When should I begin my Decent?

benb172

benb172

Pre-takeoff checklist
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Benb172
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Norwood Airport, KOWD
I was flying today and forgot to ask my instructor this. If I am 10 miles away from the airport and at 2500', when should I begin my decent? What should the rate be? Is it normally 500fpm? Thanks for the help.
 
For a light unpressurized aircraft, 500 FPM is a nice rule of thumb to keep your passengers comfy.

How far away is determined by your ground speed. At 120 knots, that's two miles per minute. So, to descent to a 1000 foot pattern altitude from 2500, you need 3 minutes, or 6 miles. At 90 knots, it's 1.5 miles per minute (so 4.5 miles).

In a 172, you can get away with "stop'n'drop" (pull the power to idle and descend at Vno or Va), but it's poor technique and will not generalize to a high performance aircraft.
 
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Great. Thanks for the help. I am in a 172 so we usually travel at around 100-120 (as you stated).
 
benb172 said:
Great. Thanks for the help. I am in a 172 so we usually travel at around 100-120 (as you stated).
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Don't forget he said "groundspeed". :wink2:
Another tip: it's a good idea to adjust power so that you don't speed up much during this initial descent... makes things easier once you are down low, looking for your initial point to enter the pattern (and any local traffic).
 
Just don't let your decency descent to lower levels.;)

José
 
It depends on your speed. Your goal should be to be at pattern altitude about 3-5 miles out so you stay under the heavy/jet pattern normally 500 above and a bit wider than the light plane pattern. If you're arriving at 2500 MSL and the TPA is 1000 MSL, with a 500 ft/min descent rate it will take you 3 minutes to make the descent. At a typical 120 knots, that means it will take you 6 miles (3 minutes times 2 miles/min) to descent, so you'll need to start down 9 miles out. You can do the math for other altitudes and speeds.
 
Ron Levy said:
It depends on your speed. Your goal should be to be at pattern altitude about 3-5 miles out so you stay under the heavy/jet pattern normally 500 above and a bit wider than the light plane pattern. If you're arriving at 2500 MSL and the TPA is 1000 MSL, with a 500 ft/min descent rate it will take you 3 minutes to make the descent. At a typical 120 knots, that means it will take you 6 miles (3 minutes times 2 miles/min) to descent, so you'll need to start down 9 miles out. You can do the math for other altitudes and speeds.
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Great, thanks for the help.
 
I presume you have some way of determining TIME to Destination????

If so - assuming a 500fpm rate of descent you simply double the altitude and thats the time you start down. e.g. You are at 6500, the airport is at 500. You start down 12 minutes out [6000 feet to lose divided by 500 = 12]

This formula works for constant groundspeed. I've found that beginning a descent at 90% of that works best so in the above example - wait till 11 minutes.

Its simple enough - altitude to lose / rod = time.

3-5 miles out seems like an awful long time in a Skyhawk to be plodding along 700 AGL. I plan generally on reaching 1000AGL 2 miles out which if you use my 90% formula above can be an idle descent at 700-800fpm.
 
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Ever timed a pattern? a 1/4 mile upwind, 1/4 mile base, 1 1/4 mile downwind and 1/2 mile final is 2.5 minutes.

If I'm in a low winged single, I want to be at 800 agl for 2.5 minutes before pattern entry so I can see each aircraft. The easiest way to do that is 4 miles out be at pattern altitude and start scanning the horizon.

If I'm in a high winged single, I want to be at 1000 agl for 1.5 minutes before the pattern entry for the same reason that's 2.25 miles or 1.5 minutes.

So I want to be a TPA about 4 miles from the airport....assuming I'm moving at 90.
Plan the arithmatic accordingly.
 
comanchepilot said:
3-5 miles out seems like an awful long time in a Skyhawk to be plodding along 700 AGL. I plan generally on reaching 1000AGL 2 miles out which if you use my 90% formula above can be an idle descent at 700-800fpm.
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You definitely don't want to be 700 AGL 5 miles out coming in to KVLL from the SSW. Not even 1000 AGL. :no:
 
If you have a G430W...

Vertical Speed Required-VSR. Its on NAV Page 7 (last page) and you usually set it up by modifying target altitude (typically 1000 ft above WPT) and position of how far out from the airport you want to be. You set a vertical Speed profile thats comfortable for you and your passengers. Typically 400 or 500 fpm.

My GRT EFIS shows a green arc on descent to show where I will be when I reach my preset altitude.

Both of these are handy with a faster, higher flying plane.
 
When navigating by pilotage it is useful to stay higher a little longer to be sure you ID the field prior to descending.
 
And it's ok to set up a descent further out say 200 ft/min but with power and a higher speed in order to make up some of the time lost on the front end for the climb... But that's for a longer XC not returning from the practice area.
 
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Ron Levy said:
It depends on your speed. Your goal should be to be at pattern altitude about 3-5 miles out so you stay under the heavy/jet pattern normally 500 above and a bit wider than the light plane pattern. If you're arriving at 2500 MSL and the TPA is 1000 MSL, with a 500 ft/min descent rate it will take you 3 minutes to make the descent. At a typical 120 knots, that means it will take you 6 miles (3 minutes times 2 miles/min) to descent, so you'll need to start down 9 miles out. You can do the math for other altitudes and speeds.
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I've never heard of this... Is this a thing?
 
bbchien said:
Ever timed a pattern? a 1/4 mile upwind, 1/4 mile base, 1 1/4 mile downwind and 1/2 mile final is 2.5 minutes.

If I'm in a low winged single, I want to be at 800 agl for 2.5 minutes before pattern entry so I can see each aircraft. The easiest way to do that is 4 miles out be at pattern altitude and start scanning the horizon.

If I'm in a high winged single, I want to be at 1000 agl for 1.5 minutes before the pattern entry for the same reason that's 2.25 miles or 1.5 minutes.

So I want to be a TPA about 4 miles from the airport....assuming I'm moving at 90.
Plan the arithmatic accordingly.
Click to expand...

Interesting, so you are below pattern altitude in a low wing (assuming 1000 AGL) coming into the pattern. I'll have to think about that. It makes some sense.
 
Ron Levy said:
It depends on your speed. Your goal should be to be at pattern altitude about 3-5 miles out so you stay under the heavy/jet pattern normally 500 above and a bit wider than the light plane pattern. If you're arriving at 2500 MSL and the TPA is 1000 MSL, with a 500 ft/min descent rate it will take you 3 minutes to make the descent. At a typical 120 knots, that means it will take you 6 miles (3 minutes times 2 miles/min) to descent, so you'll need to start down 9 miles out. You can do the math for other altitudes and speeds.
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Ron:
Your post makes a huge amount of sense. My concern would be wake turbulance avoidance and if you pass 500 feet below the pattern altitude of a big and heavy, I would think given a perfect storm situation you could experience it given your recommendation, and so need to be real cognizant as to where the big and heavy is in the pattern so as to avoid its wake turbulance.
 
douglas393 said:
Ron:
Your post makes a huge amount of sense. My concern would be wake turbulance avoidance and if you pass 500 feet below the pattern altitude of a big and heavy, I would think given a perfect storm situation you could experience it given your recommendation, and so need to be real cognizant as to where the big and heavy is in the pattern so as to avoid its wake turbulance.
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Chances are the aircraft that will be large enough to create wake turbulence that will be a concern for you won't be flying the pattern, they'll just do a straight in.


MultiMediaWill said:
I've never heard of this... Is this a thing?
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Yep, usually they will have the TPA for the twins and jets at 1500ft AGL. This is mainly because of the speed difference.
 
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If you've got a GPS it's easy. Take your cruise altitude and your destination altitude and allow 2 minutes for every 1000' of difference. When your ETA gets within a couple of minutes of that time, start down at 500 FPM. Reduce power as not to pick up too much speed (though I've found that I don't tend to speed up in the descent so much that it skews the ETA of note).
 
John221us said:
Interesting, so you are below pattern altitude in a low wing (assuming 1000 AGL) coming into the pattern. I'll have to think about that. It makes some sense.
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Agreed. I had not heard that suggestion before either, but it makes some sense. Did all of my private and instrument training in a high wing and then bought low wing but I still just pop into the pattern at 1000 AGL unless otherwise noted/instructed.
 
I just pull the power and then add it back I'm when I'm at altitude and hope for the best :).

J/K. I generally use Comanchepilots method with ROD in FPM and then calculate how far out (time wise) I need to start my descent.

Also, if you're near your home base, you'll learn the more you fly that you sort of just know what altitude you need to be at at different points on your arrival.
 
RV10flyer said:
If you have a G430W...

Vertical Speed Required-VSR. Its on NAV Page 7 (last page) and you usually set it up by modifying target altitude (typically 1000 ft above WPT) and position of how far out from the airport you want to be. You set a vertical Speed profile thats comfortable for you and your passengers. Typically 400 or 500 fpm.

My GRT EFIS shows a green arc on descent to show where I will be when I reach my preset altitude.

Both of these are handy with a faster, higher flying plane.
Click to expand...

I use this, when the VSR hits about 400 I trim the nose down and aim for 500fpm, the extra speed usually works out that the VSR climbs up to nearly 500 by the time I have everything set up.
 
MachFly said:
Chances are the aircraft that will be large enough to create wake turbulence that will be a concern for you won't be flying the pattern, they'll just do a straight in.
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And even if they fly the pattern, they'll be flying a wider pattern and shallower final approach, so you won't be directly under their path.
 
Sometime after takeoff, and sometime before landing.

You're welcome.
 
Three times the altitude you want to lose = your distance in miles.

Say we're at 5000 feet, TPA is at 1000 feet. That's 4000 feet to lose, drop the zeroes, and multiply it by 3. 12 miles out will give you a 500ish fpm descent.

Just a quick mental calculation to get you in the ballpark.
 
Arbiter419 said:
Three times the altitude you want to lose = your distance in miles.

Say we're at 5000 feet, TPA is at 1000 feet. That's 4000 feet to lose, drop the zeroes, and multiply it by 3. 12 miles out will give you a 500ish fpm descent.

Just a quick mental calculation to get you in the ballpark.
Click to expand...
That works for about 90 knots, but not otherwise.
 
MachFly said:
Chances are the aircraft that will be large enough to create wake turbulence that will be a concern for you won't be flying the pattern, they'll just do a straight in.
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Possibly but just last week while I was doing a straight in to 22 at my home port, and Allegient jet was doing a pattern entry(was turning base from downwind when I called the tower). So while I agree it is unlikely it does occur and needs to be something to be concerned about.
 
Put me squarely in the camp of "I paid for that altitude going up, I'm going to reap the rewards coming down."

No initial reduction in throttle, just nose down trim to produce a 500FPM descent. Then, during descent, incremental reductions of the throttle to maintain the initial MP and incremental increases in mixture to maintain a benchmark EGT.

This typically places me at the bottom of, or slightly in, the yellow arc.

This is a smooth air technique only...first minor bump...game's over.
 
Arbiter419 said:
Three times the altitude you want to lose = your distance in miles.

Say we're at 5000 feet, TPA is at 1000 feet. That's 4000 feet to lose, drop the zeroes, and multiply it by 3. 12 miles out will give you a 500ish fpm descent.

Just a quick mental calculation to get you in the ballpark.
Click to expand...
That's ALMOST my technique, but instead of 3, I use my expected ground speed, divided by 30, rounded up. 3 would be for a descent rate between 60 and 90 kts. If I expect to be descending at 130 kts (what I usually shoot for), I use 5.

The reason I don't like to use my GPS ETD is that it's based on your current ground speed, which is likely to change as you descend. So I use a conservative factor valid over a 30 kt range of ground speeds.

But then, I've a strong motivator as I've hurt my ears more than once doing a too rapid descent. YMMV.
 
I'll chime in with a slightly different thought. Practice making your climb, cruise, and descents at precise IAS (pitch/power); fly by the numbers. Descents should be made at a minimum of 500 fpm as when you eventually enter the IFR system that will be the minimum (know P/P for 1000 FPM as well). I personally recommend learning to descend at ~650 fpm; this is the typical ILS/GPS glideslope. While sometimes it is nice "make up" some time by descending with power, it would be best to learn what p/p provides a decent at 650 FPM with no change in IAS.
Some day you may fly high-performance and/or retractable gear aircraft; learning to control airspeed during decent is an important skill in order to stay ahead of the airplane. The best way to do this is to fly by the numbers; start early and you’ll have no issue with transitioning.
 
Feet in thousands you want to lose (4..5..10.. whatever) x 3 = miles out.

Half your groundspeed, then add a zero and that is your vertical descent rate.

Example.. you are at 11,000 feet, pattern is at 1000 feet. You have to lose 10 thousand feet.. 10 x 3 = 30 miles out.

Ground speed 150 kts? half is 75. X10 = 750 fpm descent. If you push over, instead of pull power, use the ground speed in your descent to determine this, not the ground speed in cruise. Or push over further out.
 
timwinters said:
Put me squarely in the camp of "I paid for that altitude going up, I'm going to reap the rewards coming down."

No initial reduction in throttle, just nose down trim to produce a 500FPM descent. Then, during descent, incremental reductions of the throttle to maintain the initial MP and incremental increases in mixture to maintain a benchmark EGT.

This typically places me at the bottom of, or slightly in, the yellow arc.

This is a smooth air technique only...first minor bump...game's over.
Click to expand...

Bingo

Harder in a 172 though because of the FP prop, yet another CS advantage
 
douglas393 said:
Possibly but just last week while I was doing a straight in to 22 at my home port, and Allegient jet was doing a pattern entry(was turning base from downwind when I called the tower). So while I agree it is unlikely it does occur and needs to be something to be concerned about.
Click to expand...

Oh yeah, I was in a pattern with a SouthWest 737 once.
 
timwinters said:
Put me squarely in the camp of "I paid for that altitude going up, I'm going to reap the rewards coming down."

No initial reduction in throttle, just nose down trim to produce a 500FPM descent. Then, during descent, incremental reductions of the throttle to maintain the initial MP and incremental increases in mixture to maintain a benchmark EGT.

This typically places me at the bottom of, or slightly in, the yellow arc.

This is a smooth air technique only...first minor bump...game's over.
Click to expand...
I have a similar technique, but it is good for smooth or rough air.

I reduce MP to bottom of the green arc and trim nose down until I get the descent stabilized at my cruise speed. That way, I am not (yet) slowing down at all but am keeping my ETA (per ForeFlight) the same while I burn less gas and recover all that altitude energy. IAS stays comfortably in the green arc the whole time.

When I get to the pattern I just trim a couple of turns nose up and voila, I am right around 90 knots.
 
Morne said:
I have a similar technique, but it is good for smooth or rough air.

I reduce MP to bottom of the green arc and trim nose down until I get the descent stabilized at my cruise speed. That way, I am not (yet) slowing down at all but am keeping my ETA (per ForeFlight) the same while I burn less gas and recover all that altitude energy. IAS stays comfortably in the green arc the whole time.

When I get to the pattern I just trim a couple of turns nose up and voila, I am right around 90 knots.
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You shouldn't have to trim to maintain cruise speed in descent. When in bumpy air, I just pull the throttle and for every inch of MP reduction, with the same RPM, a 100FPM descent is established, i.e. reduce the MP by 5" and I'll be in a 500FPM descent at the same cruise speed, no change in trim required.

Same goes for RPM, I'll descend 100 FPM FOR every 100 RPM reduction while maintaining constant MP. But that's harder to set up.

This works in MY 182...I can't attest that it works in anything else.
 
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If you don't have GPS, a good starting point for most singles is to multiply the number of thousand feet you want to lose by 4. Your ground speed will be about 2 miles per minute at 120kts, and you descend at 500 FPM, or 4 miles per thousand. You can then tweak your descent rate as you pass each thousand feet. For a twin with ground spped of about 180 kts, the fudge factor would be 6. In an aircraft with a ground speed between 120 and 180, use 5.
 
Unless told by the tower "cleared to land" you are not shooting for the runway, but pattern altitude. At KMYF, approaching from the north with clearance through Bravo, SoCal will usually instruct you to maintain 2,500' until south of hwy 152. By then you will have switched over to the tower frequency. After 152 you will be cleared to enter the downwind for 27 R or L. You will find yourself having to lose a lot of altitude fast.

I usually started engaging the flaps to full while putting the nose down to keep my descent speed slower. When I reached pattern altitude and leveled off, I brought my flaps back up and entered the downwind at the proper speed (80kts).

You can also approach from any altitude above most airports with a spiraling descent down to pattern altitude, or with clearance, all the way in.

500' pm descent is normally what is used for flight planning in trainers. You have to know your ground speed. Do not assume that your airspeed indicator is close to your ground speed, with a headwind or tailwind the two speeds have little in common.

Always know the pattern altitude of the airport you are approaching to land, that is a whole lot more critical than correctly guessing when to start your descent.

If you undershoot, just level off at pattern altitude and continue your approach. If you overshoot, depending on how much, get the nose down, extend flaps, whatever it takes to get to pattern altitude. Then adjust your speed for the pattern if you have not already done it.

Use flaps, watch your speed, do not just dive down to pattern altitude. remember, in a dive, you accumulate energy, you are going to have to lose that energy before you land. It is best to try and not accumulate it in the first place.

In other words, except for planning purposes, it is not a huge deal as long as whatever you do is done correctly and safely.

-John
 
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