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Precision and Non Precision Approaches, What's the Difference?

IFR approaches can be broken down into three categories: Precision Approaches (PA), Approaches with Vertical Guidance (APV) and Non-Precision Approaches (NPA). Precision approaches and APV approaches both have vertical guidance, but precision approaches abide by stricter standards. We will discuss APVs later in the article. The most common precision approach you’ll encounter is an ILS, but there are others such as a Precision Approach Radar (PAR) or a GBAS landing system (GLS). A Non-Precision Approach is an approach that provides course deviation information but no glidepath deviation information, examples of non-precision approach would be a VOR approach or an NDB approach.

Vertical Guidance is Key

Now let’s say you’re flying IFR to your home airport, planning to fly the ILS there. As you get close to the initial approach fix, you see the inop flag on the glideslope pointer of your HSI. You elect to fly the localizer approach corresponding to that ILS instead. What kind of approach are you flying? Since your glideslope is inoperative, you cannot fly the ILS and have to find a non-precision alternative, in this case the localizer only approach. A localizer only approach doesn't provide any vertical guidance, and therefore can't be an APV either.

Approach with Vertical Guidance

APVs, just like precision approaches, have vertical guidance, so why does the FAA have two different kinds of approaches with vertical guidance? Well, precision approaches have to meet the standards of ICAO annex 10. This can be expensive and cumbersome. So to reduce the cost and allow for the creation of approaches using new technology, the FAA and ICAO came together and agreed on the APV. Some of the most common APV approaches are the GPS approaches with vertical guidance such as those using Localizer Performance with Vertical Guidance (LPV) and LNAV/VNAV. These terms are familiar from the minimums section on many GPS approaches.

An LPV approach has vertical guidance but is not considered a precision approach, it’s an APV approach. GPS-WAAS approaches may not be a precision approach, but they are extremely accurate thanks to the WAAS system. They provide minimums that are close to ILS minimums.

Decision Altitude versus Minimum Descent Altitude

A big difference between approaches with vertical guidance and approaches without is whether they involve a decision altitude (DA) or minimum descent altitude (MDA). An approach with vertical guidance or precision approach will have a DA. This DA tells you when you have to execute the missed approach: you’re descending along the glidepath, and when you get down to the DA if you can’t land you have to execute the missed approach. On the other hand, when flying a non-precision approach there will be an MDA instead of a DA. An MDA is the lowest altitude you can descend to. Unlike a DA you often should not go missed right away when you are at the MDA, you must stay there until you arrive at the missed approach point, at which point you will have to execute the missed approach if you cannot continue below.

Many WAAS enabled GPS units allow for something called LNAV+V, which provides an advisory glidepath on a non precision GPS approach. The FAA doesn’t consider an advisory glidepath proper vertical guidance, and it should be treated as an aid to situational awareness. An LNAV approach, even one where you have an advisory glidepath, is a non-precision approach and does not have a DA but an MDA.

Putting it All Together

There are three different types of approaches:

Precision approach:

1.    Has vertical guidance

2.    Meets the approach standards of ICAO annex 10

3.    Includes ILS, PAR and GLS

4.    Uses a DA

Approach with vertical guidance (APV):

1.    Has vertical guidance

2.    Includes LPV and LNAV/VNAV

3.    Uses a DA

Non-precision approach:

1.    No vertical guidance

2.    Includes VOR, NDB and LNAV

3.    Uses an MDA

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