GTN 750 Fuel On Board (resolved)

[bpcw001 22]

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

 

 

[RXP 2,338]

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?

[bpcw001 22]

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

 

 

 

 

[RXP 2,338]

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 February 17, 2019 by RXP

[bpcw001 22]

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 February 18, 2019 by bpcw001

[bpcw001 22]

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 February 18, 2019 by bpcw001

[RXP 2,338]

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

[bpcw001 22]

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.

[RXP 2,338]

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!

[bpcw001 22]

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.

 

[RXP 2,338]

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.

[bpcw001 22]

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.

[ark 1,288]

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 February 21, 2019 by ark

[fppilot 4,358]

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?

[ark 1,288]

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 February 21, 2019 by ark