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.

Thursday, March 15, 2012

More on Procedure Turns

One of the things I often encounter is confusion over when a procedure turn may be started. This generally occurs when an approach has both a DME arc and a procedure turn. Consider the ILS-27 at Duluth (DLH). The government-issue approach plate is shown below.

The procedure turn is shown outside of the DME arc. I believe it is depicted this way for chart clarity, so it doesn’t get too cluttered. But depicting it this way often leads pilots to believe they must be beyond the 15-mile DME arc before doing the procedure turn. This however is not true. The only requirement is that the standard procedure turn be completed within 15 nm of ANDOE, the OM, and it must be done on the north side of the localizer. Other than that, the point at which to start the turn, as well as type and rate of turn are pretty much left up to the pilot. This of course is not true if the procedure turn is accomplished in a holding pattern or teardrop pattern. Those must be flown exactly as depicted.

A glance at the profile view confirms that the distance requirement is simply remain within 15 nm of ANDOE, the OM. If the procedure turn were a holding pattern, the profile view would say something like “One minute holding pattern”. If it is a teardrop pattern, the profile view would say something like “Teardrop within 12 nm of LNK VORTAC”. See the ILS-18 for LNK (Lincoln).

There are some other things worth noting on ILS-27 for DLH. In big bold letters on the plan view there is the statement DME OR RADAR REQUIRED. Anytime you see a note like this on the plan view, it is talking about the requirements to get from the enroute environment to an IAF. This approach has three IAFs. The two ends of the DME arc can be identified with DME of course (or GPS). The OM, ANDOE, has the word RADAR above it, meaning ATC can identify this fix for you.

Another item to note on this approach is the existence of an alternate holding fix. It is PYKLA, which is an LOM for the ILS-9 at DLH. Alternate holding fixes are becoming more common. They exist in most cases when the navaid for the primary hold is not the same as the navaid for the approach and it is out of service. In this case, the primary hold is based on the DLH VOR. If that is down however, PYKLA becomes the holding fix. There are no charted directions on how to get to the alternate holding fix. In the event however that the primary navaid is out of service, directions to the alternate will generally be found in NOTAMS or of course ATC can give them to you.

Jeppesen shows this same approach in a better manner or least a less confusing manner. The procedure turn is shown inside the DME arc. The Jeppesen chart is shown below.

For a longer discussion of procedure turns, you can go to the May 2007 archive.

One final note on procedure turns. For the standard procedure turn the AIM states the type and rate of turn is left up to the pilot. So you can fly the charted 45° turn outbound, do a 90°-270° turn or whatever works for you. One thing to remember is that on the 45° turn outbound, when you are ready to do a 180° turn to come back to the course, you may turn either right or left. It does not matter. If the charted procedure turn shows the standard right-hand 180° turn and you are getting close to the limit specified in the profile view, turn left! It’s perfectly legal to do so.