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Strange calculation after Top of Descent (TOD)


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Hello guys, it's me again :P

I'm on 1.23 and flying PMDG's B777-300ER, and noticed a strange calculation after TOD in most of my flights. In the image below you can see some items I highlighted. In this case, I computed a flight from SKCL to SKBO, runways 01 and 13R respectively, SID ULQ6B, STAR ABL3B, being the full route ULQ6B ULQ W1 ABL ABL3B. Alternate is SKRG; both routes were calculated automatically by PFPX.

 

569358181f462_ErroneousSTARcalc.thumb.jp

 

Description of highlighted items:

 

Distance between waypoints (green): Here you can see the distance between the last waypoint and the current one.
Total remaining (orange): Here you can see the total distance remaining, from the current waypoint to the destination.
These should be inverted (red): After analyzing the flightplan format and if my interpretation is correct, I'd say that the titles in this column should be inverted, as in each line the distance between waypoints (which I assume would be DIST) is on top of each waypoint's line, and the remaining distance to destination (which I assume would be REM) is below it.
Time between legs (light blue): Here you can see how much time passed between the last waypoint and the current one.
Elapsed time (purple): Here you can see the aggregate time that has passed since the beginning of the flight.

 

Here's what happens: if you take a look, up until TOD everything is fine: 13.4 tonnes of fuel remaining, 4.2 used, 5 minutes from the previous waypoint (TOLIM) to the TOD, and 18 minutes of elapsed time since takeoff. However, for the forthcoming waypoints fuel remaining, fuel used, time between legs and elapsed time figures do not move at all. Distance between legs (DIST) and total remaining (REM) do update, though.

 

According to flightplan, from TOD to BOG VOR there is a distance of 40NM, but fuel remaining is still 13.4 tonnes, fuel used is still 4.2, there is only a one minute difference, making the elapsed time 19 minutes. Then, from BOG to the runway, there's a 0.1 tonnes difference in fuel remaining, a 0.2 tonnes difference in fuel used, and a 3 minutes difference in time between legs and logically, elapsed time. This makes no sense and ends up in an erroneous release fuel and flight time calculation. In this case I got a 22 minutes EET, while this flight lasts in average 29-31 minutes. I'm not sure when did this start to happen, but I know for a fact it did not happen in the past.

 

Can you guys please take a look into this? Thank you in advance.

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If you go on this page (and the ones before) youll find plenty of templates:

http://forum.aerosoft.com/index.php?/files/category/48-pfpx/

When you have a new template, with PFPX not running, you go to (Win7) :

C:/ users / public / public documents / pfpx data / flightplan templates

and you copy the new template(s) there.

It could be slightly different with your OS, especially in a different language.

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Thanks for your reply, I tried changing from "EDS" to PFPX's default, which I by the way did not know was still an option, thought that "EDS" one was the new & only option by default, but this topic made me realize: 

 

, However, I got the same results:

 

5699d1a4d57ee_Newtemplate.thumb.jpg.90d6

 

I supposed that the template did not have anything to do with the issue but I wanted to try just in case, plus I wanted to revert to the classic one which is way better than that "EDS" one. At least I achieved something, lol.

 

Any ideas on what the main issue could be, though?

 

Thanks!

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Well I tried two different routes. One of them seemed okay, but the other one (SKCL - SKRG, route ULQ6B W3 MATRO MATR3A) still seems suspicious to me, and the suspicious part comes also just after TOD:

 

CLOMDE.thumb.jpg.58b1ab72562f626d7444d68

 

If you take a look, at the beginning of the flight PFPX computes 5 minutes and 1.5 tonnes of fuel to travel 31NM. However, after TOD it computes just 3 minutes and 0.1 tonnes of fuel to travel 43NM, which doesn't make much sense. Total flight time is 24, which is still too little, as this flight is basically the same distance as SKCL - SKBO, which should give 28-32 minutes. Could you please try computing SKCL - SKRG? In the afternoon when I come back from work I'll uninstall and reinstall to see if that helps.

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4 hours ago, JohnsonPBX said:

If you take a look, at the beginning of the flight PFPX computes 5 minutes and 1.5 tonnes of fuel to travel 31NM. However, after TOD it computes just 3 minutes and 0.1 tonnes of fuel to travel 43NM, which doesn't make much sense.

 

Why not? When descending the engines are most of the time in idle mode. It also looks like that you are getting less headwind after TOD.

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Thank you guys for your replies, and thank you JP for your flightplans.

 

5 hours ago, Tom A320 said:

 

Why not? When descending the engines are most of the time in idle mode. It also looks like that you are getting less headwind after TOD.

 

I understand that fuel consumption is reduced during descent as engines are idle most of the time. However there's something that still doesn't make sense. Even when in idle, engines do burn fuel, so 0.1 tonnes for 43NM just does not seem realistic. However, my main concern is not really fuel consumption, but flight time (both are linked at the end of the day anyway).

 

If we go back to my first screenshot (re-attached below), you can see that there's 41NM from TOD all the way down to BOG VOR. If we take a look at fuel figures, they remain at 13.4 tonnes, which just can't be for a distance of 41NM, even with engines at idle. Furthermore, if we look at time figures, we can see that PFPX calculates just one minute from TOD to BOG, which makes even less sense.

 

Let's suppose there's less headwind, as you stated in your reply; even then, we have to keep in mind that ground speed decreases as you descend, because drag increases as surrounding air becomes denser. BOG VOR is the approach IAF, and due to speed restrictions C/D category aircraft must be 190 KIAS or slower when reaching it. Then again, let's suppose I never reduce my speed and I keep cruise speed until reaching the VOR. According to the calculation, let's say I'll keep 452 knots GS. This would mean I'd be travelling 7.5333^ miles per minute, and therefore, it would take me at least 5.44 minutes to travel 41NM from TOD to BOG. Therefore, and that's where my argument lies, the fact that PFPX calculates just one minute for this distance just makes no sense at all, as this implies that I'd be flying at 2460 knots GS. That said, there has to be something wrong in the calculation. As I stated in the past, I used to get normal figures and my flight time was calculated around 28-31 minutes, which is what it takes me in the 777 and even in real life, as I have flown this route maaany times.

 

Again.thumb.jpg.b4606126ed54c1ca74067296

 

Later on today I will do another test flight on the 777, and post screenshots of my instrument readings and the computed flight times / fuel figures so there can be a better comparison.

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Alright, so here's a flight report with screenshots:

 

1. OFP:

 

569b3bf59c864_1.OFP.thumb.jpg.0e48c4bc4e

 

2. Release fuel:

 

569b3bf85ed13_2.Release.thumb.jpg.529625

 

3. Fuel loaded to the plane:

 

569b3bfb4ceaf_3.Loadedfuel.thumb.jpg.6b4

 

4. Over ABL VOR (I accidentally deleted the screenshot from TOD right before starting descent -_-):

 

569b3bfe8c57d_4.ABL.thumb.jpg.277663f696

 

5. Over SILUM:

 

569b3c01751b6_5.SILUM.thumb.jpg.d3d38b58

 

6. Right after passing BOG VOR:

 

569b3c046dddf_6.BOG.thumb.jpg.b15090b50b

 

7. Runway vacated:

 

569b3c0785683_7.Runwayvacated.thumb.jpg.

 

Conclusion: According to PFPX, it should have taken me just one minute from ABL to BOG VORs (28NM), and 0.0 tonnes of fuel, while in the actual plane it was minutes and 0.5 tonnes. This is definitely not normal. When I wake up I'll try uninstalling and re-installing PFPX to see if that helps.

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I wonder if this is an issue with the calculations within PFPX being affected by a destination which is not close to sea level?

 

The elevation of SKBO is over 8300 ft and looking at the other routes you describe they are all into high airports. We Europeans aren't used to those kind of elevations, and I guess most PFPX/PMDG users won't be operating to or from such high airports so not seeing a similar issue.

 

I've just run that route and the times and distances generally look OK, it's the fuel which is off. Johnson you'll be at low weights with such a short sector but the performance file suggests that burn between ABL and BOG descending from roughly 21000 to 13000 ft (from my plog) should be around 250kg.

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Well I don't think elevation has anything to do with it, as PFPX computed these routes perfectly fine in the past. I don't know if something got broken in 1.23 or what might have happened, thing is I will be reinstalling soon to test again. Even 250kg is acceptable compared to 0.0 in the above flight plan...

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Now there is something else puzzling me inside the aircraft file and I enclose an exemple.

For the descent and whatever the type of descent used (so I enclosed only one type) the figures are the same even with different weights and ISA+20 / ISA-20.

 

May be this is normal with the 777, I do not know. So worth investigating.

 

JP

Boeing 777-300 PW4090 DESCENT.txt

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What you mention is something I have no knowledge about, so it would be nice to have an input from developers in that regard.

 

Regarding the other issue, I tried with a different template (first I was trying with the Boeing 777-300ER, and now with the 777-200LR), and got the same results on the same route:

 

569fd7b5a4516_Again1.thumb.jpg.79bc93895

 

I even tried a different runway (switched from 13R to 31L), same results:

 

569fd7bae2c1d_Again2.thumb.jpg.763d008bf

 

However, I tried a different route, this time SKCL - TNCM, and the calculations seem correct:

 

569fd7bdab3ec_Again3.thumb.jpg.15bac7f86

 

In this case I wouldn't really think it's something related to the template but to the route itself, as the last one computed properly. I don't know if it has something to do with the current AIRAC, but it would be greatly appreciated if developers look into this, as this did not happen in the past.

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OK I tried the same route with two different aircrafts : B737 and Q400.

These two planes have different performances but with the 737 it gives similar bad results than with your 777.

However the Q400 behaves normally regarding time and fuel consomption. Something is wrong somewhere but what?

There is one thing that I know for sure: the Q400 was made by Jon (BW901) and it is spot on as made with the real figures.

Q400.jpg

737.jpg

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Hi JP. I do believe that descent into a high elevation airport is causing a calculation error on the leg(s) immediately following TOD.

 

As for those 777 descents.........not exactly what you'd call "precise"! You'll have to ask the author how come weight, temperature, engine variant, etc don't affect performance, and right across the board, not just on the 777.

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Well today I tried a different AIRAC version to see if this could be the root cause. Tried 1513 and even 1503, same results. I don't know if it's a profile-related issue (which wouldn't make much sense as the flight to TNCM computed properly), or an airport elevation-related bug, which makes more sense. A input from developers would be greatly appreciated.

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Hello Jon,

 

I fly mostly VQPR, VNKT which are not really at sea level!!!  But I fly only the Majestic Q400 with your very good profile and I never have any problem with the fuel. In fact on the JohnsonPBX routes, the Q400 is giving good results as well so maybe the problem is coming from the altitude which has not been integrated in the aircraft profile as you mentioned.

 

Could be worth raising a ticket to see what the developpers are thinking.

 

JP

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Rather than the altitude not being integrated I'd say something was broken at some points in one of the updates to PFPX, so it would be nice indeed to have this checked by devs. By the way, I did not create the aircraft and/or any of the profiles haha, nicks are just a coincidence. Thank you nonetheless for your help as well, JP.

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