The FAA sorts instrument approaches into three groups by the guidance they give: precision approaches, approaches with vertical guidance and nonprecision approaches. The group decides whether you fly to a decision altitude or a minimum descent altitude, and it decides how low the weather minimums can go.
This guide walks through each group, names the common approaches in it and explains the rules that apply at the bottom of the approach. It also covers the visual and contact approaches, which are not instrument approach procedures in the same sense. Everything here comes from the FAA's Aeronautical Information Manual (AIM) and 14 CFR.
The Three Groups of Instrument Approach
The AIM lists the types of instrument approach procedures for which the FAA's criteria (TERPS) are written. A precision approach uses a navigation system that gives course and glidepath deviation information to the standards of ICAO Annex 10. Examples are the ILS, precision approach radar and GLS. An approach with vertical guidance (APV) gives course and glidepath information but does not meet the precision standards. Examples are Baro-VNAV, LNAV/VNAV, LDA with glidepath and LPV. A nonprecision approach gives course information but no glidepath. Examples are VOR, NDB and LNAV.
Common instrument approaches by group| Approach | Group | Guidance comes from | Flown to |
|---|
| ILS | Precision | Localizer and glide slope | Decision altitude |
|---|
| GLS | Precision | GBAS (ground-based augmentation of GPS) | Decision altitude |
|---|
| Precision approach radar (PAR) | Precision | Radar controller | Decision altitude |
|---|
| RNAV (GPS) with LPV minima | APV | WAAS GPS | Decision altitude |
|---|
| RNAV (GPS) with LNAV/VNAV minima | APV | WAAS GPS or barometric VNAV | Decision altitude |
|---|
| RNAV (GPS) with LNAV or LP minima | Nonprecision | GPS (lateral only) | Minimum descent altitude |
|---|
| VOR, NDB and localizer (LOC) | Nonprecision | Ground-based navaid | Minimum descent altitude |
|---|
An approach on one chart can offer several lines of minima. The AIM says RNAV (GPS) approaches with LNAV, LP, LNAV/VNAV and LPV lines of minima using WAAS are charted as "RNAV (GPS) RWY" followed by the runway number, and the pilot flies whichever line the aircraft equipment supports.
ILS: The Precision Approach
The instrument landing system provides an approach path for exact alignment and descent on final approach. Its basic components are the localizer, the glide slope and an outer marker or a substitute. Where a runway is equipped for Category II or III operations, it also has an inner marker. The AIM divides the system into guidance (localizer and glide slope), range information (marker beacon and DME) and visual information (approach lights, touchdown and centerline lights, runway lights). Where the outer marker is not installed, a compass locator, radar, a DME, VOR or NDB fix, or a suitable GPS fix can substitute for it.
Category II and III ILS approaches need special authorization, and 14 CFR 91.189 sets the flight crew, equipment and ground-component requirements for them. At airports where the localizer is offset, the AIM says the ILS loses Category II and III capability and the decision altitude rises by 50 feet.
GPS Approaches: LPV, LNAV/VNAV, LNAV and LP
Most new instrument approaches in the US are RNAV (GPS) procedures. A WAAS-capable receiver can fly LPV and LNAV/VNAV minima using a WAAS electronic glide path, which avoids errors from barometric altitude. The AIM says LPV minima use the same TERPS criteria as ILS approaches, and can have a decision altitude as low as 200 feet height above touchdown with visibility as low as a half mile when the terrain and airport infrastructure support it. These are APV approaches and not precision approaches under FAA terminology. LNAV approaches give lateral guidance only. A barometric Vertical Navigation (baro-VNAV) function in some aircraft can fly LNAV/VNAV minima when the approach allows it.
For how GPS, VOR and the other navigation systems work, see our guide to aviation navigation systems. The AIM also describes GLS, which uses GBAS to give three-dimensional guidance similar in benefit to an ILS or LPV.
Nonprecision Approaches: VOR, NDB and Localizer
Nonprecision approaches use a ground-based radio navaid for course guidance and give no glidepath. The VOR approach is flown using radials from a VHF omnidirectional range station, the NDB approach uses a nondirectional beacon, and a localizer-only approach uses the ILS localizer without the glide slope. The AIM notes that some nonprecision procedures publish a vertical descent angle (VDA) to help the pilot fly a stabilized descent, but that does not turn them into APV procedures, because the approach is still flown to a minimum descent altitude and has not been evaluated with a glidepath.
Decision Altitude or Minimum Descent Altitude
Approaches with vertical guidance end at a decision altitude (DA), and nonprecision approaches end at a minimum descent altitude (MDA). 14 CFR 91.175 sets what the pilot needs to continue below either one. The aircraft must be continuously in a position to descend to a landing on the intended runway at a normal rate of descent, the flight visibility must be at least that prescribed for the approach, and at least one of the listed visual references must be distinctly visible and identifiable. The list includes the approach lighting system, the threshold, the touchdown zone, the runway or its markings, the runway lights and a visual glideslope indicator. Using the approach lights alone, the pilot may not descend below 100 feet above the touchdown zone elevation unless the red terminating bars or red side row bars are also visible.
The authorized DA or MDA is the highest of the figure on the procedure, the figure prescribed for the pilot in command and the figure appropriate for the aircraft equipment. If the visual requirements are not met at the DA or the missed approach point, the pilot flies the missed approach. The AIM says obstacle protection for the missed approach assumes it starts at the DA, DH or missed approach point, and requires a climb gradient of at least 200 feet per nautical mile unless a higher one is published.
Straight-in and Circling Approaches
A straight-in minimum is published when the final approach course is within 30 degrees of the runway alignment and a normal descent can be made from the IFR altitude shown on the chart. When either condition fails, only circling minimums apply, and the pilot flies the approach to a circling MDA and then maneuvers visually to the runway. Circling minimums depend on the aircraft approach category, which 14 CFR 97.3 sets by speed: Category A is under 91 knots, B is 91 to under 121, C is 121 to under 141, D is 141 to under 166 and E is 166 or more. The guide to the types of planes shows which aircraft fall in each.
Visual, Contact and Charted Visual Approaches
A visual approach is conducted on an IFR flight plan and lets the pilot proceed visually and clear of clouds to the airport once the airport or the preceding aircraft is in sight. ATC must authorize it, and the reported weather must be a ceiling at or above 1,000 feet and visibility of 3 miles or more. It is an IFR procedure in visual conditions. A contact approach must be requested by the pilot, who must be clear of clouds with at least 1 mile of flight visibility, to an airport with a published instrument approach. ATC cannot initiate it. A charted visual flight procedure is a charted visual approach, mainly for noise or traffic reasons at towered airports, and it is not an instrument approach and has no missed approach segment.
RNP AR and Other Special Approaches
RNP AR (authorization required) procedures need an FAA authorization through operations specifications, management specifications or a letter of authorization, with no exceptions. The AIM compares it to the special authorization needed for Category II and III ILS procedures. Most general aviation pilots will never fly one. For the approaches covered above, the rating that lets a pilot fly them in the clouds is the instrument rating, and the private pilot privileges guide explains where a private certificate stops. For weather minimums, see the weather guide for private pilots.
Low cloud and fog are when instrument approaches matter most. Seattle averages 141 hours of fog a year and Charlotte 62 hours, according to our SEA airport weather and CLT airport weather pages.
Frequently Asked Questions
What are the main types of instrument approach?
The FAA groups them as precision approaches (such as the ILS), approaches with vertical guidance (such as LPV and LNAV/VNAV) and nonprecision approaches (such as VOR, NDB and LNAV).
What is the difference between precision and nonprecision approaches?
A precision approach gives course and glidepath deviation information that meets ICAO Annex 10 standards. A nonprecision approach gives course guidance only, so the pilot flies to a minimum descent altitude.
Is LPV a precision approach?
Under FAA terminology it is an approach with vertical guidance (APV), not a precision approach, but it uses the same TERPS criteria as an ILS and can have a decision altitude as low as 200 feet above touchdown.
What is the difference between DA and MDA?
A decision altitude applies on approaches with vertical guidance and an MDA applies on nonprecision approaches. At either, the pilot needs the required visual references to continue below it, or must fly the missed approach (14 CFR 91.175).
What is a circling approach?
An approach that ends with a visual maneuver to a runway that is not aligned with the final approach course, flown to a circling minimum. A straight-in minimum is published only when the final course is within 30 degrees of the runway and a normal descent is possible.
What is a visual approach?
An IFR procedure ATC may authorize when the ceiling is at least 1,000 feet and visibility at least 3 miles, in which the pilot proceeds visually to the airport after seeing it or the preceding aircraft.
Do I need special authorization for a Category II or III ILS approach?
Yes. 14 CFR 91.189 sets the crew, equipment and ground-component requirements, and the AIM says they need special authorization.
Related guides: How to Read and Decode a METAR, With a Worked Example.
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