Descent rate determinations

[Skybrd]

I found from a google search a way to determine descent rate. I am not sure if it's accurate but something to try. Many times while descending from a high altitude to the airport, it would be nice if I knew what descent rate to use that would get me at pattern altitude just before arriving at the airport. I have a Garmin 495 gps but don't think it has a vertical descent indicator. The formula I got off the google search is: Descent Rate = (altitude to loose * mph) % (60 * distance in miles)  Any of you use something like this or use some other formula.

[jetdriven]

a quick method for descending with power as in a Mooney is 250-300 feet per mile.    300 feet per mile is the standard ILS glideslope and in jets in turboprops it works out great, but in a M20J the IAS gets a little high and the descent rate exceeds 700 FPM, so I use 250 feet per mile. 

[Hank]

I don't fly at "high altitude," prefering the breathable stuff around 8-10K. I figure my descent rate to be 500 fpm unless instructed otherwise. It's easy to count on my fingers from Pattern to Cruising Altitudes on my fingers in thousands, then double it, and start down that many minutes out plus 2-3, since the descent will accelerate me somewhat.

If IAS gets away, then pull back the throttle a little bit--I find a 2" reduction in MP works well. If I start with a 2" reduction as I nose over, speed has rarely been a problem, but nosing over and maintaining WOT causes much more acceleration, even richening the mixture to maintain EGT on the way down.

[carusoam]

Skybrd,

Check your garmin for vertical guidance options....

I have a 196,

it can select:

   [1] What altitude you want to be at...  (1000' agl)

   [2] How far out from the airport you want to be at that altitude... (1 mile)

   [3] What rate you want to get there at...  (400 fpm)

It gives you:

   [1] a vertical nav timer (how long you have before you have to start down)

   [2] Glide slope bar on the HSI page that you can follow.

I use it all the time for guidance in terms of when to start down.  You won't believe how valuable this is.  Again it is for reference and situational awareness only.  Before I got the garmin (many years ago) I would routinely show up too high and to fast at my destination.  Too conservative maintaining safe altitude for single engine ops.....

For IFR operations, It gives me a hint when to ask ATC to let me start my descent. 

I like 400 fpm descent rate....good sustainable mooney speeds downhill over a long distance.  Adjust power to keep safety margin between airspeed and redline.

I also have a similar function on my IFR GPS KLN90B.  The calculation is a clumsy version of angle of descent....go figure...

Best regards,

-a-

 

[Skybrd]

Quote: jetdriven

a quick method for descending with power as in a Mooney is 250-300 feet per mile.    300 feet per mile is the standard ILS glideslope and in jets in turboprops it works out great, but in a M20J the IAS gets a little high and the descent rate exceeds 700 FPM, so I use 250 feet per mile. 

[OR75]

I calculated a while ago that avoid shock cooling (~330fpm), I needed about 7 nautical miles per 1000 ft.  Speed around 160kts. 

Terrain and ATC can change that quite a bit.

[Skybrd]

Quote: carusoam

Skybrd,

Check your garmin for vertical guidance options....

I have a 196,

it can select:

   [1] What altitude you want to be at...  (1000' agl)

   [2] How far out from the airport you want to be at that altitude... (1 mile)

   [3] What rate you want to get there at...  (400 fpm)

It gives you:

   [1] a vertical nav timer (how long you have before you have to start down)

   [2] Glide slope bar on the HSI page that you can follow.

I use it all the time for guidance in terms of when to start down.  You won't believe how valuable this is.  Again it is for reference and situational awareness only.  Before I got the garmin (many years ago) I would routinely show up too high and to fast at my destination.  Too conservative maintaining safe altitude for single engine ops.....

For IFR operations, It gives me a hint when to ask ATC to let me start my descent. 

I like 400 fpm descent rate....good sustainable mooney speeds downhill over a long distance.  Adjust power to keep safety margin between airspeed and redline.

I also have a similar function on my IFR GPS KLN90B.  The calculation is a clumsy version of angle of descent....go figure...

Best regards,

-a-

 

[FAST FLIGHT OPTIONS LLC]

Quote: OR75

I calculated a while ago that avoid shock cooling (~330fpm), I needed about 7 nautical miles per 1000 ft.  Speed around 160kts. 

Terrain and ATC can change that quite a bit.

[Immelman]

I use another angle on descent planning: I determine the fpm I want (usually 500) and work from there. 500 is comfortable, keeps the engine from cooling rapidly, the airspeed from being too high, and is easy on the mental math: I look at my altitude above the destination in thousands of feet, multiply by 2, and then have the ETE (minutes) at which I'll start my descent.

Example: cruise alt 10,500, airport elevation 500 = 10,000 ft to lose, * 2 = 20 minutes to descend @ 500fpm.

Things like wind and terrain will require adjustments.. but that is my basic method, and I'm happy with it.

[jelswick]

Mine is similar to Jesse's and I find it easy to remember.  I just look at how many thousands of feet I need to lose and multiply that by 2 for a 500 fpm descent rate.  I use 500 fpm with any passengers on board, but much prefer about 1,000 fpm and manage the manifold pressure to maintain acceptable airspeeds by myself for the fun feeling of flying.  Love the Mooney too because if at a typical longer range cruise altitude, by the time you cross the Ohio state line, it's about time to start descending to come into Columbus at the center of the state. 

[jelswick]

Not sure why the double post, but sorry about that...

[jlunseth]

I usually fly in the teens or low flight levels, and often have a tail wind, sometimes a very big tailwind, driving the plane during the descent.  I shoot for 500 fpm, I generally don't reduce power or deploy speedbrakes I just tip the nose down to get the 500.  My method involves some guesstimating because I am going to pick up a significant amount of speed on the way down.  I will usually just multiply the amount of altitude I need time two  (i.e. 18000/500 fpm = 36 minutes) and then just watch the ETE to destination on the 430.  I will add a few minutes to that, because of the speed I pick up in the descent, so if I am starting with 36 minutes I will use say, 40 minutes.  At 40 minutes I will ask for my descent.  I add the time because the flight timer is using my cruise speed to determine how fast I will get to my destination, and in the descent I am going to get there significantly faster so the 36 minutes ETE becomes more like 32 or 30, so I start with 40 minutes because I know the time will shrink but the amount of altitude to lose will stay the same. 

The slickest way is to use the Garmin vertical speed function.  It is the last of the Nav pages (turn the small knob to the right while on the Nav mode with the big knob).  It will ask you to set a rate of desired descent, a destination, a desired altitude to descend to, a distance before destination, and probably a couple of other things I am forgetting.  Then it computes moment by moment what descent rate you will need to get down to, say, 4,000 feet, 10 miles before the airport.  Now I just set that up and wait until it hits about 450 fpm, then ask for my descent.  I know it is going to go to 500 pretty quickly as I pick up speed in the descent. 

Very useful when ATC says, "Mooney 381SP, descend and maintain 5,000 at or before 40 miles from the KFCM airport."  On a couple of occasions I have dialed the numbers in and found a 1,000 fpm or better descent rate.  That is just too much for the passengers so I will ask ATC if they can modify it a little, and they will usually give me a point closer to the destination airport.  Either that, or I know I am going to have to throttle way back and deploy the brakes.

Not uncommon to start a descent 75 miles out, or even 100 on a big tailwind day. 

[jetdriven]

Quote: OR75

I calculated a while ago that avoid shock cooling (~330fpm), I needed about 7 nautical miles per 1000 ft.  Speed around 160kts. 

Terrain and ATC can change that quite a bit.

[jetdriven]

Quote: Skybrd

 Good idea Jetdriver, but that won't work from 30 miles out. One example for me is a descent on a cross country flight from Oceano to Bakersfield after crossing the two mountain ranges and being at 10,500 feet. Gps distant to Bakersfield might be 30 miles out and need to descend to 1500 msl for BFL pattern altitude. By using the formula I found, descent rate = (Altitude*Speed)%(60*Distance). altitude is 10,500 minus 1500 = 9,000 feet.  speed = aproximately 150 mph. distance = 30 miles. (9000 * 150) % (60 * 30)  equals 750 feet per minute descent rate 

[Awful_Charlie]

For cruise descents I just use 5nm per 1000' as it keeps the arithmetic easy.  Bump it up a bit with a good tail wind, shorten it a bit for a headwind.

One or my regular legs is a FL180 cruise, I need to be below 2000' for my local pattern.  16,000 feet to lose, = 80nm.  To start with I'll maintain the altitude and pull a couple of inches at say 82nm, then call ATC for descent.  It my descent gets delayed, then I'll sneak another inch or two in a few minutes and repeat as required. 

Assuming the descent comes immediately, just pitch down to towards the top of the green, and in a Bravo you get something like 165KIAS at 24" and 700FPM.  If you've been stealing MP 'cos they delayed your descent, nothing less than say 18" and pitch down for the same IAS.  If you got *really* delayed, then it's speedbrake time.

In the descent keep stealing the odd inch and monitor distance to run, and when it comes in perfect it ends up levelling with 5nm to run, 18" and the speed coming back to about 100-120, good enough to drop the gear overhead on on downwind.  I nornally open the cowl flaps when I drop the gear, as otherwise I find the CHTs on the rise again.

Do watch out for your terrain clearance though!

[jelswick]

Very good last point Charlie!  Right after I'd completed my PPL, I decided to fly myself to Florida for a work assignment with a stop in Atlanta for refuel.  It was night and I was very pleased with myself for my descent rate planning when I asked the controller to start down until he reminded me of the terrain height in the area north of Atlanta.  I decided to stay high a little longer, chastised myself for what stupid move I'd almost have made and so far have not made that simple, but critical mistake again.  Glad to have terrain alerting in the aircraft now too, but don't even want to bank on just that.

[N33GG]

I prefer more of a cruise descent that is a bit easier on the ears in the cabin.  Using a 200 feet per minute descent, it takes 5 minutes per 1000 feet.  Easy math... determine how many thousand feet you want to come down, and multiply times 5, and that is how many minutes before your arrival at that altitude to begin your descent.  Works great for me, and I have been doing it for decades.  May not be appropriate for all aircraft, and if you are IFR, don't forget to let ATC know that you will be making a slow descent.  I have never had the request refused.  Also easy on turbo engines...no shock cooling issues.  This approach is especially easy with GPS that gives you time to fix, but I used to use it long before GPS was around as well.  Of course, if the situation warrants it, I am fully capable of making more rapid descents,.

[jlunseth]

If I did 200 fpm from 23000 it would take nearly two hours to get down to 1,000.  Gotta go a little faster than that.

[N33GG]

Quote: jlunseth

If I did 200 fpm from 23000 it would take nearly two hours to get down to 1,000.  Gotta go a little faster than that.

[jax88]

My Garmin 496 has a descent profile that works well.  It allows me to tell it what altitude and what distance from the waypoint I want to reach said altitude.  In addition, I use the techniques outlined above, cruise - pattern divided by 500 multiplied by miles per minute with 3 additional minutes and 5 additional miles added to allow for time to slow the aircraft once I get to pattern altitude.

Once I have the calculations I create a waypoint for the descent in my flight plan.  When flying I will typicaly start my descent at the waypoint and use the built in vertical profile of the 496 as a cross reference.