Intercepting and Tracking Navigational Systems and DME Arcs

The VOR must be maintained under an approved procedure, or operationally checked within the preceding 30 days and found within limits — ±4° for ground-based checks and the dual-VOR cross-check, ±6° airborne — and whoever checks it logs date, place, bearing error, and signature (91.171). The check methods, their preference order, and the VOT procedure are covered under Task II.C.

1. Orient — determine the radial you're on (center the CDI with FROM)
2. Parallel the desired course to keep left/right straight
3. Take the difference between your radial and the one to intercept, then double it for the intercept angle — not less than 20°, not more than 90°
4. Set the OBS to the desired course, turn to the intercept heading
5. Hold that heading until the CDI centers, leading the turn as you gain experience (IFH ch. 9)

Bracketing. Follow the needle with an initial 20° correction; when it re-centers, take half the correction out. Each time the needle drifts, re-correct and halve again until you find the wind correction angle that holds the course. Chasing the needle without a reference heading turns tracking into homing (IFH ch. 9).

With the OBS set to the reciprocal of your intended course, the CDI deflects away from the course — correcting toward the needle takes you farther off. The fix is procedural: always set the OBS to agree with your intended course, inbound or outbound (IFH ch. 9).

The azimuth card is slaved to heading, so the needle always shows bearing directly: the head of the needle shows magnetic bearing TO the station, and the tail of the needle shows the radial you're on (bearing FROM). For a fixed-card ADF, add relative bearing to heading: MH + RB = MB to the station (IFH ch. 9).

DME (UHF, paired with the VHF facility frequency) times interrogation pulses and displays slant range — the straight-line distance from your antenna, not ground distance. Directly over the station it reads your altitude in NM. The error is negligible when you're at least 1 NM from the station for every 1,000 feet above it. Displayed groundspeed is accurate only tracking directly to or from the station (IFH ch. 9).

• Track to the station on the lead radial; at 150 knots groundspeed or less, lead the turn by 0.5 NM
• Turn approximately 90° and roll out with the bearing pointer on the wingtip
• Let the pointer drift 5°–10° behind the wingtip, then turn toward the station to put it 5°–10° ahead — a series of short, straight legs
• Correct 10°–20° toward the facility for each half-mile outside the arc; staying slightly inside keeps the arc turning toward you

The ACS tolerance is ±1 NM (IFH ch. 9; IR.V.A.S6).

Receiver Autonomous Integrity Monitoring — the receiver's self-check that the satellite geometry supports the integrity required for the phase of flight. It needs 5 satellites, or 4 plus baro-aiding, to detect an anomaly; fault detection and exclusion (FDE) needs 6 satellites (or 5 with baro-aiding) to also isolate and remove the bad one. Without RAIM you have no assurance the GPS position is trustworthy (AIM 1-1-17, IFH ch. 9).

Use the receiver's own prediction function, a manufacturer tool, the FAA Service Availability Prediction Tool (SAPT), or ask FSS — briefers give RAIM status for 1 hour before to 1 hour after your ETA unless you ask for a window, and you should also request one for the departure airport when flying a GPS departure. If RAIM is predicted unavailable: rely on other approved navigation, re-route, delay, or cancel (AIM 1-1-17, IFH ch. 9).

For a basic (non-WAAS) IFR GPS:

• En route: ±5 NM full scale
• 30 NM from the airport the receiver annunciates arm — armed, it scales to ±1 NM terminal sensitivity
• 2 NM before the final approach waypoint (FAWP) the approach mode goes active and sensitivity ramps smoothly to ±0.3 NM at the FAWP

If the approach mode never arms, sensitivity never ramps down — the equipment flags, and you fly to the missed approach waypoint (MAWP) and go missed. WAAS receivers switch to ILS-like angular scaling on LNAV/VNAV and LPV finals (AIM 1-1-17, 1-1-18; IFH ch. 9). What WAAS itself adds — accuracy, integrity, availability, and the TSO approvals behind it — is covered under Task II.B.

• Airspeed: ±10 knots
• Altitude: ±100 feet
• Headings: ±5° — tighter than the ±10° used elsewhere
• Course: within ¾-scale CDI deflection
• DME arc: ±1 NM

These are the tolerances for this task (IR.V.A.S5–S6). If navigation, approach, or communication equipment fails under IFR in controlled airspace, report it to ATC — aircraft ID, equipment affected, degree of impairment, and assistance desired (91.187; IR.V.A.S7).

Rotate the OBS until the CDI centers with a FROM indication — the course index shows the radial you're on; the inbound course is its reciprocal. One station only puts you somewhere on a line: cross-cut a second NAVAID (or use DME) to pin down a fix. Near the station, expect the needle to fluctuate in the zone of confusion — rapid or fluctuating movement means station passage is imminent, and positive passage is the TO/FROM flip (IFH ch. 9).

Ground-station course alignment is generally within ±1°. Receiver sensitivity check: with the CDI centered, rotate the OBS until the needle rests on the last dot — that should take 10°–12° or less. Two quirks worth knowing: certain prop RPM settings can modulate the signal and swing the CDI up to ±6° (change RPM before writing up the radio), and terrain can cause brief course roughness (IFH ch. 9).

Sensitivity. The localizer course is only about 5° wide, so full-scale deflection is roughly 2.5° either side of centerline — several times touchier than a 10–12° full-scale VOR. Use shallower intercept angles and half-standard-rate turns near the course, and start corrections the moment the needle moves (IFH ch. 9). Full localizer anatomy — frequencies, course width, service volume — is covered under Task VI.B.

No RMI: use the OBS-and-CDI method — after the 90° intercept turn, center the CDI; your heading sits near the left/right (90°/270°) reference of the card. Hold heading, and each time the CDI drifts 2°–4° from center, re-center it and note the new reference heading, using DME to fine-tune in or out.

Leaving the arc: at 150 knots or less the lead radial is under 5° — no different from intercepting a radial from a straight course. When a charted lead radial gives 7° of lead onto a localizer, use a half-standard-rate turn until the needle starts moving toward center (IFH ch. 9).

An IFR-certified GPS may substitute for ADF/DME to:

• Identify a DME fix
• Fly a DME arc
• Navigate to/from or hold over an NDB/compass locator
• Identify fixes made of an NDB bearing or DME distance

Conditions: the fix or facility must come from the current database (no manual lat/long), integrity monitoring must be working, and the CDI set to terminal (±1 NM) sensitivity in the terminal area. GPS cannot substitute for the lateral guidance source on the final approach of a DME- or ADF-based approach (IFH ch. 9).

An updatable IFR database is required. For en route and terminal operations you may fly with an expired database if you verify the data are correct (waypoints unchanged); for approaches the database must be current, or you must verify the procedure hasn't been amended since it expired. Approaches must be retrievable from the database by name — building one from user waypoints is prohibited, and flying point-to-point doesn't give you approach RAIM or 0.3 NM scaling (AIM 1-1-17; IFH ch. 9).

Two flavors: "not enough satellites" — RAIM can't monitor at all, so the position may be fine but its integrity is unknown; and "integrity alert" — RAIM detected an error exceeding the limit for the phase of flight. Either one on an approach means don't descend: fly to the MAWP and execute the missed. En route, revert to your alternate means of navigation and start actively monitoring it — that's the one time monitoring the backup NAVAID becomes mandatory rather than good practice (AIM 1-1-17; IFH ch. 9).

• Onboard electronics — certain receivers, transceivers, mobile radios, and portable receivers can cause signal interference, and some VHF transmissions cause "harmonic interference". Isolate it by relocating portables, changing frequencies, or turning off suspects while watching the receiver's signal quality page
• Antenna shadowing — the airframe can block the GPS antenna, notably while banked; terrain does the same in valleys surrounded by high ground
• Constellation status — with fewer than 24 operational satellites, GPS may be unavailable at certain locations; satellite status is published through the NOTAM system

Any of these can cost you signal or integrity with no ground station to call about it (IFH ch. 9).

Know what the box is flying before it flies it. Verify the active waypoint and leg — the same waypoint can appear more than once in a procedure (IAWP, FAWP, MAWP on a procedure turn), and skipping fly-over waypoints can leave the receiver sequenced to the wrong leg; some segments demand manual sequencing or a manually set course (IFH ch. 9). Also confirm the autopilot's annunciated roll and pitch modes after every selection — armed is not captured, and the AFM's procedures and limitations govern its use (IPH ch. 4).

The evaluator may have you fly course intercepts with the autopilot (IR.V.A.S9).

These factors erode situational awareness (IFH ch. 11):

• Distractions
• Unusual or unexpected events
• Complacency
• High workload
• Unfamiliar situations
• Inoperative equipment

In this task the classic trap is fixation — staring at one needle until heading and altitude wander, or chasing the CDI instead of flying reference headings. The warning sign is a reactive mindset: the airplane keeps doing things you didn't anticipate. Recovery: fly the airplane first, then reassess and rebuild the picture from additional sources — navigation instruments, the multi-function display (MFD)/moving map, or ATC (IFH ch. 7, ch. 11).