Descent Calculations

[Captain]

Ummm, no. 10 minutes, 20 miles.

See? Easy math!

 

[timwinters]

 

[txflyer]

Follow the yellow brick road.

This thread has become superfluous.

Time for some Miley.

 

[EdFred]

But then, I am a perfectionist.

And yet you continue to live in Wisconsin.

 

[flyingcheesehead]

And yet you continue to live in Wisconsin.

Yeah, well, your beloved UP was attached to Wisconsin for a reason.

 

[azure]

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.

Ummm, no. 20 miles.

The formula I use in practice (really!) is

(altitude to lose)/1000 * ceil(GS/30) --> distance in nm for 500 fpm descent.

If I'm VFR, that's what I use to plan my descent. If I'm IFR, I get a little antsy if ATC still hasn't cleared me lower when I'm closer than that. Yes, I'm one of those folks that will ask for lower.

Politely.

Once. And then cancel, if I don't really need to be IFR.

 

[timwinters]

Yeah, well, your beloved UP was attached to Wisconsin for a reason.

Zing!

 

[midlifeflyer]

Ummm, no. 20 miles.

The formula I use in practice (really!) is

(altitude to lose)/1000 * ceil(GS/30) --> distance in nm for 500 fpm descent.

If I'm VFR, that's what I use to plan my descent. If I'm IFR, I get a little antsy if ATC still hasn't cleared me lower when I'm closer than that. Yes, I'm one of those folks that will ask for lower.

Politely.

Once. And then cancel, if I don't really need to be IFR.

The 500 FPM rule of thumb is the one I use for comfort. Actually go a little further, with pre-calculated GS/30 numbers (i.e. at 120 kts, it's 4 NM/1000' to lose) . For anything more complicated than that I use a calculator.

 

[dtuuri]

Anyone know of a good equation for descent? Altitude to loose, distance to cover, and time out?

Simple math for me, not any of that higher math stuff: Just keep figuring your descent ratio as you descend to maintain your personal favorite (mine was 5:1 in my Twin Comanche). Turbojet pilots (and ATC) usually use 3:1. For crossing restrictions it's wise to plan a level-off a few miles short of the fix. Begin the descent when, say, 3 times the thousands of feet you need to lose matches your distance out i.e., 3 X 8 thousand = 24 miles. FWIW.

dtuuri

 

[Geico266]

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.

Garmin 496, 396, does that same thing. Nice tool when traveling at altitude.

 

[onwards]

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.

Exactly right. Plus 170kt in the 182, even if for just a couple minutes, is plain fun.

 

[dell30rb]

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.

Who flies a J3 on a three degree glide slope anyways? :wink2:

 

[dell30rb]

Exactly right. Plus 170kt in the 182, even if for just a couple minutes, is plain fun.

Do you like your wings attached? I thought Vne was lower than that.

Edit, looks like some 182's have Vne 175-180 kts

 

[coma24]

Also worth mentioning the VNAV page on the Garmin 430/530. Of course, you can live without it, but it can be helpful to set that up as well...here's an example:
http://www.youtube.com/watch?v=z02TeOzgMGE

In that case, the instruction was to cross 5NM prior to a fix at 9k.

Ignore the self-shot at the end, a few people asked me why I'm never in the video, so I figured I'd turn the cam around for once.

Big fan of the VNAV page. Even bigger fan since I discovered you can have it alert you when you hit top of descent

 

[denverpilot]

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.

Use it or lose it? Heh.

 

[0luke1]

Fly the downwind leg over the active at 7000, split s 10 seconds after the numbers, drop flaps and gear, land.

 

[denverpilot]

Fly the downwind leg over the active at 7000, split s 10 seconds after the numbers, drop flaps and gear, land.

You're into the spilt S thing huh? Two posts two threads, two spilt S's.

 

[BillTIZ]

Also worth mentioning the VNAV page on the Garmin 430/530. Of course, you can live without it, but it can be helpful to set that up as well...here's an example:
http://www.youtube.com/watch?v=z02TeOzgMGE

In that case, the instruction was to cross 5NM prior to a fix at 9k.

Ignore the self-shot at the end, a few people asked me why I'm never in the video, so I figured I'd turn the cam around for once.

Big fan of the VNAV page. Even bigger fan since I discovered you can have it alert you when you hit top of descent

The VNAV top of descent today would have sent me through a ridge line 30nm before the airport. 9500 cruise to 2200 airport elevation, 7000 ridge.

 

[Captain]

The VNAV top of descent today would have sent me through a ridge line 30nm before the airport. 9500 cruise to 2200 airport elevation, 7000 ridge.

Not IFR it wouldn't have. There's minimum altitudes and stuff.

 

[dtuuri]

The VNAV top of descent today would have sent me through a ridge line 30nm before the airport. 9500 cruise to 2200 airport elevation, 7000 ridge.

Not IFR it wouldn't have. There's minimum altitudes and stuff.

Maybe he was VFR-on-top. Can't ATC issue him an IFR cruise clearance? Or make a mistake? I think his point is to refrain from being a slave to VNAV and continue to think.

dtuuri

 

[Bill]

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.

Even in my non-draggy plane I keep the throttle in and come down at 180kts.

• (Cruise alt - TPA)*2 is the number of minutes at 500fpm
• # of minutes*3 is miles out
• I then add 10-15miles and ask for lower that number of miles out as sometimes you don't get lower immediately.

 

[EdFred]

Yeah, well, your beloved UP was attached to Wisconsin for a reason.

Yep. Every ****bucket needs a handle to carry it with.

 

[denverpilot]

Yep. Every ****bucket needs a handle to carry it with.

ROFL!!!

 

[flyingcheesehead]

Yep. Every ****bucket needs a handle to carry it with.

'cept in this case, you can't tell the difference between the bucket and the handle until you draw a line on the side...

Really, for the most part WI and MI are *very* similar. Lots of trees, lakes, animals, people shooting animals, etc. Both states are full of people who appreciate the outdoors, even when it is ****ing cold out and snowing. And we all hate Ohio State.

 

[EdFred]

'cept in this case, you can't tell the difference between the bucket and the handle until you draw a line on the side...

Really, for the most part WI and MI are *very* similar. Lots of trees, lakes, animals, people shooting animals, etc. Both states are full of people who appreciate the outdoors, even when it is ****ing cold out and snowing. And we all hate Ohio State.

There's more of a demarcation between the UP and Wisconsin with the Brule and Montreal Rivers than, say, the straight line separating WI from, IL. And we all know how much you love IL and their drivers.

But we can agree on the Ohio thing.

 

[Ghery]

Since this was posted in the Instrument flying subforum, my answer is to start my descent when ATC tells me.

or . . . ask for a pilot discretion if you don't want to burn all that fuel down low leveled off!

PD works for me. Once I know I'm clear of the rocks east of home I'm happy to start down. And for my wife's and my comfort, no more than 500 fpm. Much easier on the ears.

 

[flyingcheesehead]

There's more of a demarcation between the UP and Wisconsin with the Brule and Montreal Rivers than, say, the straight line separating WI from, IL. And we all know how much you love IL and their drivers.

Bah! State Line Road separates WI from IL. It's one of the more visible state lines from the air.

Plus, there is that long segment between the UP and WI that's just a straight line, too.

I think we can agree on Illinois AND Ohio!

 

[RotorAndWing]

The VNAV top of descent today would have sent me through a ridge line 30nm before the airport. 9500 cruise to 2200 airport elevation, 7000 ridge.

You can't put a constraint in as a waypoint?

 

[comanchepilot]

It's ALWAYS fun coming back from Vegas and the northeast into the LA Basin through Cajon Pass - its very hard to maintain speeds below yellow line in the descent and still make the airport - you have a 4500'MSL pass elevation and most of the rocks are in the 6-7k range - until you get to the basin - even the guys in the airline seats are using spoilers from 10k to 3k to get down low enough to land -

You simply cannot descend until you are past the rocks and assuming you can slow it down enough over the mountains you can start it down -

When its just me I'll put it into a full rudder deflection slip and that'll get me 2k fpm down which is enough at GA speeds to get me down to pattern altitude for POC by the time I get close to Cable - then I level off and slow down from 200mph to gear extension speed which generally puts me inside the Class D . . .its a real balancing act- there are times when you need to use a couple of 360's . . .