IFR Navigation
AIM 1-1 / IFH Ch 9 • VOR, GPS/WAAS, ILS, DME/ADF, Airways & Charts
Navigation System Currency
VOR checks are required every 30 days for IFR flight (14 CFR §91.171), and GPS databases must be current. Always verify RAIM availability before a non-WAAS GPS approach, and know your equipment — not every GPS can fly every GPS approach.
Key Takeaways
- A VOR gives 360 radials defined FROM the station; full-scale CDI deflection is 10° off course (≈2°/dot).
- The OBS sets the course to fly, not a heading — and TO/FROM shows which way that course points relative to the station.
- IFR VOR checks (§91.171) every 30 days: VOT and ground ±4°, airborne ±6°, dual-VOR cross-check ±4° — all logged.
- Navigate only within the published service volume, and only after identifying the station's Morse/voice ident.
VOR Principles of Operation
- VHF Omnidirectional Range — operates 108.0–117.95 MHz (shared band with localizers on odd tenths)
- 360 radials radiating outward from the station like spokes of a wheel
- Two signals transmitted: Reference (omnidirectional, constant phase) and Variable (rotating, phase varies with direction)
- Receiver compares phase difference between the two signals to determine the radial you are on
- Radials are always defined as FROM the station — "on the 270 radial" means west of the VOR
- Line-of-sight reception: Signal blocked by terrain, limited by altitude and distance from station
- Identify the station by Morse code or voice — never navigate off an unidentified VOR
AIM 1-1-3 / IFH Ch. 9
CDI Interpretation & Tracking
- CDI (Course Deviation Indicator) shows position relative to the selected course
- Full-scale deflection = 10° off course (each dot ≈ 2°)
- OBS (Omni Bearing Selector) sets the desired course — this is NOT your heading, it is the course to fly
- TO/FROM indicator: TO means the selected course takes you toward the station; FROM means away
- Reverse sensing: If you set a FROM radial while flying TO the station, the CDI deflects opposite to your position
- Station passage: TO/FROM flag flips, CDI deflects full-scale briefly, then settles — normal behavior
- Tracking TO: Set the course TO the station on OBS; correct into the CDI deflection; bracket wind correction
- Tracking FROM: Set the radial FROM the station; correct into the CDI deflection in the same way
IFH Ch. 9 / AIM 1-1-3
VOR Accuracy Checks (14 CFR §91.171)
- Required within the preceding 30 days to use VOR for IFR navigation — see the tolerance table below
- Log four things every time: date, place, bearing error, and signature
- No valid check in the last 30 days? The aircraft is NOT legal to navigate by VOR under IFR
| Check | Error | Notes |
|---|---|---|
| VOT | ±4° | CDI centers on 360° FROM / 180° TO. §91.171(b)(1) |
| Ground checkpoint | ±4° | Designated point on the airport surface. §91.171(b)(2) |
| Airborne checkpoint | ±6° | FAA-designated airborne position. §91.171(b)(3) |
| Airway in flight | 6° | Over a prominent ground point, preferably >20 NM from the VOR. §91.171(b)(4) |
| Dual VOR | 4° apart | Independent units, same station. A difference between the two indications — not a ± band against a known reference. §91.171(c) |
Any one of these satisfies the 30-day requirement — you do not need all of them.
14 CFR §91.171
VOR Service Volumes
- Terminal (TVOR): 1,000–12,000 ft AGL, 25 NM — found on and near airports
- Low Altitude (LVOR): 1,000–18,000 ft AGL, 40 NM — supports Victor airways up to 18,000 ft
- High Altitude (HVOR): 1,000–14,500 ft (40 NM), 14,500–60,000 ft (100–200 NM) — supports jet routes
- Signal unreliable outside published service volume — do NOT navigate off a VOR beyond its volume
- Check NOTAMs for VOR outages before every IFR flight — may require alternate routing
- VOR MON (Minimum Operational Network): FAA retaining ≈590 VORs as a GPS backup network
AIM 1-1-8
Intercepting & Tracking Radials
- To intercept a radial: Turn to a heading that creates an intercept angle (typically 30–90°)
- As the CDI centers, turn to track the radial — apply wind correction to keep CDI centered
- Bracketing technique: If CDI drifts left, turn left (toward the CDI); if it stays centered, hold heading
- If CDI starts moving again, adjust wind correction angle in smaller increments — bracket until stable
- Outbound tracking is harder than inbound — corrections feel "reversed" until you develop the habit
- Near the station: CDI becomes very sensitive (small position change = large CDI movement)
- Time/distance to station: Turn 90° off course, note time for a 10° bearing change; TTS = time × 60 / degrees
IFH Ch. 9
Key Takeaways
- IFR GPS needs a current database and RAIM (5 satellites; 6 for fault exclusion); WAAS replaces the separate RAIM check.
- CDI sensitivity auto-scales: ±5 NM en route, ±1 NM terminal, ±0.3 NM on approach — a downgrade off approach mode means you can't continue the GPS approach.
- WAAS (TSO-C145/146) enables LPV to a 200-ft DA; non-WAAS (TSO-C129) is LNAV only.
- GPS may substitute for VOR/NDB/DME/ADF (AC 90-108) but never for an ILS localizer or glideslope.
GPS Fundamentals
- Global Positioning System: 24+ satellites in 6 orbital planes at ~20,200 km altitude
- Requires minimum 4 satellites for 3D fix (latitude, longitude, altitude)
- RAIM (Receiver Autonomous Integrity Monitoring) requires 5 satellites (6 for fault detection AND exclusion)
- GPS accuracy: ~15 meters (standard); ~1 meter with WAAS correction
- Must have a current database for IFR navigation — expired database = NOT legal for IFR
- TSO-C129: Basic IFR GPS (non-WAAS) — can fly LNAV approaches only
- TSO-C145/C146: WAAS GPS — can fly LNAV, LNAV/VNAV, LP, and LPV approaches
AIM 1-1-17 / IFH Ch. 9
GPS CDI Sensitivity (Auto-Scaling)
- En route: ±5.0 NM full-scale deflection — broadest sensitivity
- Terminal (within 30 NM of airport): ±1.0 NM full-scale deflection
- Approach (within 2 NM of FAF): ±0.3 NM full-scale deflection — tightest sensitivity
- Sensitivity transitions automatically based on flight phase — watch for annunciation changes
- If GPS downgrades from approach to terminal mode: You cannot continue the GPS approach
- Some units allow manual arming of approach mode — verify before reaching the IAF
- CDI source selection: Verify you are navigating on GPS, not VOR/LOC, during a GPS approach
AIM 1-1-17
WAAS (Wide Area Augmentation System)
- Ground-based reference stations measure GPS errors and broadcast corrections via geostationary satellites
- Improves accuracy to ~1 meter horizontal, ~1.5 meters vertical
- Enables LPV approaches with DA as low as 200 ft — equivalent to Category I ILS performance
- WAAS integrity monitoring replaces RAIM — no separate RAIM check needed with WAAS equipment
- WAAS availability varies by location and satellite geometry — check NOTAMs for outages
- Not all airports have LPV approaches — availability depends on obstacle survey and procedure design
AIM 1-1-17
GPS Approach Types
- LNAV: Lateral navigation only — non-precision, uses MDA; stepdown fixes common
- LNAV/VNAV: Lateral + barometric vertical navigation — uses DA; requires baro-VNAV or WAAS
- LPV: Localizer Performance with Vertical guidance — uses DA; WAAS required; best GPS approach
- LP: Localizer Performance (lateral only) — uses MDA; WAAS required; no vertical guidance
- LNAV+V: Advisory glidepath only — still uses MDA; the +V is for situational awareness, NOT a precision approach
- Approach minimums decrease: LNAV (highest) > LP > LNAV/VNAV > LPV (lowest)
- Always verify your equipment can fly the approach type — "GPS" in the title does not mean any GPS works
AIM 5-4-5 / IPH Ch. 4
GPS Substitution & Limitations
- GPS can substitute for VOR, NDB, and DME per AC 90-108 — with current database and IFR-approved unit
- GPS can substitute for ADF for NDB approaches and compass locator fixes
- GPS CANNOT substitute for ILS localizer or glideslope — an ILS requires an ILS receiver
- RAIM check required before non-WAAS GPS approach — check availability at ETA ±15 minutes
- If RAIM is not available: You cannot fly the GPS approach — have an alternate plan
- GPS for position awareness does NOT require current database — but IFR navigation does
- Monitor approach annunciations: "LPV," "LNAV/VNAV," "LNAV" — know what your box is giving you
AC 90-108 / AIM 1-1-17
Key Takeaways
- The localizer is ±2.5° full-scale (much tighter than a VOR); the glideslope auto-tunes with it and is typically a 3° path.
- Marker beacons: Outer (blue, dashes, ~4–7 NM), Middle (amber, dot-dash, ~3,500 ft), Inner (white, dots, Cat II/III).
- Cat I is 200 ft / RVR 2400; the lower categories (II/III) need special aircraft, training, and OpSpecs.
- Always intercept the glideslope from BELOW — false glideslopes exist at multiples of the 3° angle; mismatched crossing altitudes mean go missed.
ILS Components
- Localizer: Lateral guidance, 108.10–111.95 MHz (odd tenths only), ±2.5° full-scale deflection
- Localizer antenna located at the far end of the runway — signal narrows as you approach
- Glideslope: Vertical guidance, 329.15–335.00 MHz, paired with localizer frequency (auto-tunes)
- Glideslope antenna located ~1,000 ft from threshold, offset from centerline — typically 3° descent angle
- Outer Marker (OM): Blue light, continuous dashes at 400 Hz — approximately 4–7 NM from threshold
- Middle Marker (MM): Amber light, alternating dot-dash at 1300 Hz — approximately 3,500 ft from threshold
- Inner Marker (IM): White light, continuous dots at 3000 Hz — used on Cat II/III approaches
- Approach lighting system (ALS/MALSR/ALSF): Visual transition from instruments to runway environment
AIM 1-1-9 / IFH Ch. 9
ILS Categories & Minimums
- Category I: DH 200 ft AGL, visibility 1/2 SM or RVR 2400 — standard GA ILS
- Category II: DH 100 ft AGL, RVR 1200 — requires special aircraft certification and crew training
- Category III-A: DH < 100 ft, RVR ≥ 700 — autoland or HUD required
- Category III-B: DH < 50 ft or no DH, RVR ≥ 150 — autoland required with rollout guidance
- Category III-C: No DH, no RVR minimum — zero visibility, full autoland; not yet in use in the US
- Most GA pilots operate Cat I only — lower categories require specific equipment, training, and OpSpecs
- Some airports publish lower-than-standard Cat I minimums with enhanced lighting or terrain clearance
AIM 1-1-9
Flying the ILS
- Intercept the localizer: ATC typically vectors you at a 20–30° intercept angle
- Set the final approach course on the CDI/HSI — must match the published course (e.g., ILS RWY 14 = 140°)
- As localizer centers, turn to track the final approach course; apply wind correction to keep centered
- Glideslope intercept: From below, capture the GS as the needle centers — then follow it down
- Maintain the published approach speed; small, smooth corrections for GS and LOC deviations
- At DA/DH: If runway environment is in sight, continue to land; if not, execute missed approach
- ILS is the most precise approach available to GA pilots — full-scale CDI = only 2.5° off course
- Cross-check: Altimeter at known fixes (OM, MM) should match published crossing altitudes ±50 ft
IFH Ch. 9 / AIM 5-4-5
Localizer-Only & Back Course Approaches
- LOC approach: When glideslope is out of service, only lateral guidance is available
- LOC approach uses MDA (not DA) — higher minimums than the full ILS
- LOC CDI sensitivity is the same as ILS (±2.5°) — much more sensitive than VOR
- Back-course (BC) localizer: Approach from the opposite end of the runway
- BC has reverse CDI sensing (needle deflects opposite to your position from course)
- HSI corrects for reverse sensing automatically — standard CDI does not
- No glideslope on back course — step down altitudes and MDA apply
- SDF (Simplified Directional Facility): Similar to localizer but wider beam (6° or 12°); lower precision
AIM 1-1-9
False Glideslope & Common ILS Errors
- False glideslopes exist at multiples of the published angle (e.g., 6°, 9° for a 3° GS)
- Always intercept the glideslope from below — intercepting from above may capture a false GS
- If crossing altitudes at fixes don’t match, you may be on a false glideslope — go missed
- Common errors: Chasing the localizer needle (over-correcting), diving for the glideslope, late configuration
- Maintain approach speed ±5 knots — speed excursions cause GS/LOC deviations
- Wind correction: Crab into the wind; do not use wings-level sideslip on an ILS
- Go-around/missed approach: Applies any time you cannot maintain the approach profile or lose the needle
IFH Ch. 9 / AIM 1-1-9
Key Takeaways
- DME measures slant range, so it overreads close-in and high — directly overhead it shows your altitude in NM.
- Fly a DME arc by keeping the bearing pointer on the wingtip and turning ~10° toward the station for each 1 NM of drift; lead onto the arc by ~0.5 NM.
- The ADF needle points TO the station: Magnetic Bearing = Relative Bearing + Magnetic Heading.
- Victor airways are 8 NM wide (4 each side), 1,200 AGL to 17,999 MSL; MOCA guarantees obstacle clearance but VOR reception only within 22 NM.
DME (Distance Measuring Equipment)
- Measures slant-range distance to the station (not ground distance)
- At high altitude close to the station: DME reads your altitude in NM (pythagorean error), not zero
- Slant range error becomes negligible at distances > 1 NM per 1,000 ft altitude
- Paired with VOR or ILS — auto-tunes when the VOR/ILS frequency is selected
- Ground speed: Calculated from rate of distance change — shown on many DME displays
- Time-to-station: Also computed by DME — accurate when tracking directly TO the station
- DME arc: A curved path at constant DME distance from a NAVAID — used on arrivals and approaches
AIM 1-1-7 / IFH Ch. 9
Flying a DME Arc
- DME arcs are published on approach and arrival procedures (e.g., "10 DME arc northeast of XYZ VOR")
- Technique: Turn to place the bearing pointer on the wingtip (90° from the station)
- Lead turns: Approximately 0.5 NM before the desired DME distance to avoid overshooting the arc
- During the arc: Periodically turn 10–20° toward the station to maintain constant DME distance
- Rule of thumb: For each 1 NM you drift outside the arc, turn 10° toward the station
- Watch for the lead radial — the radial marking where you turn inbound for the approach
- DME arcs require practice — they are tested on the IFR checkride under ACS IV.A
IFH Ch. 9 / ACS IV.A
ADF/NDB (Automatic Direction Finder / Non-Directional Beacon)
- NDB operates 190–535 kHz (low/medium frequency)
- ADF needle always points TO the station — relative bearing indicator
- Magnetic Bearing to station = Relative Bearing + Magnetic Heading
- No course deviation indication — requires constant mental computation for tracking
- Subject to interference: Night effect, terrain/mountain effect, thunderstorm deflection, shoreline refraction, precipitation static
- Being phased out but still tested on ACS and found at some airports
- NDB approaches are the least precise instrument approaches still in use
AIM 1-1-2 / IFH Ch. 9
RNAV & RNP Concepts
- RNAV (Area Navigation): Point-to-point navigation using GPS, DME/DME, VOR/DME, or INS
- Eliminates need to fly directly to/from ground-based NAVAIDs — more direct routing
- RNP (Required Navigation Performance): RNAV with onboard monitoring and alerting capability
- RNP value = total system error in NM that must be maintained 95% of the time (e.g., RNP 0.3)
- RNP AR (Authorization Required): Special curved approaches with lower minimums — requires OpSpecs
- T-routes and Q-routes: RNAV airways replacing some Victor (VHF) and Jet routes
- Most IFR-approved GPS units meet RNAV requirements for published procedures — verify your equipment
AIM 1-2-1 / IFH Ch. 9
Federal Airways & Route Structure
- Victor airways (V-routes): 1,200 ft AGL to 17,999 ft MSL — based on VOR radials, 8 NM wide (4 NM each side)
- Jet routes (J-routes): 18,000 ft (FL180) to FL450 — based on VOR radials
- T-routes: RNAV replacements for Victor airways — GPS-based
- Q-routes: RNAV replacements for Jet routes
- MEA (Minimum En Route Altitude): Ensures NAVAID reception and obstacle clearance on the airway
- MOCA (Minimum Obstruction Clearance Altitude): Obstacle clearance guaranteed; NAVAID reception only within 22 NM of VOR
- MCA (Minimum Crossing Altitude): Required altitude at specific fixes, often at airway intersections
- Changeover Point (COP): Where you switch from one VOR to the next on an airway (default: midpoint)
AIM 5-3-2 / IFH Ch. 1
Key Takeaways
- En route altitudes: MEA (reception + obstacles), MOCA (obstacles; reception within 22 NM), MCA, MAA, MRA, and OROCA / Grid MORA off-route.
- MSA gives 1,000 ft obstacle clearance within 25 NM of the NAVAID — for emergency use only, not navigation.
- The barbed arrow shows the procedure-turn side; remain at or above PT altitude until established inbound (max 200 KIAS).
- NoPT means no procedure turn — when you're vectored, a TAA applies, or a straight-in entry is expected.
IFR En Route Chart Symbology
- VOR: Compass rose with hexagonal symbol; VORTAC includes DME channel
- Airways: Thin black lines with route identifier (V-xxx), MEA, MOCA (*altitude), and distance between fixes
- Changeover points: Arrow symbol with distance from each facility
- Intersections: Triangle (compulsory reporting) or open triangle (non-compulsory)
- DME fix: Indicated by a "D" symbol with distance from the NAVAID
- Minimum reception altitude (MRA): Lowest altitude at which an intersection can be identified
- ARTCC sector boundaries: Blue serrated lines with center name and sector frequency
IFH Ch. 1 / AIM
Instrument Approach Chart Layout
- Pilot Briefing (top): NAVAID info, frequency, approach course, runway length, TDZE, airport elevation
- Notes section: Non-standard takeoff minimums (▲T), non-standard alternate minimums (▲A)
- Plan View: Overhead view showing procedure, fixes, altitudes, holding patterns, MSA circle
- Profile View: Side view showing descent path, FAF, stepdown fixes, missed approach altitude
- Minimums section: DA/DH or MDA, visibility requirements by aircraft category (A/B/C/D)
- Airport Diagram: Runway layout, lighting, taxiways, elevation
- MSA (Minimum Safe Altitude): 1,000 ft obstacle clearance within 25 NM of the NAVAID — emergency use only
IFH Ch. 1 / IPH Ch. 4
Charted IFR Altitudes
- MEA (Minimum En Route Altitude): NAVAID reception + obstacle clearance along the entire airway segment
- MOCA (Minimum Obstruction Clearance Altitude): Obstacle clearance only; NAVAID reception within 22 NM of VOR
- MCA (Minimum Crossing Altitude): Minimum altitude required to cross a specific fix (often at intersections)
- MAA (Maximum Authorized Altitude): Highest altitude on an airway ensuring adequate NAVAID reception
- MRA (Minimum Reception Altitude): Lowest altitude to receive signals to identify a fix
- OROCA (Off-Route Obstruction Clearance Altitude): 1,000 ft clearance in non-mountainous terrain, 2,000 ft in mountainous
- Grid MORA (Minimum Off-Route Altitude): Similar to OROCA but on Jeppesen charts
AIM 5-3-2 / IFH Ch. 1
Procedure Turn & Course Reversal
- Procedure turn: Barbed arrow on approach chart shows which side the PT is flown
- Standard PT: Outbound on approach course, 45° turn away, fly 1 min, 180° turn back to intercept
- Alternatives: 80°/260° turn, racetrack pattern, teardrop — pilot’s choice unless specified
- PT distance: Normally within 10 NM of the fix (5 NM for Cat A, up to 15 NM for high-performance)
- Maximum speed in PT: 200 KIAS
- NoPT: "No Procedure Turn" — indicated when vectors are provided, TAA applies, or direct entry is expected
- Holding in lieu of PT: Published hold at an IF or FAF serves as the course reversal
- Remain at or above PT altitude until established inbound on the approach course
AIM 5-4-9 / IFH Ch. 1
Go Deeper
Question about VOR tracking, GPS approach types, or reading an en route chart? Ask Kit for a CFI-level explanation.
“Explain GPS CDI sensitivity scaling from en route to approach”
Training aid only — verify all data against your POH and current FAA publications.