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 needle on the CDI.
The receiver answers a positional question: which side of the selected course line contains the aircraft? Turning the heading without changing position cannot change that geometric relationship. This is why checking the selected course matters before responding to the needle.
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.
Check the reciprocal explicitly. On the 030 radial, a selected 030 course gives FROM; selecting 210 gives TO. The aircraft can face either way without changing those indications. Near the boundary between TO and FROM, use the position picture and the instrument flag rather than treating the flag as a turn instruction.
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.
The display measures an angle. At 60 NM, a 2° error represents about 2 NM sideways; at 6 NM, the same angle represents about 0.2 NM. Less lateral movement is therefore needed to move the needle as you approach the station.
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 needle is deflected, you are off course on the opposite side (needle 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 needle 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.
Allow for the time the aircraft takes to turn. At a nominal 3° per second, changing heading by 45° takes 15 seconds. During those seconds you still travel toward and then across the course, so begin the capture turn before the needle reaches the centre. Use the aircraft procedure and observed closure rate to choose the lead.
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 needle.
The rule that keeps the CDI sensible is simple:
Build the position picture first: intended course, aircraft side of that course, and intended direction of travel. Select the course in the direction you plan to follow. Only then use the CDI as a left/right tracking cue; selecting the reciprocal reverses the conventional needle sense.
In plain terms: the OBS setting should agree with your direction of flight within ±90°. If it doesn't, the needle sense flips. A needle 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.
The HSI helps you see the aircraft heading relative to the selected course. Treat the course arrow as part of a map-like picture; a lateral needle displacement alone is not a universal instruction to turn in a particular direction. A failed heading input removes an important part of that picture.
An HSI places the selected course and deviation bar on the heading display. That common picture makes the geometry easier to interpret, but does not select the intended course for you. Check the course arrow and TO/FROM indication, and follow the installed equipment guidance.
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 needle 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.
Further reading
FAA Pilot’s Handbook, navigation and glossary and FAA AIM, navigation aids. These links support the underlying concepts; the explanations and examples above are AvioQuiz study material.