As a pilot, I hate to do math. Before Foreflight, it was a toss up of which was worse, doing math in my head or using the manual E6B. After Foreflight, my math and E6B skills have deteriorated somewhat. Even worse than doing math while pre-flight planning, is doing math in my head, in the airplane, with all the noise of the engine and radio. The math skills just don’t click as well with all that going on.
Enter in all of our modern GPS units. Among many other wonderful things that modern GPS units give us, the ability not to have to do math is a big one. One function where this shows up is descent planning.
Descent planning becomes more critical the higher the cruise altitude is. If your normal cruise altitude is 1500 AGL (I’m looking at you, tailwheel pilots), then your descent planning is easy. See the airport, go down. If your normal cruise altitude is 28,000 feet (typical turboprop cruising altitude is the service ceiling of the airplane, which, depending on the plane, can be between 27,000-34,000), then there’s a lot more planning that needs to take place. And you can’t always trust ATC to start you down in time.
Both Avidyne and Garmin GPS units have descent planning functionality built in to them. On the Avidyne IFD units, the descent profile is built on the flight plan by inputting a specific altitude at a specific waypoint (or setting in an offset), then the IFD plans out a descent based on a pre-programed descent angle for that particular type of airplane.
The functionality of VNAV on a Garmin GPS unit varies, but the bones are all the same:
- Pick a Waypoint
- On a 530 unit, there is a VNV button; on a 430 unit, it’s the last page on the Map chapter
- On a 750/650, tap the altitude column next to the Waypoint
- On a G1000/Garmin Perspective, using the cursor, highlight the altitude column next to the desired Waypoint
- Input the altitude that you want to descend to at the waypoint
- Select a mileage offset if desired
- On the G1000/Perspective, in order to do this, you have to highlight the waypoint first, then press the ATK OFST soft key at the bottom of the MFD
- On the 750/650, it will be on the VNAV screen
- Once all this is set in, then the GPS will calculate your TOD and BOD points and away you go
If ATC hasn’t given you a lower altitude by the time you get to your Top Of Descent (TOD), request lower.
If using a Garmin Autopilot (GFC 500, 600 or 700) and a GTN 750/650 or G1000/Perspective, the autopilot will fly the VNAV profile for you. Just wait to hear the “Vertical Track” chime (this indicates 1 minute before the TOD), then it’s a 2 step process. Press VNV on the autopilot and set your altitude selector for the altitude you want to descend to. When you arrive at your TOD, watch the magic happen.