A Review of the Garmin VNAV Functionality, Part I
by Brandon Ray, CFAI+, CSIP, Master CFI, ATP, Gold Seal CFI, CFII, MEI, AGI
The VNAV function on a G1000 or Garmin Perspective system is a great resource to assist pilots in planning descents. It takes a lot of the math out of the process. Here are some advanced tips and tricks for utilizing the VNAV function properly.
If you are a beginner with descent planning or VNAV, you may find it helpful to read this article first: https://www.flyhpa.com/2010/10/g1000-descent-planning/
The VNAV profile (found in the lower right of the FPL page on the MFD) sets a predetermined FPA (Flight Path Angle) of 2.5˚ in the Columbia / Cessna Corvalis (1.7˚ in the Cirrus aircraft). In the Cessna Corvalis, 2.5˚ results in a VS Target (a function of your current groundspeed) of around 700-800 fpm at typical cruise speeds, or 500-600 fpm at slower speeds for transitioning to an approach. Works pretty well in most cases, but you can change the VS target or the FPA (each one affects the other) if desired. You will see that this affects the position of the TOD (top of descent) along your route. In an unpressurized piston aircraft, 500 fpm may be a reasonable target for passenger comfort.
WARNING: VNAV does not take into consideration terrain or obstacles. If you plan a descent through a mountain, you will not receive any warnings. Please keep terrain in mind when planning your descent, especially for airports that are in valleys. For these airports, you may need to consider a steeper descent angle than normally necessary.
Along Track Offset
Try out the ATK OFST (Along-Track Offset) feature sometime for enroute descents or with crossing restrictions. Works well, especially since you can couple it with the GFC700 (if you have a VNV key). This function is found as a softkey on the FPL page on the MFD.
The 5 Minute Rule
In order for the autopilot to capture the vertical path (VPTH), the VNAV has to be armed/acknowledged within 5 minutes prior to reaching the TOD. This can be accomplished by pressing the VNV key on the autopilot or by adjusting the altitude bug.
BEST PRACTICES TIP: It’s best to wait for the 1-minute “Vertical Track” alert. This “Vertical Track” callout occurs 1 minute prior to reaching the TOD. That’s when the altitude should be selected and “VNV” should be armed (in that order). Arming VNV once is all that is required for multiple subsequent step-downs. (If you press VNV again, you may actually un-arm it). Check your autopilot status bar at the top of the PFD.
VNAV and GP
VNAV & GP can both be armed simultaneously without causing any issues (as long as you know when to expect each one). VNAV typically works up to one fix prior to FAF. Then the glidepath becomes active. (If applicable, the GP is armed by pressing the APR key on the autopilot).
Which Altitude will the VNAV Use?
VNAV will always level off at the higher of the 2 altitudes: Either the selected altitude or the VNAV altitude. Take a look at the subtle change from ALTS to ALTV in the autopilot status bar if they are in disagreement… This is how you can know where it will level off.
One more trick for you advanced VNAV users… Have you ever noticed the VNV Direct softkey at the bottom of the FPL page on the MFD? Try it out next time you fly. It is just a simple way of moving the TOD up to your current position. Useful when ATC says to start your descent NOW, and you still want a reference. VNAV direct does the same thing for you vertically that the direct-key does for you laterally.
BEST PRACTICES TIP: Once you activate VNAV-Direct, the TOD will be at your position almost immediately, so you might consider selecting the altitude and arming VNAV, prior to pressing the enter key, this way you don’t fly past the vertical path without the autopilot having a chance to capture it. If you do accidentally fly through it, no problem. Just re-activate VNAV-Direct and it will move the TOD to your current position once again.
Vertical Speed Required (VSR)
BEST PRACTICES TIP: If you’re going to use VNAV often, it would be a good practice to make VSR (Vertical Speed Required) one of your MFD data bar fields at the top of the screen. This way, you always know what the current vertical speed requirement is.
Need a practical example? In my next article, I’ll describe using VNAV with an actual approach.
How it Started
One of my best friends in high school, (Doug Gray) was a private pilot. He offered to take me up for a flight in a 1967 Cessna 150, N6228S. We took off from Calhoun, Georgia, and he took me on a scenic tour of the area, I was hooked. I later found out that my English teacher, (Jan Haluska) was also a flight instructor and the school was offering a ground school course the next year, which he taught, along with flight training with the goal of becoming a private pilot. I managed to talk my dad into funding the training, at the time the total cost was right around $500. Cessna 150 rental rates where $12 an hour, and the 172 we used for cross country was $15 an hour including fuel.
It started August of 1977. The fall semester rolls around, and I am enrolled in ground school, and if memory serves me, we met twice a week. First came the paperwork for my student pilot certificate, which at the time was included with the 3rd class medical. I loved ground school, especially learning navigation, plotting courses on the sectional, again this was before iPads and GPS, so we did a lot of dead reckoning and VOR navigation for cross countries. At the end of the course, I made an 83% on the Private Pilot written exam, remember this was before we had the question-and-answer books that on future test I would consume before taking a written.
The fun really began on August 22, 1977, it was my first flight. To my surprise we used the same Cessna 150, N6228S that my friend Doug Gray had taken me for a ride in. The Cessna 150 had a standard VFR instrument panel, with one NAV/COM, and one VOR CDI. No intercom, so no headset, at the time I didn’t know what I was missing. I didn’t get my first headset until I started my instrument training in 2003. I was a big guy, I was 6’ 4” and weighed 245 lbs. but I do not remember being uncomfortable in the 150 even with the instructor in the right seat.
First flight lasted .8 hours, and we accomplished orentation, shallow turns, stability, and effects of flight controls. Over the next few months we added steep 720’s, slow flight, stalls, turns around a point, S-turns, emergencies, landings, (short field, soft field and normal).
At this point I want to talk about a training experience that still stands out. We were close to solo and were practicing takeoffs and landings. Turning base to final Jan got very upset at the way I was cross controlling the aircraft. Cross control is when you are in, say a left turn, and use opposite rudder to line the nose up with the runway. So, he had me depart the pattern and head east to the practice area, and climbing up to 5500 feet MSL, he had me slow down to just above stalling speed, start a left shallow turn and add right rudder. As the airplane stalled I had the strangest sensation, no roller coaster has ever come close, instead of blue sky in the windscreen I was looking at brown ground, and as far as I could tell we were upside down, through the terror of the moment Jan talked me out of the spin, controls neutral, oposite rudder, pull slowly out of the dive. As we leveled off he asked me what altitude we were at, as I remember it was around 2,800 feet MSL. Then he asked me what would happen if I experenced this on base to final in the traffic pattern. The answer was obvious, I would be a pile of wreckage off the end of the runway with a very short-lived aviation career. Needless to say this cured my cross-control tendencies.
Another training event the sticks out in my mind was the first time we did takeoffs and landings at the High School runway. The runway was 1,500 feet long, not sure of the width, but it seems like we had about 5 feet on each side of the wheels when on the center line of the runway. Landing from the south you also had to go between a cutout in the trees to be able to stop in time on the runway. After getting confortable landing here every runway since seemed huge. I remember landing at Chattanooga (KCHA) on a night cross country and commenting to Jan, that I felt like I could of landed sideways on the runway, it felt that large.
Very soon after this I started wearing old shirts to all my flight lessons, the reason for this occurred on February 8, 1978. The lesson that day was stalls, takeoffs and Landing. As we were taxing back in Jan told me it was time for my first solo. I was very excited, and after some last minute advice from Jan, including watch for floating on landing the plane will be light with him not in the right seat, complete 3 takeoffs and landings to a full stop. It was a blast, and at the end of my shirt was shorter in the back because Jan cut the tail out of it signed and dated my solo [an aviation tradition]. Total flight time accumulated on the day of my solo was 18.1 hours. I was now officially a pilot with solo priveleges.
In my next article I start cross country training, when Jan decided that it would be best accomplished this in the 172. Checking out in N5970R was like moving into a 747, this started a love affair with one of my faviriote planes to date. Since this time I have flown over 900 hours in many different models of the 172 and feel like I am stepping into an old friend every time.
More about Randy here: https://www.flyhpa.com/team/randy-delong/
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