Precision Approach

The ACS defines it as a standard instrument approach procedure flown to a published decision altitude (DA) using approved vertical guidance (FAA-S-ACS-8C, Area VI Task B). The ILS is the classic case. An LPV or LNAV/VNAV line is technically an approach with vertical guidance (APV) in AIM terms rather than a precision approach (AIM 5-4-5), but it's flown to a DA with approved vertical guidance — from the pilot's seat an LPV "looks and flies like an ILS" (IPH ch. 4).

The localizer, the glideslope, and the outer marker — plus an inner marker where installed for CAT II/III procedures.

Authorized substitutes for the outer marker:

• A compass locator
• PAR or ASR radar
• A DME, VOR, or NDB fix authorized in the procedure
• A suitable RNAV system used with a fix identified in the procedure (91.175(k))

• Transmits on 108.10–111.95 MHz, odd tenths only; antenna at the departure end of the runway (IFH ch. 9)
• Course width is very narrow — normally 5° total, so full-scale deflection means only 2.5° off centerline; a quarter-scale deflection or less keeps you aligned with the runway
• Identified by a three-letter ident preceded by "I" (e.g., I-SGF); always positively identify it
• Usable to 18 NM from the antenna within the coverage volume (IFH ch. 9)

• Projection angle normally 2.5° to 3.5° above horizontal (up to 4° where obstacles require); it intersects the middle marker at about 200 ft and the outer marker at about 1,400 ft above runway elevation (IFH ch. 9)
• The path is only 1.4° thick full-up to full-down — needle sensitivity is high, so make small corrections early
• Radiates only on the front course — no vertical guidance on a back course
• Transmitter sits 750–1,250 ft down the runway, offset 400–600 ft from centerline (IFH ch. 9)

GS facilities inherently produce additional courses at higher vertical angles — the lowest false course occurs around 9°–12°. Getting established on one produces reversed needle indications or demands an absurd descent rate. The protection is procedural: fly the published altitudes so you intercept the glideslope from below — at charted approach altitudes the false courses are never encountered (IFH ch. 9).

• OM — blue light, low tone, continuous dashes; located 4–7 NM out, roughly where you intercept the glidepath at the published altitude
• MM — amber light, intermediate tone, alternating dots and dashes; about 3,500 ft from the threshold, glideslope about 200 ft above the touchdown zone elevation (TDZE)
• IM — white light, high tone, continuous dots; marks CAT II decision height (IFH ch. 9)

With all components operative — minimums given as decision height (DH) and runway visual range (RVR) (IPH ch. 4):

CAT I needs only an instrument-rated, current pilot and an appropriately equipped aircraft; CAT II and III require special certification for pilots, aircraft, and ground equipment (IPH ch. 4; IFH ch. 9).

Localizer performance with vertical guidance — a WAAS-based approach flown to a DA, with lateral guidance equivalent to a localizer and angular scaling that tightens toward the runway. Minimums can be as low as 200 ft HAT and ½ SM at qualifying airports — ILS-equivalent — with no ground-based transmitters, no ILS critical areas, and a signal that's actually more stable than an ILS. Requires WAAS-LPV avionics with an AFM statement supporting LPV (IPH ch. 4).

An APV line flown to a DA using approved vertical guidance — WAAS or approach-certified Baro-VNAV — with vertical and lateral integrity limits larger than a precision approach or LPV. The visibility can be higher than the LNAV line because the DA point on the vertical path sits farther from the threshold than the LNAV missed approach point (IPH ch. 4). Baro-VNAV also lives on the barometric altimeter — altimeter setting and temperature limits on the chart matter.

Establish a predetermined rate of descent at the point where vertical guidance begins — one that approximates what's needed to follow the guidance — rather than chasing the needle down. Get the rate from the rate-of-descent table in the back of the TPP using the published angle and your groundspeed (AIM 5-4-5), then hold attitude and power and make small corrections (IFH ch. 9).

Tolerances: from the final approach fix (FAF) to DA, no more than ¾-scale deflection of either the lateral or vertical indication, and airspeed ±10 knots (VI.B S12).

At DA: immediately initiate the missed approach unless the required visual references for the runway are unmistakably visible and identifiable (VI.B S13). A DA is a decision made at the altitude — the design allows a momentary descent below it while transitioning to the climb (IFH ch. 1).

The ILS reverts to a localizer approach — glideslope-out minimums are published on the chart as the S-LOC (localizer) MDA (IFH ch. 10). That only works if you briefed the LOC minimums and the timing or fix that defines the LOC MAP before you started down, so brief them every time. Recognize the failure (flags in view, needle behavior) and take appropriate action (VI.B S5); otherwise, fly the missed approach and advise ATC.

Working from the chart top-down (IPH ch. 4):

• Approach name, airport, chart date — confirm you're both on the same page
• Frequencies — localizer/NAVAID tuned and identified, comms in sequence
• Final approach course
• FAF crossing altitude and glideslope intercept altitude
• DA (or MDA) and TDZE/airport elevation
• Missed approach procedure — verbalize it so it's fresh, even when the tower will likely give instructions
• MSA, notes, required equipment (e.g., "ADF or DME required"), lighting, visibility required
• Runway length, planned configuration, and taxi expectation

By the OM, drift correction should be nailed down well enough that heading changes of no more than 2° complete the approach. As the localizer narrows toward the runway, corrections must shrink proportionately. The heaviest demand comes between the OM and MM: hold the localizer, adjust pitch for the descent rate, adjust power for airspeed, cross-check the altimeter, and prepare for the land-or-miss decision — all at once (IFH ch. 9). Small, early, trimmed corrections are the whole game.

Convert the RVR minimum to ground visibility per 91.175(h):

So a standard CAT I "RVR 2400" ILS needs ½ SM when RVR isn't available (91.175(h)).

Part 91: yes — reported weather doesn't bar you from continuing, and at the DA the 91.175(c) test is flight visibility plus the required visual references. If flight visibility is below the prescribed minimum, go missed regardless of what's reported (IFH ch. 10; 91.175(c)).

Part 121/135: crews may not proceed past the FAF unless the latest report is at or above minimums, with relief only if the report arrives after the FAF (IPH ch. 4).

Deteriorating weather is also where stabilized-approach discipline pays off: holding the parameters steady is what makes the abnormal indications of windshear stand out (IPH ch. 4). The broader divert-or-hold decision is covered under Task VI.A.

The published minimums assume every component and visual aid works. Raise them for (IFH ch. 10; 91.175(b)):

• Inoperative components or visual aids — apply the TPP Inoperative Components Table; e.g., ALSF-1 approach lights out on an ILS raises visibility by ¼ SM. Multiple failures: apply only the highest single increase.
• NOTAMs amending the procedure.
• Aircraft equipment — the authorized DA/MDA is the highest of: the procedure's, the pilot's, and the one appropriate to the equipment available and used (91.175(b)).
• Approach category — flying final faster than your category's band means the higher category's minimums.

Because at a 100-foot DH or below there's no time to assess and react using ordinary skills and equipment — so the FAA requires special certification for the pilot, the aircraft/avionics, and the ground equipment before anyone uses those minimums (IFH ch. 9; IPH ch. 4). For CAT II, the TDZ RVR system is required and controlling — no substitutions, unlike CAT I where midfield RVR can substitute when TDZ RVR is unavailable (IPH ch. 4). As a Part 91 instrument pilot you fly CAT I.