Non-precision Approaches (NPA) & Approaches with Vertical Guidance (APV)

Unfortunately, ICAO has just proposed changes to the approach classification stuff, so it will not be current for long.

Approach operations will be classified as 3D or 2D according to whether vertical guidance (i.e. a vertical deviation indicator) is used or not. 3D ops are further divided into Type A (DH > 250 ft) and Type B (DH < 250 ft) with Cat I, Cat II and Cat III subdivisions.

Approach runways will be classified as Non-instrument runway, Non-precision Approach runway, Precision Approach Cats I, II and III.

Finally, Instrument Approach Procedures will be classified as NPA (NDB, LOC, VOR, LNAV), APV (LNAV/VNAV, LPV) and PA (ILS, MLS, GBAS, LPV).

Clearly, only certain combinations are possible, but the terminology will help.

So you could have a Type A 3D operation using a NPA (LNAV with Garmin's +V), to a non-precision approach runway?

Bookworm,

You list LPV in two categories, APV and Precision. Is this intentional?

JasonC wrote:

So you could have a Type A 3D operation using a NPA (LNAV with Garmin's +V), to a non-precision approach runway?

LNAV is 2D. +V is an invention of the manufacturer and is advisory only, it is not official vertical guidance and a LNAV procedure has a MDA, not a DH. In the US, some operators are authorized to use a DH in lieu of a MDA, but in those cases, normally the DH is a derived DH by adding 50 feet to the MDA. A part 91 operator does not have this authority. In the US, now days, if the runway doesn't qualify for a LPV, either a LNAV or LP is charted, so in many cases, these approaches are not suitable for vertical guidance to the runway. Also, in these cases, the dive and drive method is more appropriate. 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. For Turbojet aircraft operated by the airlines, they have criteria that determine if vertical guidance to a runway is permitted, so even though the approach may be NPA, they have evaluated the approach for obstacle criteria.

At many GA airports in the US, the vast majority in fact, are uncontrolled airports without a control tower, there is no approach lighting, and the runway length can be 3000 feet or shorter. The only criteria for these airports are that the MDA must be 250 feet or higher above the highest obstacle found in the area evaluated. Many are not suitable for a glidepath or approach at night when it is impossible to see unlit obstacles. For many of these, a dive and drive is appropriate and particularly in low visibility, the constant angle approach leaves the aircraft too far to see the runway or there are obstacles along the path once below the MDA.

JasonC wrote: So you could have a Type A 3D operation using a NPA (LNAV with Garmin's +V), to a non-precision approach runway? LNAV is 2D.

NCYankee, I do know it is a manufacturer feature - I said it in my question! However it does provide vertical deviation guidance and I think the new structure may distinguish between the operation (one with vertical guidance) from the approach type NPA ie with an MDA rather than a DH. I make no value judgement as to whether someone should use the vertical guidance.

But maybe this is not possible and it will be treated as a 2D approach anyway. I was just asking.

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. Here in the US, you will never see an approach chart with LNAV+V, on it. You can't tell by looking at the approach chart if the manufacturer will provide +V or not, although you can guess. LNAV+V is an annunciation when an advisory glidepath is provided by the manufacturer and never appears on the approach chart. On the GNS530W, GTN, and G1000 systems, only one of the following is possible to be annunciated: LPV, LP, L/VNAV, LNAV+V, or LNAV. If a failure in integrity occurs, the annunciation will change to LNAV without any vertical guidance (LPV to LNAV, LP to LNAV, L/VNAV to LNAV, or LNAV+V to LNAV). In the last case, there are no changes in the minimums or procedure.

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".

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.

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.

NPA accidents

PA versus NPA