Descent Calculations

akpilot907

akpilot907

Pre-takeoff checklist
Joined
May 14, 2013
Messages
398
Location
Anchorage, Alaska
Display Name
Display name:
citabriav8tr
Anyone know of a good equation for descent? Altitude to loose, distance to cover, and time out?
 
What works for me:

(My altitude - (Field Altitude + 1000)) / 500 = Minutes out to start descent at 500fpm.
When the GPS says I'm that many minutes out (or a couple more as a cushion) I'll pull the power back a bit and let it come down at it's trimmed speed at 500fpm.
 
if you're in a plane that descends at roughly 120kts and you want to do a standard 500FPM descent then it's:

twice the thousands of feet that you need to descend in minutes or four times in miles.

i.e. if you're cruising at 7500' and TPA is 1500' then you need to descend 6,000' and you'd start your descent 12 minutes or 24 miles out.

I descend at about 135 to 140kts and I use the "miles out" formula. I just descend at 600fpm instead of 500.

(ps the minute calculation is the same regardless of cruise speed for 500 fpm to work)
 
Last edited:
I cheat, when the 430 says about 400fpm decent profile I nose over, building speed brings the needed decent to about 500fpm and down I go.
 
timwinters said:
if you're in a plane that descends at roughly 120kts and you want to do a standard 500FPM descent then it's:

twice the thousands of feet that you need to descend in minutes or four times in miles.

i.e. if you're cruising at 7500' and TPA is 1500' then you need to descend 6,000' and you'd start your descent 12 minutes or 24 miles out.

I descend at about 135 to 140kts and I still use this formula. I just descend at 600fpm instead of 500.

(ps the minute calculation is the same regardless of cruise speed for 500 fpm to work)
Click to expand...

+1 This is the one I use.
 
This is what I use:
(Altitude to lose / 1000) * 5 = distance in NM to start descent
 
Immediately above the airport, chop the throttle to idle, and do a steep spiral down to final and do a straight-in with no radio calls.
 
JeffDG said:
Immediately above the airport, chop the throttle to idle, and do a steep spiral down to final and do a straight-in with no radio calls.
Click to expand...

But what if I'm not flying a Grumman or RV?
 
I always wonder why folks pull the power back. No need until you're joining the flow of the pattern, other than airspeed limitations of the aircraft or legal speed limitations.

Admittedly the 182 is a draggy truck, but I just push the nose down in smooth air. Might as well make up some of that time and speed lost to the climb.
 
denverpilot said:
I always wonder why folks pull the power back. No need until you're joining the flow of the pattern, other than airspeed limitations of the aircraft or legal speed limitations.

Admittedly the 182 is a draggy truck, but I just push the nose down in smooth air. Might as well make up some of that time and speed lost to the climb.
Click to expand...

+1,000 !!!!

Just roll the trim forward to 500 FPM and go down. Gradually reduce the throttle to keep the MP from climbing and/or to keep the speed below yellow line.

As my primary instructor used to say "you paid for that altitude now cash in"

Caveat: the game ends at the first bump.
 
how fast you want to go down?

500/fpm is 2X the altitude to be lost.

altitude 9500. Field elevation 1200. Pattern Alt 2200. need to losde 7000' times 2 = 14 min out. Thats ez with GPS. 700fpm? 10 min. Decide how fast you want to come down. Maintain your cruise ground speed with throttle and you will arrive over the airport or fix or whereever - at the stated time and altitude ....
 
timwinters said:
+1,000 !!!!

Just roll the trim forward to 500 FPM and go down. Gradually reduce the throttle to keep the MP from climbing and/or to keep the speed below yellow line.

As my primary instructor used to say "you paid for that altitude now cash in"

Caveat: the game ends at the first bump.
Click to expand...

I usually plan for a 250ft/min descent for just this reason, unless asked to descend by ATC.
Thenformula is ..
thousands of altitude to lose times 4. so 4000 feet = 16 minutes to descend.

if I want distance then 1.5 @ 90 kts = 24miles
2 at 120 kts = 32 miles out

and I can keep the power setting fairly high, especially if I have a fixed pitch prop and the decent is very comfortable for me and my passenger.

Brian
 
Since this was posted in the Instrument flying subforum, my answer is to start my descent when ATC tells me.:D
 
denverpilot said:
I always wonder why folks pull the power back. No need until you're joining the flow of the pattern, other than airspeed limitations of the aircraft or legal speed limitations.
Click to expand...

Probably like a lot of pilots I was taught to pull the power back and descend at cruise speed, but I recently changed to leaving the throttle mostly alone and pitching down. It is the only time I can get the C-152 speed into the yellow arc.

But - in a fixed pitch prop plane - I wonder how wise it is to lose altitude at a speed that pushes the RPMs much above the tach's red line.

Oh - and as to the OP's question - lately I've been using (and enjoying) the feature in WingX that gives me a constant update on the descent rate I would need to reach my destination's field elevation from my current elevation.
 
If you're Barb then you plan to climb to pattern altitude about 5 miles out.
 
I use my current groundspeed + 10 or so as a ballpark descent groundspeed, and usually round to the nearest 1/4 mile per minute (IE 150 knots = 2.5nm/min, 165 knots = 2.75nm/min, 180 knots = 3nm/min etc).

Then I double the thousands of feet to get down to field elevation - That's the number of minutes at a pax-friendly 500 fpm descent. 7500 feet and a field near 500 feet = 7000 feet to descend = 7*2=14 minutes to descend.

14 minutes * 3 nm/min = 42nm from the destination as a top of descent. If I'm IFR and haven't heard from ATC by 35 or so, I'd start asking for lower, unless I was going to have to shoot an approach. Then, I'd be calculating based on the IAF and its minimum altitude rather than from the field.
 
to get a very rough estimate i just multiply my atitude by 3. so at 5000 feet ill start descending at around 15 miles
 
Last edited:
I don't use any formula...I work out how long it's going to take me to complete that descent (as a function of time), then when my ETA to the fix matches that number, it's time to descend.

I might start a minute or two early to allow for the increase in speed associated with the descent (I don't pull power for my descents).

So, assuming I need to lose, say, 8000ft...that's going to take 16 minutes. How far away is that? Who cares...I just deal with time. :)
 
I set 600fpm down/arrive at 1000' msl above waypoint three miles out in my 496 and start down when it tells me to.

If it messes up, then I do the chop power spiral dive thing another guy mentioned .... :goofy:
 
I can't believe how many of you depend on you gps. It's very simple. 3 time your altitude loss is how far out you start down. 5 times your groundspeed is how fast you come down. And guess what, it keeps you on a perfect 3 degree glide slope.
 
Last edited:
J3 Driver said:
I can't believe how's my of you depend on you gps. It's very simple. 3 time your altitude loss is how far out you start down. 5 times your groundspeed is how fast you come down. And guess what, it keeps you on a perfect 3 degree glide slope.
Click to expand...



You are the Bodhizafa of descent pilots ... :rolleyes:
 
Fearless Tower said:
Since this was posted in the Instrument flying subforum, my answer is to start my descent when ATC tells me.:D
Click to expand...

or . . . ask for a pilot discretion if you don't want to burn all that fuel down low leveled off!
 
comanchepilot said:
or . . . ask for a pilot discretion if you don't want to burn all that fuel down low leveled off!
Click to expand...
I haven't had that problem yet.....I'm usually left high.....like a few weeks ago when Chattanooga Approach kept me at cruise altitude (9000') until I was literally over the field.
 
Fearless Tower said:
Since this was posted in the Instrument flying subforum, my answer is to start my descent when ATC tells me.:D
Click to expand...

That's a good one, caught me off guard... That's a winner! I was looking for a good descent equation in the unfortunate event I loose comm. A good tool to have on standby if, ATC, becomes unavailable.. :rockon:

Fearless Tower said:
I haven't had that problem yet.....I'm usually left high.....like a few weeks ago when Chattanooga Approach kept me at cruise altitude (9000') until I was literally over the field.
Click to expand...

That happened to me once, I was flying a GA-8 down to Talkeetna, AK from Fairbanks a few years back and center kept me at the MEA (10,000ft) until I was 11 miles out from the airport... I was like, greeaaaat....
 
Last edited:
Simple. Watch the ND course line, when you see an arrow symbol with a down pointed slant, verify on FMS Flightplan page the T/D, dial in the altitude on the FCU and push the knob to manage the descent. Monitor progress on the PFD and FMS page PROG.

For the GA crowd several good recommendations already given.
 
Waiting for ATC to start you down is poor form. The pilot is responsible and a good pilot should have a plan. If ATC starts you down way too soon you should recognize that and act accordingly. If they forget about you (it happens) you should recognize that too and request down. If you overfly the field at cruise altitude YOU look dumb, not ATC.

This touched a nerve with me as it happened to me within the last 12 months. FO flying and we're 100 miles away at FL350. I ask the FO what his descent planning is and he responds, "Oh, I just go down when ATC tells me." Great. This guy is collecting a pay check for this wonderful airmanship.
 
Captain said:
Waiting for ATC to start you down is poor form. The pilot is responsible and a good pilot should have a plan. If ATC starts you down way too soon you should recognize that and act accordingly. If they forget about you (it happens) you should recognize that too and request down. If you overfly the field at cruise altitude YOU look dumb, not ATC.

This touched a nerve with me as it happened to me within the last 12 months. FO flying and we're 100 miles away at FL350. I ask the FO what his descent planning is and he responds, "Oh, I just go down when ATC tells me." Great. This guy is collecting a pay check for this wonderful airmanship.
Click to expand...
Didn't say the pilot shouldn't have a plan....but I would counter that 'bad form' are the guys that are repeatedly/constantly bitching at ATC for lower when ATC has a need to keep them high.
 
I've been told to start down by ATC while IFR and have asked them if it would be possible to get the descent as a crossing restriction, rather than a 'descend and maintain'. That way, I stay as high as possible for as long as possible.

In other cases, I've requested lower because I'm approaching the top of descent that I've already calculated, and ATC still has me at cruise altitude.
 
Fearless Tower said:
Didn't say the pilot shouldn't have a plan....but I would counter that 'bad form' are the guys that are repeatedly/constantly bitching at ATC for lower when ATC has a need to keep them high.
Click to expand...

Agree. If I'm started down substantially before I want or fuel is tight'ish ill request once pilot discretion. If I get it, great. If not its not the end of the world...I did my part and asked.

As for being kept high longer than I want, that's not my MO. I prefer a flight idle descent for fuel savings anyway so rare is the day ATC wants me steeper than I already want. If I'm asking for lower they either forgot about me or something odd is going on. Either way, if asked to stay high ill gladly oblige.
 
Last edited:
J3 Driver said:
I can't believe how many of you depend on you gps. It's very simple. 3 time your altitude loss is how far out you start down. 5 times your groundspeed is how fast you come down. And guess what, it keeps you on a perfect 3 degree glide slope.
Click to expand...

I love those 75,000fpm descents and 1:5 descent profiles!
 
Captain said:
Waiting for ATC to start you down is poor form. The pilot is responsible and a good pilot should have a plan. If ATC starts you down way too soon you should recognize that and act accordingly. If they forget about you (it happens) you should recognize that too and request down. If you overfly the field at cruise altitude YOU look dumb, not ATC.

This touched a nerve with me as it happened to me within the last 12 months. FO flying and we're 100 miles away at FL350. I ask the FO what his descent planning is and he responds, "Oh, I just go down when ATC tells me." Great. This guy is collecting a pay check for this wonderful airmanship.
Click to expand...

That's why your paycheck is bigger. ;) ;) ;)
 
Captain said:
Waiting for ATC to start you down is poor form. The pilot is responsible and a good pilot should have a plan. If ATC starts you down way too soon you should recognize that and act accordingly.
Click to expand...

Yup - I got discretion from 10,000 to 4,000 recently when I was still 125nm away from my destination! The controller had to cancel that when I was still at 10,000 and he had to hand me off to the next facility. :rofl:
 
J3 Driver said:
I can't believe how many of you depend on you gps.
Click to expand...

I can't believe how many of you depend on rules of thumb that don't apply in all situations. :rolleyes: If you can do the math in your head, do it (I do). If you need a rule of thumb, do the math on paper for typical performance numbers for your own airplane.

It's very simple. 3 time your altitude loss is how far out you start down.
Click to expand...

That's great if you're in a pressurized plane. In the Mooney I'd be coming down at over 1,000 fpm. Not nice on passengers' ears.

5 times your groundspeed is how fast you come down.
Click to expand...

So, 180 knots GS = 900 fpm or 200 knots GS = 1000 fpm. Again, not good for pax in an unpressurized plane.

And guess what, it keeps you on a perfect 3 degree glide slope.
Click to expand...

1 nm = 6076.1 feet
1 knot = 6076.1 feet/hour = 101.2683 feet/min forward.
1 knot * 5 by your formula = 5 feet/min down.
tan-1(5/101.2683) = 2.8266º glideslope. Not perfect 3 degrees...

Let's try the other one. 3x altitude loss is how far out to start the descent (i.e. 3nm/1000 feet). That means 18,228.3 feet horizontally for each 1000 feet vertically. tan-1(1000/18228.3) = 3.14º. Also not perfect.

But then, I am a perfectionist. ;)
 
If'n you fly an unpressurized airplane then 500 fpm should be a max number for comfort. Every 60 kts GS is a mile per minute. So if your GS is 120 that's two miles a minute. Easy math with the 500fpm requirement. 5,000 foot descent equals 10 miles out start your descent.
 
Captain said:
If'n you fly an unpressurized airplane then 500 fpm should be a max number for comfort. Every 60 kts GS is a mile per minute. So if your GS is 120 that's two miles a minute. Easy math with the 500fpm requirement. 5,000 foot descent equals 10 miles out start your descent.
Click to expand...

Ummm, no. 10 minutes, 20 miles.
 
You must log in or register to reply here.
Back
Top