External Tanks Switch

Hey all I have tested this quite a few times and am seeing some strange behavior.

I flew a couple times and stopped the EXT tanks from feeding then transferred just to see the behavior. Anyways for some reason fuel consumption went WAY UP. I thought maybe it was due to ALT Throttle setting etc. Figured it was just me. I decided to just test it on the ground at idle.

So I loaded the plane cold on the ground with 75% internal fuel and a Full Center line tank. Start both turbines wait for APU auto shut off.

Watching fuel burn and A/C weight it goes down around a pound every few seconds with engines idling. If I then switch the fuel tanks too Override all of a sudden the A/C weight goes from dropping a pound every few seconds to 10-20LBS a second and fuel total is descending at same rate. Not just the EXT Total from transfer the entire fuel load is dropping almost like its in full Mil power. Even switching back to NORM doesn't stop this. Its almost acting as if the Fuel dump is on. Even once the EXT tanks have transferred all their fuel to internal tanks the fuel burn rate is still off the charts at idle power and the A/C Weight is dropping at a high rate with no throttle changes.

I am doing this all on the ground with 0 throttle changes with Park Brake on and just hitting the Tank select switches are causing the fuel consumption to go off chart and plane weight is dropping almost like Fuel Dump is on.

Here is a video of the issue.

In the video, total fuel is dropping faster than internal fuel is increasing, losing about 3/4 of the fuel. This is probably what's causing the weight loss.

You should consider making a track file of the problem and posting. Such track files are useful in creating automated tests.

I'm also seeing a probably related issue. Three tanks, I set the centerline tank to stop, and when I've looked back at the fuel display center is empty.

From NATOPS. Page 85.

2.2.4.1 External Tank Fuel Control Switches. Two EXT TANKS fuel control switches, labeled WING (for external wing tanks) and CTR (for centerline tank), are on the FUEL panel.

NORM: With the external tank(s) pressurized, external fuel transfers to any internal tank that accepts it.

STOP: With the external tank(s) pressurized, external fuel does not transfer until FUEL LO caution display is on.

ORIDE: Pressurization of and fuel transfer from all installed external tanks is provided (HOOK handle must be up in F/A-18A/B aircraft). The other external tank fuel control switch must be in STOP if fuel transfer from its tank(s) is not desired.

Basically what you are doing is Pressurize external fuel tanks and sending fuel to internal tanks while they are full. Guess what will happen to extra fuel?

It will be dumped in the engine.

Keep it at NORM positions and the regulator will control the fuel transfer to the internal tanks by the amount consumed by the engines.

What I do if I have 3 Tanks I stop the CNTR Tank as soon as I'm airporn. After I consume all the fuel in wings tank I set the CNTR tank to NORM and drop the wings tanks, to improve the aerodynamics.

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I will try to make a track but its easy to reproduce.

Knives when you say its just getting dumped into the engine that would mean the FADEC etc would just be allowing Fuel to dump in and stall or flameout the turbine this makes no sense.

ORIDE: Pressurization of and fuel transfer from all installed external tanks is provided (HOOK handle must be up in F/A-18A/B aircraft). The other external tank fuel control switch must be in STOP if fuel transfer from its tank(s) is not desired.

Clearly they can both be on ORIDE position its just saying that if you dont want flow from a tank to set it to STOP.

From NATOPS. Page 85.

 

2.2.4.1 External Tank Fuel Control Switches. Two EXT TANKS fuel control switches, labeled WING (for external wing tanks) and CTR (for centerline tank), are on the FUEL panel.

 

NORM: With the external tank(s) pressurized, external fuel transfers to any internal tank that accepts it.

 

STOP: With the external tank(s) pressurized, external fuel does not transfer until FUEL LO caution display is on.

 

ORIDE: Pressurization of and fuel transfer from all installed external tanks is provided (HOOK handle must be up in F/A-18A/B aircraft). The other external tank fuel control switch must be in STOP if fuel transfer from its tank(s) is not desired.

 

 

Basically what you are doing is Pressurize external fuel tanks and sending fuel to internal tanks while they are full. Guess what will happen to extra fuel?

 

It will be dumped in the engine.

 

Keep it at NORM positions and the regulator will control the fuel transfer to the internal tanks by the amount consumed by the engines.

 

What I do if I have 3 Tanks I stop the CNTR Tank as soon as I'm airporn. After I consume all the fuel in wings tank I set the CNTR tank to NORM and drop the wings tanks, to improve the aerodynamics.

 

Sent from my H8266 using Tapatalk

No. Fuel transfer from external tanks is disabled with weight on wheels with the switch in NORM. When ORIDE is selected, this forces fuel transfer. This is why the fuel transfers from the external tanks quickly, it's replenishing internal fuel burnt during start up.

The Checklist page should show gross weight regardless of whether fuel is internal or external, so shouldn't drop like that just because it's transferring. Definitely looks like it could be a bug.

Old thread I know, but what are the use case scenarios for using these switches - as what advantage does it convey? I'm looking over the NATOPS manual now which is very clear but doesn't indicate a use scenario.

Also on a separate point. I presume the "INTR WING INHIBIT" is for use when a damaged wing is loosing fuel?

Old thread I know, but what are the use case scenarios for using these switches - as what advantage does it convey? I'm looking over the NATOPS manual now which is very clear but doesn't indicate a use scenario.

I can think of a couple. Maybe of limited use in DCS though, as I'm not sure how developed the damage model is:

• ORIDE will force fuel transfer if the WoW switch fails to ground mode (which would otherwise prevent fuel transfer from external tanks).
  
• ORIDE will force fuel transfer with the probe out (which again would otherwise prevent fuel transfer from external tanks).
  

Also on a separate point. I presume the "INTR WING INHIBIT" is for use when a damaged wing is loosing fuel?

Correct. Or indeed, stops fuel from flowing into the damaged tank during refuelling.

Thanks for the quick reply.

Why does the weight on the wheels prevent the external tanks being used? Is there an advantage / safety feature that would mean that when on the ground you wouldn't want to use the ext tanks?

Why does the probe being out prevent transfer from the external tanks?

Also, if they are two ORIDE reasons - any reasons to use the STOP for external tanks?

Sorry for all the questions, just trying to get a deeper understanding - out of curiosity!

I would imagine the WOW sensor for the external tanks would be a safety feature in the event that there's a problem in the transfer lines between the external tanks and the wing tanks. That way it couldn't leak fuel on the carrier and risk a fire, the problem would only manifest after takeoff.

As far as the STOP switch; let's imagine you're headed back to the carrier after a sortie. You can't get there with the fuel you have left, so you hit the tanker. The range you have to go to get back doesn't require you to top off both internal and external tanks, so you engage the STOP switch to ensure that you won't take on any more fuel than necessary. Otherwise you'd have more fuel than you need and can't land because you're too heavy, so you either have to waste time burning it, or dump it. Or it can be useful when loaded asymmetrically (double ugly) and you only want fuel transferred to one external tank over the other.

Also, if they are two ORIDE reasons - any reasons to use the STOP for external tanks?

Personally, when I have 3 tanks I use STOP on the centerline one to burn the wing tanks first, usually on the way to the target, so that I can drop them when I'm more likely to meet the enemy and have to maneuver, without losing too much fuel (if any).

Thanks Tholozor and Gianky.

Tholozor for the reason you describe could you not just leave it on and take on required fuel spread out across all tanks? Is there an advantage to skipping the ext tanks? Are you not meant to land with fuel in ext tanks on carrier?

Also as I write this reply, I just happened to be on the page of manual covering the refueling probe and it says for EXTEND;

Extends the air refueling probe, turns on the probe light, if the exterior lights master

switch is on, and depressurizes the fuel tanks. The external tanks will not

transfer unless either external tank fuel control switch is in ORIDE.

So it seems that by default the external fuel tanks don't fill during air-to-air refueling?

Thanks again.

And then the next paragraph;

2.2.11.3 External Tank Fuel Control Switches.

NORM Permits selected external tank(s) to be refueled.

Completely confused!

Thanks for the quick reply.

 

Why does the weight on the wheels prevent the external tanks being used? Is there an advantage / safety feature that would mean that when on the ground you wouldn't want to use the ext tanks?

 

Why does the probe being out prevent transfer from the external tanks?

 

Also, if they are two ORIDE reasons - any reasons to use the STOP for external tanks?

 

 

Sorry for all the questions, just trying to get a deeper understanding - out of curiosity!

Re WoW I suspect it's because catapult launches are prohibited with partially filled external tanks. By preventing transfer on the ground you ensure they stay full.

Fuel transfers from external tanks by pressurised air forcing it out of the tanks. This pressure would also work against the tanker trying to fill your tanks; this is why tanks depressurise when you extend the probe.

Thanks Tholozor and Gianky.

 

Tholozor for the reason you describe could you not just leave it on and take on required fuel spread out across all tanks? Is there an advantage to skipping the ext tanks? Are you not meant to land with fuel in ext tanks on carrier?

 

Also as I write this reply, I just happened to be on the page of manual covering the refueling probe and it says for EXTEND;

 

Extends the air refueling probe, turns on the probe light, if the exterior lights master

switch is on, and depressurizes the fuel tanks. The external tanks will not

transfer unless either external tank fuel control switch is in ORIDE.

 

So it seems that by default the external fuel tanks don't fill during air-to-air refueling?

 

Thanks again.

Yes there are limits on landing with fuel in the external tanks.

External tanks will fill during air refueling. When it says it won't transfer, it means external fuel won't transfer from the external tanks to internal, for the reasons I describe above.

Thanks guys re-read all this with a refreshed head in the morning and it makes a lot more sense!

I think I misinterpreting "transfer"; hence my confusion.

Thanks for all the replies and info :thumbup::thumbup: - very interesting and certainly satisfying my deeper understanding itch.

Re WoW I suspect it's because catapult launches are prohibited with partially filled external tanks. By preventing transfer on the ground you ensure they stay full.

Disabled external tank pressurization is pretty common and extends outside carrier based aircraft. -15s, -16s, -18s are all that way, and it’s probably not limited to those three. I don’t know the exact reason, but if I had to take a stab at it...it would be because of one or more of three possibilities.

- To maximize ECS airflow to things like avionic equipment

- Just not needed due to low fuel flow rates. In addition to point, external tanks building pressure tend to push gas out of the stand pipes and piss fuel due to the slow xfer rates of gas out of the tanks.

- I would say “danger” could be a possibility, just because pressurized tanks on the ground around people poses risk, but it has to be done for ops checks and theirs no real emphasis put on it being “dangerous” when we did it...but it’s a possibility I guess.