How does CDI work when flying to VOR without DME?

Let's show what I mean on an example.

Flying towards a VOR (not knowing the distance), HSI course pointer pointing
towards the VOR. Let's say the plane is really far from the VOR and we turn the course pointer 5 degrees (no matter what way) CDI will move to one side (far because of the distance). If the plane was close to the VOR and we turned the course pointer 5 degrees CDI would not move as much (or should not because the deviation is really small).

based on this example CDI needs to know distance from VOR to work properly, yet it works without knowing the distance. How?

(If I'm wrong about something, quickly correct me)

• I suppose you are looking for something more specific than "The receiver does some magic and converts it into a number of dots of deflection on the indicator" : )
– fooot
Aug 24, 2015 at 21:49
• @fooot, I believe the key point in this question is what the VOR CDI indicates. Aug 24, 2015 at 21:50
• When the plane is sufficiently far from the VOR, you can determine distance without DME by setting a timer, turning perpendicular to the radial and reintercepting a different radial.
– rbp
Apr 25, 2016 at 14:55

2 Answers

The VOR and ILS CDI indicates difference, in degrees, between the radial selected and the radial sensed.

If the plane was far from the VOR and we turn the course pointer 5 degrees, the CDI will move to one side. If the plane was close to the VOR and we turned the course pointer 5 degrees, the CDI would move exactly as much, because it indicates the angular difference in degrees.

This is in contrast to a CDI slaved to GPS, which indeed indicates lateral distance from the selected track in miles (en-route; GPS approach has to simulate ILS).

VOR does nothing with DME. DME is completely independent piece of equipment that was introduced later than VOR. The only relation is that there is a standard correspondence between VOR and DME channels and the DME receiver automatically tunes to the DME frequency corresponding to the VOR or ILS frequency tuned on your VOR/ILS receiver.

• This explains alot... Thanks Just for clarification: I was thinking if you had VOR/DME then you know how far you are and CDI could show the derivation like the one slaved to GPS... But yeah... I've been expecting too much magic :) Aug 24, 2015 at 22:43
• @IsawU, on approach, the angle is what you want to see, because there you need the sensitivity to increase as you approach the runway. And en-route you just know that closer than some distance you just don't need to bother and just maintain heading for the few remaining miles. Aug 25, 2015 at 10:05
• Maybe this gif (en.wikipedia.org/wiki/VHF_omnidirectional_range#/media/…) will help you understand how VORs work a little better. Here you can see what Jan explained in his question. VOR work by measuring the time difference between two signals, one is a beam pulsating 20 times per second, the other rotates 20 times per second, showing the aircraft what course it is on seen from the VOR. Aug 25, 2015 at 19:22

The change in "sensitivity" of the CDI needle as you approach a VOR has nothing to do with the DME distance - it depends on your lateral distance from the VOR.
You could be 5 miles straight up from the VOR (showing 5 miles on the DME), but the VOR receiver's sensitivity will depend on your lateral distance from the VOR.

This is because the VOR is displaying a difference in angular position: how many degrees of arc you are away from the desired course. The OBS knob on your CDI is selecting a roughly one-degree wide corridor that you want to be within, which is a pie-shaped wedge around the radial you've selected.

That wedge extends out from the VOR, getting wider as you get further from the station.
To illustrate, look at the 50-degree wide corridor highlighted in yellow below:

This has two practical sets of effects on VOR navigation:

1. The CDI needle sensitivity varies inversely with distance from the station

• Far from the VOR station the CDI needle has low sensitivity: A large variation in position (miles) is a small number of degrees of arc away, so you could be a mile off the centerline of your selected course with the needle still nearly centered.
• Close to the VOR station the CDI needle is extremely sensitive and "twitchy" - a tenth of a mile left or right could be full-scale deflection, because it represents being several degrees of arc off course.
• VERY Close to the VOR station the CDI needle twitches erratically to the point of being nearly unusable because the VOR decoder in your aircraft can't determine which radial you're actually on. (Shortly after this you will usually fly past the station, and eventually the needle will calm down again.)
2. The OBS knob sensitivity appears to vary directly with distance from the station
As Jan noted an ideal VOR (transmitter and receiver) would vary the same number of dots of deflection for the same number of degrees dialed on the OBS, whether you're 10 miles from the station or 10 feet. With real world hardware though the OBS knob's sensitivity is also affected by distance from the VOR, but in the opposite way from the CDI needle.

• Very Close to the VOR station the distance between each radial gets small enough that they are practically overlapping, and your radio may wind up decoding the VOR signal as indicating any one of those overlapping radials. Spinning the OBS knob appears to have less of an effect here, particularly with mechanical meter-movement CDIs, because the needle is twitching enough that what you're seeing is an average of what's being decoded.
• Far from the station - far enough that the VOR receiver can definitively decode which radial it thinks you're on - the CDI appears to be more sensitive: The signal is definitely 065 degrees, and when you change the OBS value to 070 the needle will move to reflect the difference between the selected and detected value, with no erroneous decodings to mess it up. At these distances the VOR behaves more-or-less ideally, as Jan described.