Non-Precision Approach Procedures
Non-precision approaches differ from precision approaches in that only lateral guidance, not vertical guidance, is provided by the navigational signal upon which the approach is based. This typically leads to particular fixes on the approach at which the pilot must descend to an appropriate minimum altitude for that segment, levelling off until the next fix, and repeating until the pilot reaches the Minimum Descent Altitude (MDA).
Another primary difference is that the missed approach point is not necessarily the point at which the pilot reaches the MDA. It is not a decision height, so the pilot should level off and motor along at the MDA (and not a foot lower!) until reaching the Missed Approach Point. At that point, a missed approach must be executed if the runway environment is not in view.
A variety of non-precision approaches are enumerated below.
VOR approaches are widely used non-precision approaches. The minimum required navigational equipment is one VOR receiver, though some also require DME. VOR approaches can provide MDAs as low as 250 ft above the runway, and include a minimum of 300 ft of obstacle clearance in final app area.
There are a variety of the VOR approaches, including VOR/DME with and without a DME arc. Here are a few examples (all of which will be out of date when you read this, so don't use them to fly an actual approach):
Example (out of date!): SYI VOR 18
Example (out of date!): SYI VOR/DME 18
Example (out of date!): GHM VOR/DME 2 (arc)
The GPS approach requires an IFR certified (TSO C129a) GPS receiver to fly. Most common among these are the Garmin 530 and 430, G1000, Garmin 480, and King KLN 94B, though there are many others.
When flying IFR, in general, GPS (or a suitable RNAV system) can be used in lieu of a VOR, TACAN, DME, NDB, or compass locator. It can be used to perform holds, determine fixes, navigate to or from a station, or fly a DME arc. According to the AIM section 1-2-3, the primarily limitation of substituting GPS for another navigational facility is that "Pilots may not substitute for the navigation aid providing lateral guidance for the final approach segment. This restriction does not refer to instrument approach procedures with “or GPS” in the title when using GPS or WAAS..." Thus, the approach must say "or GPS" in order to use GPS past the FAF.
So what is the difference between an RNAV approach and a GPS approach? Early, stand-alone GPS approaches were labeled as such, but newer procedures are identified with RNAV in the heading, recognizing the fact that some more sophisticated aircraft are equipped with flight management systems (FMS) in which GPS may be only one of several navigation sensors. As of 2006, stand-alone GPS approaches were identified as RNAV (GPS). Over the next several years, the FAA will slowly adopt a single terminology, renaming all GPS approaches as RNAV. For more information, read the AOPA Air Safety Foundation's Safety Advisor, GPS From the Ground Up, or take the foundation's free interactive course, GPS for IFR Operations. (Source, AOPA GPS articles)
Any approach with GPS in the approach name, such as GPS RWY 2, NDB or GPS RWY 4 are non-precision approaches. The RNAV (GPS) approach may have up to four types of minima: LPV, LNAV/VNAV, LNAV, or circling. If you have an approach approved GPS but it is not a WAAS TSO C146a GPS, you can only fly RNAV (GPS) approaches to the LNAV MDA or the circling MDA. (Source, John Collins)
Many GPS approaches now incorporate Terminal Arrival Areas (TAA) which depict boundaries of specific arrival area and the MSA for those areas. The objective of a TAA is to provide a seamless transition from the en route structure to the terminal environment for arriving AC equipped with Flight Management System (FMS) and/or GPS. When TAA is published it replaces the MSA for that app procedure.
a) basic "T" design
b) modified basic "T"
c) modified "T" app to parallel runways
Example (out of date, do not use in an actual approach!): M91 RNAV 04
The Localizer (LOC) can provide both precision and non-precision app capabilities. It is part of the ILS system, on which the localizer provides horizontal guidance for a precision approach. Unlike a VOR approach, the OBS knob will have no effect on the selected course for a LOC approach or one of its derivatives.
Four separate applications of the localizer:
1. localizer approaches
2. localizer/dme approaches
3. localizer back course approaches
4. localizer-type directional aid (LDA) approaches
Remember to set an HSI to the front course when flying a back course to avoid reverse sensing (a phenomenon in which the direction the selected course is located is reversed). Also, like a localizer, the back course does not offer a glide slope, but remember that back course can project a false glide slope signal and glide slope should be ignored.
Example (out of date!): SYI VOR 18
Localizer-type directional aid (LDA) is a navaid that provides non-precision approach capabilities. A LDA is essentially a localizer, but it is termed LDA because the course alignment with the runway exceeds 3°. Also, does not offer a navigable back course. There are currently less than 30 LDA installations in the US.
Straight-in minimums may be published where alignment does not exceed 30 degrees between the course and runway. Circling minimums only are published where this alignment exceeds 30 degrees.
A very limited number of LDA approaches also incorporate a glideslope. These are annotated in the plan view of the instrument approach chart with a note, "LDA/Glideslope." These procedures fall under a newly defined category of approaches called Approach with Vertical Guidance (APV) described in paragraph 5-4-5
Hint: Remember it as "Localizer Different Angle".
Simplified Directional Facility. Similar to a LOC, an SDF rovides final approach course similar to that of ILS. Does not provide glide slope, and it utilizes essentially the same techniques as ILS/LOC except the SDF course may not be aligned with runway and course may be wider resulting in less precision. The transmitter is fixed at a course width of either 6° or 12°, whereas a LOC is typically between 3-6°.
Hint: Remember these as SDF = "Stuff Don't Fit" :)
Example (out of date!): THA SDF 18
3 Main types:
- PAR - Precision Approach Radar (covered under precision approaches here)
- ASR - Airport Surveillance Radar
- No gyro
ATC may also offer radar approach options to aircraft in distress regardless of weather conditions or as necessary to expedite traffic. Regardless of type of radar approach in use, ATC monitors the aircraft position and issues specific heading and altitutde info throughout the approach. Here are some examples discussed in greater detail.
ASR approaches are the non-precision equivalents of a PAR approach. They are typically only approved when necessitated for an ATC operational requirement, or in an unusual or emergency situation. This type of radar only provides heading and range info. All altitudes given will be advisory only, but ATC will provide altitude to the pilot at each mile of the approach if requested.
The final approach course for an ASR approach is aligned with runway centerline for straight-in approraches and aligned with center of airport for circling approaches. The pilot is guaranteed a minimum of 250 feet obstacle clearance.
An example of an ASR instruction issued by a controller: "On course, five miles from the airport, descend to your minimum descent altitude."
The greatest benefit of radar approaches is the ability to use radar to execute a "no-gyro" approach. Assuming standard rate turns, ATC can indicate when to begin and end turns in order to bring down a pilot with a vacuum pump failure (or similar partial panel circumstances). The pilot should make all turns at standard rate (unless otherwise instructed) and should execute the turn immediately upon receipt of instructions.
- Setting OBS to reciprocal resulting in FROM indication and reverse sensing
- Improper wind correction
- Not briefing the missed approach
- Setting improper frequencies into nav or com
- Not leveling at MDA and/or immediately executing missed approach at MDA
- Failing to time from the FAF to the missed
- Going below MDA
- Selecting the wrong minimums based on aircraft category
ACS Test Standards
- Must execute two different non-precision approaches from this list: NDB, VOR, LOC, LDA, SDF, GPS, RNAV
- Altitude +/- 100 feet
- Maintain MDA within +100 and -0 feet to MAP
- Airspeed +/- 10 knots
- Heading +/- 10°
- Reach MDA prior to missed approach point
- Circle to land / Straight in
- CDI within 3/4 scale deflection