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Do you use the VNAV feature with Garmin GPS?


Runtoeat

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Absolutely.

 

On my 496 I have it programmed to have me 1,500' agl 3 miles from my destination. That gets me about 500' above the pattern on my overfly of a non-towered airport.

 

I just fly the needle like a faux glideslope.

 

At low altitudes, barely a nicety. But for high up, it's quite an aid. In my Cirrus at 17,500', I might spend about 30 minutes in a 500 fpm descent. That meant starting down around 100 miles out from the destination. Descending too soon made for a bumpy ride down low when I was based in FL. Descending too late makes for a pretty radical descent at the end.

 

Hardly an essential tool, but definitely in the "nice to have" category.

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On every trip. (796)

I have mine set for 1,000 above airport elevation from two miles out.

 

Also, got some first time use from the obstacles and terrain warning feature in some marginal wx on a return trip from the NY Finger Lakes.

 

What's next in Tech?

 

Rich

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Today in history...

 

VNAV management did not start with modern GPS's.

 

Here's how we used to do it, and teach students to do it:

 

I'm cruising at 8,500' westbound.

 

My destination field elevation is about 1,500'.

 

Pattern would then be about 2,500'.

 

Add 500' to that for an overfly, for a target of about 3,000'.

 

So, 5,500' to lose.

 

At 500 fpm, that would take 11 minutes.

 

Get out the whiz wheel, and plug in 11 minutes and the approximate groundspeed in descent to find out how how far out to begin descent.

 

Get out the plotter and mark that distance on the sectional and find a landmark to identify when to start down.

 

WHEW!

 

You youngin's don't know how good you got it!

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It wasn't that hard out west in the Skyhawk. You cruise around 10,000' ( give or take ) and you make 2 miles / minute.

 

On almost every flight I would start a 500'/min descent at 40 miles out. And we had DME.

 

That's for the trip there the the trip home it doesn't matter, we don't have to descend, we live in the sky.

 

4 miles to descend 1,000' is math you can do in your head, not sure I ever had a whiz wheel.

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When the bar is centered on my 496 HSI I should be descending 500'/min to keep it there. On many flights whey you are at this point there is a huge mountain in front of you, Mammoth to Stove Pipe Wells comes to mind.

 

What do you do in similar circumstances? Fly farther and descend at 500'/min or see how steep you can go down?

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CT, not sure I get the layout and relevant altitudes. How far are you from the airport and at what altitude when you start following the VNAV profile? Is the mountain in your path and do you need to clear this before you can start your 500 fpm decent?

 

Dick,

 

The vertical profile below is from Mammoth to Stove Pipe Wells, The redline is an approximation of what the default glide path that that Garmin 496 gives me, you can see the terrain conflict. It is typical for a number of flights that I do in California.

 

post-6-0-70456900-1379801037_thumb.png

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OK, got it. Great graphic. Hmmm, how about setting a VNAV profile that you intercept after the last peak? How high is the last peak and how far from the runway? Let's say the peak is 12,500' agl and it's 13 miles from the runway. Assume 80kt approach speed which = 92mph = approx. 1.5 miles/min. With a VNAV of 500' and one mile you would need to decend 12,000' in the next 12 miles to hit your VNAV point. If I understand how the VNAV works, it appears that your VNAV profile would require a decent rate of 1,500 fpm during the 8 minutes it would take to get to your VNAV target point. Now, those mountains look to be closer than 13 miles. If they are higher and/or closer than in my example, I can imagine that this can really get interesting. Question, do you make a lot of extreme diving approaches to the airports you frequent??

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Question, do you make a lot of extreme diving approaches to the airports you frequent??

 

I never get close to Vne, even on smooth days penetrating layers that fast isn't comfortable. Prolonged slips are uncomfortable so I usually settle for 1,200fpm descent at the highest comfortable speed.

 

There are a couple of steep ones at the coast that tend to involve weather as well.

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I have and do once in a while just to cut down on my cabin heat on hot days. I did once into Yuma International once from 6500 to 1200' to get into the required air space. 3300' pm descent and no flaps at 160 mph. Smooth as silk. 1100- 1500 pm descent is not that uncommon for us out in our group in the hotter months.

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Real life usage from yesterday's leg from Peru, IN to Danville, KY.

 

Cruising along at 9,500' to take advantage of a nice tailwind. Note the "glideslope" just starting down from the top of the "HSI" on the Garmin:

 

9895467326_a77c752325_z.jpg

 

Note: I had done the mental math: field at 1,000', pattern at 2,000', flyover at 2,500', 7,000' to lose, start about 14 minutes out @ 500 fpm.

 

The Garmin then gives you a head's up:

 

9895479864_7c29fb86b6_z.jpg

 

Just about time to follow that "glideslope":

 

9895445965_bfa4cba616_z.jpg

 

Screaming down the "glideslope":

 

9895484874_2b74fd479c_z.jpg

 

Bear in mind that 120+ knots GS may be ho-hum in a CT - it's a very rare occurrence in a Sky Arrow!

 

Did I mention it was a beautiful day for flying?

 

In and around Lake Cumberland, KY:

 

9895479386_541d4fa6ef_z.jpg

 

9895588853_7c9706e094_z.jpg

 

Life is good!

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All you guys keep in mind with the Garmin and with the mental math, you have to account for the fact that when you point the nose down you are going to gain airspeed and as you gain airspeed you increase lift which means you have to push the nose down farther...... Garmin or mental math, I like to swag in a couple of minutes lead time. That is only me. Others will have methods that work for them. There are many ways to do this and it's good to see that we have many who employ both the old and the new and some use both.

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Yes, the (usually) increased groundspeed and changing winds aloft with altitude will always throw off one's calculations a bit. The increased groundspeed will usually mean a bit more than 500fpm average, in my case, to keep the needle centered*. Still, close enough for government work!

 

Not mentioned in my post was some scattered clouds that I had to maneuver around and I had to increase my rate of descent for a while to maintain clearance. Leveled off for a while shortly thereafter to "catch up" with the needle.

 

These are all just hints and guidelines and aids. I've seen lots of pilots poorly plan descents - better to say they did not plan the descent at all - and end up bumping along down low or dive bombing the field at the end. Any of the ways mentioned so far are an improvement over that!

 

 

*click on the below "screaming down" photo and notice the Dynon is indicating 650fpm - which is still in most people's comfort zones.

 

9895484874_2b74fd479c_c.jpg

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You can always make your decent with power instead of pitch. If you pull power back, your airplane will seek its trimmed airspeed for the new power setting, which will put you in a gentle descent. Your speed will stay about the same, and you'll save fuel! This is how I usually manage descents unless I'm in a hurry to get somewhere.

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Hi Dick,

Your aircraft should not have any flutter to our published VNE and our real VNE is much higher. When I do things like this learning to feel the plane through the stick and hull are important. This is well within the aircraft flight envelope, just maybe out of somes comfort zone. Just blindly doing anything to a plane is never good. If you want to see some altitude loss you should see the original FD video. I just pull the power back and lower the nose and it stays as smooth as silk. Even the stick bump that occurs sometimes in level flight goes away.

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Hi Dick,

Your aircraft should not have any flutter to our published VNE and our real VNE is much higher. When I do things like this learning to feel the plane through the stick and hull are important. This is well within the aircraft flight envelope, just maybe out of somes comfort zone. Just blindly doing anything to a plane is never good. If you want to see some altitude loss you should see the original FD video. I just pull the power back and lower the nose and it stays as smooth as silk. Even the stick bump that occurs sometimes in level flight goes away. 1500' pm isn't even considered sever.

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