The OBS button
The OBS button does double duty, selecting the OBS course to a GPS waypoint (OBS mode) or Suspending (and Unsuspending) leg sequencing on your flight plan. Since these are entirely different things, examples of each will explain the differences.
Perhaps you’re aware that when on a leg to a GPS waypoint you can go into OBS mode by pushing that button, then selecting the course to that waypoint as though it were in fact a VOR station. This only works with legs that end with a waypoint, and not an altitude, intercept, radial, etc. In the GTN navigators the OBS button no longer displays “OBS” in those cases, and instead will display “UNSUSP”. The GNS 430/530’s don’t do that since they can’t even create such legs.
To illustrate the OBS mode, in Fig 1 below the GTN 750 shows a flight plan (left) from KSBP to the PRB VOR, then continuing on V113. After takeoff we’re asked to join V113, which goes from MQO to PRB on a 358° course. So just push OBS, which brings up a screen to put in the OBS course (by default it will be the 342° direct course) and announces OBS in green. In the GNS 430/530 you’ll enter the OBS course with the OBS knob on your CDI or HSI. The screen on the right shows the new course, now clearly aligned with the airway. Note that the flight leg starts off screen, and continues past PRB as a white line on the same course, whereas your flight plan makes a left turn to remain on V113.
Figure 1. A graphically-created flight plan (left) from KSBP through PRB, continuing on V113 enroute to KMOD. The course to PRB is changed using the OBS button (right) and then selecting 358°. The OBS mode is displayed in green below the button.
The question now is, what happens at PRB? Your flight plan requires a left turn, but the autopilot guidance and white course line is straight ahead. Our plan is shown in Fig 2 (left), showing us on the 358° course to PRB and in the OBS mode. To get out of that mode, so that the next leg of your flight plan on V113 can be tracked, push the OBS button and the green OBS annunciation goes away. If you do that after reaching PRB, leg sequencing will be suspended (SUSP is displayed in magenta) and you need to push the OBS button again (now an UNSUSP button) to restore sequencing. You push OBS twice after PRB with both the GTN’s and GNS’s.
Figure 2. Flight plan (left) from KSBP to KMOD along V113, with the first leg converted to an OBS course along a 358° course. Note that leg sequencing is suspended after PRB when you push OBS to exit the OBS mode.
If you want to make an OBS course to any waypoint except the next one, first go D-> to the waypoint, then go into OBS mode to select the course. Again, this will suspend sequencing when you reach that waypoint. To avoid that, choose the course to a waypoint within the D-> function. There is a place to select the course after you push D->, and the default will always be a course from present position. But you can change that to any course you like. If the waypoint is on your flight plan, sequencing will be automatic by this method.
Next, lets look at the use of the OBS button in the context of leg sequencing and suspension of that sequencing. We just saw one example in Fig 2 (right) when the first push of OBS was to exit the mode, but the second push was to restore sequencing. Our example is from the Coastal 3 departure from RW 02 at KBAF, which has a vector leg along the runway heading until cleared D-> HFD. Since there is no end to a vector, sequencing is suspended during takeoff, and will not be restored until you push the OBS button. Then, wherever you are, you’ll sequence to a leg which is D-> HFD (the first waypoint on the departure). Fig 3 shows the before and after (pushing OBS) screens on a GNS 530.
Figure 3. Sequencing is suspended on the Vector leg (manual sequence to the next leg) until the OBS button is pushed. SUSP is announced above the OBS key until it is pushed, then you will go D-> HFD as shown.
Things look different for this example when using a GTN navigator. Fig 4 shows the flight plan, which in this unit has an additional leg added before the Vector, a HDG to altitude (VA leg).
First, you have to sequence from the VA leg to the VM leg, which is automatic (on reaching 1700 ft altitude) if you have altitude information sent to the unit from an Air Data system, say from a G500 or Aspen. Otherwise, push UNSUSP to sequence as in Fig 5 (left). Now, you’re on the VM leg and need to push UNSUSP again to sequence to the leg D-> HFD (right).
Figure 5 Sequence to the Vector leg on passing 1700 ft either automatically, or if no HDG input to the 750, manually by pushing UNSUSP. Sequence to D-> HFD by pushing the button again.