Vertical Navigation Question
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backpacker
If a Cessna doing a GS of 90 kts wants to lose 5000 feet to be over the airfield at a certain height. And using 1500 fpm average which we do in the faster stuff.
The 3times rule would say approx. 15 miles.
That would take you 10 minutes.
At 1500 fpm you would be at your planned altitude after 3.33minutes. Well before the planned time.
In this case you would need only 500fpm to be there at your planned height and time.
Therefore the three time rule will not work for an aircraft at 90 knots. But try the same at 270 knots and it works fine.
What i am trying to get across is that, if you use a fixed formula (3times), you need a constand ROD and speed ('ish) or it wont work. The 3 times is really for faster aircraft as the above example dictates.
Fright levels one seems to be quite good though.
The 3times rule would say approx. 15 miles.
That would take you 10 minutes.
At 1500 fpm you would be at your planned altitude after 3.33minutes. Well before the planned time.
In this case you would need only 500fpm to be there at your planned height and time.
Therefore the three time rule will not work for an aircraft at 90 knots. But try the same at 270 knots and it works fine.
What i am trying to get across is that, if you use a fixed formula (3times), you need a constand ROD and speed ('ish) or it wont work. The 3 times is really for faster aircraft as the above example dictates.
Fright levels one seems to be quite good though.
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Mr Kirk
As backpacker has said what we are looking for is a 3 degree descent path and the three times rule will give you this.
To achive the target altitude at the aircraft's normal decent speed you have to set the correct R.O.D for that speed, I cant see why you seem to have an issue with this.
With piston engine aircraft you should be going down with some power on to avoid shock cooling (check the BGA website for the most in depth reserch on this) and with a jet you will be descending with the throtles shut.
Most piston aircraft will descend at about 5-600ft/min this works for both engine cooling and passenger comfort, with a pressurised jet both of these factors are not an issue normaly.
I went flying twice yesterday and descended both times using the three times rule to descend and both aircraft got to the point in space that I required with little effort on my part. And just in case you are wondering the first sector was on an A320 and the second on a DR400-180 so in practice it works at both ends of the speed range.
I do have to question all those who turn this into a "black art" if they think that it is worth the effort? After all brain power that is being used for this in depth calculation cant be used to check for things like the MSA.
To achive the target altitude at the aircraft's normal decent speed you have to set the correct R.O.D for that speed, I cant see why you seem to have an issue with this.
With piston engine aircraft you should be going down with some power on to avoid shock cooling (check the BGA website for the most in depth reserch on this) and with a jet you will be descending with the throtles shut.
Most piston aircraft will descend at about 5-600ft/min this works for both engine cooling and passenger comfort, with a pressurised jet both of these factors are not an issue normaly.
I went flying twice yesterday and descended both times using the three times rule to descend and both aircraft got to the point in space that I required with little effort on my part. And just in case you are wondering the first sector was on an A320 and the second on a DR400-180 so in practice it works at both ends of the speed range.
I do have to question all those who turn this into a "black art" if they think that it is worth the effort? After all brain power that is being used for this in depth calculation cant be used to check for things like the MSA.
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A & C
yes, i think this thread has gone into the depths of mathematics.
Can you just agree, from the calculation listed by me about two posts up that it may not be suitable for light aircraft.
I am in favour of this rule of thumb stuff to save time and make life easier.
But can somenone be adult enough to look at the calculations and say 'it probably is not suitable for this purpose'. The calculation is very clear and relevant to light aircraft flying. And I totally agree with you about light aircraft descending at 5-600fpm. And in that case the 3 times clearly is not suitable.
But, please do the same calculation as i did for your A320 and your DR400 with exactly the same figures. 5000 feet to lose using the 3times rule and transpose it to the DR400 and let me see what the ROD is for both for the time/distnace covered to the destination.
If the figures are close together then i stand corrected. If not, then please do the same.
I am not looking for a debate and I too use this for my RHS job and as an FI.
Can you just agree, from the calculation listed by me about two posts up that it may not be suitable for light aircraft.
I am in favour of this rule of thumb stuff to save time and make life easier.
But can somenone be adult enough to look at the calculations and say 'it probably is not suitable for this purpose'. The calculation is very clear and relevant to light aircraft flying. And I totally agree with you about light aircraft descending at 5-600fpm. And in that case the 3 times clearly is not suitable.
But, please do the same calculation as i did for your A320 and your DR400 with exactly the same figures. 5000 feet to lose using the 3times rule and transpose it to the DR400 and let me see what the ROD is for both for the time/distnace covered to the destination.
If the figures are close together then i stand corrected. If not, then please do the same.
I am not looking for a debate and I too use this for my RHS job and as an FI.
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jamestkirk, when you find yourself in a hole - stop digging!
Somehow I think you have missed the point here, and are trying to complicate something that from the outset is very straight forward and simple.
You cannot escape the fact that the 3 x rule works for just about every aircraft, including the "fast ones" that you may fly. Even at the perhaps not so fast 350 kts that I typically descend at it works perfectly, so much so that I rarely use the FMS to calculate a TOD.
As for pressurisation, and passenger comfort, the rule works well over all speed ranges as well. Those descending at a more modest groundspeed (un-pressurised types) will rarely exceed 500 fpm change in the in the cabin, while pressurised types descending at say 3,000 fpm will still permit passenger comfort due to the fact that their cabins typically have a TOD cabin pressure equivalent to not more than 8,000 ft in the first place - and therefore have less pressure change to make in the descent.
Sooty
Somehow I think you have missed the point here, and are trying to complicate something that from the outset is very straight forward and simple.
You cannot escape the fact that the 3 x rule works for just about every aircraft, including the "fast ones" that you may fly. Even at the perhaps not so fast 350 kts that I typically descend at it works perfectly, so much so that I rarely use the FMS to calculate a TOD.
As for pressurisation, and passenger comfort, the rule works well over all speed ranges as well. Those descending at a more modest groundspeed (un-pressurised types) will rarely exceed 500 fpm change in the in the cabin, while pressurised types descending at say 3,000 fpm will still permit passenger comfort due to the fact that their cabins typically have a TOD cabin pressure equivalent to not more than 8,000 ft in the first place - and therefore have less pressure change to make in the descent.
Sooty
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But, please do the same calculation as i did for your A320 and your DR400 with exactly the same figures. 5000 feet to lose using the 3times rule and transpose it to the DR400 and let me see what the ROD is for both for the time/distnace covered to the destination.
5000 feet to lose according to the 3X rule requires 15 nm in both cases. Something the A320 covers in 3.6 minutes and the DR400 in 9.5 minutes at their given speeds.
For the A320, three degree slope (or 5000 feet to lose in 3.6 minutes) leads to 1388 fpm. Entirely doable in a pressurized jet (I assume - I've never flown those).
For the DR400, three degree slope (or 5000 feet to lose in 9.5 minutes) leads to 528 fpm. Entirely doable in a non-pressurized piston single.
Now of course, if for some reason, instead of descending at 528 fpm, you start descending at 1500 fpm, you will end up at your target altitude way before the planned point. Simply because you flew about a 9 degree slope instead of 3 degrees. If you routinely use descent rates of 1500 fpm in light aircraft then you might want to think about applying a 1X rule instead of 3X. And have a permanent eardrum perforation, otherwise it won't be comfortable or healthy in the long run.
But why would you want to use an ROD of 1500 fpm in a small aircraft?
(*) A&C, sorry, the DR400 I fly is a -120 with 95 knots. I know your -180 is a lot faster.
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I think DP's answer was the correct exam question answer.
I use time as the reference...and plan my ROD.......so if I need to lose 10'000feet and I want a 500fpm ROD I'll start my descent 20 mins out. How far out you are depends on your speed of course. At 120kts this is 40nm, 240kts 80 miles.
Or I setup the VNAV profile on the GPS
I use time as the reference...and plan my ROD.......so if I need to lose 10'000feet and I want a 500fpm ROD I'll start my descent 20 mins out. How far out you are depends on your speed of course. At 120kts this is 40nm, 240kts 80 miles.
Or I setup the VNAV profile on the GPS
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Goodness me - how complicated!
If the question is an exam one, then the calculations are correct.
That most of us wouldn't do it that way is a given. Those who are good at mental arithmetic while flying single pilot IFR will have no problems. Me, I get into brainfade, so I have a simple rule.
I climb at 500fpm (until the poor aeroplane says "enough"), and I descend at 500 fpm (unless there's a good reason that requires something else).
Conveniently, my Arrow flies close enough to 120 knots that 500 fpm allows me to use 4 miles per thousand feet as my standard. KISS is the only way for old lags like me.
Then, I learned the "numbers" that go with that. For example, if I'm cruising at 23/23, reducing MP to 17 inches will set me up neatly for 500fpm down. KISS again.
If the question is an exam one, then the calculations are correct.
That most of us wouldn't do it that way is a given. Those who are good at mental arithmetic while flying single pilot IFR will have no problems. Me, I get into brainfade, so I have a simple rule.
I climb at 500fpm (until the poor aeroplane says "enough"), and I descend at 500 fpm (unless there's a good reason that requires something else).
Conveniently, my Arrow flies close enough to 120 knots that 500 fpm allows me to use 4 miles per thousand feet as my standard. KISS is the only way for old lags like me.
Then, I learned the "numbers" that go with that. For example, if I'm cruising at 23/23, reducing MP to 17 inches will set me up neatly for 500fpm down. KISS again.
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Perhaps i've missed something here but I was always taught the 3 x height to lose method. The rate of descent required to achieve the 3/1 profile was 5 x groundspeed.
So, turboprop at 300 kts g/s descending from 20k ft to 5k ft = 3 x 15 which equals 45 miles. Rate of descent is 5 x 300 = 1500 fpm
Cessna/piper travelling at 120 kts at 5k feet descending to 1k feet = 3 x 4 which equals 12 miles. rate of descent is 5 x 120 = 600 fpm.
For those that want convincing, then go work it out with a calculator, it works out prety much on the nose.
So, turboprop at 300 kts g/s descending from 20k ft to 5k ft = 3 x 15 which equals 45 miles. Rate of descent is 5 x 300 = 1500 fpm
Cessna/piper travelling at 120 kts at 5k feet descending to 1k feet = 3 x 4 which equals 12 miles. rate of descent is 5 x 120 = 600 fpm.
For those that want convincing, then go work it out with a calculator, it works out prety much on the nose.
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sooty615
you stated that the 3times rule works for pretty much every aircraft.
At what ROD?
What ROD do you use in your airliner and would it be the same when you fly light aircraft for the same distance to cover?
All I trying to get across is that the ROD will change for different GS's. Its a methematical certainty.
If yours answer is 'differing' ROD then how can you use the 3times rule.
If its the same ROD then it will not work for slower aircraft.
Could you confirm that the above two sentences a factual.
Could I politely ask for you to read the last post and check my mathematics. If they are wrong, please let me know.
Anyone coming back to me on this seems not to want to confirm the formula that myself or others have posted, which are 100% correct and written in most PPL books.
Backpacker - You keep missing the point.
At what ROD?
What ROD do you use in your airliner and would it be the same when you fly light aircraft for the same distance to cover?
All I trying to get across is that the ROD will change for different GS's. Its a methematical certainty.
If yours answer is 'differing' ROD then how can you use the 3times rule.
If its the same ROD then it will not work for slower aircraft.
Could you confirm that the above two sentences a factual.
Could I politely ask for you to read the last post and check my mathematics. If they are wrong, please let me know.
Anyone coming back to me on this seems not to want to confirm the formula that myself or others have posted, which are 100% correct and written in most PPL books.
Backpacker - You keep missing the point.
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Backpacker - You keep missing the point.
Obviously with a 1500 fpm descent rate and a groundspeed of, say, 100 knots, you are not going to have a three degree glidepath but something way steeper, and thus the 3X rule does not apply.
After all, the only thing the 3X rule will calculate for you is the distance required to descend x feet, based on a three degree glidepath. How to achieve that flightpath, powersettings, ROD etc., will differ from aircraft to aircraft. (If you use five times the groundspeed as your ROD you will get a reasonably accurate three degree glideslope though, as others have pointed out.)
And indeed, if your desire is not a three degree flightpath, but to be x feet lower in y nm, there are other formula that you can use. We've already established that long ago.
But I have asked before, and will ask once again: why do small aircraft need a 1500 fpm descent rate, leading to a glide path of approximately nine degrees? What's wrong with a three degree glide path at 500 fpm? Particularly if they are able to plan their descent beforehand?
Unless the point you're trying to make is different, or until you are able to answer that question, I'm just going to enjoy watching you dig an even bigger hole for yourself...
(Must admit that alongside the enjoyment, I am a bit worried too. You are an FI and you fly RHS in an airliner. There's four people here on this forum, one of which a long time LHS and FI, two other airline pilots, and a PPL with a bachelors degree in math, all trying to tell you you are wrong. Yet you maintain you are right and claim that the math prove without a shred of doubt that you are right - without giving the actual proof. Then someone runs the numbers for you, proves that your math is incorrect and that the formula lead to plausible numbers for all relevant scenarios. And you try to debunk that with a simple "you are missing the point" rebuttal. You sound very reluctant to admit you're wrong, and in your position, that worries me.)
Last edited by BackPacker; 17th Aug 2008 at 19:32.
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No, No, No
What I am saying is that for the 3times rule the ROD needs to change for different speeds. Do you agree with that? If you do then we are not in dispute.
The original post I replied to was someone saying they use the airline model ( i.e 250 kts ish and apply it for slower aircraft) The average for that is 1500fpm, which would not work for 90 kts. Your first post which had some figures on it did not include distance. I was saying that you must take distance into account for descent planning. Is that correct?
Admittedly you added it on a later post.
I have done those calculations back and you have not said that the maths are wrong. Please have a look again. I think I did a 5000 foot descent thing at 90 kts.
On a slightly different note. On occasion you can can descent light aircarft at 1500fpm without to much trouble. In some situations it may be necessary.
The reason I said you are missing the point is that I was only using the 1500fpm in light aircraft to compare the figures from the original post not as a general way of descending light aircraft.
The original post I replied to was someone saying they use the airline model ( i.e 250 kts ish and apply it for slower aircraft) The average for that is 1500fpm, which would not work for 90 kts. Your first post which had some figures on it did not include distance. I was saying that you must take distance into account for descent planning. Is that correct?
Admittedly you added it on a later post.
I have done those calculations back and you have not said that the maths are wrong. Please have a look again. I think I did a 5000 foot descent thing at 90 kts.
On a slightly different note. On occasion you can can descent light aircarft at 1500fpm without to much trouble. In some situations it may be necessary.
The reason I said you are missing the point is that I was only using the 1500fpm in light aircraft to compare the figures from the original post not as a general way of descending light aircraft.
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start descent @ 3 x distance to drop
ROD 5 x g/s
eg need to drop from 8000' to 3000'= 5000' to lose
3 x 5 = start descent 15miles out
rod = 5 x g/s g/s=100kts
rod= 5 x 100 = 500fpm
ROD 5 x g/s
eg need to drop from 8000' to 3000'= 5000' to lose
3 x 5 = start descent 15miles out
rod = 5 x g/s g/s=100kts
rod= 5 x 100 = 500fpm
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What I am saying is that for the 3times rule the ROD needs to change for different speeds. Do you agree with that? If you do then we are not in dispute.
The original post I replied to was someone saying they use the airline model ( i.e 250 kts ish and apply it for slower aircraft) The average for that is 1500fpm, which would not work for 90 kts.
Anyway, I think we've now established that the 3x rule works for all types of aircraft, assumes a three (point thirteen) degree flightpath, and to achieve that three degree flightpath you need five (point five) times the groundspeed as your ROD. Agreed?
On a slightly different note. On occasion you can can descent light aircarft at 1500fpm without to much trouble. In some situations it may be necessary.
I do not recommend that technique if you find yourself 1000 feet too high on an IFR approach though.
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For all of those who think TOD is 3 x Alt / 1000 then I hope none of you a) fly high performance unpressurised aircraft and b) have problems with your ears
My aircraft descends at 180Kts = 3nm / minute. If cruising at 12,000ft then by many of the opinions here then TOD is 36nm away. So - you come down at 1000fpm.
If you've got kids in the back, as others have mentioned, they are going to find that a touch on the uncomfortable side.
ROD in that case is the best way to go, and with that in mind a more comfortable 500fpm cruise descent (as I really don't want to replace my IO540 just yet) means the formula is 6 x Alt / 1000 - 72nm.
3 x altitude is great for pressurised airliners or C152's doing 90Kts. It is not a universal panacea as some here seem to think.
My aircraft descends at 180Kts = 3nm / minute. If cruising at 12,000ft then by many of the opinions here then TOD is 36nm away. So - you come down at 1000fpm.
If you've got kids in the back, as others have mentioned, they are going to find that a touch on the uncomfortable side.
ROD in that case is the best way to go, and with that in mind a more comfortable 500fpm cruise descent (as I really don't want to replace my IO540 just yet) means the formula is 6 x Alt / 1000 - 72nm.
3 x altitude is great for pressurised airliners or C152's doing 90Kts. It is not a universal panacea as some here seem to think.
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backpacker
These types of posts do sometimes get a bit over compliacted and mis-construde.Thats the beauty of pprune.
A few years back i did the AOPA aerobatics thing in a 152. Spent most of my time in descents trying to get enough energy to loop it. Not the most graceful of aircraft to do it in.
A few years back i did the AOPA aerobatics thing in a 152. Spent most of my time in descents trying to get enough energy to loop it. Not the most graceful of aircraft to do it in.
