Descent calculation question(s), advices needed :-)

[DaH]

hallo all,
I´m new to the 733 an I like it very much, especially that you have to do your own descent calaculations.
I´ve read the forums and watched Jan videos, still have some questions because I don´t descent that smooth (LVL CHG), at this point it´s not a linear descent but more in waves (speed up/down trying to match the figures in Jans table. I hope that it "only" needs a lot of practice, I only did 5-6 flight, so I am really the beginning :-)

My test flight is EDDL-EDDS at FL230 to ILS07/25 - for the landing on 25 I fly over the IAF LBU than vectors to final | for the landing on 07 I delete the IAF LBU and I fly directly from the clearance limit "BADSO" into final (see attached screenshot).

Reaching CRZ FL230 the FMC gives me a ToD at 82nm EDDS.
I do the following calculation, hope I got that right:
EDDS airport elevation nearly is at 1300ft - I think Jans table describes the altitude above MSL.
So I substract 1.300ft from 23.000ft, wich gives mir 21.700ft / 300 = 72,33nm, round it to 73nm.
Is that the way to go if any airport is higher than MSL?

While cruising I decided to give me a direct to "BADSO" and delete the IAF "LBU" and head by myself to the final approach ILS07.
Track miles were now shortened and the FMC calculates a new ToD at 75nm EDDS, is that right? With less track miles, the ToD should come more to the airplane, and stretch the distance to the arrival airport, or do I think in a wrong way?
Besides that the new calculated ToD (75nm) is 2nm ahead of my own calculated ToD (73nm):
In my flight I´ve overflown the TOD at 75nm to descent at 73nm - so I was high on the profile from the first second on.
In this case I think I should have gone with VNAV and descent and then check the figures in the table if I match the correct distances at the given altitudes.
(When the VNAV profile would not correspond to the table figures, the I could still use LVL CHG to correct the desend, right? 
The whole descend is like playing piano, compare figures and push the keys, isn´t it?)
Being high from second one on, I chased the path and I got into that "wavy descent" ;-)
Not a nice descent but I could manage it, to reach 3000ft approx @180kts 5nm prior to the FDP "VATER" and land safely.
 

I´m looking forward to your help, very satisfying if the descent works how wanted it to be, really glad that the 733 (at least the IXEG) does not have a backup when you screwed up and have to head for delay vectors :-)

Cheers and greetings from Hahn-Airport (EDFH)
Daniel


 

 

 

 

 

[Litjan]

Hi Daniel,

I did not check your calculation step by step, but it sounds about right. Naturally if you change your remaining distance to the airport you also need to change your angle of descent - the best way to do that is using FL CHG mode. VNAV will have a hard time regaining the path - as it descend steep to get back onto the path, it will overspeed and then pitch up - that is the "wavy" descent that you observe.

VNAV descent will ONLY work well if you fly exactly on the planned path and never get too high - otherwise you need to revert to a more basic mode and do the math in your head (remaining distance x 3.3 is a good approximation).

Viele Grüße in den Hunsrück, Jan

 

[DaH]

Hi Jan,

thank you for the clarification, the "wavy" descent however was flown by using LVH CHG, but doesn´t matter, I think I got the whole thing now way more clear in my mind!
Gonna keep up trying and flying!

(Danke!)
Daniel

Hi Jan,

thank you for the clarification, the "wavy" descent however was flown by using LVH CHG, but doesn´t matter, I think I got the whole thing now way more clear in my mind!
Gonna keep up trying and flying!

(Danke!)
Daniel

[Litjan]

Did you see the "descent helper" sheet that is available for the avitab?

[DaH]

Yes! Fantastic document
I cross-checked the table during the flight I described above.