Really? there's a difference between FL/3 = dist. to start decent & FLx3 /10.......LOL!

Not quite sure what you're trying to say, you both calculate the same thing lol

 

Im at FL300 and should be at 10'000ft for an approach at a specific point, so therefore I need to calculate when to initiate my descent in order to descend 20'000ft.

 

means (20'000 /1000) * 3 =  roughly +60NM (just as Nathan said),

 

now to calculate what FPM you need by using your current airspeed:

 

Cruising at 430kts means 430/0.2 = 2150 fpm for optimal rate of descent 

 

 

 

Now with what you said;

 

FL / 3 = dist. to start decent

ex:cursing at FL240, 240/3=80, start decent 80 nm out

 

 

FL200/3 = 66NM

 

Sinkrate = half the GS x 10,  ex: my GS is 240kts, 240/2x10=1200 f/m,

or 24/2=12 & just add two 00's =1200 f/m

430/2 = 215x10 = 2150 fpm

 

 

Any thoughts?

I have a thought...

 

They do not calculate the same thing. The flight level change / 3 method results in a distance of 66NM for an altitude change of 20,000 ft. The altitude change *3 method results in a distance of 60NM for an altitude change of 20,000ft.

 

Nathan's method is closest to the true 3 degree glidepath (it's actually a 3.14 degree slope, but pretty close). Rodd's method produces a 2.82 degree slope. While Nathan's method is closer to the true 3 degree slope, Rodd's results in a more conservative distance, giving some extra breathing room. So in the end, they both achieve the same goal, albeit slightly different numbers.

raysalmon, on 12 May 2013 - 5:21 PM, said:

 

I have a thought...

 

They do not calculate the same thing. The flight level change / 3 method results in a distance of 66NM for an altitude change of 20,000 ft. The altitude change *3 method results in a distance of 60NM for an altitude change of 20,000ft.

 

Nathan's method is closest to the true 3 degree glidepath (it's actually a 3.14 degree slope, but pretty close). Rodd's method produces a 2.82 degree slope. While Nathan's method is closer to the true 3 degree slope, Rodd's results in a more conservative distance, giving some extra breathing room. So in the end, they both achieve the same goal, albeit slightly different numbers.

 

Exactly, and thus my statement: "You basically just re-quoted what he said? lol "

 

Both will calculate the same thing = get you close enough to where you want to be at a specific altitude,

 

you will never be spot on due to wind changes, un-even airspeeds and so on unless you are using an A/P to have the green banana calculate it for you on the go

Exactly, and thus my statement: "You basically just re-quoted what he said? lol "

 

Both will calculate the same thing = get you close enough to where you want to be at a specific altitude,

 

you will never be spot on due to wind changes, un-even airspeeds and so on unless you are using an A/P to have the green banana calculate it for you on the go

 

Yup...6 of one half a dozen of the other.

I've not tried it myself yet but I am pretty sure you can use IAS mode too, that will adjust pitch to keep a set speed, if you pull the PL's back to almost idle you should get a good rate of descent with a speed set at 220 to 240 or so. Guess what I am going to try now in FSX, lol.

 

So when ATC tells me to descend the best and easiest mode to use is IAS, esp when I have not set up an approach to use VNAV? Then when I get assigned a STAr and runway I can switch to VNAV for landing?

I use the FlyBy E6b app on the iPad and it seems to do a pretty good job getting me to meet altitude restrictions whenever I need to descend, as well as takes input as to how much altitude I need to lose, average groundspeed that I estimate, as well as vertical speed suggested. I can't seem to get the hang of the FMC VNAV, but that might just be due to lack of use, I'm a little bit more comfortable doing it the manual way. And plus, it's a little bit more hands on and fun in my opinion!

Which part of the app do you use I just bought it will try it out? Can you give an example of how you use it appreciate it megs8888.

Hi,

 

If you use VS in descent, WATCH THE SPEED. Use of vertical speed mode has resulted in many crashes/incidents over the years, either due to stall in climb, or because the autopilot didn't capture the altitude at low level in descent. It will also allow you to overspeed the aircraft.

 

In this aircraft more than many, you must watch the ALT SEL. Every time you move that little black wheel, check the FMA, and press ALT SEL for good measure, checking the FMA again. Nearing the level-off altitude, ensure the altitude captures. The real aircraft can fail to capture more often than is liked, and the sim is no exception.

 

VNAV can be quite useful, but requires that you stay ahead of the aircraft. The Q400 is one fast turboprop, so you need to be thinking more in terms of jet speeds when you're preparing for descent. You need to be sure you prepped the FMS in advance and that you know your altitude constraints. It is great for computing ToD, but IMHO adds to the workload.

 

Because of the large props, the Q400 (like many turboprops) has the advantage that it can stay higher, longer, and descend quicker than a jet aircraft might, especially from the higher altitudes. If you're on top of things, you can comfortably use 2 x your altitude for descent distance, especialy if you let the aircraft accelerate during descent, which is particularly useful over mountainous terrain. It also means the aircraft does a pretty decent job of decelerating whilst descending, too.

 

IMHO the best mode for climb or descent is IAS. You select a speed (~160 kts for Vx, ~180/210 kts for Vy in climb, 240 kts for descent) and either open or close the power to adjust vertical speed.

 

Best regards,

Robin.

Yes, as Robin said IAS works great in descent. I too select 160, works great. Although I believe most airlines that operate the Q400 use VS mode in descent.

 

True about the capability to decelerate rapidly too, either in descent or on approach. Q400's have been known to approach at 240 knots, and not decelerate until 6 miles out. The huge drag from those props at Disc serve as superb speed brakes.

To use the VNAV you need to pull power off (sometimes quite sharply at TOD) to keep out of overspeed (Especially at FL250 where VNE is down around 245kts and your cruise speed 240kias.

 

possibly a good idea to pull back on speed before you go over the hill.

 

Of course the good ol' Altitude minus 3 zeroes times by 3 = Top of Decent is always a good trick. Groundspeed times 5 = decent rate.

Remember to subtract airfield elevation from the -3x3 calculation.

Hi Hopskip would you kindly set a scenario for those calculations you posted descent etc so that my thick head can absorb it better please regards Wayne.

I  lower the speed around 190 before I reach TOD. With VS around 1500 or less depending wind/weather. I decent with  minor throttle adjustment  . Before  I level-off  on long final .  I will be around 150 and ready to lower the flaps.

Wow that is really really slow, you should be able to bring it down at 260 knots down to 12000 then slow to 240 and keep it there till 6 mile final. The dash 8 has no problem slowing down at all even in a steep descent. Actually you will probably want to slow to 230 below 8000, but you can really keep this thing going until short final.

I spotted what I think was a neat trick yesterday. I'd set up a VNAV descent from FL230 to cross a fix at FL90. Once the aircraft was in descent (1500fpm), I switched to IAS 240kts. The VNAV page will continues to show the required rate of descent needed to cross the fix at the pre-programmed Altitude/Level, and the VNAV magenta vertical marker operates as normal, so all that's needed is small tweaks of power to adjust rate of descent to stay on the correct profile. I found it easier than trying to manage the power to give me the required speed, as I do in full VNAV mode.

Eugene

IAS mode is IMHO the easiest to use. You don't have to worry about speed, only vertical profile.

 

Best regards,

Robin.