[BUG/WIP] The fuel feed switch is without function

[Germane]

The fuel feed switch dont work. No matter which position is selected, the engines get always fuel from all tanks.

Edited February 2, 2021 by Germane

[Golo]

Edit: You are on to something here actually. I went through it again a little in NATOPS and there is some discrepancies between it and actual behaviour in game.

 

Fuel feed switch:

When switched to AFT position - Left feed of aft and left system starts to decrease quantity and R feed of forward, and right will increase its quantity (if it was lower than max) or or will hold max.

When switched to FWD position Left feed of left and aft system will increase its quantity (if it was lower than max) or or will hold max, and R feed of forward and right system will start to decrease.

And there is no transfer of fuel between Aft and left and Forward and right systems, not that is obvious to me.

 

What should happen if I understand the description correctly, is that whatever position you select on Fuel feed switch, the fuel feed of that selected side should remain full and there should be transfer of fuel from opposite side to selected side.

 

 

Further more, I was testing AB effect on feed groups.

High engine fuel demands should cause the fuel in box beam tanks to decrease creating pressure drop in beam box tanks that will cause fuel to be drawn from tanks no.2 and no.5 into sump tank, instead of drawing it from box beam tanks.

That should result in dropping fuel quantity of L/R feed group somewhere around some 1200lbs and hold as any further fuel is drawn from tanks no.2 and no.5.

However in game the fuel in L/R feeds will always decrease to 300lbs and is not affected by any altitude change (tested same on the deck and 30k ft). 

 

 

Edited January 26, 2021 by Golo

[Germane]

Hallo Golo, so it is a bug, or not? I checked it again. Nevertheless in which position the switch is, the fuel feed is always the same.

[Golo]

Based on my knowledge I would say it a bug. There are even more discrepancies in fuel system between what NATOPS says and what I experience in game, Ill write it up as im done testing it.

[IronMike]

Thank you, I added it to the tracker, we'll investigate.

[Golo]

In game fuel system function review for F-14B, in comparison to NATOPS 01-F14AAP-1, 1 AUG 2001.

 

According to NATOPS Fuels system section 2.14:

 

2.14.1.1 Sump Tanks: My main concern were WARNING notes - Zero or negative-g flight longer than 10 seconds in AB or 20 seconds in MIL or less will deplete the fuel sump tanks (cells 3 and 4), resulting in flameout of both engines.

                                                                                                       - Afterburner operation in the 0g to –0.5g regime may result in air ingestion into the fuel boost pumps, causing possible afterburner blowout or engine flameout.

                                                                                                       - With fuel in feed group below 1,000 pounds, AB operation could result in AB blowout.

 

In game results - Fuel press warning light and subsequent engine flameout in AB at about -1g, 10s, 13s, 12s, above 4.5k lbs of fuel. 17s, 13s, 17s below 4.5k lbs of fuel. I would conclude that this is correct given some margin of error.     

                           - Fuel press warning light and subsequent engine flameout in MIL at about -1g, 59s, 74s, 70s, at around 7k lbs of fuel. I would conclude that this is incorrect even given some margin of error. Furthermore if test is run to only show L/R fuel pressure low without engine flameout (set positive g to clear L/R fuel pressure low warning) negative g flight can be repeated immediately to the same time duration (this is major BUG in my opinion as I think there is no way of replenishing fuel supply in sump tank that quickly).

 

AB/MIL engine operation in 0g to -0,5g should in my opinion result in intermediate indication of L/R engine fuel low, due to fact that fuel in sump tank would be in floating freely mixed with air and there should be possibility to ingest air into the boost pump. However I did not see L/R fuel pressure low blink even once during around 0g flight. 

 

AB engine operation with less than 1k lbs of fuel in 1g flight. I could not get AB to blowout/engine to flameout even with 300 lbs in sump tank.

 

2.14.2.1 Fuel Quantity Indicators: L feed group should indicate 1500lbs of fuel according to specification. Right feed group should indicate 1600lbs of fuel. AFT group contains 4400lbs of fuel, FWD group contains 4700lbs of fuel. Total fuselage fuel amount is 12200lbs. 

 

In game - Both AFT and FWD group indicate correct fuel amount on tapes. Both feed groups indicate 1800lbs of fuel (where are those extra 500lbs of fuel come from? They are not accounted in total fuel qtty as far as I can tell, total amount of fuselage fuel would be 12 700+4000 of fuel in wing tanks that would be 16 700lbs of fuel, indicator shows 16 200?)

 

2.14.2.2 FUEL LOW Caution Lights: Works correctly as it illuminates at specified fuel qtty.

 

2.14.3 Engine Feed:  Situation 1: a. Fuel in cell Nos. 2 and 5, b. FEED switch in NORMAL, c. Normal engine fuel flow (MIL thrust or less).

In game results - Works correctly as far as I can tell

 

                                  Situation 2: a. Fuel in cell Nos. 2 and 5, b. FEED switch in NORM, c. High-engine fuel demands (afterburner).

Fuel in box beam tanks should deplete to sump tank via ports A,B only as far to create pressure drop (to amount of aprox. 1200 fuel in a feed goup), this low pressure in box beam tank will hold remaining fuel in box beam tank and the majority of fuel to sump tank should be drawn via ports C,D from tanks no.2 and no.5.   

In game results - Incorrect operation. Feed groups will deplete to 300lbs of fuel and hold.

 

                                 Situation 3: a. Fuel in either cell No. 2 or 5 has been depleted, b. FEED switch in NORM, c. Any normal engine demand.

When the low-level thermistor in either cell No. 2 or 5 is uncovered, both box-beam tanks are vented and the sump tank interconnect valve is opened. The two groups become a common system and will seek a common level to equalize the static pressure head. Fuel will flow through the open sump tank interconnect valve only as a function of differential pressure. With open vent valves, the fuel in both box-beam tanks has a positive vent pressure, forcing the fuel into the respective sump tank through interconnect A or B.

 

In game results - Incorrect operation. In my experience the fuel levels equalize at 300lbs of fuel. This would indicate to me that you used low fuel level termistors in tanks no.2/no.5 and L/R box beam tank. Which would correlate to 300lbs of fuel left in sump tank. The correct operation would be to open L/R box beam tank vents and sump tank interconnect at the depletion of fuel in tanks no.2 or no.5 which would leave full feed groups fuel qtty to equalize (Left feed 1500lbs and Right feed 1600lbs, or more if any more fuel is present)

 

2.14.3.2 Engine Fuel Feed During Afterburner Operations: ...

In game results - In nose high (30°-60°) 0g to -1g flight I could not get the left engine AB to blow out/L engine to flameout first. They both flamed out simultaneously. With full Fuselage fuel 0g, 20s, 20s, 20s to flameout of both engines. With 9k lbs of fuel -1g, 11s, 10s, 9s to flameout both engines.

 

2.14.5 Fuel Quantity Balancing: ...

In game results - Does not work at all for me.

 

Note: Check engine oil distribution too please. During all of my negative flight time some of as long as 90s low engine oil pressure did not light up even once, and oil pressure indicator for L/R engine only showed drop of 10psi from nominal value in 1g flight. I bellive this is also not correct as there is no negative flight oil tank in F-14 as far as I know. That thing should light up almost as soon as you go negative and the pressure should drop drastically from what I know.

 

Edit: This is of course only what I find out, I might be wrong on some of this. Ive been starring at that fuel system for days now and to say its complex would be a major understatement.

 

Edited February 3, 2021 by Golo

[Cobra847]

Thanks! The Fuel system is one of the oldest components in the jet. Some things may have broken along the way. We'll look into it.

[Germane]

Hey @Cobra847, what's the current status of the issue? 

[FlyingTaco21]

The feed switch balances fuel by forcing depletion of the selected set of tanks forward and right or rear and left . There is no transfer, its just that one both engines feed off of one set until a near balance is achieved and then the in the norm feed setting one engine is fed by each set of tanks. I have noticed the forward and right always seems to have more fuel but when I set feed to that tank set and forgot about it for a bit I definitely drained that tank set. More than rear and left. 

[Golo]

4 hours ago, FlyingTaco21 said:

The feed switch balances fuel by forcing depletion of the selected set of tanks forward and right or rear and left . There is no transfer, its just that one both engines feed off of one set until a near balance is achieved and then the in the norm feed setting one engine is fed by each set of tanks. I have noticed the forward and right always seems to have more fuel but when I set feed to that tank set and forgot about it for a bit I definitely drained that tank set. More than rear and left.

That is not quite correct, there is a transfer. Read "fuel quantity balancing" section of the F-14 NATOPS manuals.

[Germane]

vor 18 Minuten schrieb Golo:

That is not quite correct, there is a transfer. Read "fuel quantity balancing" section of the F-14 NATOPS manuals.

What I still don't quite understand is. The manual talks about left and right tanks. But what is meant by this? Does this also include the wing tanks and external tanks? In order to compensate for an imbalance, that would make sense, wouldn't it?

[Golo]

1 hour ago, Germane said:

What I still don't quite understand is. The manual talks about left and right tanks. But what is meant by this? Does this also include the wing tanks and external tanks? In order to compensate for an imbalance, that would make sense, wouldn't it?

I would consider ext. and wing tanks separate (excluded) system in regards to fuel balancing. I dont think there is a way to get fuel into them except from refueling.

As for fuel balancing there are, FWD/RIGHT - forward fuselage tanks (fuel cells 1, 2) and right box beam tank (fuel cell 4), AFT/LEFT - aft fuselage tanks (fuel cells 5, 6, 7, 8) and left box beam tank (fuel cell 3). All fuel balancing is done between those tanks, so its mostly shifting fuselage fuel fwd or aft. Lateral imbalance in this case is really not a thing unless there would be a failure to extract fuel from a one wing tank.

   

[Germane]

vor 1 Stunde schrieb Golo:

I would consider ext. and wing tanks separate (excluded) system in regards to fuel balancing. I dont think there is a way to get fuel into them except from refueling.

As for fuel balancing there are, FWD/RIGHT - forward fuselage tanks (fuel cells 1, 2) and right box beam tank (fuel cell 4), AFT/LEFT - aft fuselage tanks (fuel cells 5, 6, 7, and left box beam tank (fuel cell 3). All fuel balancing is done between those tanks, so its mostly shifting fuselage fuel fwd or aft. Lateral imbalance in this case is really not a thing unless there would be a failure to extract fuel from a one wing tank.

   

Hmm, there may not be a way to get fuel into the tanks (wings and ext) but the option of which tanks to take fuel from.  It is possible to choose whether fuel is taken from the central or external tanks first. So maybe also from which side?? That would be enough to compensate for an imbalance. Otherwise, I honestly don't see the point of the switch.

[sLYFa]

Besides shutting off fuel transfer from one of the fuselage tanks, the FEED switch has another function. It opens the transfer selector valves, allowing fuel from either wing tank to flow into either box beam tank. This is useful if one of the transfer shutoff valves in one of the box beam tanks closes despite fuel being left in the respective wing tank (remember that the wings tanks feed into their respective box beam tank). Thus, fuel from either wing tank can flow into either box beam tank. So if e.g. the left transfer shutoff valve fails closed with fuel left in the left wing tank, placing the FEED switch into AFT or FWD will allow fuel from the left wing tank to flow into the right box beam tank (provided the right shutoff valve is not closed). I don't think those kinds of failures are modelled though.

As for transferring fuel between fuselage feed groups, this is not directly possible but can happen under some conditions.(e.g. heavy nose up/down attitude with FEED out of the NORM position).