Vertical speed advice

[Constantine 16]

Hi guys

just wanted to seek your expertise on set up and use of V/S  just wondered if b747-4 real world pilots would use V/S straight from top of descent, and if so how far would they continue to decsend in V/S  and secondly how do they calculate their V/S rate of descent ?

 

Many Thanks

Richard

[pschlute 37]

V/S is a basic A/P mode that does not allow any automated control against overspeed. Most descents will be conducted using VNAV or Level Change there the throttles will be at flight idle and the aircraft will descend at a set airspeed somewhere around 300 KIAS. For small changes in altitude as instructed by ATC, V/S can used. It may also be used in gusty conditions as it will give a smoother ride for the passengers.

[vadriver 247]

richard

 

to answer your post, if & when you/they use v/s (more often than peter suggests) ...........

 

if distance to go (to the next altitude restriction) is targeted at 320' / nm (3..0deg glide) then 500 fpm / 100kt of ground speed.

 

if steeper, tweak it a bit but start thinking a slower speed ( & FLCH for idle thrust) & maybe an orbit !

 

so broadly speaking ............ from TOD, 2250fpm decreasing to 1500fpm at FL100 (AGL), then to about 700fpm just before the threshold.

[Wink207 78]

In all modern transport aircraft with sophisticated AP systems, VS mode is used in conjunction with autothrottle speed control and is the most used descent lineup. VNAV typically does not provide sufficient speed control where required by ATC and FLC can "hunt" on descent rate in favor of speed, providing a bumpier ride. Both VNAV and FLC are most useful during climbs, and VS during descent.

[3Green 80]

Here is a simple rule for Vertical Speed:
Note:  V/S is dependent upon Ground Speed.
Vertical Speed = Ground Speed (KTS) X 5

V/S - Vertical Speed
GNDSPD - Ground Speed (Knots)
V/S = GNDSPDx5

Or use this table:

GND
SPD
KTS V/S

550 2750
540 2700
530 2650
520 2600
510 2550
500 2500
490 2450
480 2400
470 2350
460 2300
450 2250
440 2200
430 2150
420 2100
410 2050
400 2000
390 1950
380 1900
370 1850
360 1800
350 1750
340 1700
330 1650
320 1600
310 1550
300 1500
290 1450
280 1400
270 1350
260 1300
250 1250
240 1200
230 1150
220 1100
210 1050
200 1000
190 950
180 900
170 850
160 800
150 750
140 700 <--do these Vertical Speeds look familiar during landing? At 140KTS, V/S of 700 ft/min
130 650 <--do these Vertical Speeds look familiar during landing?
120 600 <--do these Vertical Speeds look familiar during landing?
110 550
100 500



Top of descent Calculation:

ToD     - Top of descent
FL      - Flight Level
RALT    - Required Altitude or Elevation
FLDiff  - Flight Level Difference (FL-RALT)

ToD = (FL-RALT)x3/1000

Or use this table.

FLDiff ToD
45,000 135
44,000 132
43,000 129
42,000 126
41,000 123
40,000 120
39,000 117
38,000 114
37,000 111
36,000 108
35,000 105
34,000 102
33,000 99
32,000 96
31,000 93
30,000 90 <-- At 30,000ft you should be 90nm from the airport (easy numbers to remember)
29,000 87
28,000 84
27,000 81
26,000 78
25,000 75
24,000 72
23,000 69
22,000 66
21,000 63
20,000 60 <-- At 20,000ft you should be 60nm from the airport
19,000 57
18,000 54
17,000 51
16,000 48
15,000 45
14,000 42
13,000 39
12,000 36
11,000 33
10,000 30 <-- At 10,000ft you should be 30nm from the airport
9,000 27
8,000 24
7,000 21
6,000 18
5,000 15
4,000 12
3,000 9
2,000 6


As an example.
Your Flight Level is 36,000ft (FL360)
Airport Elevation is 5,000ft
Your Ground Speed is 430 KTS

Flight Level Difference is 31,000ft (36,000ft-5000ft)

Top of Descent:
ToD   = (FL-RALT)x3/1000
93nm  = (36,000ft-5000ft)x3/1000

Your Top of Descent should start no later then 93nm from the airport and your initial vertical speed should be set to 2150 ft/min.

If you make a copy of these tables, you can quickly determine if you're on the correct rate of descent with reference to your DME.

Note:
If you're landing on a runway in the opposite direction then your present heading, you'll most likely fly 15nm past the airport, turn base leg and fly another 15nm on final.  This adds another 30nm for you to fly.  
In this case, your TOD should start no later then (93nm-30nm) 63nm from the airport at an initial descent rate of 2150 ft/min.

 

Again, as VADriver said, you must adjust your vertical speed as your ground speed changes.

RJ







KTS     V/S     FL      NM
550    2750    45,000    135
540    2700    44,000    132
530    2650    43,000    129
520    2600    42,000    126
510    2550    41,000    123
500    2500    40,000    120
490    2450    39,000    117
480    2400    38,000    114
470    2350    37,000    111
460    2300    36,000    108
450    2250    35,000    105
440    2200    34,000    102
430    2150    33,000    99
420    2100    32,000    96
410    2050    31,000    93
400    2000    30,000    90
390    1950    29,000    87
380    1900    28,000    84
370    1850    27,000    81
360    1800    26,000    78
350    1750    25,000    75
340    1700    24,000    72
330    1650    23,000    69
320    1600    22,000    66
310    1550    21,000    63
300    1500    20,000    60
290    1450    19,000    57
280    1400    18,000    54
270    1350    17,000    51
260    1300    16,000    48
250    1250    15,000    45
240    1200    14,000    42
230    1150    13,000    39
220    1100    12,000    36
210    1050    11,000    33
200    1000    10,000    30
190    950    9,000    27
180    900    8,000    24
170    850    7,000    21
160    800    6,000    18
150    750    5,000    15
140    700    4,000    12
130    650    3,000    9
120    600    2,000    6
 

[Constantine 16]

Hi Guys thank you so very much for the detailed yet highly understandable information which I'm sure will help so many others too.

PGreen thank you for the table an amazing resource thanks!!

May I ask if B747-4 pilots would alternate between between SPD FLCH & V/S in the descent e.g. commencing TOD in FLCH then around FL100 reverting to V/S I read V/S provides a smoother descent through levels. 

If FLCH is selected for the entire descent could you give me an example of how we would perform this in the simulator, say from a CRZ ALT 3400 to threshold in terms of FL & Knots in the descent.

Again thank you once again for the amazing support

Richard

[skelsey 765]

Hi Richard,

 

Generally speaking it would be usual to use FLCH or VNAV for the initial descent. Your aim is to deliver the aircraft at the final approach fix on speed and on profile.

 

This -- particularly in a big aeroplane like the B747 -- is an exercise in energy management. Now, you could practically write a book about this, and I suspect someone has, but I'll try and highlight the salient points.

 

The most economical descent profile from a fuel burn point of view is one where you close the thrust levers at T/D and don't open them again until you have to -- i.e. at 1000ft on final approach. Obviously in real life ATC will invariably get in the way of achieving this, but we'll put that to one side for a moment.

 

Because you start at high altitude and high speed, and you want to end up at low altitude and a relatively low speed, you need to decelerate as you descend. Think of it a little bit like a rollercoaster -- with no (effective) acceleration force (because the engines are idling), you exchange airspeed for altitude and altitude for airspeed. If you steepen the descent you will speed up; if you shallow the descent off you will slow down.

 

So you start off with the calculation mentioned above - multiply your flight level by three to get a rough top of descent point. If you want to be technical about it you should also adjust this for the wind: if you are descending in to a headwind this will reduce your speed over the ground and thus make the angle of descent steeper, whereas a tailwind will have the opposite effect. The rule of thumb is to add or subtract 1/3 of the headwind or tailwind (add for a tailwind, subtract for a headwind) to your basic calculation (1/3 because your average descent takes about 20 minutes... 1/3 of an hour).

 

Set yourself some gates for speed and altitude so you can monitor your progress. It is important to be aware of speed as well as altitude for the reason mentioned above -- they are interchangeable. If you are at FL100 30-40NM out at 250 knots you are about on profile. If you are at the same place and altitude but doing 350 knots then you are _high_ -- because you will need to shallow the descent sooner or later to lose the excess speed.

 

As a general rule, I like to be at FL200 70NM out at whatever the ECON descent speed calculated by the FMC is. FL100 at 250 kt and 30-40NM. 6000ft and 220kts at 20nm, and 3000ft/180kt/10NM.

 

As you descend, monitor your progress against these gates and adjust as needed. If you are high then you have two options: increase drag (speedbrake) or increase your speed, which will increase your rate of descent and increase drag (remember drag increases with the square of speed). If you are really, really high then you can do both, and the 747 will do a very good impression of a brick with speedbrake out and the speed wound up to the barber pole.

 

The way I tend to work is to use VNAV (or more commonly FLCH) at the higher levels. If I'm high I'll wind the speed up to increase the rate of descent. Once I start getting towards the intermediate approach and I start wanting to slow down, I'll tend to use V/S to finesse the last stage of the descent (say from around FL80 down) generally to enable me to maintain a steady rate of descent whilst slowing down (as an example -- if you're bombing down in FLCH at 250kt and suddenly wind the speed back to clean speed, FLCH will bring your rate of descent back to almost nothing, a couple of hundred feet per minute perhaps, to get the speed off before increasing the rate again. If instead you go in to V/S you can maintain a rate of descent that is less than you were doing before (and thus the aircraft will slow) but more than FLCH would be giving you).

 

It's hard to explain on paper but hopefully you get the gist!

 

Edit to add: to provide perhaps a better, visual example: here's one from a full-sized 747 sim during Worldflight in November that I was particularly proud of

 

https://youtu.be/zg3bZTPB7DY?t=1h40m35s

[G550flyer 1,013]

Hi Guys thank you so very much for the detailed yet highly understandable information which I'm sure will help so many others too.

PGreen thank you for the table an amazing resource thanks!!

May I ask if B747-4 pilots would alternate between between SPD FLCH & V/S in the descent e.g. commencing TOD in FLCH then around FL100 reverting to V/S I read V/S provides a smoother descent through levels. 

If FLCH is selected for the entire descent could you give me an example of how we would perform this in the simulator, say from a CRZ ALT 3400 to threshold in terms of FL & Knots in the descent.

Again thank you once again for the amazing support

Richard

From an operational standpoint, its all ready been stated the limitations revolving around VNAV. In the climb, VNAV functions similar to FLCH as long as there isn't any restrictions. VNAV is most helpful for SIDs when departing an airport with intermediate altitude restrictions. When climbing right up to altitude, it functions like FLCH, using max climb power and varying pitch. Here is the problem with VNAV. In busy airspace, your are given altitudes levels on the way up. In this environment, you would be constantly in and out of VNAV and changing the predicted climb profile. This is also the same on the way down. You only get the descend VIA a STAR every blue moon. Usually they clear you a STAR, but give you intermediate altitudes on the way down. Again, this jacks up the profile requiring you to keep adjusting it. Once you are cleared off your flight plan route, VNAV is useless. In busy airspace, you are vectored for you climb and descents every flight. FLCH is alot better, but it gets a little pitchy after MACH changeover. Any little variance in temp and the nose will start pitching either getting you too fast or slow for the climb. The nice thing about FLCH is its speed protection especially When in IAS mode. Here's the thing about V/S, you have to watch it like a hawk to keep adjusting it to stay on speed. So the best answer is that you use a mixture of all three. During climbs, I initially start in V/S to accommodate a nice smooth pitch up. You slowly roll the vert wheel up for a smooth transition. I continue up to the desired V/S. I always V/S before using FLCH. I smoothly use the vert wheel up until the engines reach max climb power. Once the engines reach climb power, i hit the FLCH button. This technique minimizes large pitching moments. There is no noticeable pitching using FLCH with this technique. If the engines are at a lower power setting when selecting FLCH, the engines will surge forward and the plane will pitch up quickly to maintain the speed. While in FLCH, if i hit temperature spots, as soon as I see the indicated MACH/SPEED bump, i quickly hit the V/S button to hold what I got. Once the speed settles, i V/S the correction and return to FLCH. Sometimes you can VNAV up when in non busy areas, but you still have to watch it. After MACH changeover, some guys will stay in V/S. On the way down, unless cleared to descend VIA a STAR, most guys will FLCH down. As I stated before, you will use a mixture of all three, maybe VNAV the least. 

 

Not a 747 guy, but this is pretty much how it works out in all the heavies i flew and in the jets i currently fly.

 

Rick   

[ghiom 2]

Very interesting.

In FLCH at high altitude and FMC calculated speed, my iFly 747 will sink like a stone. I don't see the same in the posted video at 1h40+

Interesting...

[Constantine 16]

Guys thank you for the great advice and step by step explanations and rationales I must admit as ghiom said above I did a flight from EGkk to VHHH at flight level 36400  I reduced speed to the fmc econ descent speed 310 then 5nm to TD idle throttles about 70%  and the plane descends like a rock eventually speedbrakes slow it down.

 

I'll continue this method I went down to FL 80 with flch then from 8000ft used v/s 1500 to 2000ft then v/s 700 are any of you guys experiencing this suden drop from initial descent? or am i doing something wrong at this stage?

again thank you for all the past posts with superb pointers !

 

Many thanks

 Richard

[pschlute 37]

 I did a flight from EGkk to VHHH at flight level 36400  I reduced speed to the fmc econ descent speed 310 then 5nm to TD idle throttles about 70%  and the plane descends like a rock eventually speedbrakes slow it down.

 

 

At FL360 in cruise you should be flying at a Mach speed. Your IAS would have been below 300 at this time, so how can you "reduce" speed to 310 IAS ?

 

Normal procedure is to start your descent at the same mach speed you are cruising at and to use that speed (or FMC mach descent speed) until the crossover point (about FL280, or when your IAS has increased to your FMC descent IAS)  when you switch to using IAS.

 

If you are using FLCH you should let the autothrottle control the thrust setting (idle)

[Wink207 78]

Guys thank you for the great advice and step by step explanations and rationales I must admit as ghiom said above I did a flight from EGkk to VHHH at flight level 36400  I reduced speed to the fmc econ descent speed 310 then 5nm to TD idle throttles about 70%  and the plane descends like a rock eventually speedbrakes slow it down.

 

I'll continue this method I went down to FL 80 with flch then from 8000ft used v/s 1500 to 2000ft then v/s 700 are any of you guys experiencing this suden drop from initial descent? or am i doing something wrong at this stage?

again thank you for all the past posts with superb pointers !

 

Many thanks

 Richard

Two suggestions. RW pilots abhor using speed brakes. Considered a screwed up descent if speed brakes are needed. Second, since FLCH reduces throttles to idle on descent, that's what happens. Ergo, that is why V/S  mode is used mostly on descent. One exception may be an ATC instruction to "...make immediate descent to FL XXX...". Then FLCH would be used since it will give the quickest descent. The challenge is to arrive at your desired geographical point or NAVAID at assigned altitude and on assigned speed, such as fix xyz at 10000 ft, speed 250 kts in a smooth and professional manner.

Two suggestions. RW pilots abhor using speed brakes. Considered a screwed up descent if speed brakes are needed. Second, since FLCH reduces throttles to idle on descent, that's what happens. Ergo, that is why V/S  mode is used mostly on descent. One exception may be an ATC instruction to "...make immediate descent to FL XXX...". Then FLCH would be used since it will give the quickest descent. The challenge is to arrive at your desired geographical point or NAVAID at assigned altitude and on assigned speed, such as fix xyz at 10000 ft, speed 250 kts in a smooth, economical, and professional manner.

[A32xx 129]

"RW pilots abhor using speed brakes. Considered a screwed up descent if speed brakes are needed."

 

I've flown around the real world four times and seen speed brakes in use on every type that has them fitted. They wouldn't be there if they weren't needed. And they are needed because in the real world ATC often needs to make last-minute changes to maintain safe separation on approaches.

[Constantine 16]

Hi guys thanks again, great advice. yes sorry Peter I was at FL340 .86 around 293kts so are you suggesting hit flch at TD at my crz speed until the aircraft reaches the fmc econ descent speed, and that I don't need to physically idle my throttles

then just descend to my cleared altitude in flch before selecting V/S at a later stage ? 

My main concern was that initial plummet after selecting FLCH on the FMC econ descent speed once under control my aircraft met its levels and I selected V/S around 800ft V/s-900 down to 2000ft then vs-700 on active GS and it came in nicely,

just interested in how RW pilots would comence & continue the descent all the way in VS would mean massive monitoring and FLCH would bring about a very hasty descent so I was confused, not hard for me, thanks guys just wanted to clarify best practice on initial decent.

Many Thanks

Richard

[ghiom 2]

Thanks for all the follow up.

Why isn't the Simfest 747 simulator plummetting upon initial FLCH descent?