Instrument Flight notes

Lesson 14

1. Purpose of ILS

Precision approach: gives lateral guidance (localizer) + vertical guidance (glideslope), converging to a nominal 3° glidepath to the runway. Does not fly you to a random point — it's a defined 3D path to touchdown.

LPV (GPS/WAAS) is functionally identical — same lateral+vertical guidance concept, different signal source.

2. Three Components — Memorize Verbatim (instructor explicitly flagged this)

"GRV" — Guidance, Range, Visual

• Guidance: localizer + glideslope (physically steers you)

• Range: marker beacons, DME, GPS, cross-radials (tells you where you are along the course)

• Visual: approach lighting, runway/threshold lights, PAPI/VASI (transition from instrument → visual phase)

⚠ Instructor was explicit: be able to rattle this off cold, in any order, as long as all three land.

3. Localizer

• VHF, same band as VOR

• 90 Hz on left / 150 Hz on right (facing inbound course). Equal volume = centered. Louder 90 = drifted right; louder 150 = drifted left.

• Full-scale deflection = 700 ft total width (350 ft either side) at the runway threshold — always, regardless of runway length

• Sensitivity scales with runway length: short runway → course narrows to full-scale faster (more sensitive sooner); long runway (e.g., Montgomery) → narrows more gradually but stays sensitive longer

• Service volume (matches AIM 1-1-9, transcript numbers check out): 35° either side of course out to 10 NM; narrows to 10° either side from 10–18 NM

• Back course: 90/150 flip — flying inbound on BC gives reverse sensing unless BC approach is loaded in the G1000 (in which case the HSI auto-corrects sensing to read normally)

LDA (Localizer-type Directional Aid)

• Used where terrain won't allow normal LOC alignment (example given: mountainous terrain, needs a turn to align with runway)

• Misaligned 3°–30° off runway centerline

• No glideslope

• Higher minimums than a standard LOC approach — oral trap: don't call an LDA a "localizer approach"

4. Glideslope

• UHF (paired channel to the LOC frequency)

• 90 Hz top / 150 Hz bottom

• Standard glidepath = 3°, but not universal — steep-terrain approaches can require much higher angles (Aspen-type example cited, ~7°); always check the charted glidepath angle, don't assume 3°

• 3° ≈ 450 fpm descent in the C172; steeper angles push you past ~500 fpm, harder to hold precisely in this airplane

• Always intercept glideslope from below — intercepting/chasing from above risks capturing a false glideslope (signal reflection artifact)

• GS antenna is physically offset near the touchdown zone (not at threshold), aimed to guide you to touchdown ~750–1250 ft down the runway (near the 1000-ft markers)

• Service volume ~±0.7° about the nominal glidepath (transcript numbers were garbled here — verify exact figure against AIM/FOM before oral)

5. Marker Beacons

Mnemonic instructor used: Blue → Amber → White, outer to inner.

• ILS identifiers always start with "I" (Morse: dot-dot). If the ident you're hearing doesn't start with that, you're tuned to the wrong thing.

• Range info doesn't require a marker/DME — can be a cross-radial fix off another VOR (e.g., "on the localizer, crossing the XYZ 090 radial" = fix identification without dedicated range equipment).

6. ⚠ Key Oral Trap: LOC FAF vs. ILS FAF

• Maltese cross symbol = localizer (LOC-only) final approach fix

• Lightning bolt symbol = ILS final approach fix (defined by glideslope intercept altitude)

• Usually co-located — not always.

• Example worked in lesson (Gwinnett-type LOC/ILS): crossing the outer marker while established on glideslope put the aircraft below the LOC step-down altitude that would otherwise still apply. Point: if flying the LOC-only procedure (glideslope unused/inop), you fly the charted LOC step-downs and the LOC FAF — you do NOT get to descend early just because you happen to be intercepting where the glideslope would be.

• This is a legitimate DPE trap: confusing which FAF and which minimums apply depending on whether you're flying the full ILS or reverted to LOC.

7. Visual Component — Ties Directly to 14 CFR § 91.175

• Approach Lighting System (ALS): bridges instrument → visual phase. CAT II/III fields get denser systems.

• Sequenced flashing lights = "the rabbit"

• Red terminating bars / red side row bars (ALSF-1-type systems): if distinctly visible, they authorize continued descent to 100 ft above TDZE using the ALS as the sole reference — without them, ALS alone caps you at 100 ft AGL as a visual reference floor.

§ 91.175(c) — get this verbatim, per instructor

Three conditions, all required, to descend below DA/DH or MDA:

1. Aircraft continuously in a position to make a normal descent to landing on the intended runway using normal maneuvers.

2. Flight visibility is not less than the visibility prescribed for the approach being used.

3. At least one of these visual references for the intended runway is distinctly visible and identifiable:

   • Approach light system (100-ft AGL descent floor unless red terminating/side row bars also visible)

   • Threshold, threshold markings, threshold lights

   • REIL

   • VASI/PAPI

   • Touchdown zone or touchdown zone markings/lights

   • Runway or runway markings

   • Runway lights

§ 91.175(e) — missed approach

• Once at DA/DH/MDA or the MAP without the visual references above → immediate missed approach. No second-guessing after that point.

• Circling: must go missed if an identifiable part of the airport is lost from view at/above MDA — except momentary loss from a normal bank angle during the circling maneuver itself is not disqualifying.

8. CAT I/II/III — Use With Caution

Transcript numbers here were unreliable (audio garbled specific RVR/DH figures). General shape, verify exact values against AIM Table 1-1-9-1 / FOM before oral, do not cite these numbers cold:

• CAT I: DH ~200 ft, visibility ~½ mile / RVR ~1800–2400 depending on lighting/equipment

• CAT II: DH ~100 ft, RVR ~1200 (lower with autoland/HUD-equipped ops)

• CAT III: sub-100 ft or no DH, RVR down into low hundreds or zero depending on subcategory

• Aircraft with autopilot/flight director/HUD can sometimes fly to reduced minimums (referenced 1800 RVR w/ FD) even on an otherwise-standard CAT I approach

• Note: CAT II/III is Part 121/135/large-aircraft territory, not really applicable to C172 Part 91 ops — but DPEs ask conceptually to test understanding of the DH/RVR relationship, not because you'll fly it.

9. Equipment Malfunctions

• Glideslope INOP → approach reverts to LOC-only; fly charted LOC step-down altitudes, LOC minimums (Maltese cross FAF), not ILS minimums

• Localizer INOP → no usable lateral guidance from that approach at all

• Signal distortion sources (oral fodder): large vehicles/objects/aircraft near the localizer/GS antennas or between the antennas and threshold, aircraft below 5000 ft AGL between the outer marker and runway

• ILS Critical Area: protected/marked area near the antennas; ATC holds other aircraft/vehicles clear of it during low-visibility ops to prevent signal distortion

10. Currency (61.57(c)) — tangent covered in this session

• 6 instrument approaches + holding procedures + intercepting/tracking courses, within the preceding 6 calendar months

• Legal interpretation (referenced, dates matter — confirm current guidance): tasks completed today count toward currency starting immediately, not delayed by a month — resolves the "if I fly today am I current tomorrow" confusion

• If currency lapses (6–12 months since last current): can requalify with a safety pilot under simulated instrument conditions, but cannot act as PIC under IFR/actual IMC until current again

• Beyond 12 months lapsed: requires an Instrument Proficiency Check (IPC) with a CFII — described as similar format to a checkride but lower stakes, doesn't count as pass/fail against your record