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
AIM 1-1-3 / IFH Ch. 9- 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
CDI Interpretation & Tracking
IFH Ch. 9 / AIM 1-1-3- 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
VOR Accuracy Checks (14 CFR §91.171)
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.
VOR Service Volumes
AIM 1-1-8- 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
Intercepting & Tracking Radials
IFH Ch. 9- 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
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
AIM 1-1-17 / IFH Ch. 9- 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
GPS CDI Sensitivity (Auto-Scaling)
AIM 1-1-17- 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
WAAS (Wide Area Augmentation System)
AIM 1-1-17- 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
GPS Approach Types
AIM 5-4-5 / IPH Ch. 4- 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
GPS Substitution & Limitations
AC 90-108 / AIM 1-1-17- 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
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
AIM 1-1-9 / IFH Ch. 9- 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
ILS Categories & Minimums
AIM 1-1-9- 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
Flying the ILS
IFH Ch. 9 / AIM 5-4-5- 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
Localizer-Only & Back Course Approaches
AIM 1-1-9- 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
False Glideslope & Common ILS Errors
IFH Ch. 9 / AIM 1-1-9- 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
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)
AIM 1-1-7 / IFH Ch. 9- 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
Flying a DME Arc
IFH Ch. 9 / ACS IV.A- 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
ADF/NDB (Automatic Direction Finder / Non-Directional Beacon)
AIM 1-1-2 / IFH Ch. 9- 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
RNAV & RNP Concepts
AIM 1-2-1 / IFH Ch. 9- 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
Federal Airways & Route Structure
AIM 5-3-4 / IFH Ch. 1- 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)
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
IFH Ch. 1 / AIM- 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
Instrument Approach Chart Layout
IFH Ch. 1 / IPH Ch. 4- 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
Charted IFR Altitudes
P/CG / AIM 5-3-4- 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
Procedure Turn & Course Reversal
AIM 5-4-9 / IFH Ch. 1- 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
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.