bpcw001

GTN 750 Fuel On Board (resolved)

Recommended Posts

Hi,

 

I am using the GTN 750 (2.5.12.0) with P3D v4.4 and the Flysimware Lear 35A. Single GTN Unit, weights configured to be lbs, fuel type JETA.

No matter if I use the Shadin Fuel Data computer or not, the Fuel on board reported by the GTN is always about 11% too low.

The Lear35A maxes out at 6238 lbs, which I can load and use via P3D. However, the GTN displays 5619.9 lbs.

The value is consistently off by 11% on every fuel loadout.

I don't know how accurate that stuff is supposed to be in the real world, but 11% seems a lot to me.

Units in P3D are "hybrid", so I put in the weights in kg, which are correctly calculated in lbs in the Lear35 fuel loadout.

Any ideas?

 

Thanks

 

 

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Hi,

this might be due to either a fuel density difference in the calculation, or, a 'trick' this aircraft is using with the fuel to workaround a FltSim limitation, or indeed a bug in our code.

What Kg amount have you entered to give what Lb amount?

Share this post


Link to post
Share on other sites
15 hours ago, RXP said:

Hi,

this might be due to either a fuel density difference in the calculation, or, a 'trick' this aircraft is using with the fuel to workaround a FltSim limitation, or indeed a bug in our code.

What Kg amount have you entered to give what Lb amount?

P3D Vehicle->Fuel and Payload menu. All tanks empty except for the center tank. Entered 100 kg for the center tank.

Lear35A load manager correctly shows all tanks empty except for the center tank which shows the fuel quantity correctly as 220 lbs.

GTN shows 197.5 lbs

 

 

 

 

Share this post


Link to post
Share on other sites

Thank you. Here is what this is about:

The GTN request the simulator host fuel quantity in 'litres'. The value is directly coming from Prepar3d own internal conversions from your input in Kg to l. Then, the GTN gets both a fuel volume and a fuel density of your choice (AVGAS, JET-A etc...).

The issue therefore is in Prepar3d and how it converts Kg to litres...

You might want to cross check:

  • the aircraft.cfg file, [fuel] section, fuel_type = ?.
  • configure Prepar3d for fuel in litres instead of Kg and compare how it converts one to the other. 

[ADDENDUM] 

 (197.5lbs) (GTN Jet-A) = 111.413 liters

 

Edited by RXP

Share this post


Link to post
Share on other sites
12 hours ago, RXP said:

Thank you. Here is what this is about:

The GTN request the simulator host fuel quantity in 'litres'. The value is directly coming from Prepar3d own internal conversions from your input in Kg to l. Then, the GTN gets both a fuel volume and a fuel density of your choice (AVGAS, JET-A etc...).

The issue therefore is in Prepar3d and how it converts Kg to litres...

You might want to cross check:

  • the aircraft.cfg file, [fuel] section, fuel_type = ?.
  • configure Prepar3d for fuel in litres instead of Kg and compare how it converts one to the other. 

[ADDENDUM] 

 (197.5lbs) (GTN Jet-A) = 111.413 liters

 

Thanks for the analysis. Will try and check.

I can see that if there are too many conversions involved, inaccuracy builds up with every step  🙁

Edited by bpcw001

Share this post


Link to post
Share on other sites

Hi RXP,

if I put in 111.413 litres in P3D, the GTN displays 176.6 lbs. That doesn't even match for AVGAS which should be around 182.5 lbs.

The Lear 35 load manager tool displays 197 lbs which is expected for Jet A.

in aircraft.cfg, fuel_type = 2

 

Calculator used: http://mye6b.com/Fuel/

 

Edited by bpcw001

Share this post


Link to post
Share on other sites

I'll have to cross check this again. Thank you for your feedback.

Share this post


Link to post
Share on other sites

To leave you with something other than complaints 😉, I can tell you that I thorougly enjoy the RXP GTN. Very stable experience, no crashes.

Cannot say the same about a competitor's product which I ceased to use.

  • Like 1

Share this post


Link to post
Share on other sites

Hi,

I've just review this and I find no errors:

  • P3D4 Options | General...
  • Application | International | Unit of Measure: Metric
  • GTN | System | Units | Kg

P3D4 Beech King Air 350 - it's fuel_type is JETA (I've kept Beech aircraft in P3D4.4):

  • Vehicle | Fuel and Payload
  • Display fuel quantity as weight: unselect
  • Change Fuel: all empty but left set to 111 liters
  • Display fuel quantity as weight: select - it shows 89.1 kg (expected)
  • GTN | Utilities | Fuel Planning | User Sensor Data (enabled)
  • Fuel on Board shows: 89.2 kg (expected)

P3D4 Beech Baron 58 - it's fuel_type is AVGAS (I've kept Beech aircraft in P3D4.4):

  • Vehicle | Fuel and Payload
  • Display fuel quantity as weight: unselect
  • Change Fuel: all empty but left set to 111 liters
  • Display fuel quantity as weight: select, it shows 79.8 kg (expected)
  • GTN | Utilities | Fuel Planning | User Sensor Data (enabled)
  • Fuel on Board shows: 79.8 kg (expected)

 

Now try this with B58:

Change P3D4 Unit of Measure to: U.S. System

GTN | System | Units | Pounds

  • Vehicle | Fuel and Payload
  • Display fuel quantity as weight: select
  • Change Fuel: all empty but left set to 220 pounds
  • Display fuel quantity as weight: unselect, it shows 36.66 Gallons (expected)
  • GTN | Utilities | Fuel Planning | User Sensor Data (enabled)
  • Fuel on Board shows: 220 lb (expected)

 

Therefore it seems the Flysimware Lear 35A is supposed to use JETA density but is in fact using AVGAS!

 

2 hours ago, bpcw001 said:

Cannot say the same about a competitor's product which I ceased to use.

Thank you for your kind words!

Share this post


Link to post
Share on other sites
6 hours ago, RXP said:

Hi,

I've just review this and I find no errors:

  • P3D4 Options | General...
  • Application | International | Unit of Measure: Metric
  • GTN | System | Units | Kg

P3D4 Beech King Air 350 - it's fuel_type is JETA (I've kept Beech aircraft in P3D4.4):

  • Vehicle | Fuel and Payload
  • Display fuel quantity as weight: unselect
  • Change Fuel: all empty but left set to 111 liters
  • Display fuel quantity as weight: select - it shows 89.1 kg (expected)
  • GTN | Utilities | Fuel Planning | User Sensor Data (enabled)
  • Fuel on Board shows: 89.2 kg (expected)

P3D4 Beech Baron 58 - it's fuel_type is AVGAS (I've kept Beech aircraft in P3D4.4):

  • Vehicle | Fuel and Payload
  • Display fuel quantity as weight: unselect
  • Change Fuel: all empty but left set to 111 liters
  • Display fuel quantity as weight: select, it shows 79.8 kg (expected)
  • GTN | Utilities | Fuel Planning | User Sensor Data (enabled)
  • Fuel on Board shows: 79.8 kg (expected)

 

Now try this with B58:

Change P3D4 Unit of Measure to: U.S. System

GTN | System | Units | Pounds

  • Vehicle | Fuel and Payload
  • Display fuel quantity as weight: select
  • Change Fuel: all empty but left set to 220 pounds
  • Display fuel quantity as weight: unselect, it shows 36.66 Gallons (expected)
  • GTN | Utilities | Fuel Planning | User Sensor Data (enabled)
  • Fuel on Board shows: 220 lb (expected)

 

Therefore it seems the Flysimware Lear 35A is supposed to use JETA density but is in fact using AVGAS!

 

Thank you for your kind words!

Thanks for your investigations.

I checked with my P3D installation and found all the values to be correct for other aircraft just as you did.

Will make Flysimware aware of this.

 

Share this post


Link to post
Share on other sites

Thank you for the confirmation.

When in doubt, it is always best to first remove all plugins/modules/third party aircraft from the equation. It is easier to isolate the causes using a default aircraft too, otherwise there are so many possible points of failure along the entire chain...

The B58 (XPlane and FltSim) is my favourite test aircraft for the GTN and the GNS V2: simple yet well equipped, fast enough for multiple touch & goes in a short time, light enough to load quick.

Share this post


Link to post
Share on other sites
23 hours ago, RXP said:

Thank you for the confirmation.

When in doubt, it is always best to first remove all plugins/modules/third party aircraft from the equation. It is easier to isolate the causes using a default aircraft too, otherwise there are so many possible points of failure along the entire chain...

The B58 (XPlane and FltSim) is my favourite test aircraft for the GTN and the GNS V2: simple yet well equipped, fast enough for multiple touch & goes in a short time, light enough to load quick.

 

Well, I could make the GTN pick up the correct fuel quantity, but with a detour:

Went into the GTN itself, System setup, units section. 

Swapped units for fuel from lbs to kg, and lo and behold, I got the correct fuel weight in kilograms. Swapped back to lbs, and finally got the correct fuel weight in lbs.

 

Sadly, that doesn't stick. I have to do the unit swapping trick every time I load the aircraft for the GTN to pick up the correct fuel weight.

Share this post


Link to post
Share on other sites

I have found that if the GTN750 is started up in the mode with the fuel units in pounds when the FSW Lear35 is loaded, the GTN displays the wrong value. But if the GTN is started up in the mode with the fuel units as gallons, the GTN displays the correct value, and if you then switch the units to pounds, the value will be correct.

I then tried the default Beech King Air 350 and get the same results. If the GTN750 is initially in the fuel pounds mode when the a/c is loaded, the GTN displays the wrong value.

The Beech Baron58, which uses AVGAS and not JETA like the Lear35 and King Air 350, did NOT have the above problem -- the GTN displayed the correct fuel weight in pounds.

So the problem seems to show up when the a/c is using JETA fuel and the GTN is initially (when first started) using pounds for the fuel units.

I used Win10/P3Dv4.4 for the above tests.

Al

Edited by ark

Share this post


Link to post
Share on other sites
1 hour ago, ark said:

The Beech Baron58 did NOT have the above problem -- the GTN displayed the correct fuel weight in pounds.

Interesting findings. Did you happen to shut down the simulator and start fresh before checking this aircraft?

Share this post


Link to post
Share on other sites
11 minutes ago, fppilot said:

Interesting findings. Did you happen to shut down the simulator and start fresh before checking this aircraft?

Why do you ask? Doesn't make any difference either way.  Did you get different results?

Al

Edited by ark

Share this post


Link to post
Share on other sites
25 minutes ago, ark said:

Why do you ask? Doesn't make any difference either way.  Did you get different results?

Al

Not challenging you at all.  I have in the past encountered issues that when restarting the simulator disappear. You toggled the settings and that worked. Sometimes a restart of the simulator also corrects issues.   Been at this for many many years.  It can and does sometimes make a difference.  Obviously cannot say in your case, but it is a legitimate step to complete for testing and reporting any such issue, not just yours.  So please relax.

I do not have the Lear. I have not experienced the issue with other aircraft models.

Edited by fppilot

Share this post


Link to post
Share on other sites
4 minutes ago, fppilot said:

Not challenging you at all.  I have in the past encountered issues that when restarting the simulator disappear.  Been at this for many many years.  It can and does sometimes make a difference.  Obviously cannot say in your case, but it is a legitimate step to complete for any such issue, not just yours.  So please relax.

I do not have the Lear. I have not experienced the issue with other aircraft models.

You miss understand the intent of my question -- I was curious if there was something I was missing and you had different results.  So I have no need to 'relax', although some days I would certainly welcome the opportunity.

Based on the numbers, my hypothesis is that when the GTN starts up in pound fuel units, it defaults to using the weight of AVGAS (about 6 lbs/gallon) instead of the weight of JETA (about 6.7lbs/gallon). Thus the Baron58 numbers are correct, but not the Lear35 or King Air 350 numbers. But if you start the GTN in some other fuel unit besides pounds, when it then switches to pounds it uses the correct lbs/gal value for JETA.

Al 

Share this post


Link to post
Share on other sites
27 minutes ago, ark said:

But if you start the GTN in some other fuel unit besides pounds, when it then switches to pounds it uses the correct lbs/gal value for JETA.

Understand. So knowing Jean-Luc he will give this a hard look.

Share this post


Link to post
Share on other sites
8 hours ago, ark said:

I have found that if the GTN750 is started up in the mode with the fuel units in pounds when the FSW Lear35 is loaded, the GTN displays the wrong value. But if the GTN is started up in the mode with the fuel units as gallons, the GTN displays the correct value, and if you then switch the units to pounds, the value will be correct.

I then tried the default Beech King Air 350 and get the same results. If the GTN750 is initially in the fuel pounds mode when the a/c is loaded, the GTN displays the wrong value.

The Beech Baron58, which uses AVGAS and not JETA like the Lear35 and King Air 350, did NOT have the above problem -- the GTN displayed the correct fuel weight in pounds.

So the problem seems to show up when the a/c is using JETA fuel and the GTN is initially (when first started) using pounds for the fuel units.

I used Win10/P3Dv4.4 for the above tests.

Al

Exactly my obervations!

Share this post


Link to post
Share on other sites

Hi and thank you for your thorough investigation.

Now we'll have to find out whether this is solely a GTN trainer bug or not, but now that you've narrowed it down it will be easier for us to try repro and cure.

Somehow, if you also have the GNS V2 and it shows the same issue, there is a great chance the problem is in our code somehow.

Edited by RXP

Share this post


Link to post
Share on other sites
2 hours ago, RXP said:

Hi and thank you for your thorough investigation.

Now we'll have to find out whether this is solely a GTN trainer bug or not, but now that you've narrowed it down it will be easier for us to try repro and cure.

Somehow, if you also have the GNS V2 and it shows the same issue, there is a great chance the problem is in our code somehow.

Unfortunately Jean-Luc I do not have the GNS V2 to check. My guess is that if the GTN is initially started (powered up) in the pounds fuel mode,  for some reason it does not read the fuel type from the aircraft.cfg (or initially uses the wrong pounds/gallon value for JETA) and in effect "defaults" to AVGAS as the initial fuel type. The fuel numbers I was getting seem to indicate the GTN on power up was using 6lbs/gal instead of 6.7lbs/gal although the fuel type was JETA .

Al

Edited by ark

Share this post


Link to post
Share on other sites

@ark and you are perfectly right! It is a 'who's first' initialization issue with the fuel type prior powering up. We've overlooked this one obviously. Thank you all for your astute help with this! Unfortunately it will have to wait for the next update for a fix.

Share this post


Link to post
Share on other sites
1 hour ago, RXP said:

@ark and you are perfectly right! It is a 'who's first' initialization issue with the fuel type prior powering up. We've overlooked this one obviously. Thank you all for your astute help with this! Unfortunately it will have to wait for the next update for a fix.

Thanks for the information. At least the issue has been identified now.

Looking forward to the next update!

  • Like 1

Share this post


Link to post
Share on other sites
7 hours ago, RXP said:

@ark and you are perfectly right! It is a 'who's first' initialization issue with the fuel type prior powering up. We've overlooked this one obviously. Thank you all for your astute help with this! Unfortunately it will have to wait for the next update for a fix.

Jean-Luc -- I sincerely appreciate your willingness to take the time to investigate issues reported by your customers. Thanks for the outstanding support.

Al

  • Like 1

Share this post


Link to post
Share on other sites

Hi,

v2.5.14 is now released with a fix for this and other things!

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now