA is suitable for using a DA in lieu of a MDA, it reads as "Only authorized operators may use VNAV DA(H) in lieu of MDA(H)". Absence of the note means that for the approach the MDA should be used.
Bookworm, per your request, I got them from MITRE employee Dr. S. Vincent Massimini that was involved in the work. I searched on the web and was unable to find the attached summaries. Since Dr. Massimini and I participate on the same FAA groups, I contacted him directly and he willingly sent them to me.
Constant angle descents are appropriate for Turbojet aircraft and are preferred. MITRE did a study and clearly demonstrated that a vertically guided approach is much safer for this category of aircraft. They also did a similar study for piston GA aircraft and the results were there was no improvement in safety for this class of aircraft; in fact the dive and drive had a lower accident rate than vertically guided approaches for this class of aircraft, although the difference was not statistically significant.
Please would you point me at the MITRE study? I've been banging on about this for years and I'm glad someone has finally done the research.
Because it is a manufacturer invention, it carries no official weight and is a 2D procedure. A 3D procedure has to be evaluated for use by a GS, a 2D does not.
With respect NYCYankee, you miss the point of the change, which arises because everyone, in the commercial world at least, uses FMS generated vertical guidance to fly NPAs.
There is (or will be when the ICAO change comes into effect) no such thing as a "2D procedure" or "3D procedure", only a "2D operation" or "3D operation". The type of operation is about how the flight crew flies the procedure, not the underlying system or runway.
So Jason's example is spot on. An LNAV procedure flown with LNAV+V advisory guidance to a non-precision approach runway is considered a 3D type A operation, and is done with a DA not an MDA. What doesn't change are the minima, and unless the approach is coded for LNAV/VNAV, such a 3D operation is flown to the same LNAV minima values as if a 2D operation were used. But as far as I can see, if the operation is 3D, the flight crew will be authorised to treat the minima line value as a DA without increment.
The Instrument Approach Procedures are actually classified according to the operations they are designed to support. If they support only type A operations (DH >= 250 ft) then they are classified as NPA or APV. If they also support type B operations (DH < 250 ft) then they are classified as PA. Thus an LPV can be either APV or PA, depending on the minima. But the Annex 10 terminology is not "LPV", but rather "SBAS vertical guidance".
There are some interesting changes in the way that minima are treated too.
The operating minima for 2D instrument approach operations using instrument approach ?procedures shall be determined by establishing a minimum descent altitude (MDA) or minimum descent ?height (MDH), minimum visibility and, if necessary, cloud conditions. [Note no mention of RVR.]
The operating minima for 3D instrument approach operations using instrument approach ?procedures shall be determined by establishing a decision altitude (DA) or decision height (DH) and the ??minimum visibility or RVR.
And, there are some amendments associated with runway classification.
A non-precision approach runway is intended for type A operations and a visibility greater than 1000 m. "The rationale for the 1 000 m visibility or greater for non-precision approach runways stems from an extrapolation of the values for precision approach runways (rounded up for ease of recollection) that provides a 75 m (250 ft) MDA/H."
Finally "this amendment allows aircraft to fly an instrument approach procedure to a non-instrument runway, so long as the minima is established at or above VMC. The runway infrastructure requirements need not be any different than visual procedures to non-instrument runways since the final segment will always have the same VMC requirements".
