Thursday, March 22, 2012

When Is It Time to "un-SUSP"?

“To be or not to be, that is the question.”

So wrote the Bard in the beginning of Act 3 of “Hamlet”. But for the instrument pilot, the more relevant question is

To press or not to press OBS when it shows SUSP, that is the question.

As pilots familiar with flying approaches with the popular Garmin 400/500 GPS units know, waypoint sequencing is handled automatically. As you fly over (or by) one waypoint, the VOR head or HSI momentarily shows a FROM indication and then promptly reverts to a TO indication as the GPS automatically picks up the next waypoint in FPL0 and makes it the active waypoint. This holds true for both the enroute environment as well as approaches – to a point.

GPS waypoints are generally classified as fly-by or fly-over waypoints. Almost all of them are fly-by, meaning the GPS unit anticipates when you should start turning to the next course heading in order to roll out on the new course. Consequently you fly by the waypoint rather than directly over it. The exception to this paradigm is the MAP waypoint, which is most definitely a fly-over waypoint. When you fly over the MAP waypoint, two things happen. First, automatic sequencing of waypoints is suspended. The MAP waypoint remains the active waypoint. Second, to catch your attention, the GPS displays SUSP in bright green letters over the OBS button. This is telling you that automatic sequencing has stopped, or is suspended, and the unit is waiting for instructions from you.

The normal sequence of actions is for the pilot to press the OBS button. This tells the GPS unit that the pilot wants to go to the waypoint for the published hold. It removes SUSP and the next waypoint in FPL0 becomes the active waypoint. Generally this is where you will hold, but there may be some intermediate waypoints before the hold. Once the OBS button is pushed, the unit starts providing navigation information to the next active waypoint. And therein lies the rub. You may not be ready at that particular moment to start toward the next waypoint. This situation most commonly occurs when the next waypoint is not straight, ahead and the missed approach instructions direct you to climb to a certain altitude before turning to navigate to the next waypoint.

An example of this can be found in the NDB or GPS approach for runway 34 at Cambridge (KCBG). The approach plate is shown below.

The important thing to note here is that after reaching the missed approach point, which is the runway threshold as far as the GPS is concerned, you are at a minimum of 1480’ altitude and you need to climb straight ahead a thousand feet before starting a right-hand turn back to the NDB for the hold.

The really important point here is that the pilot should NOT hit OBS to reactivate waypoint sequencing until the altitude of 2500’ has been reached. Because once you do so, the GPS immediately wants to guide you back to the holding waypoint from your current location. So don’t press OBS until you are ready to turn. Once you are climbing to 2500’ and before pressing OBS, the GPS will look like this.

Depending on wind and aircraft, you could be two or three miles north of the NDB before you have reached 2500’ and are ready to turn. Once you press the OBS button, SUSP is cleared, automatic waypoint sequencing starts up again, and the unit provides guidance to the published holding waypoint.

One thing worth mentioning is that when you review the waypoints put into FPL0 by loading the approach, you will see an extra CBG between the MAP point, RW34, and the holding waypoint CBG. The only explanation I can come up with is that this is an overlay GPS approach of an old NDB approach. Because the NDB is on the field, there is no FAF. In examining the profile view, you can see that once you are established inbound, you simply start down to MDA. This works fine if you are flying it as an NDB approach. However the GPS unit needs a waypoint to tell it when to be at approach sensitivity. So it creates what is known as a procedural waypoint. In this case it is DUMDY. After the procedure turn, two miles before DUMDY the GPS unit starts scaling down to approach sensitivity. To the GPS unit, the MAP is at the runway threshold, but the NDB is actually about mid-point on the runway. I think this is the source and explanation for the extra CBG waypoint in FPL0. So what do you do with that extra CBG waypoint? Easiest thing is to simply delete it. Highlight it with the cursor and press CLR. Confirm that you want it gone, and it will vanish.


James David said...

Linda, your blog is quite informative. Right timing on when to push buttons in flying should be known to pilots. Thanks for this post. It is really helpful.

James David teaches people how to buy single engine airplanes & has a passion for the Cessna 177

Rydair said...

Great content here Linda!
Check out my blog to se my way to becoming a flight instructor.

Anonymous said...

DUMDY is a computer navigation fix which serves no purpose for pilot/controller communication. It is only for GPS sequencing. That is to say that the GPS is a To-To navigation unit and it doesn't know whether you are outbound to execute the PT or inbound for the approach so DUMDY simply acts as a reference to sequence the GPS waypoints correctly. A CNF will be identified by an "x" and a 5 letter name in parenthesis. Refer to AIM 1-1-19.