A VOR intercept is one of the first procedures every instrument student learns, and it stays deceptively easy — right until the CDI starts deflecting the wrong way. That's reverse sensing, and it catches experienced pilots too, usually after a rushed frequency swap or an OBS knock. Understanding why it happens takes ten minutes and removes the whole failure mode.
What the VOR actually gives you
A VOR station radiates 360 "radials" — magnetic bearings FROM the station. You tell the airborne equipment which radial you're interested in by turning the OBS (Omni-Bearing Selector) knob. The receiver then compares the radial the aircraft is actually on to the one you selected, and displays the difference as a course deviation bar on the CDI.
"This instrument displays VOR information … The indicator indicates the displacement of the aircraft with respect to a bearing (to or from the VOR station) which has been selected on the Course Selector Knob or OBS." — Oxford ATPL Radio Navigation, §8, "VOR Deviation Indicator"
In plain terms: the CDI's job is to tell you how far off the selected course line you are, not how far off the one you intended.
Radial vs. bearing, TO vs. FROM
- A radial is a magnetic track FROM the station (QDR).
- A bearing TO the station (QDM) is that radial's reciprocal.
- The TO/FROM flag is set automatically: if the radial the aircraft is actually on lies within 90° of the OBS setting, FROM appears; if more than 90°, TO appears.
"If the actual radial, which the aircraft is on, is within 90° of the Set Course then FROM will be shown. If the actual radial, which the aircraft is on, is more than 90° from the Set Course then TO will be shown." — Oxford ATPL Radio Navigation, §8, "CDI (Course Deviation Indicator)"
CDI deflection geometry
- One dot = 2° on a standard CDI. Full-scale deflection = 10°.
- The deflection is purely a function of aircraft position and OBS setting — not aircraft heading.
- Approaching the station the radials converge, so the needle gets more sensitive — expect it to move faster as DME drops.
"As the VOR is approached, the radials converge and the VOR needle becomes more sensitive." — Oxford ATPL Radio Navigation, §8, "Station Passage"
Intercepting a course
The standard intercept is a four-step procedure.
- Set the OBS to the course you want to fly (inbound or outbound).
- Observe the CDI. If the bar is deflected, you are off course on the opposite side (bar left = course is to your left = fly left).
- Choose an intercept angle. Typical values are 30° to 45°; steeper angles intercept faster but require a bigger recovery turn. A 45° intercept is the workhorse for en-route tracking.
- Fly the intercept heading until the CDI begins to move in from full-scale deflection (roughly 1–2 dots to go), then start the turn to roll out on course.
The key cue is when to start the turn. If you wait for the bar to centre before rolling, you'll overshoot — the aircraft needs lead equal to the bank you're about to establish. For a standard-rate 45° turn the overshoot without lead is several degrees.
"Commencing the turn shortly before making good the radial so as not to over-shoot it … A heading of 090°(M) is selected to allow for starboard drift inbound. So the turn is through 45° taking about 15 seconds." — Oxford ATPL Radio Navigation, §8, "Radial Interceptions"
Reverse sensing — why the needle "lies"
Reverse sensing is what happens when the OBS is set to the reciprocal of the track you are actually flying. The CDI is a purely geometric instrument: it has no idea which way the aircraft is pointing. It only compares the radial the aircraft is on to the OBS setting and draws a bar.
The rule that keeps the CDI sensible is simple:
"To keep the deviation bar sense correct when flying a track to or from a VOR station, the aircraft's heading should be about the same as the track selected on the Omni-bearing Selector (plus or minus any drift allowance)." — Oxford ATPL Radio Navigation, §8, "VOR Deviation Indicator"
In plain terms: the OBS setting should agree with your direction of flight within ±90°. If it doesn't, the bar sense flips. A bar deflected LEFT now means the course is to your right — you'd correctly turn right, but the instinctive "fly toward the needle" turn takes you the wrong way.
This is the same reason a back-course localiser displays reversed on a non-reversing HSI unless the pilot tells the box about the back-course: the sensing is geometric, not intention-aware.
The mental rule that prevents it
Set the OBS to the track you intend to make good, not the reciprocal.
- Inbound to the station on the 180° radial → you want to fly heading 360°, so set OBS 360° (TO).
- Outbound on the 180° radial → you want heading 180°, so set OBS 180° (FROM).
- Either way, OBS within 90° of aircraft heading = normal sensing.
On an HSI this problem disappears because the instrument rotates the course arrow with the compass card — the arrowhead always points toward the selected course on the map, so "fly toward the needle" is always correct regardless of heading. On a bare CDI (or an HSI whose heading input has failed), you have to enforce the rule manually.
"Aircraft heading is taken into consideration in displaying a fly left or fly right indication. However, as the instrument includes heading, it is able to determine the best direction to turn to achieve the required radial." — Oxford ATPL Radio Navigation, §8, "HSI (Horizontal Situation Indicator)"
Common mistakes
- Setting the OBS to the radial instead of the course. If you want to track inbound on the 090° radial, OBS goes to 270° (the reciprocal) so it matches your heading. Setting 090° gives reverse sensing.
- Chasing a sensitive needle close to the station. Inside ~10 NM the bar moves fast. Smaller heading corrections; don't over-bank.
- Forgetting to re-centre after station passage. Overhead the VOR the needle swings hard, the TO/FROM flag flips, and the new outbound course may need a fresh OBS setting.
- Leading the turn by too much. 45° intercepts need about 5° of lead, not 20°. Lead depends on groundspeed, bank, and intercept angle — start with a dot or two and calibrate.
Why it matters
Reverse sensing is a routine exam question on DGCA and EASA theory papers, and it is a real-world trap on backcourse approaches, circling NDB/VOR approaches where the inbound and outbound segments share a course line, and after any missed frequency or course change in a busy cockpit. The cost of setting the wrong OBS is usually a wasted minute and a confused trainee; on a non-precision approach at minimums it is much more expensive. The fix is mechanical — match OBS to flight direction — but only if you know the failure mode exists.