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-   -   CDFA Calculation in Charts (https://www.pprune.org/tech-log/660864-cdfa-calculation-charts.html)

galaxy flyer 15th August 2024 20:42


Originally Posted by EXDAC (Post 11717592)
How well do you think that would work in Nepal? Did you look at the required glidepath angles for VNKT?

It clearly wouldn’t, but the initial post was KLAX 24, not a special aircraft with special crew qualification where one would expect this information would be trained and any required tables issued. Few are going to run Excel spreadsheets on final approach.

In any case, the 1:60 rule says the 5.4* path is 540 ft/nm times ground speed in nm/min. 150 knots equals about 1350 fpm.

EXDAC 15th August 2024 21:07

Why wouldn't one fly an approach like this in FPA mode? Is it that few aircraft have an autopilot FPA mode or that the mode is there but crews are not trained to use it. (MD-11, B-717, and MD-10 all had an FD/AP FPA mode which is ground referenced, not airmass referenced. No need to observe GS and compute required VS. Just dial in FPA -5.4 at the FAF and change it to -3.0 at 5.5 DME. Yes, of course altitude monitoring is still required.)

swh 16th August 2024 00:45

We flew the VOR 02 in NAV-FPA, as an early stabilized approach, landing config and Vapp at the FAF, make Vapp a speed constraint at the FAF. The approach path was too steep to fly the Airbus CDA where you configure during descent to reach Vapp at 1000 ft. Even being fully configured at the FAF, often with 20 kts of tailwind, you are using a fair bit of speed brake to prevent a flap over speed.

john_tullamarine 16th August 2024 10:08

the 1:60 rule

(With the risk of my being labelled a pita pedant), one needs to keep in mind that the 1:60 rule is a pilot-friendly approximation of the 1:57. (whatever decimals float your boat) rule.

mabg88 16th August 2024 10:53

Thank you TeeS, I will keep your reference in my notes!!!

mabg88 16th August 2024 10:56

Thank you all for the replies.

I will take a pic of the actual data base for LOC 02 at VNKT since I'm going there in a few days. We stopped shooting the LOC there cuz the coding of the mcdu leaves you high on the approach flying a A333 with 1T bellow max landing weight (usually). Unlike the VOR which has the correct data in the mcdu to xcheck the chart alt and distance with the actual flight path of the aircraft. As mentioned in another post, we fly it NAV-FPA (No FLS installed) and xchecking the recommended altitudes of the chart while the mcdu has a different coding reduces your situation awareness. Any how, I just wanted to make sure that I was not missing something regarding the math. I'll post the pic of the coding once I have it for those who are interested.

mabg88 16th August 2024 11:11


Originally Posted by galaxy flyer (Post 11717676)
It clearly wouldn’t, but the initial post was KLAX 24, not a special aircraft with special crew qualification where one would expect this information would be trained and any required tables issued. Few are going to run Excel spreadsheets on final approach.

In any case, the 1:60 rule says the 5.4* path is 540 ft/nm times ground speed in nm/min. 150 knots equals about 1350 fpm.

I made the post to confirm I was not missing any other data for the calculations of the altitudes in the charts to identify and understand if the chart of the data of the mcdu is wrong. Plenty of training and years of going there 2 or 3 times per month for the past 9 years. This is not about flying the approach with less than 10ft of precision while on NAV-FPA. As mentioned before this is about understanding the math and making sure I was not missing any other criteria for the calculations. Thank you for your time and replies.

swh 17th August 2024 08:18


Originally Posted by mabg88 (Post 11717961)
Thank you all for the replies.

I will take a pic of the actual data base for LOC 02 at VNKT since I'm going there in a few days. We stopped shooting the LOC there cuz the coding of the mcdu leaves you high on the approach flying a A333 with 1T bellow max landing weight (usually). Unlike the VOR which has the correct data in the mcdu to xcheck the chart alt and distance with the actual flight path of the aircraft. As mentioned in another post, we fly it NAV-FPA (No FLS installed) and xchecking the recommended altitudes of the chart while the mcdu has a different coding reduces your situation awareness. Any how, I just wanted to make sure that I was not missing something regarding the math. I'll post the pic of the coding once I have it for those who are interested.

The coding I have
GURAS /+11500
C022º 5NM
CL02/+9300
C022º TRK022º 3-5.3º
FF02/+9000
C022º 1-5.3º
LOC11/+8420
C022º 3-5.3º
75LOC/+6720
C022º 2-5.3º
55LOC/+5590
C022º 1-5.3º
45LOC/+5280
C022º 2-5.3º
KTM/4750

This corresponds to the charted altitudes. Linear regression through the altitude/distance table on the Jepp chart works out that they used 5.21 degrees.


That to me looks like the coding inside 55LOC is wrong, should be -3.0º, looked at a machine with xLS, the approach is not in the data base.

This time of year its ISA+25 or more, so your true altitude at the FAF is more like 9500ft despite it indicating 9000ft on the altimeter, the non xLS machines are not temperature compensated. Something like FPA -5.9 will need to be flown to meet the profile. This should be flown in LOC-FPA.

TeeS 17th August 2024 09:23

Hi SWH
I'm no expert on the coding but that almost looks like someone used the gradient of 5.3% in place of the 3° as that kept the numbers constant?
Cheers
TeeS

swh 17th August 2024 13:58

The chart has 5.31 degrees between FF02 and 55LOC, then 3 degrees from 55LOC to 3.1 DME for the VDP/MDA. If you were to draw a straight line from the FAF to the KTM VOR it would be a FPA of 4.25 degrees. The chart is designed so that the last 1200 feet is flown at 3 degrees from 55LOC.

Cargo rates into KTM are pretty good, and fuel is expensive (tanker), so it’s common to land near MLW. With the higher density height, TAS is up around 155-160 kts, plus a tailwind, on a 3 degree path you are pushing 900 fpm V/S on short final.

TeeS 17th August 2024 18:38

Hi swh
Sorry, I was a bit rushed with my last response. I was commenting on your statement of: "That to me looks like the coding inside 55LOC is wrong, should be -3.0º" (which I agree with from what you have shown). I was wondering whether some manual data input was involved (unlikely, I know) and a gradient of 5.3%, which equates (roughly) to 3° was copied and pasted in error, just a thought.
Cheers
TeeS


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