Planning descents

[MATTS875]

what kind of descent planning are you all doing? I ahve a friend that flys and I believe I understood him correct but he said that if he is flying say 5000'  he takes the 5000' divide by 2= 2500 and starts his standard 500' minute descent about 25 miles out.I think I understood him correctly on this.

[dcrogers11]

I never pay a lot of attention to the miles out, but instead look at minutes from destination on my Garmin.  For a 500 fpm decent my simple math is Cruise Altitude - Pattern Altitude / 500.  (ex. (8000-1500)/500=13 minutes out that I start my 500 fpm decent.  Or easier yet, just subtract the cruise and pattern alts and multiply the number by 2 and take the resulting number's two first digits as the minutes.  (ex. (8000-1500) x 2= 13,000).  After doing it a few times it becomes pretty easy.  

[stevesm20b]

That works if your ground speed is 150 in the descent. If your going 180 in the descent you have to start your descent 30 miles out if you want to drop 5000ft at 500fpm. I just divide my groundspeed I'm traveling in the descent by 60, and that will give me the miles per min I'm traveling. Then I take the time it's going to take to descend. 5000ft divided by 500fpm is 10 min. Multiply the two numbers together and you get the miles out you have to start your descent. 

[Ned Gravel]

I plan 3 mins per 1000' of descent (about 300 fpm) for cruise descents.  For a 5000' descent that works out to about 41 nm away when travelling at 150 kts - so slightly longer than your friend.  But he is planning for a 500 fpm descent from the numbers you have given (I think).

I generally like a shallower descent because the higher descent rate pushes my airspeed closer to Vne unless I throttle back to less than 20 inches.

There is an old saying about Mooneys being able to either slow down or go down - just not at the same time.

 

[xftrplt]

All good techniques; however, since you appear to have a 430, how about using your Garmin-given vertical nav capability?

Page 58 of http://www8.garmin.com/manuals/GNS430_PilotsGuide.pdf

[Hank]

I'll have to play with the Garmin vnav function. So far, it's been very easy to count thousands of feet from TPA to cruise, double that number, add 2 or three, and start down that many minutes out. If you've got a GPS that will give ETE, "distance out" is not needed, you've got time left and a descent rate in feet per minute--converting minutes to miles is not easy to do in your head, and it varies tremendously with wind direction/strength.

If you want less than 500 feet per minute, triple the thousands of feet number. The key is to hold your descent rate pretty close to target, let the speed build [i walk the throttle and mixture during descent to maintain cruise MP/EGT all the way down]. From 10,000 cruise to 1600' pattern is 8½ thousand feet, or 17 minutes, so I'll start down around 20 minutes out, which is generally 50+ miles.

If you're up in turbo territory, this may not work as well, but it works pretty good for me. When IFR, you can ask for descent, but it's pretty controlled in my limited experience.

[John Pleisse]

Quote: MATTS875

what kind of descent planning are you all doing? I ahve a friend that flys and I believe I understood him correct but he said that if he is flying say 5000'  he takes the 5000' divide by 2= 2500 and starts his standard 500' minute descent about 25 miles out.I think I understood him correctly on this.

[Hank]

Quote: N4352H

 And Hank multiplied. Your math is fuzzy. It should be:

5000-1000(arrive at TPA). To loose 4000 feet going two miles a minute, choose your decent rate or distance out. 1000 feet per minute 4 mins/8 miles out, 500fpm 8 min/16 miles out. Faster/slower..interpolate.

As mentioned earlier in the thread, if you have a G430, the V-nav function kicks butt. I put it in first Nav window of my 430.

[jetdriven]

I think your speed is a little low for Mooney speeds, perhaps closer to 3 miles per minute.

Quote: N4352H

 And Hank multiplied. Your math is fuzzy. It should be:

5000-1000(arrive at TPA). To loose 4000 feet going two miles a minute, choose your decent rate or distance out. 1000 feet per minute 4 mins/8 miles out, 500fpm 8 min/16 miles out. Faster/slower..interpolate.

As mentioned earlier in the thread, if you have a G430, the V-nav function kicks butt. I put it in first Nav window of my 430.

[jetdriven]

\

Quote: Hank

I'll have to play with the Garmin vnav function. So far, it's been very easy to count thousands of feet from TPA to cruise, double that number, add 2 or three, and start down that many minutes out. If you've got a GPS that will give ETE, "distance out" is not needed, you've got time left and a descent rate in feet per minute--converting minutes to miles is not easy to do in your head, and it varies tremendously with wind direction/strength.

If you want less than 500 feet per minute, triple the thousands of feet number. The key is to hold your descent rate pretty close to target, let the speed build [i walk the throttle and mixture during descent to maintain cruise MP/EGT all the way down]. From 10,000 cruise to 1600' pattern is 8½ thousand feet, or 17 minutes, so I'll start down around 20 minutes out, which is generally 50+ miles.

If you're up in turbo territory, this may not work as well, but it works pretty good for me. When IFR, you can ask for descent, but it's pretty controlled in my limited experience.

[MATTS875]

Thanks , I do have a garmin 430w and looked in the book about the VNAV feature on the Nav page.I think this should do the trick.Will take me a little time to get use to it but looks pretty basic.Thanks

[xftrplt]

You're welcome, Matt.  It's easy to use.

Sure, there dozens of ways of computing TOD (top of descent), but, if you've got a nav system that will do it (as well as remind you approaching TOD and update you on your progress in the descent), it seems, to me, good airmenship to use it--if for no other reason than to check one's mental math--oops, mental counting.

Furthermore, it's been my experience that the more tasks you can accomplish ahead of time, the less likely you are to get overloaded when unforseen events (wx, mechanical, ATC, etc.) occur.

[MATTS875]

Dick, I did not know it but I also have a garmin 496 that has the vnav feature on it as well. so that even helps more.thanks again

[jetdriven]

Garmin aera has that as well.

[eldeano]

I tend to to the more technical, oh look I'm 20 miles out, guess I should start thinking about getting down now.  unless IFR of course.

[Carl S]

I was IFR and Shreveport kept my up high (for my Mooney -- 9000') despite my requests for lower.  When they finally cleared me for descent I fugured, "Oh well, it's just me and my instructor." 

Speedbrakes really do work to help get you down fast. ;-) 

[TonyPynes]

I was out about 21 miles this week when Greensboro Approach said I could start my descent from 4500 to pattern atitude 1900.

I was shocked and hesitated before replying. That never happened before!

I guess he saw the M20F coming in blazing and figured I would need help with reentry.

At least that is what I told myself, even as I was surrounded by airliners.

Tomorrow I will be descending to real sea level as we fly to North Myrtle Beach for a Steve Miller Band concert.

I love my Mooney!

[John Pleisse]

When I need lower and I know they forgot about me..I quietly whisper my tail number....that's it... works every time.

[TonyPynes]

Quote: N4352H

When I need lower and I know they forgot about me..I quietly whisper my tail number....that's it... works every time.

[Carl S]

Quote: N4352H

When I need lower and I know they forgot about me..I quietly whisper my tail number....that's it... works every time.

[John Pleisse]

One time I was vectored out over the Atlantic during a shift change and it didn't work ......40 miles later...

[John Pleisse]

Quote: N223MM

 Any hand movements with that? 

(I don't think he forgot about me - I think he was ignoring me.   To be almost fair, DFW had been closed down and lots of iron was diverting to SHV that night.)

[gsengle]

Quote: MATTS875

Thanks , I do have a garmin 430w and looked in the book about the VNAV feature on the Nav page.I think this should do the trick.Will take me a little time to get use to it but looks pretty basic.Thanks

[Mooneymite]

Okay, this thread has been way too peaceful with way too much agreement.

I just have to ask:  What MAP/RPM/mixture do you set on descent and what IAS do you use?

 

[jetdriven]

Full, 2300-3500, 15 LOP at altitude, (same as cruise setting)  around 50-75 LOP below 3000', to enter the pattern, 90 LOP.   180-185 MPH IAS. (whatever is required for 500-700 FPM.)  On downwind, reduce MP to 17", reset mixture to 1" from full rich, gear, flaps, etc.