16.1 VOR Navigation

ACS PA.VI.B.K1 is ground-based navigation: orientation, course determination, equipment, tests, and the regulation. PHAK Chapter 16 and AIM 1-1-3 are the how. This section is VOR. GPS and radar services are 16.2.

What you are actually receiving

A VOR (VHF omnidirectional range) transmits in 108.0–117.95 MHz, line-of-sight. The ground station is the hub of a 360-spoke wheel. Each spoke is a radial: the magnetic course FROM the station. The 090 radial is the line of positions east of the facility. Fly that line outbound on heading about 090°. Fly it inbound on heading about 270°. The radial did not flip; only your direction along the same line flipped.

Collocated distance is how the chart labels the facility:

• VOR — azimuth only.
• VOR/DME — azimuth plus civil DME.
• VORTAC — VOR azimuth plus TACAN (civil aircraft still take DME from the TACAN).

AIM 1-1-3: the only positive identification is the Morse identifier, or the recorded voice that says the name followed by the word “VOR.” Do not treat an FSS voice overlay as the ident — one FSS may talk on several VORs. During maintenance the facility may radiate TEST (− ● ●●● −) or drop the ident. Either way, do not navigate on it. A receiver that decodes the letters still needs you to match them to the chart.

OBS, CDI, TO/FROM, and the 10° scale

The omnibearing selector (OBS) is the course you are asking about. The course deviation indicator (CDI) shows how many degrees you are left or right of that course. On a conventional VOR head, full-scale deflection is 10°; five dots at 2° per dot is the usual picture. A one-dot fly-right is about 2° off the selected course, not 2 miles. Angular error becomes linear miles as you get farther from the station — 6° at 60 NM is about 6 NM off the radial.

The TO/FROM flag answers a different question: if you flew the OBS course, would the station be ahead (TO) or behind (FROM)? It is not a command to turn around, and it is not a heading bug.

Worked picture. You are west of PODUNK VOR. Tune, identify, and rotate the OBS until the CDI centers with a FROM flag. The OBS now reads the radial you are on — say 270. You are on the 270 radial, west of the station. To go to the station along that same line, set the reciprocal 090 so the flag flips to TO, then fly a heading near 090°. If you leave OBS on 270 and fly east anyway, you have just selected reverse sensing.

Reverse sensing is the classic PAR fail. The needle always tells you where the selected course is, not where the nose should go. If the OBS says 090 and you fly heading 270, left-needle now means the 090 course is actually to your right. Chasing the needle takes you farther off. The cure is mechanical: set the OBS to the course you intend to fly, then fly a heading on that side of the compass. TO inbound, FROM outbound.

An HSI (horizontal situation indicator) puts the same CDI on a heading card. Set the course pointer to the desired course and fly toward the head of the pointer; the picture matches the world and reverse sensing is hard to miss. Set the tail of the pointer and fly that heading and you can still reverse-sense an HSI. The instrument does not forgive a reciprocal you selected on purpose.

Intercept, track, station passage, cone of confusion

To intercept a radial outbound: identify, set the radial in the OBS, confirm FROM as you approach the line, take a 30°–45° intercept heading toward the needle, then roll out on the radial heading when the CDI comes in. Do not wait for a perfectly centered needle at 45° of intercept or you will overshoot. To track inbound, set the inbound course (reciprocal of the radial), TO flag, intercept toward the needle, then hold the heading that keeps the needle centered — that heading is the wind-corrected inbound course, not necessarily the OBS number.

Station passage is a FROM flip after a solid TO, often with a CDI swing. That is the fix, not the first twitch of the flag. Overhead and nearby is the cone of confusion: azimuth is unreliable close to the station, the needle may wander, and the flag may flicker. Hold heading, wait for a stable FROM, then resume tracking outbound. AIM also warns that certain propeller RPM settings can make the CDI hunt ±6°; change RPM a little before you write up the VOR.

Service volumes and the 30-day check

AIM 1-1-8 still publishes the three original standard service volumes (SSV), measured above transmitter height (ATH). Reception below about 1,000 feet ATH is not part of the guaranteed volume.

The VOR Minimum Operational Network added VL (VOR Low) and VH (VOR High) with longer ranges at and above 5,000 feet ATH (VL: 40 NM below 5,000, 70 NM from 5,000 to 18,000). A PAR item that says “terminal VOR” still wants 25 NM / 12,000. Beyond the published SSV you may hear the Morse and still have a bent azimuth — do not treat “I have a needle” as a service-volume waiver. Line-of-sight still wins: a hill between you and a low-altitude VOR is a better explanation than a failed receiver.

14 CFR 91.171 is titled for IFR. No person may operate under IFR using VOR unless the equipment is maintained under an approved procedure or has been operationally checked within the preceding 30 days and found inside these bearing-error limits:

• VOT or designated ground checkpoint: ±4°. AIM 1-1-4 VOT: CDI centered, OBS 000° FROM or 180° TO. An RMI on a VOT reads 180° on any OBS.
• Designated airborne checkpoint or an airway check over a prominent point (preferably more than 20 NM from the VOR): ±6°.
• Dual independent VORs tuned to the same station: ±4° between the two indicated bearings.

Log the date, place, bearing error, and signature in the aircraft record. VFR Part 91 does not make that entry a legal precondition to a sunny $100 hamburger. A private-pilot written that asks “must you?” wants the IFR rule. A private-pilot DPE who asks “should you?” wants you to verify the receiver anyway — a 6° error at 40 NM is 4 NM off the line you thought you were on.

PAR scenario: Chart shows you need the 180 radial inbound to PODUNK. That is the line south of the VOR. Inbound course is 360°. Set OBS 360, wait for TO, intercept on a heading near 030° or 330°, then track the heading that centers the needle. If you set 180 and fly north, the TO/FROM flag and the needle will both lie to you — that is reverse sensing, not a broken VOR. Identify Morse first. If the ident is silent, pick another facility.