When to start decent

RalphInCA

RalphInCA

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RalphInCA
I know I could probably look this up but I thought I would ask here: is there an easy rule of thumb/app/slide rule that I can use to determine when I should start my decent to pattern altitude?

For example if I am flying at 5500MSL and pattern altitude is 2250 feet and my groundspeed is 100 kts and I want to do a 500 fpm decent, how far out should I begin the decent?
 
RalphInCA said:
I know I could probably look this up but I thought I would ask here: is there an easy rule of thumb/app/slide rule that I can use to determine when I should start my decent to pattern altitude?

For example if I am flying at 5500MSL and pattern altitude is 2250 feet and my groundspeed is 100 kts and I want to do a 500 fpm decent, how far out should I begin the decent?
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4 1/2 to five minutes before the traffic pattern ???:dunno::rolleyes:
 
RalphInCA said:
I know I could probably look this up but I thought I would ask here: is there an easy rule of thumb/app/slide rule that I can use to determine when I should start my decent to pattern altitude?

For example if I am flying at 5500MSL and pattern altitude is 2250 feet and my groundspeed is 100 kts and I want to do a 500 fpm decent, how far out should I begin the decent?
Click to expand...

So roughly 5500-2500=3000 feet to lose.
at 500 feet per minute it will take you 6 minutes to descend so start your descent 6 minutes out.

if you want distance
@ 90 kts is 1.5 miles per minute so 6 minutes = 9 miles out
@ 120 kts is 2 miles per minute so 6 minutes = 12 miles out.

Brian
CFIIG/ASEL
 
RalphInCA said:
I know I could probably look this up but I thought I would ask here: is there an easy rule of thumb/app/slide rule that I can use to determine when I should start my decent to pattern altitude?

For example if I am flying at 5500MSL and pattern altitude is 2250 feet and my groundspeed is 100 kts and I want to do a 500 fpm decent, how far out should I begin the decent?
Click to expand...

Normally I have a GPS running so I cheat a little and let it show me time to destination (ipad does this too) then I will round your pattern altitude up to and even number (IE 2500) and subtract it from current altitude (5500 - 2500 = 3000). So this makes it easy 3000ft divided by 500FPM is 6 minutes. (Which comes to about 10 miles at 100kt gs. )

Since I will probably pickup speed on descent I would start at 10 minutes.:dunno:
 
In my opinion 5,500 MSL-2,250 pattern altitude =3250 feet to descend divided by 500 feet per minute descent = 6.5minutes or .11 hours X 100kts= 11 nautical miles until you reach pattern altitude.
 
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More experienced pilots than me will chime in, but I was taught to be at pattern altitude no closer than three miles before the pattern. So, depending on my altitude and descent rate I do the math backwards. In my Cherokee, my cruise speed is approx two miles per minute. At 500ft descent per minute, that means I'm descending about 1000ft every two minutes (or four miles).


So, for example, if I'm flying at 5000ft and heading to my pattern altitude of 2400ft, and want to descend at a comfortable 500ft/min for my pax:

I need to drop about 2500ft, which would take me about five minutes at 500ft/min. How far does my plane travel in five min? About 10 miles. When do I want to BE at this altitude? About 3 miles out from the pattern. So, I would begin my 500ft/min descent about 13 miles out.


In reality, I often descend quicker and start later - especially when I'm alone. Same example at 5000ft and my pattern altitude of 2400ft, but descending at 1000ft/min:


I need to drop about 2500 feet at 1000ft/min - so about 2.5 minutes. How far do I travel in 2.5 minutes? about 5 miles. Add my 3 miles out from the pattern and I would start my descent at around 8 miles out.

Anyway...works for me.
 
4 miles for every thousand feet, seems to work out pretty well, Cessna 170. That's a pretty rough rule of thumb, to be more exact that works out to 120 knot groundspeed & 500 fpm, tweak it as required :D
 
4 miles / thousand feet was good in the Skyhawk and in my light sport.

My Garmin portable 496 gives me bars across the faux HSI when I intercept the vertical profile, that happens if you have a destination entered. Too easy.
 
RalphInCA said:
I know I could probably look this up but I thought I would ask here: is there an easy rule of thumb/app/slide rule that I can use to determine when I should start my decent to pattern altitude?

For example if I am flying at 5500MSL and pattern altitude is 2250 feet and my groundspeed is 100 kts and I want to do a 500 fpm decent, how far out should I begin the decent?
Click to expand...


A little estimating math is in order. Remember that 60PMH is a mile a minute. At 100 Knots you are traveling very roughly a mile and a half per minute. To lose 2250 feet will take about 4 1/2 minutes, so you need to start the descent about 4 1/2 minutes times 1 1/2 miles for an estimated 6.75 miles out from the point where you want to be at pattern altitude.

Do that kind of rough math and give yourself a little margin and make it 7 or 8 miles.

No need for an app or a calculator, just a little simple and less than accurate math in your head.

That is my process and it works out pretty well for me.

Hope this helps.
 
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Vance Breese said:
Part of my flight plan a waypoint for where I start my descent.
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That's good planning! Nothing beats thinking ahead. One more less thing to worry about in flight. Who knows, you might be preoccupied with the radio or flying in rough air or something that will distract you. Anything you can do ahead of time is a good thing IMHO.
 
MBDiagMan said:
That's good planning! Nothing beats thinking ahead. One more less thing to worry about in flight. Who knows, you might be preoccupied with the radio or flying in rough air or something that will distract you. Anything you can do ahead of time is a good thing IMHO.
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Thank you for the kind words.

Flight planning is like aviation foreplay for me; the more I plane the nicer the experience.

It is also a way to spend time flying without spending money.

I have typically flown a flight four times in planning before I touch the aircraft controls.

I find the exercise of looking at flying as a three dimensional puzzle to be instructive.

Reviewing the plan for wind correction just before the flight helps me to be confident that I am ready to make the flight.

If I am flying to an unfamiliar airport I call ahead to find out about the most likely pattern entry and the best reporting points.

For me finding each waypoint is like finding a hidden treasure.

I love it when a flight plan comes together.
 
eman1200 said:
you should also plan for curveballs that throw you off of your plan :)
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That is part of what I love about a good flight plan.
I can see when a deviation begins and address it much sooner.
Having the information on my alternate airports is comforting when things aren’t working out.
I love following my progress on the chart.
I feel like a flight plan expands my freedom because I have clearly defined my limitations and options for that flight.
I fly for fun and often make deviations from the plan to exercise that freedom.
 
I use the same rule of thumb that John Saubak uses, only I am in a C-150 so I usually use 3 miles per 1,000 ft.
 
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"In preparing for battle I have always found that plans are useless, but planning is indispensable."

Dwight Eisenhower
 
simple.....look at your GPS. Dial in the airport.....6 minutes ETA start your descent. With a 500 fpm rate you'll be right where you need to be.

Working in the "time" continuum is easy peasy....much more than the distance continuum.
 
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Don't forget about local terrain...all that "rule of thumb stuff" goes out the window if your airport is on the other side of locally high terrain... Just say'n!:dunno:
 
Provided your spam trap descends at a forward speed between 2-2.5NM/min, you can double the altitude to lose in thousands of feet for a 500ft/NM gradient or quadruple the altitude to lose for a 250ft/NM gradient. At 2NM/min that correlates to 1000ft/min and 500ft/min VS respectively (or 1250ft/min and 625ft/min in the case of the 150KTGS descent).

So, multiply x2 or x4 as required. Done. That's a quick wag when to start the descent miles wise and you don't have to do gorilla math 60-to-1 rule every single time. It's not like you're a turbofan equipped ship that can take advantage of the fuel savings over 100NM of spooled-down/idle descent. Even with a tailwind, the variance on our category airplanes is not gonna be that great. If in doubt, point at the ground and tell the pax to suck it up for a minute or two :D
 
60knots is one nautical mile per minute. 120knots is 2. 90 knots is 1.5

Figure it at 60 first that way if its 5 miles out, then its 5 minutes. 2500' would be 500fpm at 60 (2500/5). Now if you are flying at 90, it would be 750fpm (1.5 times as much). 120 1000fpm (2 times as much).
 
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RalphInCA said:
is there an easy rule of thumb/app/slide rule that I can use to determine when I should start my decent to pattern?
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WingX is an app that has this feature. See my WingX screen capture attached. Look at the yellow text at the top center area of the screen. It shows your distance to destination and calculated rate of descent needed for your destination airport. It just so happens my screen capture was made when it was calling for 500 fpm. I believe that information is not visible until you get close enough to display a calculated 200 fpm descent or higher and the closer you get to your destination the calculated descent rate increases accordingly. I usually wait until I see 400 fpm then start preparing for my descent and track it down at 500 fpm. It works amazingly well. I love it!
 

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If you have a 430 in the plane you can rotate all the way to the right nav page that shows vertical descent profile. I have mine set to get me to 1000ft agl, 2 miles out and then I just watch occasionally for the 500fpm.
 
MickYoumans said:
WingX is an app that has this feature. See my WingX screen capture attached. Look at the yellow text at the top center area of the screen. It shows your distance to destination and calculated rate of descent needed for your destination airport. It just so happens my screen capture was made when it was calling for 500 fpm. I believe that information is not visible until you get close enough to display a calculated 200 fpm descent or higher and the closer you get to your destination the calculated descent rate increases accordingly. I usually wait until I see 400 fpm then start preparing for my descent and track it down at 500 fpm. It works amazingly well. I love it!
Click to expand...


Interesting. I wonder if ForeFlight has this? Will have to check.


Sent from my iPhone using Tapatalk
 
MickYoumans said:
WingX is an app that has this feature. See my WingX screen capture attached. Look at the yellow text at the top center area of the screen. It shows your distance to destination and calculated rate of descent needed for your destination airport. It just so happens my screen capture was made when it was calling for 500 fpm. I believe that information is not visible until you get close enough to display a calculated 200 fpm descent or higher and the closer you get to your destination the calculated descent rate increases accordingly. I usually wait until I see 400 fpm then start preparing for my descent and track it down at 500 fpm. It works amazingly well. I love it!
Click to expand...


Thanks for posting and explaining.

This is an interesting feature, but not one that would make a difference for me either way when making a Foreflight/WingX decision. I've been making this simple mental calculation in my head with no trouble. For others, it might have much value.

I do plan on examining WingX further before the next time my FF subscription is due.

Thanks again.
 
RalphInCA said:
Interesting. I wonder if ForeFlight has this? Will have to check.


Sent from my iPhone using Tapatalk
Click to expand...


I have no idea why they haven't implemented something like this. It would be very easy to do.
 
Along with AUDIO TERRAIN ALERTING! Really wouldn't be that hard and could save someone's life!
 
The rule of thumb I use for descent is:

Thousands of ft to lose x 3 = NM out to start down

Descent rate = ground speed x 5

It's roughly a 3 degree descent.
 
depends on the aircraft. on a capable plane you can always dive bomb to pattern altitude. not that it is a good idea.:dunno:
 
hindsight2020 said:
Provided your spam trap descends at a forward speed between 2-2.5NM/min, you can double the altitude to lose in thousands of feet for a 500ft/NM gradient or quadruple the altitude to lose for a 250ft/NM gradient. At 2NM/min that correlates to 1000ft/min and 500ft/min VS respectively (or 1250ft/min and 625ft/min in the case of the 150KTGS descent).

So, multiply x2 or x4 as required. Done. That's a quick wag when to start the descent miles wise and you don't have to do gorilla math 60-to-1 rule every single time. It's not like you're a turbofan equipped ship that can take advantage of the fuel savings over 100NM of spooled-down/idle descent. Even with a tailwind, the variance on our category airplanes is not gonna be that great. If in doubt, point at the ground and tell the pax to suck it up for a minute or two :D
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I'm really not a fan of this advise. It's not like GA is robust and the world is full of people itching to go fly these things. And your suggestion is to combine fear with pain. Lovely combination that is sure to send those pax running.

No unpressurized plane carrying passengers should ever descend at more than 500 fpm. If you can't manage that and figure out when to start down then don't carry pax until you learn this basic skill. If there is an emergency and you feel like a greater descent is needed then I suggest it be a big one...on fire for example.

And if you're doing greater than 500fpm with other pilots in the plane brief it first and make sure everybody is cool with your plan. Early in my post ppl days some cat took me up in his plane and dive bombed me at over 3,000fpm and I was not expecting it. I wanted to punch him but the pain in my ears prevented me from doing anything. Don't be that guy.
 
Again, as has been mentioned, this is where Garmin handhelds shine.

I set in the vertical descent profile to descend at 500 fpm so as to arrive 1,500' AGL above my destination 3 miles out. That's because I like to overfly my destination if non-towered 500' above pattern altitude.

As you approach the profile, you get a message, then if you have the faux HSI selected a needle starts down from the top. When it's centered vertically, just begin a 500 fpm descent and then vary it to keep the needle centered.

Easy Peasey.
 
MyassisDragon said:
Is this a field you can add to the main Nav page? Or do you have to go to the last page every time?
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Mine is set to show ETE on the map screen and the CDI page. It's easy to count from pattern altitude to cruise; needn't be exact, round the pattern to the closest 500'. It's then 2 minutes per 1000' to descend, plus a minute to two to slow to pattern speed (I descend at cruise power to get free speed from the altitude that I climbed at high fuel burn).

Example: cruising along at 9500 msl. Pattern at home was 1600 msl (one of my first "real" trips). That's 8000' to lose, so 16 minutes. When ETE showed 18:00, I pushed on the yoke for 500 fpm and trimmed to hold it. As I come down, periodically walk throttle back and mixture forward to maintain cruise MP and EGT values. If I recall, this point was 56 nm out, well over the horizon and certainly beyond visibility for much of the descent.

This does require the ability to fly at pattern altitude for 3-5 miles from the airport, not an issue in my part of WV.
 
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