744 horizontal stab fuel

[JammedStab]

How come it is not installed on the freighter aircraft.

[tdracer]

Freighters don't fly long enough legs to need the extra fuel. Freight operators would rather carry more payload than more fuel.

The typical 747 freighter missions doesn't even use center tank fuel - wing fuel is all that's needed.

[JammedStab]

Used CWT fuel yesterday, 9 hour freighter flight. Very close alternate. 6 tons in the CWT. That being said, the payloads are usually pretty heavy and the CWT is so big that I can see why the stab tank would not be used very often as you mentioned. But some flights can be quite long. Thinking of Singapore to the US. I suppose it is cheaper to fuel up enroute with the bigger payload than cut into payload for longer range.

Were stab fuel tanks ever a freighter option or never offered. And it appears that converted freighters have had their stab tanks removed.

[SMOC]

There's no such thing as having or not having a stab tank per se, the stab is the tank just like the wings are. During manufacture the basic stab just doesn't get the full treatment to make it qualify as a tank, and a BCF just has as much of the equipment that makes it function as a tank removed.

The classic on the other hand had flight control cables running through the structure of the stab therefore requiring the -400 to have a redesigned flight control system relocating the cables to the outside thereby allowing the stabilizer to be sealed and function as a tank.

[JammedStab]

There's no such thing as having or not having a stab tank per se, the stab is the tank just like the wings are. During manufacture the basic stab just doesn't get the full treatment to make it qualify as a tank, and a BCF just has as much of the equipment that makes it function as a tank removed.


Thanks,

I guess then, that it could be described as a wet stab instead of the usually stated wet wing description we hear as a type of fuel tank. Therefore, it is there as a potential fuel tank but the plumbing obviously is not as no vent or dripsticks can be seen on the freighters that I have flown. Have never flown a BCF.

[By George]

My log book shows one flight ORD-NKG (Chicago to Nanjing) in a 744F with a flight time of 14.1 hours. Admittedly back-loads to China were usually very light, it still demonstrates the Freighter still has considerable 'legs' even without stab-tanks.

[Airclues]

It's not only freighters that don't have the stab tank plumbed in. BA have four 747-400 "Lite" aircraft, G-CIVF, G-CIVG. G-CIVH and G-CIVI. They have a higher ZFW and lower fuel capacity.

[tdracer]

To the best of my knowledge, stab fuel was never offered on the 747-400F (or the 747-8F). We book-keep a "Typical" freighter mission at about 7 hours, and stab fuel only comes in to play when you're up around 16 hours. BTW, stab fuel had to be deactivated for initial 747-8I deliveries due to a flutter issue (that took quite a bit of work to resolve). New -8I deliveries have stab fuel active, and there is a Service Bulletin to activate it on already delivered airplanes.

While SMOC is technically correct that all -400s (and -8s) have a stab tank, the associated claptrap of pumps, fuel lines, seals, etc. is not insignificant in both cost and weight, so no reason to have it if you're not going to use it. Further, there are MEL implications - if something is INOP, - even optional systems - it needs to be fixed at some point per the MEL. So that provides even more incentive to simply get rid of the stab tank fuel system if you're not going to use it.

[Dan Winterland]

And it wasn't designed to be a 'trim tank' as on the A330/340. It was just another place to stuff fuel.

[dartman748]

With the freighter, yes the typical mission duration might be 7-8 hours, but remember that is artificially low due to the (in the case of the trans-pac) refuelling stops in Anchorage. If the a/c had the capability to carry the fuel, you'd see longer average sectors. In a perfect world the aircraft could take MZFW, w/ full tanks, but as we know that doesn't happen. You can have the ZFW, or the fuel, but not both. In the freighter version, operators place a higher priority in lifting the weight, and accept tech stops. You could (in theory) install a stab tank on a freighter, but it would only make sense if you know your typical loads were going to bulk out. BTW, fuel starts to go into the stab on a pax a/c at approx 130T of fuel. Broad strokes, 10-12T / hr, so sectors around 11.5-12+ hours you start seeing stab fuel loaded. Above 150T of fuel, you start restricting ZFW.

[Guam360]

"Freighters don't fly long enough legs to need the extra fuel. Freight operators would rather carry more payload than more fuel."

"The typical 747 freighter missions doesn't even use center tank fuel - wing fuel is all that's needed."


"While SMOC is technically correct that all -400s (and -8s) have a stab tank, the associated claptrap of pumps, fuel lines, seals, etc. is not insignificant in both cost and weight, so no reason to have it if you're not going to use it. Further, there are MEL implications - if something is INOP, - even optional systems - it needs to be fixed at some point per the MEL. So that provides even more incentive to simply get rid of the stab tank fuel system if you're not going to use it."


WHAAAAT?
15 hours KCVG-VHHH.

9.5 hours VHHH-PANC, 910,000 lbs, fully loaded, fuel of fuel all tanks including CWT.
NOT all -400's have horizontal stab tanks. our BCF's don't.

[JammedStab]

BTW, fuel starts to go into the stab on a pax a/c at approx 130T of fuel. Broad strokes, 10-12T / hr, so sectors around 11.5-12+ hours you start seeing stab fuel loaded.

Going from memory, at about a 5 to1 ratio with the CWT tank. Not sure of the reason why but perhaps for balance.

[SMOC]

As stated the BCFs still have the tank but it's now full of holes where the plumbing, pumps and access panels were, the vent and dipsticks etc removed and blanked, plus a new fuel panel in the cockpit.

Also the -8 can carry more fuel in its CWT & wings than the -400 can including its stab. It can load an extra 9T in each wing compared to the -400.

[tdracer]

Guam

Perhaps I wasn't clear - all 747-400/-8 aircraft structurally have a stab fuel tank. But on Freighters (and some passenger models) it's not functional as a fuel tank - the sealing, pumps, fuel lines, etc. are not installed. Further, when a passenger model is converted to a freighter (e.g BCF), the stab fuel is disabled and most of that equipment comes out to save weight.

A 747-400 with full tanks (but no stab fuel) can carry ~ 180 to 190 tons of fuel (depending on density - about 53,600 US gallons). Stab fuel adds another ~11.5 tons (3300 gallons), or about a 6% increase. I don't know why you'd need stab fuel at ~130 tons unless as JammedStab notes it's for balance.

BTW Guam, why would you need full CWT fuel (again, ~185 tons) for a 9.5 hour flight?

[dartman748]

Going from memory, at about a 5 to1 ratio with the CWT tank. Not sure of the reason why but perhaps for balance.

Good memory. The actual ratio is 5.2-1

A 747-400 with full tanks (but no stab fuel) can carry ~ 180 to 190 tons of fuel (depending on density - about 53,600 US gallons). Stab fuel adds another ~11.5 tons (3300 gallons), or about a 6% increase. I don't know why you'd need stab fuel at ~130 tons unless as JammedStab notes it's for balance.

The software in the fuel system determines when the fuel starts to go into the stab tank. It can be between 125-132T of total fuel load, that the stab will start to be used. It will then go in at the ratio of 5.2-1 (CWT-STAB) Total fuel is approx 170 T w/ the stab tank, 160 T w/o. 13.3 in each of Main 1&4, 37.3 in Main 2 &3, 51.0 in CWT, 3.8 in the reserves, plus 10T in the stab (as installed). That using a SG of 0.785.

[parabellum]

Max I ever got onto a -400 was 173tons and it came out of the ground cold. usually maxed out about 168tons.

[JammedStab]

173 tons is the number I am familiar with. I guess that there are versions that can take more. Perhaps an option.

[Intruder]

That would be with the stab tank. According to our FCOM, standard 744F carries 163 Tons, and the 744 pax bird with stab tank carries 173. The most I've seen in a 744F is around 161.

[SMOC]

For the 744 pax with stab fuel it depends what SG you're using, an SG of 0.80 comes out to 173T if you're using 0.785 it comes out to 169T, so it depends what your company considers a standard SG for their location on the planet. (For us it's 0.785 so 744 = 169T & 744F - 160T).

The only way a -400 could have more gas than standard would be the auxilliary tanks (12,151L) in the forward cargo and only for the 744ER I believe.

http://www.boeing.com/commercial/aeromagazine/aero_21/images/747ER_fig3.jpg

[tdracer]

To be clear, the numbers I posted above are the max fuel weights allowed per the AFM - turns out that's based on a density of .885 which is exceptionally high for jet fuel (but not out of line for gasoline).
Obviously with a more typical .78-.80 density the volume (53,600 gallons, plus another 3,300 gallons for stab fuel) will limit the mass of fuel carried.

[JammedStab]

To be clear, the numbers I posted above are the max fuel weights allowed per the AFM - turns out that's based on a density of .885 which is exceptionally high for jet fuel (but not out of line for gasoline).
Obviously with a more typical .78-.80 density the volume (53,600 gallons, plus another 3,300 gallons for stab fuel) will limit the mass of fuel carried.

That's interesting. My FCOM shows almost exactly the 163 and 173 ton numbers stating "Usable fuel at level attitude, fuel density = 0.8029 kilograms per liter". Not sure if that is written by Boeing or the company.

I think the 173 ton and 163 ton numbers are what you may be able to reasonably expect but the higher AFM values(I added it up to almost 184 tons for the freighter) are the maximum allowable if you happen to come across some very cold fuel available somewhere. Maybe if you are find some Jet A-1 fuel available above ground at -42 degrees C, you could exceed fuel tank weight limitations. I'll let someone else do the calculations.

[SMOC]

Low SGs below .77 have a MTOW limitation, the lower the SG the lower the max takeoff weight.

[Tinwacker]

SMOC
so it depends what your company considers a standard SG for their location on the planet.

My company doesn't have a standard SG, and would be angry if an SG was not the figure provided by the refuel company seen at the aircraft.

[SMOC]

My company doesn't have a standard SG

Of course you don't use a standard SG in real operations.

If you read the thread you would realize we are talking about FCOM/AFMs, which use a standard SG where applicable otherwise every graph / weight and limitation would have to be published for all possible SGs.

The differing figures given in this thread are as a result of companies using different standard SGs in their manuals, most likely the most realistic SG for your location on the planet and what's your standard/most common fuel Jet A or Jet A1.

I seriously doubt your manuals publish limitations on every SG between 0.719 & 0.851

Ours are based on 0.785 with a few options to see 0.78 / 0.80 / 0.82

Lastly unless you send your SG to load control every sector they will be using a standard SG for weight and balance calculations, most likely based on statistical data provided by the airport or fuel company with likely seasonal changes to the SG.

[Swedish Steve]

Lastly unless you send your SG to load control every sector they will be using a standard SG for weight and balance calculations
So what difference does it make to weight and balance if you go from one high SG to one low SG?
The fuel is loaded in kgs, so the weight is always the same.
Yes with a high SG you will get less fuel in the wings, and more in the centre, byr does that really make a difference worth worrying about?

[SMOC]

Does nobody read anymore?

The previous poster said his company doesn't use standard SGs, the point is they most likely do for load control and manual publications, aircrew obviously don't as we need accurate figures for performance, a thread like this draws on both aspects.

does it make a difference worth worrying about

If you scrape the tail during takeoff because of a fast/over rotation due to an incorrect stabilizer setting received from load control would you worry about it?

[Swedish Steve]

aircrew obviously don't as we need accurate figures for performance,
But what do you do with the SG figures?
I have refuelled loads of aircraft for long flights, Tristar B747 B777 B787 A330 and I always use the actual SG to calculate the fuel uplift.
But the crew have Never asked me for the SG, or sent it anywhere.
I present the fuel log 10 mins before departure. (B747 with 130 tons ramp fuel on 80min transit) They check it, but I have never seen anyone do anything with the SG.
So where do you put the SG figures to calculate performance? I thought everything was in KG. Once you have agreed that the fuel on board is correct, what use is the SG figure.

The aircraft sense the SG themselves and use it to calculate the fuel distribution, but no one tells them.

I am not having a go, I really am interested as it is something I have never noticed.

[rudderrudderrat]

Hi Swedish Steve,
So where do you put the SG figures to calculate performance?
Performance is based on aircraft mass so we use the Aircraft Fuel Quantity gauges (Kgs) for the fuel on board.

The uplift (litres) is converted to Kgs using actual SG (pocket calculator or mental maths if it's close to 0.8), and compared with the calculated uplift. If the discrepancy (as +/- figures or a % age) is outside FM limits, then we investigate further.

[JammedStab]

We had a little over 1500 kg uplift discrepancy listed on the fuel sheet the other day. The refueler was asked to investigate and came back with a new fuel sheet stating that the discrepancy was now around 800 kg which was within our limits based on the uplift quantity.

The reason given to me through a third party was that the fuel temperature check of the bowser had been done 4 hours earlier and when a new updated temperature was used, there was a new and supposedly more accurate fuel uplift quantity.

[Swedish Steve]

Hi Swedish Steve,
Quote:
So where do you put the SG figures to calculate performance?
Performance is based on aircraft mass so we use the Aircraft Fuel Quantity gauges (Kgs) for the fuel on board.

I give up.
If you read what I said I have been refuelling large aircraft over 30 years. I have heard of fuel gauges reading in Kgs!!!

SMOC says we should send the SG figure to the dispatcher. All I want to know is what he does with it. He already has the ramp fuel in KGs. He doesn't care about the uplift in LTs

[SMOC]

Apologies Swedish Steve, it's been a long week.

Most airlines don't trust the A/C Fuel system due to accidents/incidents (ran out of fuel) and tail scrapes etc. Due to errors.

So.

Most airlines take the fuel figure from the bowser in Liters / Gallons or Imperial gallons and using a correction plus the SG provided by your good self convert it to KG or pounds depending on if your aircraft has a metric system or imperial, some operators have both systems as they buy lease A/C from different states. As I'm sure you know.

We then cross check in my case the KG loaded plus the previous fuel on board and confirm it totals within limits (usually less than 1000KG) with what the A/C tells us. As stated above any discrepancy investigated ie a two step fueling with APU burning a few hundred KG might be a source of error.

Now I've just confirmed that our company uses standard SG of 0.78/.80/.82 to construct the load sheet weight and balance dependent on location on the planet and season to try and closely match what will be the actual SG.

This way the data provided should mean the aircraft behaves as expected

Now I just asked the refueler today what he does and the interesting thing is and I'm not sure if this is standard but he said if I order 90,000 KG not be loaded but to be the fuel onboard after fueling is complete and say we have 10,000KG onboard already and his SG is 0.8 he converts that to liters and dials up 100,000 liters on the truck and pushes the go button.

So when it boils down to it, it's a bit redundant that he converts weight into liters and we convert liters back into weight. All we've done is check our button pushing abilities.

I think we all just assumed they push the go button till the A/C says 90,000KGs and then the refueler read off the liters that went through the truck.

[Intruder]

Steve is correct. The fuel verification ONLY gives a rough check of the fuel quantity system and gives the beancounters an indication that they're not getting cheated by the refuelers.

The FQ system already accounts for fuel density, so with an operational FQ system the fuel mass is already calculated. No corrections to performance need be made.

The exception (and I've NEVER seen it) is when the SG is outside the normal limits for the aircraft. In that case an adjustment to max ZFW and/or TOGW may have to be made.

[NSEU]

Now I just asked the refueler today what he does and the interesting thing is and I'm not sure if this is standard but he said if I order 90,000 KG not be loaded but to be the fuel onboard after fueling is complete and say we have 10,000KG onboard already and his SG is 0.8 he converts that to liters and dials up 100,000 liters on the truck and pushes the go button.

I think we all just assumed they push the go button till the A/C says 90,000KGs and then the refueler read off the liters that went through the truck.

Bizarre... I've never heard of any fueller doing this for aircraft with automatic fuelling capabilities. Even when the automatics are faulty, we (engineers) open and close the tank refuel valves manually according to the Kg or Lbs reading on the aircraft refuelling panel gauges. The aircraft have their own densitometers and compensators for computing weight.

[JammedStab]

The aircraft have their own densitometers and compensators for computing weight.

But how accurate are they? We pushed back the other day with more fuel in tanks 2 and 3 than what was in 1and 4 so the override pumps were on for 2 and 3. During the climb tanks 2 and 3 both displayed 5 or 600 kg less than 1 and 4(although there was no EICAS message) so the override pumps were selected off for those tanks with 1 and 4 crossfeeds closed. After levelling off, 2 and 3 now unexpectedly had more fuel than 1 and 4 again so we went back to the original configuration.

Which makes me think that these quantity guages are not very accurate in the climb. Theoretically are they supposed to compensate for aircraft pitch.

And is the Xfeed Config message inhibited during climb?

[main_dog]

I have noticed that the 744 fuel totalizer tends to slightly over-read in descent and under-read in a climb. Next time at TOD take a look at the totalizer, it occasionally creeps up by a couple hundred Kgs as you pitch down. In a go-around, conversely, it begins to under-read.

I once made the mistake of glancing at the totalizer on the lower Eicas during a go-around and was a bit concerned to see almost a ton less than I was expecting... as soon as we levelled off at MAP altitude it started to creep back to where it should have been. I do believe this effect is more pronounced for tank 2 and 3 readings.

[JammedStab]

I have seen this on another aircraft type as well and it was also an'80's design. Perhaps the newer aircraft types have eliminated this problem.

[Intruder]

I suspect it's not practicable to design an FQ system that reads accurately in all pitch and roll conditions. If the FQ reading is accurate on the ground and in level flight over the normal speed range, that should be sufficient.

The capacitance probes in modern FQ systems inherently compensate for fuel density, so they are as accurate as you can get with current technology.

In the 747 specifically, a high pitch attitude at low FQ is NOT a good situation, since the forward boost pumps will cavitate. There is an FCOM warning regarding go-arounds with FQ <900 Kg in a tank.

[JammedStab]

In the 747 specifically, a high pitch attitude at low FQ is NOT a good situation, since the forward boost pumps will cavitate. There is an FCOM warning regarding go-arounds with FQ <900 Kg in a tank.

This happens quite frequently during initial climb with full climb thrust at light weights(for flights of 1.5 hours or less). During the climb, forward pumps cavitate with associated EICAS messages. And this is with quite a bit more than 900 Kg in those fuel tanks.

[JammedStab]

I have noticed that the 744 fuel totalizer tends to slightly over-read in descent and under-read in a climb. Next time at TOD take a look at the totalizer, it occasionally creeps up by a couple hundred Kgs as you pitch down. In a go-around, conversely, it begins to under-read.

I once made the mistake of glancing at the totalizer on the lower Eicas during a go-around and was a bit concerned to see almost a ton less than I was expecting... as soon as we levelled off at MAP altitude it started to creep back to where it should have been. I do believe this effect is more pronounced for tank 2 and 3 readings.

Perhaps this is what the FCTM refers to as "Fuel Sloshing".

Taken as only part of a procedure for determining if a Fuel Leak exists:

"Some fuel-related checklists (for example, FUEL IMBAL) list reasons that a fuel leak should be suspected. This list is not exhaustive and, in all cases the flight crew should use their knowledge of the fuel system and current operating conditions to determine whether a fuel leak should be suspected. Some reasons are:
• The total fuel remaining on EICAS is less than the planned fuel remaining. The total fuel can be less than planned fuel for a number of reasons, such as a fuel leak, unforecast headwinds, fuel sloshing (such as from high angles of pitch). Sloshing fuel would be a temporary effect. Flight crews should consider these when deciding whether or not to suspect a fuel leak.
• An engine has excessive fuel flow. A faulty fuel flow meter......"