OBS stands for Omni-Bearing Selector, the knob on a CDI that lets a pilot select the radial he intends to track from a VOR ground station. By turning the outer ring, the OBS knob simultaneously rotates the compass card and swings the course deviation indicator, giving a clear steering command without the need to retune the radio.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is OBS in aviation?
The OBS (Omni-Bearing Selector) knob allows the selection of the radial (course) via the outer ring, and it is located on the CDI. This knob works off of the NAV radio OBS button on a GPS unit, and it enables a pilot to select which direction they wish to fly towards or away from a VOR navigational beacon or GPS waypoint.
An OBS (Omni-Bearing Selector) controls the pilot’s navigational instrument. Initially, OBSs were connected to a NAV indicator that worked with a VOR receiver. These navigational radios receive VHF (Very-High Frequency) signals from a VOR (VHF Omnidirectional Range) ground station. The locations of VOR stations are shown on aeronautical charts, along with their frequencies. The pilot can input the frequency and then navigate using that ground station. The OBS controls the display in the cockpit to tell the pilot different things about the station. For example, the instrument can tell the pilot the course to fly either directly toward or away from the VOR by turning the OBS.
How does an OBS work in aviation?
The Omni-Bearing Selector is a course-selector knob on the panel-mounted navigation radio. Turning the knob rotates a compass card inside the VOR indicator; the pilot uses the card to pick the radial he wishes to track to or from the VOR ground station. Once the radial is selected, the instrument shows lateral deviation from the VOR radial and the TO/FROM flag announces whether the selected course will take the airplane toward or away from the station; actual airplane heading is irrelevant because the system only works in the horizontal plane. The same knob works with modern GPS units: pressing the OBS button places the receiver in OBS mode, suspends automatic leg sequencing on the flight plan, and causes the GPS to act like a VOR receiver. In OBS mode the pilot can choose any course line to the loaded GPS waypoint, needle and flag then respond as if a VOR station were sitting at the waypoint, giving continuous guidance until the pilot pushes the OBS button again to unsuspend sequencing and resume normal GPS navigation.
How to use an OBS in aviation?
To use an OBS in aviation follow the instructions given below.
- Use OBS mode for procedure turns
- Use OBS mode for missed approaches
- Use GPS unit in OBS mode at holding patterns
- Use GPS unit in OBS mode at missed approaches
- Set outbound course into OBS
- Use OBS mode for procedure turn on V113 leg
- Bring up screen to input OBS course
- Select OBS course to GPS waypoint
- Do double duty with OBS button
To use an OBS, first start by rotating the knob. On a VOR navigator head, the OBS knob allows selection of the radial you wish to track from a VOR ground station; rotate the knob until the desired course is centered under the course-deviation bar, then fly the indicated heading to maintain that radial. In GPS installations, pushing the OBS button brings up a screen to enter an OBS course, instantly turning the GPS into a VOR tracker that delivers left-right guidance along the selected bearing. The same button suspends automatic leg sequencing, so the unit will not advance to the next waypoint until the pilot again pushes the OBS button to release the hold. Pilots engage OBS mode for procedure turns, missed approaches, or when cleared to track a specific outbound course. Once established, they can set the inbound course and, with reverse sensing, fly the outbound leg while still watching the same needle. When the procedure is complete, another press of the OBS button resumes sequencing along the flight plan, allowing the airplane to transition to the next leg on V113 or any other airway.
What are the differences between an OBS and a CDI in aviation?
The difference between an OBS and a CDI is that the Omni-Bearing Selector (OBS) is not a separate instrument, while the CDI is a standalone instrument. The OBS is the knob on the left side of the Course Deviation Indicator (CDI) that lets the pilot select the radial to be tracked. That radial is the course to or from the VOR ground station. By rotating the outer ring, the OBS determines which of the 360 spokes will be compared with the airplane's present position.
The CDI itself is a standalone avionics instrument that shows only course deviation. It consists of the OBS knob, a single left/right needle, and a TO/FROM indicator. After the radial is chosen, the needle moves proportionally to the aircraft's distance from that invisible line, each dot on the scale equals two degrees off course. The direction of deflection tells the pilot which way to turn to intercept the selected radial, while the TO/FROM flag shows whether following that line takes the aircraft toward or away from the station.
