Descent Rate Options

[turbojet1937]

I'm 100 miles from touchdown, 350 knots at 35k. How do I determine my descent rate to reach the end of runway?

[effte]

I'm 100 miles from touchdown, 350 knots at 35k. How do I determine my descent rate to reach the end of runway?

100 miles will take you 100/350 hours = 10/35 hours, or 600/35 minutes.

If you're planning on a continuous descent approach, rather than reaching an intercept altitude, you need to loose 35k feet in those 100/350 hours, or 600/35 minutes.

35/(100/350) thousand feet per hour = 35*350/100 = 35*3.5 roughly equals 120.000 feet per hour. Divide by 60 to get 2000 or so fpm.

Usually you can achieve good enough results through mental arithmetics, but if you want a solid figure bring out the E6B. First you calculate the time needed, then divide the altitude loss with this time and the descent rate will pop out, just as the speed would if you were doing the classic distance/time problem.

However, you will not be able to maintain 350 KTAS all the way. Your TAS will reduce with decreasing altitude, and chances are you will be limited to 250 KIAS below FL100 (or 10k feet, if in the US or some other place with a high TA). Descending through the wind layers adds a further complication.

A good rule of thumb for calculating top of descent in most commercial aircraft is to start descending when the remaining track miles are the altitude you need to loose in thousands of feet times three. You need to get from FL350 to 2000 feet? 35-2 = 33. 33 times three gives you 100 track miles. In other words, in your example, start your descent immediately following a normal profile.

With experience you will figure out if you need to adjust the multiplication constant for your aircraft. If you find yourself arriving high all the time, increase it to 3.5. If you have a headvind, you can initiate your descent a bit later, with a tailwind start down sooner. Continuously reevaluate if you are on your descent path or not, and adjust your speed or power setting if needed.

Cheers,
/Fred

[Ranger]

I'm 100 miles from touchdown, 350 knots at 35k. How do I determine my descent rate to reach the end of runway?

I'm a simple guy who gets confused easily by big numbers. Over the years I've worked hard to make stuff like this easy. I'll try to make my explanation easy for you.

Before we start on this there is one big variable that you need to take into account. Winds aloft. If you're flying into a big headwind and your ground speed is substantially reduced, start down later. If you have a hefty tailwind and a high ground speed you'll need to start down earlier.

Here goes. FL350, indicating 350 knots. We'll assume that the airport is at sea level. One thing that has to be accounted for is slowing at the transition level, regardless of what that level is. Since I'm a Yank, it'll be 10,000'. 35,000 minus the 10,000= 25,000 total feet we need to lose to reach the TL. 25X3= 75 miles. Regardless of what your indicated airspeed is at the TL the rule of thumb is 1 mile per 10 knots to lose. So, if we're still indicating 350K and we need to slow to 250K that's another 10 miles. 75+10= 85 miles. Once you've gotten down and slowed down it becomes an exercise in speed management and configuration. Given your criteria in this example we would probably not be able to get it down in 100 miles without the use of speed brakes or maybe some real early configuring with maybe the gear for drag.

Fred got most of this into his post. But he seems a lot better with big numbers than I am.

One other thing. The advent of FMCs has made most of this ridiculously easy. Make the proper entries and the FMC will figure out a very good and reliable profile descent. However, FMC's are not bullet proof. They do make mistakes, get confused and then work hard at screwing you over if you let them. Most of the time when the FMCs spit out bad info it's because either I put in bad info (this doesn't happen often) or I forgot to put in something (this seems to happen more often as I get older). In any case, this is my hard, fast rule. And I make every pilot that I fly with say this out loud.

ALWAYS USE GOOD, BASIC PILOT TECHNIQUE!

To hell with the magic. Do not believe it out of hand. As the airplane is descending, I'm looking at how far I have to go and how much altitude I need to lose. In my head. I want to be at Alpha VOR at 10,000'. I'm at 27,000' now. I need to lose another 17,000'. 17X3= 51 miles to go. I'm actually 53 miles away from Alpha. I'm in good shape.

[AirRabbit]

Hey Turbo…

Another “rule of thumb” that has worked for me over the years is the reference to Mach – where, if you’re holding M0.75, you’re averaging about 7 ½ miles a minute and if you’re lucky enough to be flying an airplane that will give you M0.84 or so, you’ll be clicking along at almost 8 ½ miles a minute. The relationship between the two numbers is a function of the number “600.” Airspeed = mach speed multiplied by 600 … and Mach speed = airspeed divided by 600 (its easier just using “6” by the way). In your example, 350 knots (we’re presuming that’s what you meant) would be a Mach speed of (350/6 = 58.33, and of course, we have to put the decimal in the proper place … so) about M0.58.

Of course, real world situations are never like the ones you imagine them to be – and your example would never happen with a continuous descent – you’d very likely have several level off points and speed adjustments during your descent, but … just for the example … to travel 100 miles at M0.58 will take you a bit more than 17 minutes. To descend 35000 feet in 17 minutes, you’ll have to descend about 2060 feet per minute – I’d round it up to 2100 fpm. If you speed up during the descent, you’ll have to increase your rate of descent … if you slow down during the descent, you’ll be able to shallow your rate of descent.

Using Ranger’s situation … you’re at 27,000 feet and need to be at 10,000 feet in 53 miles … if you’re holding M0.58 (a bit under 6 miles a minute) you’ll be over your fix in just under 9 minutes … so, to descend 17,000 feet in 9 minutes … you’d hold a descent rate of about 1900 feet per minute … and you’d get to the altitude just a bit short of the fix … just what you want to do.

[Becky_KSTS]

I gotta print this one. I will have to study this for some time yet. In the 172 I wait until I see the runway environment and go from there! Rabbit and Ranger... you are jewels to this site. I hope you are appreciated!

[dash8q300]

Oh this is great. I tried it in flight simulator a few times and I managed to get down to the runway perfectly.

Dash,

[Ranger]

Oh this is great. I tried it in flight simulator a few times and I managed to get down to the runway perfectly.

Dash,

Just keep telling yourself,

"Three times the altitude, plus one mile per 10K of airspeed. Three times the altitude..........."

And never fall into the trap of accepting the automation out of hand. It's like the girlfriend you WISH you had. It'll screw you in a heartbeat.

[turbojet1937]

Thank you all for the resoponse. Very helpful and maybe now I can stop bouncing in for a landing, Thanks again.

[BMeister]

Hey Boys and Girls,

Okay I've sat up asking myself the same question about when to descend and I came up with this Computation,

Please dont laugh at me, as Im afraid, this might be a load of cumulus!

In simple terms for a second - Forget the Speed Restriction of 250IAS below 10,000

AIRSPEED = 350KTS
ALTITUDE = 35,000FT
NM TO THE AIRPORT = 100

A Descent to the Threshold. here we go:

350 KTS (350 Nautical Miles an Hour)

350 Divided by 60 (60 minutes in 1 hour) = You fly 5.83NM per Minute at that speed.

100 NM to the airport Divided by 5.83NM Per Minute = 17.1 Minutes to Threshold.

okay so 17.1 Minutes horizontal distance at this speed (350) will get us to 100NM (airport)

35,000Feet Divided by 17.1Minutes = Tells you your required Descent rate = 2,046FT Per Minute.


Taadaaaa!! Okay start laughing lol ...

I ask myself another question, what happens if I dont want to descent that fast with passengers... at that pitch attitude, then I guess I have try and understand Your guys theory!!

Let me know your feedback on this,

to sum it up: Speed/60=Answer Distance/Answer=Minutes Altitude/Minutes=Descent rate

FlySafe

[BMeister]

Actually I think it makes Sense ^^ Efee Im confused with your theory though, or It's either late or im tired or both

[effte]

Actually I think it makes Sense ^^ Efee Im confused with your theory though, or It's either late or im tired or both

Same math as you wrote actually, just laid out the way you'd calculate it mentally rather than the way you do it on a calculator.

I will always have the "no-calculator-available" plan as my primary plan. If one happens to be around, have fresh batteries and be usable under the circumstances at hand (light available, turbulence, traffic situation, gloves needed etc etc) I may be prepared to accept it as a nice bonus.

[Fast Jet]

Did somone just cover this (am in a rush today, cant read all the posts/replies)

Are you in a jet at altititude? Seyour flight level..example FL350 times it by three that makes - wait a sec-.....duh, 105 right? So you wanna start down at 105 miles from zero feet - as that is hardly going to be the case, decide what FL you wanna descend to FL100, ok? for example, so FL350 - FL100 = 25,000 feet right? so three times twenny five equals 75 right..? So, your Top of Descent Point will be around 75 miles from the point where you wanna be = at FL100 got that?

When do you start down i.e., what is your TOD? = 3times your height = your distance.

At what Rate of Descent? = Half your Ground Speed. (If you work out the 5 times the Ground speed equation then the the difference is the same i.e. equal. E.G., Half times 350 knots = 175 = 1750 feet per minute (FPM) rate of descent (ROD) and 5 x 350 = ?

This (the above) should give you a three degree descent angle - slightly above or slightly below. the 3 degree descent angle.. Check it Out.

Use your cruise speed if not given any other by ATC, for the intial descent then bring the speed back to something suitable for your aircraft like 280 kts - avoiding overspeed and mach crit but descend at RAS so that your Mach num decreases, or you will approach Mach Crit. I.e., don`t descend at a constant mach no.

[BMeister]

uuuuuuuuh???????

We are all busy, I was busy when I tried to spend time explaining how it works!! you could atleast spend time reading it,,

I couldn't begin to understand your post fastjet,

I've posted a forumla in this read, should get you, what you want to know

regards,

Did somone just cover this (am in a rush today, cant read all the posts/replies)

Are you in a jet at altititude? Seyour flight level..example FL350 times it by three that makes - wait a sec-.....duh, 105 right? So you wanna start down at 105 miles from zero feet - as that is hardly going to be the case, decide what FL you wanna descend to FL100, ok so FL350 - FL100 = 25,000 feet right? so three times tenny five equals 75 right..? So, your Top of Descent Point will be around 75 miles from the point where you wanna be at FL100 got that?

When do you start down i.e., what is your TOD? = 3times your height.

At what Rate of Descent? Half you Ground Speed. (If you work out all the 5 times the Ground speed the difference is the same i.e. equal.

This (the above) should give you a three degree descent. Check it Out.

Use your cruise speed if not given any other by ATC, but descend at RAS so that your Mach num deceases, or there will be a very loud bang outside of the aircraft, which you won`t hear as you will be more concerned with your sudden lack of wings.

[Fast Jet]

uuuuuuuuh???????

We are all busy, I was busy when I tried to spend time explaining how it works!! you could atleast spend time reading it,,

I couldn't begin to understand your post fastjet,

I've posted a forumla in this read, should get you, what you want to know

regards,

I did not say I was (too) busy, I said I was in a rush - there is a difference. I am not in a rush today though and I have edited my post - so that you can decipher it and I am taking time, which I now have, to read your post and others. Thank you for your time.

The next subject up for discussion might be - how to avoid rushed approaches. In fact, as I now have time, I will post it, here, on FL350.com

[Fast Jet]

Hey Boys and Girls,

Okay I've sat up asking myself the same question about when to descend and I came up with this Computation,

Please dont laugh at me, as Im afraid, this might be a load of cumulus!

In simple terms for a second - Forget the Speed Restriction of 250IAS below 10,000

AIRSPEED = 350KTS
ALTITUDE = 35,000FT
NM TO THE AIRPORT = 100

A Descent to the Threshold. here we go:

350 KTS (350 Nautical Miles an Hour)

350 Divided by 60 (60 minutes in 1 hour) = You fly 5.83NM per Minute at that speed.

100 NM to the airport Divided by 5.83NM Per Minute = 17.1 Minutes to Threshold.

okay so 17.1 Minutes horizontal distance at this speed (350) will get us to 100NM (airport)

35,000Feet Divided by 17.1Minutes = Tells you your required Descent rate = 2,046FT Per Minute.
Taadaaaa!! Okay start laughing lol ...

I ask myself another question, what happens if I dont want to descent that fast with passengers... at that pitch attitude, then I guess I have try and understand Your guys theory!!

Let me know your feedback on this,

to sum it up: Speed/60=Answer Distance/Answer=Minutes Altitude/Minutes=Descent rate

FlySafe

.Determine when YOU want to descend before you leave the ground to fly the route. You missed your Top of Descent point by 5 miles. You cannot hurtle down, you could reduce speed though in this situation and maintain RoD.

[Fast Jet]

.Determine when YOU want to descend before you leave the ground to fly the route. You missed your Top of Descent point by 5 miles. You cannot hurtle down, you could reduce speed though in this situation and maintain RoD.

Avoid complex math formulas when simple does it fine always.

Be at 250kts by 10,000 feet subject to ATC requirements.

Its simple. 3 x your distance = your height. So at 105 miles you wanna be at 35,000 ft. At 20 miles you should be at 6000 10 miles at 3000 8 miles at 2400 6 miles at 1800 4.5 miles at 1350 3 miles at 900 - approaching minima? Decide. G/A or Land. You only need your 3 times table.

[Harmattan96]

Be at 250kts by 10,000 feet subject to ATC requirements.

You have to be at 250 by 10000?!?!?!
Now I understand all this screaming on frequency as I passed 10, I thought they were greeting me, I guess I'll have to answer all those very official looking letters I have been receiving requesting my attendance for flight checks (I was under the impression they were just recurrent invite...if you felt like it)! I swear, I was just fine at 250 on a 8 miles final, even though those darn flaps/slats kept getting stuck while extending, and those main gears only lock 9 out of 10 tries. I really dislike this creeking sound when you have to recycle them to force them to lock.

[Fast Jet]

You have to be at 250 by 10000?!?!?!
Now I understand all this screaming on frequency as I passed 10, I thought they were greeting me, I guess I'll have to answer all those very official looking letters I have been receiving requesting my attendance for flight checks (I was under the impression they were just recurrent invite...if you felt like it)! I swear, I was just fine at 250 on a 8 miles final, even though those darn flaps/slats kept getting stuck while extending, and those main gears only lock 9 out of 10 tries. I really dislike this creeking sound when you have to recycle them to force them to lock.

YEH, RIGHT Harmaggedon, You could have used the rope on the flightdeck as a lassoo to catch the tops of the trees - this would help to slow you down - alternately you could use an anchor from a boat - simply throw it out of the flight deck window, try not to forget to attach something to the anchor first, like a piece of string, you could also use a metre square piece of hardboard and hold it face on to the airflow - this may slow you down, but it is not guaranteed (Also, if we don`t put 250kts at 10 then some doodle bug is gonna gripe - even if we are at 280 at 8d with speedbrake - quiet impressive, thats 3.5 miles in which to lose 140 kts. doing 280. " 1,000 STABLE!!!" "..righty ho.." "Oh, I forgot, were we heavy today...?" "Wow, no speedbrakes - fancy that?!" "How come we overtook those other three aircraft also on final, they`ve got their gear down too! - Quick, there`s one just landing, lets see if we can beat him to the threshold Capt. Burk"

Being serious, once more - somebody said if you have a headwind start down later, and for a tail wind (TWC) start down later.... hmmmm, well, yeh, but not really - as it is the Groundspeed which determines the Rate of Descent calculation and GroundSpeed consists of....? Yes, you gottit, WIND!! So, if the wind is up the chuff then the Groundspeed will be greater, therefore the Rate of Descent will have to be greater.
Also, with a headwind (HWC) then the Groundspeed will be lesser, so the rate of descent will be less too. Hmmm? Ne`est pas?

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