Vertical Navigation Question

[kestrel101]

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.

[dublinpilot]

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

[A and C]

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.

[dublinpilot]

I assumed it was an exam question.

In practice I have a very simple approach too. 4 miles for every 1000ft that I want to lose ;) That's 500fpm at 120kts.

[kestrel101]

Thanks for the replies guys very helpful its actually quite simple if you look at it. Thanks again.

[DBo]

...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

[italianjon]

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!

[jamestkirk]

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.

[IO540]

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.

[A and C]

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.

[jamestkirk]

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.

[dirkdj]

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.

[BackPacker]

You seem a reasonable individual. Please don't be the 'how dare you question me' brigade.

Great reaction. Particularly since A&C is right, and thus right to question you.:ok:

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.

[jamestkirk]

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.

[BackPacker]

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:

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.

What is wrong with a 527 fpm ROD at 95 knots? Or indeed even a 333 fpm ROD at 60 knots?

[jamestkirk]

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.

[BackPacker]

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.

I might be wrong but where in the sentences like "Losing 5000 feet takes 15 nm according to the 3X rule." did I leave out the distance?

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.

[IFollowRoads]

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

[the beater]

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.

[Fright Level]

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.

[jamestkirk]

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.

[A and C]

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.

[jamestkirk]

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.

[sooty615]

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

[BackPacker]

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.

A320, assume 250 knots groundspeed. DR400, assume 95 knots groundspeed. (*)

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.

[englishal]

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 ;)

[Keef]

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.

[MIKECR]

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.

[jamestkirk]

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.

[BackPacker]

Backpacker - You keep missing the point.

Well, I have just read through all your posts on this subject again and the only point you seem to want to hammer home is that descents have to be done at 1500 fpm, regardless of the type of aircraft or its groundspeed. Or, at least, that the 3X rule is somehow linked to a ROD of 1500 fpm.

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...:ok:

(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.)

[jamestkirk]

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.

[smith]

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

[BackPacker]

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.

I've been saying in all my posts that the ROD depends on the aircraft type and speed. It was you who came up with a 1500 fpm ROD for all types of aircraft. But I'm glad that you now agree that the ROD depends on the speed. In fact, multiple people have mentioned 5x groundspeed as a good ROD. (Although if you do the calculation properly you'll find that it's more like 5.5x the groundspeed.)

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.

A&C only said that he used the 3x rule to determine his TOD point. It was you who first mentioned anything about a 1500 fpm ROD, and started applying that ROD to small aircraft, with obvious results.

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.

Tell me about it. I'm training for an aerobatics competition (basic level). When we perform a spin we have to end in a 90-degree downline and we see airspeeds of 90-110 knots, straight down, before we pull up to the horizontal. That's over 13.000 fpm. Not for long, obviously, and it's very unnerving the first few times. Fortunately we lose less than 1000 feet in such a spin so it's not that hard on the ears.

I do not recommend that technique if you find yourself 1000 feet too high on an IFR approach though.:eek:

[Chilli Monster]

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.

[jamestkirk]

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.