Vertical Navigation Question
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Vertical Navigation Question
Hi all, this is probably going to sound really simple but could someone be able to help me with the following question, its probably easy I just can't do it.
An aircraft is 15nm from the destination airfield at 3000ft QNH. It is necessary to descend to 1000ft QFE to be level 3nm before reaching the airfield, the elevation of which is 372ft. G/S in the descent will be 110knts. What ROD is required (assume 1mb=30ft).
It would be great if someone could point in the right direction for this one.Thank you for your time.
An aircraft is 15nm from the destination airfield at 3000ft QNH. It is necessary to descend to 1000ft QFE to be level 3nm before reaching the airfield, the elevation of which is 372ft. G/S in the descent will be 110knts. What ROD is required (assume 1mb=30ft).
It would be great if someone could point in the right direction for this one.Thank you for your time.
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Well you want to desent to 1372ft QNH (1000QFE plus airfield elevation of 372ft)
So you want to desent 1628ft....(3000 down to 1372). You want to do this in the space of 12nm (15nm less 3nm as you want to be level by 3nm)
So 1628ft in 12nm.
So low long will it take you to travel 12nm? Well ground speed =110kts. So 12/110kts *60 mins in an hour=6.545 mins to travel 12nm.
So you want to desent 1628 feet in 6.545 mins. or 249 feet per minute.
I hope I haven't missed anything!
dp
So you want to desent 1628ft....(3000 down to 1372). You want to do this in the space of 12nm (15nm less 3nm as you want to be level by 3nm)
So 1628ft in 12nm.
So low long will it take you to travel 12nm? Well ground speed =110kts. So 12/110kts *60 mins in an hour=6.545 mins to travel 12nm.
So you want to desent 1628 feet in 6.545 mins. or 249 feet per minute.
I hope I haven't missed anything!
dp
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TFC
You are using too much brain power with all this! most of us use altitude to loose X 3.
So if you need to descend 5000 ft you need 15 NM to do it, add or subtract a little for wind direction and that is all you have to do.
It works for all the aircraft that I fly, so please beware of people who try to turn this into a black art.
So if you need to descend 5000 ft you need 15 NM to do it, add or subtract a little for wind direction and that is all you have to do.
It works for all the aircraft that I fly, so please beware of people who try to turn this into a black art.
That's 500fpm at 120kts.
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...Or you can do what my instructor taught me - forget all the hard maths involved in trying to figure out the distance at which you should start your descent and then figuring out where that is - just do it by time.
You have an ETA calculated for your leg, and say 500fpm is a comfortable descent, you want to lose 2000 ft, so 4 minutes before your ETA set up a 500 fpm descent. No hard maths, and no need to figure out how far - or where 4 minutes at 110 knots is.
Dave
You have an ETA calculated for your leg, and say 500fpm is a comfortable descent, you want to lose 2000 ft, so 4 minutes before your ETA set up a 500 fpm descent. No hard maths, and no need to figure out how far - or where 4 minutes at 110 knots is.
Dave
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Through a little bit of experimentation... pulling the power back to 1700 rpm on my C150 results in a nice 3 miles per 1000 feet descent profile; whilst maintaining cruise trim setting. That way I do not need to think about it.
And each 100rpm equates to around 50 to 70 feet per minute... so I can throttle up and down for adjustments as I need to.
In the configuration I am also travelling at 1.5 Nautical Miles per minute; so reducing to 1400 rpm will give me a minimum of 150fpm extra rate, which gives me at least an extra 300 feet on the profile per 1000 feet, or 30% extra altitude loss; returning the power to 1700 removes the extra descent rate, and then returning to cruise power removes it all together.
it is VERY rule of thumb but works and I don't need to think about it in the descent, just do it!
And each 100rpm equates to around 50 to 70 feet per minute... so I can throttle up and down for adjustments as I need to.
In the configuration I am also travelling at 1.5 Nautical Miles per minute; so reducing to 1400 rpm will give me a minimum of 150fpm extra rate, which gives me at least an extra 300 feet on the profile per 1000 feet, or 30% extra altitude loss; returning the power to 1700 removes the extra descent rate, and then returning to cruise power removes it all together.
it is VERY rule of thumb but works and I don't need to think about it in the descent, just do it!
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A & C
That rule of thumb is used for, as you know, faster aircraft. I use it as well.
It does not work for all speeds and I think you'll find the 3times rule is for a rough 1500fpm descent rate. As you can see the three times rule would not work for Kestrels example and most others when dealing with SEP speeds.
It does not work for all speeds and I think you'll find the 3times rule is for a rough 1500fpm descent rate. As you can see the three times rule would not work for Kestrels example and most others when dealing with SEP speeds.
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I use this:
ROD required from 10nm out is 200fpm for every 1000ft to lose.
This works at 120kt.
So, if say you have 4000ft to lose then you need -800fpm.
Or, if 20nm to run, -400fpm will do it.
ROD required from 10nm out is 200fpm for every 1000ft to lose.
This works at 120kt.
So, if say you have 4000ft to lose then you need -800fpm.
Or, if 20nm to run, -400fpm will do it.
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Mr Kirk
You are just making all of this TFC! the three times rule works in my Robin DR400 just as well as it works in a B738 or an A320.
It is not perfect, it will not give you an exact TOD to get you to an exact point in space but it will get you into the ball park without using too much brain power, I have no doubt that by using a complicated formula you could much more accurately fly into a hill that you had forgotten about due to the accurate and time consuming calculation.
Remember single pilot IF is about keeping the "big picture" rather than micro managing.
It is not perfect, it will not give you an exact TOD to get you to an exact point in space but it will get you into the ball park without using too much brain power, I have no doubt that by using a complicated formula you could much more accurately fly into a hill that you had forgotten about due to the accurate and time consuming calculation.
Remember single pilot IF is about keeping the "big picture" rather than micro managing.
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A @ C
No, all am am saying is that the 3times rule only works for a constant ROD. As i previously stated, the norm is 1500 fpm.
IF you deviate fron that, then the constant does not work. As previously proved by the mathematical examples.
You did not however answer my question. The 3times ruke IS based on 1500fpm on a fast(er) moving aircraft. You know this as well as i do.
You seem a reasonable individual. Please don't be the 'how dare you question me' brigade.
IF you deviate fron that, then the constant does not work. As previously proved by the mathematical examples.
You did not however answer my question. The 3times ruke IS based on 1500fpm on a fast(er) moving aircraft. You know this as well as i do.
You seem a reasonable individual. Please don't be the 'how dare you question me' brigade.
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Unless you are already on top of the airport at 8000ft after forgetting your descent, you usually have a pretty good idea of your desired rate of descent. On unpressurized aircraft it is usually something like 500fpm for passenger comfort, on pressurized aircraft 1000fpm or more;
So if you need to loose 12000 ft (cruising altitude - final approach altitude) with no ATC restrictions and you are in a unpressurized aircraft, then you would want 500fpm, thus 120000/500= 24 minutes out. This works if you don't increase speed and make the descent by reducing power.
Couldn't be simpler and takes into account head or tailwinds since you're working with ETA.
So if you need to loose 12000 ft (cruising altitude - final approach altitude) with no ATC restrictions and you are in a unpressurized aircraft, then you would want 500fpm, thus 120000/500= 24 minutes out. This works if you don't increase speed and make the descent by reducing power.
Couldn't be simpler and takes into account head or tailwinds since you're working with ETA.
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You seem a reasonable individual. Please don't be the 'how dare you question me' brigade.
If you apply the 3X rule, what you're essentially calculating is a certain glide slope angle. A hair over three degrees, to be exact. This is an easy descent in all types of aircraft. However, the faster the aircraft flies, the higher the rate of descent will be.
Since A&C mentions a DR400 (though not the type), let's assume 95 knots. Using the 3X rule this works out to be 527 fpm. Very doable in a non-pressurized aircraft.
A larger aircraft at 300 knots can use the exact same formula but would end up with a ROD of 1660 fpm.
Now the norm for ATC for pressurized aircraft might be a 1500 fpm descent, I don't know. I don't fly pressurized aircraft. But if you apply that norm and want to calculate your TOD point based on ROD instead of a 3 degree glideslope, then dirkdj's calculation makes more sense to use than the 3X rule.
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Backpacker -you have just proved yourself wrong
the 3 times rule also takes into account distance. Something you have completely missed out on.
Your calculations have just given differing rated of descent so the 3 times rule does not come into effect.
Before you retort back. Can you just confirm that the rule of thumb is 'I have to lose 10,000' so i need about 30 miles'. If so, where is that on your calculations above. We use that for about 250 knots in the descent rate about 1500fpm. Do that at 60kts and its 333 fpm, for the same 30 miles travelled. All I am trying to get across is that it clearly is a good rule of thumb for faster aircraft but is mathematically incorrect for smaller one. The figures prove that without question.
If you use the 3times rule. What is the ROD, because you must have a second constant for it to work. You cannot have 3times the height, then vary ROD as it makes a mockery of it.
If you don't take distance into account, especially in faster aircraft, you will just fly past the airport too high/low, whatever.
For cessna's and the like i used to taech groundspeed/distance = minutes and divide by altitude to lose. So yes, I like to keep it simple when its right.
Again, just go a check the formula in a nav book somewhere that takes into account the above and then come back to me and tell me I am wrong or right.
Your last comment about specific aircraft ROD, I was not questioning anyone about.
Your calculations have just given differing rated of descent so the 3 times rule does not come into effect.
Before you retort back. Can you just confirm that the rule of thumb is 'I have to lose 10,000' so i need about 30 miles'. If so, where is that on your calculations above. We use that for about 250 knots in the descent rate about 1500fpm. Do that at 60kts and its 333 fpm, for the same 30 miles travelled. All I am trying to get across is that it clearly is a good rule of thumb for faster aircraft but is mathematically incorrect for smaller one. The figures prove that without question.
If you use the 3times rule. What is the ROD, because you must have a second constant for it to work. You cannot have 3times the height, then vary ROD as it makes a mockery of it.
If you don't take distance into account, especially in faster aircraft, you will just fly past the airport too high/low, whatever.
For cessna's and the like i used to taech groundspeed/distance = minutes and divide by altitude to lose. So yes, I like to keep it simple when its right.
Again, just go a check the formula in a nav book somewhere that takes into account the above and then come back to me and tell me I am wrong or right.
Your last comment about specific aircraft ROD, I was not questioning anyone about.
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jtk, your last post is so rambling that I don't even know where to begin answering it.
Let's go back to the beginning. We are at some altitude, say x, and we need to be at altitude y at some point in time/space. In order to reach that point, there's a few things we need to know. Top of the list will be the TOD time/place, which is the time/place we need to start our descent. Secondary to that are the ROD and speed numbers but what you will find is that the ROD and speed in the descent are a given for your aircraft type and passenger comfort. They don't vary (much) between flights.
For your average spamcan, this will mean 100 knots and 500 feet ROD, more or less. For your average pressurized airliner, this will mean 250 knots (below 10.000 feet due to speed restrictions) and 1500 fpm. Both will lead to a glideslope of approximately three degrees. And if you have anything in between (like the C414A of a friend of mine, who I spoke to just last weekend about this) you do the calculations once and then you know the ROD to achieve for your typical cruise descent on a three degree slope.
That leaves the TOD point as the only variable to calculate. That's where the 3X rule comes into play. And that's what A&C argues: the rule can be applied to anything that flies, as long as the objective is to have a three degree glide slope.
Again, how to achieve and maintain that three degree glide slope, varies from aircraft to aircraft. The faster the cruise speed in the descent, the higher your ROD. But three degrees is doable in almost any kind of aircraft (gliders, space shuttles and the like excepted).
Oh, and my calculations are simply based on working knots, miles, feet, hours and minutes back to metric values. Losing 5000 feet takes 15 nm according to the 3X rule. 5000 feet is 0.82 nm. arctan (0.82/15) = 3.13 degrees. Close enough. So the 3X rule leads you to an almost-perfect 3 degree slope. (Your example of 10.000 feet in 30nm works out the same by the way.)
ROD at 95 knots: 95 knots divided by 60 is 1.58 nm/min. The glide ratio was 5000 feet per 15 miles, so the ROD is 1.58 * 5000 / 15 = 527 fpm. That's just a number you calculate once and then use every time for that aircraft type. In fact, you will probably work out the exact RPM (or MAP) reduction you need for that ROD and remember that - see italianjons post.
ROD at 300 knots: 300 knots divided by 60 is 5 nm/min. Same glide ratio, so the ROD is 5*5000/15 = 1667 fpm.
Now we both got those same answers. So why do you say:
What is wrong with a 527 fpm ROD at 95 knots? Or indeed even a 333 fpm ROD at 60 knots?
Let's go back to the beginning. We are at some altitude, say x, and we need to be at altitude y at some point in time/space. In order to reach that point, there's a few things we need to know. Top of the list will be the TOD time/place, which is the time/place we need to start our descent. Secondary to that are the ROD and speed numbers but what you will find is that the ROD and speed in the descent are a given for your aircraft type and passenger comfort. They don't vary (much) between flights.
For your average spamcan, this will mean 100 knots and 500 feet ROD, more or less. For your average pressurized airliner, this will mean 250 knots (below 10.000 feet due to speed restrictions) and 1500 fpm. Both will lead to a glideslope of approximately three degrees. And if you have anything in between (like the C414A of a friend of mine, who I spoke to just last weekend about this) you do the calculations once and then you know the ROD to achieve for your typical cruise descent on a three degree slope.
That leaves the TOD point as the only variable to calculate. That's where the 3X rule comes into play. And that's what A&C argues: the rule can be applied to anything that flies, as long as the objective is to have a three degree glide slope.
Again, how to achieve and maintain that three degree glide slope, varies from aircraft to aircraft. The faster the cruise speed in the descent, the higher your ROD. But three degrees is doable in almost any kind of aircraft (gliders, space shuttles and the like excepted).
Oh, and my calculations are simply based on working knots, miles, feet, hours and minutes back to metric values. Losing 5000 feet takes 15 nm according to the 3X rule. 5000 feet is 0.82 nm. arctan (0.82/15) = 3.13 degrees. Close enough. So the 3X rule leads you to an almost-perfect 3 degree slope. (Your example of 10.000 feet in 30nm works out the same by the way.)
ROD at 95 knots: 95 knots divided by 60 is 1.58 nm/min. The glide ratio was 5000 feet per 15 miles, so the ROD is 1.58 * 5000 / 15 = 527 fpm. That's just a number you calculate once and then use every time for that aircraft type. In fact, you will probably work out the exact RPM (or MAP) reduction you need for that ROD and remember that - see italianjons post.
ROD at 300 knots: 300 knots divided by 60 is 5 nm/min. Same glide ratio, so the ROD is 5*5000/15 = 1667 fpm.
Now we both got those same answers. So why do you say:
All I am trying to get across is that it clearly is a good rule of thumb for faster aircraft but is mathematically incorrect for smaller one.
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backpacker
You have just proven that you cannot answer it. And a bit hypocritical calling my post rambling.
Lets try and be absolutely clear; You need to take distance into account if you are planning a descent. You seem to have a problem with that.
Every time you do a verbose claculation, you prove me right.
Please, please, please go and do some homework and then come back to me and you will see the 3times rule is not for all speeds.
I have no idea what you are so upset about 527/333 fpm. I am not saying anything is wrong with any of them. They are just numbers in a calculation.
The reason I replied back before is that you DID NOT take distance into account and you still have not given a reason why. You added on your last post but hindsight is a great educator.
If you have a PPL or ATPL book on nav, please look it up.
Lets try and be absolutely clear; You need to take distance into account if you are planning a descent. You seem to have a problem with that.
Every time you do a verbose claculation, you prove me right.
Please, please, please go and do some homework and then come back to me and you will see the 3times rule is not for all speeds.
I have no idea what you are so upset about 527/333 fpm. I am not saying anything is wrong with any of them. They are just numbers in a calculation.
The reason I replied back before is that you DID NOT take distance into account and you still have not given a reason why. You added on your last post but hindsight is a great educator.
If you have a PPL or ATPL book on nav, please look it up.
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Lets try and be absolutely clear; You need to take distance into account if you are planning a descent. You seem to have a problem with that.
Although I'm beginning to get a feel for our mutual misunderstanding. My assumption in my posts (and A&Cs posts) is that you are cruising along with the DME or the GPS telling you the distance to your target. At some point in time you need to start your descent, and you want that descent to be a comfortable one at a three degree slope. So you calculate the distance from TOD to the target using the 3X rule and at that point in time you either start your descent (using the numbers appropriate for a three degree slope in your aircraft type) or you gently remind ATC that you would like to start your descent right now.
Your assumption might be different. What I feel is that you are not calculating a TOD point in advance, but are suddenly confronted with a wish or ATC instruction to be x foot lower in y nm. In that case, indeed, the 3X rule does not work at all and you need a different calculation (either based on distance or time to the target, it doesn't matter) to calculate the actual ROD needed to be level at the desired altitude, at the desired distance. Nothing to do with the aircraft type or speed, but simply by the fact that losing x foot in y nm might need something other than a three degree glideslope.
So here's my opinion: The 3X rule can be used (and is being used) to calculate your TOD point in advance and it works for virtually all aircraft, because it leads to a three (point thirteen) degree glidepath. However, because of external circumstances you might not be able to start your descent at the optimum point. In that case you need a different calculation to calculate your actual needed ROD, based on the actual distance to cover and your actual groundspeed.
Now what I don't get in your posts is why you tie any of the calculations (both the 3X rule and yours) to specific aircraft types. You say "For cessna's and the like i used to taech groundspeed/distance = minutes and divide by altitude to lose." Absolutely correct, but the exact same method can be used for A380s as well.
Likewise, about the 3X rule you say "All I am trying to get across is that it clearly is a good rule of thumb for faster aircraft but is mathematically incorrect for smaller one. The figures prove that without question." Now I did run the numbers for you, got descent rates between 333 and 527 fpm for small aircraft, when using the 3X rule, which you find entirely acceptable. Yet you maintain that they are mathematically incorrect?
So, please, for my sake and the rest of this community, could you come up with a scenario where the 3X rule is NOT appropriate for a small aircraft, but is appropriate for a fast aircraft, all circumstances being equal? And keep in mind of course the reason for using the 3X rule: to calculate the TOD point in advance.
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I'm a bit anal about engine cooling, so I use a rule of thumb of 5 miles per 1000' - it make my poor mental maths a bit easier too.
As I'm normally interested in getting to the aerodrome rather than a bimble of a certain duration, at the descent point I will just stick the nose down and hold cruise power and take the increased airspeed. After a minute or more, if needed, I'll start to pull MP as required in steps of 1 or 2" to hopefully end up at circuit height with the speed bleeding off about 3-5nm before the aerodrome.
But presented with the original problem in the air, I would probably go:
15nm away less 3 is 12, less another 1 whilst I work this out (and while flying the plane and talking to ATC) is 11, which is close enough (for government work) to 10nm
10nm at 110kts is near enough 2nm a minute, so 5 mins
3000' to 1000' is 2000', less 400' for QFE is 1600'
1600 divided by 5mins is a bit over 300fpm
Yes, the errors can accumulate, but I wouldn't try to use any VSI that I have come across in a GA aircraft to 100fpm resolution, so working things out to the nearest 50fpm or finer is a waste of time IMHO
As I'm normally interested in getting to the aerodrome rather than a bimble of a certain duration, at the descent point I will just stick the nose down and hold cruise power and take the increased airspeed. After a minute or more, if needed, I'll start to pull MP as required in steps of 1 or 2" to hopefully end up at circuit height with the speed bleeding off about 3-5nm before the aerodrome.
But presented with the original problem in the air, I would probably go:
15nm away less 3 is 12, less another 1 whilst I work this out (and while flying the plane and talking to ATC) is 11, which is close enough (for government work) to 10nm
10nm at 110kts is near enough 2nm a minute, so 5 mins
3000' to 1000' is 2000', less 400' for QFE is 1600'
1600 divided by 5mins is a bit over 300fpm
Yes, the errors can accumulate, but I wouldn't try to use any VSI that I have come across in a GA aircraft to 100fpm resolution, so working things out to the nearest 50fpm or finer is a waste of time IMHO
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dublinpilot has it right!
It's an exam question.
So...
ATFQ!
It doesn't matter what we would do in real life, the answer (rounded up to the nearest FPM) is 249fpm. Personally, I start a descent when I feel like it. Usually at a convenient position report. I can then use a cruise descent, or faster if necessary. I can't recall when I last used the mathematics involved in answering this question in real life.
It's an exam question.
So...
ATFQ!
It doesn't matter what we would do in real life, the answer (rounded up to the nearest FPM) is 249fpm. Personally, I start a descent when I feel like it. Usually at a convenient position report. I can then use a cruise descent, or faster if necessary. I can't recall when I last used the mathematics involved in answering this question in real life.
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It can be uncomfortable (particularly for the over 50's or infants) to descend at greater than 500fpm in an unpressurised aircraft, therefore the ROD is most important to me.
I take airfield elevation, add 1,000 for the circuit, then deduct that from my current cruise altitude and multiply by 2 for my time to start descent ahead of my ETA. Eg cruise at 9,500 feet to a sea level airport, you're looking at losing 8.5 (thousand) feet. Descent should begin 17 mins prior to the eta. If you're going to use increased speed (than planned) for the descent, bung an increment on. In this case, I'd be looking at 20 mins out.
I take airfield elevation, add 1,000 for the circuit, then deduct that from my current cruise altitude and multiply by 2 for my time to start descent ahead of my ETA. Eg cruise at 9,500 feet to a sea level airport, you're looking at losing 8.5 (thousand) feet. Descent should begin 17 mins prior to the eta. If you're going to use increased speed (than planned) for the descent, bung an increment on. In this case, I'd be looking at 20 mins out.