When the CDI is centered, the OBS should read within six degrees of the selected radial

During an airborne VOR check, what should the omnibearing selector read when the CDI needle is centered?

• A. within 6° of the selected radial
• B. 0° TO, only if you are due south of the VOR
• C. within 4° of the selected radial

Answer: (A)

When the Course Deviation Indicator (CDI) needle is centered during an airborne VOR check, the omnibearing selector, also known as the OBS (Omni Bearing Selector), should read within 6° of the selected radial. This is because VOR (VHF Omnidirectional Range) accuracy standards allow for a certain amount of deviation. This 6° tolerance ensures that pilots can maintain situational awareness and navigate accurately while accounting for potential variations caused by equipment discrepancies, atmospheric conditions, or other factors. When the CDI needle is centered, it indicates that the aircraft is precisely following the selected radial, confirming that the VOR receiver is functioning correctly. In contrast, the other options suggest stricter tolerances or specific situations that would not generally apply during a standard airborne VOR check, which is a straightforward procedure to verify the operational status of the VOR navigation system.

Outline

• Hook: Navigating with confidence starts with the basics—VOR accuracy is a quiet, dependable ally.

• Explain CDI and OBS: what the needle and the Omnibearing Selector tell you.

• The key rule: when the CDI is centered, the OBS should read within 6° of the selected radial.

• Why 6°, not 0° or 4°: a practical look at tolerance and real-world conditions.

• How to perform an airborne VOR check: quick steps and what to watch for.

• Common pitfalls and tips: keeping your navigation trustworthy.

• Real-world takeaway: tying it all back to safe, efficient flight.

• Friendly close: a reminder that good navigation comes from steady checks and clear understanding.

VOR navigation, explained like a chat with a seasoned pilot

Let’s start with the basics you’ll use every time you hop in the cockpit or study the navigation systems. A VOR provides a precise line of bearing to a ground station. You can think of it as a magnetic-guidance beacon in the sky, but modern avionics add layers of checks to keep it honest. Two things matter most here: the CDI, or Course Deviation Indicator, and the OBS, the Omnibearing Selector. The CDI shows how far you are off the desired course, while the OBS lets you pick which radial from the VOR you want to fly.

When the CDI needle is centered, it isn’t just luck or a momentary calm sky. It’s a signal that you’re exactly on the selected radial—at least, within the aircraft’s tolerance. That tolerance is what we’re zeroing in on today.

The 6-degree rule, plain and practical

Here’s the core fact you’re looking for: when the CDI needle is centered during an airborne VOR check, the OBS should read within 6° of the selected radial. Why 6°? Because VOR systems aren’t perfection machines. They’re robust, but conditions—signal drift, equipment wear, atmospheric quirks, even the airplane’s own movement—introduce small variances. A 6-degree leeway gives you a realistic, safe margin that still confirms the VOR receiver is functioning properly and that you’re tracking the intended path.

This tolerance isn’t about chasing exactness to the minute. It’s about ensuring that you’re not wandering off course because of a flaky signal or a misbehaving needle. If you’re exactly on the radial to the degree, that’s great. If you’re within 6°, you’re still in the green. The key word is “within,” not “dead on.” The real world doesn’t hand pilots perfectly centered needles every time, especially when you’re moving and listening to the air around you.

Why not 0° or 4°? A quick reality check

That leaves the other options you might see tossed around: 0° TO only if due south, or within 4°. Here’s the straight answer: those are more about specific situations or stricter expectations than a standard airborne check. The 0° TO rule would imply a very particular geometry to the VOR and aircraft position that doesn’t hold as a general rule for routine checks. And 4° would be a tighter tolerance than typical aviation standards assume under normal flight conditions. In practice, you use the 6° guideline because it aligns with how VOR stations and receivers are designed to operate day to day, with enough cushion for small, inevitable deviations.

Think of it like a speed limit. A 60 mph limit is about safety and practicality; telling drivers “you must be exactly 60.000 mph” ignores real-world bumps. The 6° limit is the aviation equivalent—strict enough to keep you on course, forgiving enough to account for normal noise in the system.

How to perform an airborne VOR check without overcomplicating it

Let’s map out a simple, real-world approach. You don’t need a toolbox full of gadgets to do this well—just a few common steps, done calmly:

• Set the OBS to the desired radial you want to verify.

• Select the corresponding course on the CDI so that you’re attempting to fly that radial.

• Watch the CDI as you briefly hold the course; your goal is to see the needle center.

• When the needle centers, read the OBS. If it’s within 6° of the selected radial, you’re in the green.

• Note it down with the date, time, and weather/aircraft conditions. This keeps the check honest and traceable.

A few practical tips:

• Cross-check with another instrument or navigation aid if you can. GPS overlays or secondary VOR indicators can help confirm that you’re not chasing a phantom.

• Be mindful of multipath signals or near severe weather, which can muddy the signal and push the needle off a hair.

• If you find you’re consistently above or below the 6° mark, you’ve got a good reason to investigate the VOR receiver, antenna, or grounding. Equipment health matters just as much as technique.

Common pitfalls that trip up even seasoned flyers

Navigation is part science, part art, and a pinch of patience. Here are a few slip-ups to watch out for:

• Assuming the needle “stays centered.” VOR needles can drift with atmospheric changes. Recheck after a short maneuver or a slight change in position.

• Relying on a single reading. If you’re in doubt, re-verify using a different radial or a second VOR station—redundancy saves you in the air.

• Ignoring the OBS readout immediately after a course change. Give the system a beat to settle before you lock in the new track.

• Forgetting to log the check. A simple note with date, time, and conditions helps you spot patterns later, whether you’re reviewing for maintenance or training.

Why this matters beyond the cockpit

This isn’t just about getting a number right on a screen. It’s about maintaining situational awareness, building confidence in the aircraft’s navigation systems, and ensuring you can navigate safely in varied environments. A reliable VOR readout is like a trusted compass when you’re climbing through the clouds or threading your way through airspace with multiple radio channels. You’ll rely on it for en route planning, approach phases, and those moments when other navigation aids might be out of service or temporarily unreliable.

A note on tools, tech, and the human factor

The modern cockpit blends digital displays with classic radio navigation. You’ll see VOR indicators on primary flight displays, multi-function displays, and dedicated navigation radios. The science part is the math and the signal propagation; the art part is knowing when to trust the readouts and when to cross-check. Even with advanced automation, pilots keep a hand on the wheel a little longer than you might expect. It’s about exercising good judgment, not surrendering to automation.

If you like analogies, think of the VOR system as a city lighthouse. The light (the VOR signal) remains steady, but the weather, wind, and ocean currents can shadow its glow. Your CDI is the beam you steer toward, and the OBS is the compass you turn to align with that beam. The 6° allowance is the harbor’s safety margin—enough to keep you on course without registering a false alarm just because a squall popped up for a moment.

Bringing it all together

So, what’s the bottom line when you’re mid-flight and the CDI needle sits calmly centered? The OBS should read within 6° of the selected radial. That small window is the practical truth of VOR navigation: it confirms that your receiver is healthy, your setup is sound, and you’re tracking the intended path with reasonable confidence. It’s a quiet, constant check you perform without fanfare, but it pays off in safer, smoother flying.

If you’re curious, you’ll find that this standard isn’t a dry rule carved in stone; it’s a reflection of how navigation equipment behaves in the real world. The 6-degree tolerance accommodates minor signal variations, instrument wear, and environmental effects—while still providing a clear signal that you’re linked to the right radial. It’s a balance between precision and practicality, and it’s exactly what keeps pilots reliable when it matters most.

Closing thought: practice makes confident

You don’t need dramatic moments to become confident with VOR navigation. It’s the small, consistent checks—the centered CDI, the 6° readout, the habit of logging results—that build trust over time. And when you combine that with occasional cross-checks and a healthy respect for instrument limitations, you’re not just passing a test—you’re flying with steady competence.

If you’re revisiting these concepts, remember the key takeaway: during an airborne VOR check, a centered CDI means the OBS should read within 6° of the chosen radial. That precise, sensible rule is the thread that ties together accuracy, safety, and calm mastery in the cockpit.