Fuselage Fuel Tanks
 

What specific regulatory (and/or practical industry) restrictions are there on the use of fuselage fuel tanks (located outside of the cabin pressure vessel) in transport category aircraft?

I've seen FAR Part § 25.963, which constrains emergency landing impact/scraping specifically for fuselage fuel tanks and FAR Part § 121.255 which constrains fuel tanks that are in designated fire zones. Are there other restrictions?

I'm curious why the transport industry has such sparse use of fuselage tanks. I certainly understand the advantages of reducing the wing bending moment as to why most of the fuel is traditionally located in the wings -- but I'm surprised there are not more fuselage aux tanks. Such fuselage tanks seem to be more common in longer range business aircraft (Learjet 60XR comes to mind).

Outside of the pressure vessel? Is there any room there? Stab bay perhaps.

There's a whole lot of Jet A in the center tank of the 744.

And once you've made provision for cargo/baggage, electrical and hydraulic equipment bays and landing gear stowage, there isn't a whole lot of room under the floor of most airliners smaller than the 747.

I think he's referring to conformal tanks in wing to body fairings.

Not much room unless you make the fairing much bigger like they do on the biz jets.

24,000# in the center section of the 727, IIRC

Quote:
"And once you've made provision for cargo/baggage, electrical and hydraulic equipment bays and landing gear stowage, there isn't a whole lot of room under the floor of most airliners smaller than the 747."

I have to disagree. Unless the OP is excluding centre-section fuel tanks from the discussion, or limiting it to business jet-sized aircraft, his/her question is perplexing. From personal experience the vast majority of medium-to-large airliners have centre tanks.

Take the example of a modest-sized one, the A320: from memory the air-conditioning bays are in front of the centre tank and the main landing-gear bays are behind, with hydraulic bays in the belly fairings, wing-fillets and on the keel member between the gear bays.

No specific references, but when the Aux Tanks were fitted in the (pressurised) forward cargo areas of 747-400ERs, some titanium panels were added to either side of the tank to help protect the tanks from engine shrapnel. I would imagine external tanks would have to have similar protection.

In my question I was excluding center section fuel tanks contained within the wing's pass-thru structural center section. Even though there admittedly isn't a lot of room for fuselage tanks in most cases, I take it there aren't any regulations precluding or severely restricting such tanks?

Well yes, that's why I didn't say "there isn't any room", just not much space. :O

The other side of the coin is that many (most?) current narrow-body airliners have ACT provision, but only at the expense of baggage/cargo capacity.

As mentioned quite a few aircraft are capable of having fuse tanks fitted but for commercial passenger aircraft the loss of load capacity is not worth the extra range. The extra tanks are normally fitted inside the pressure vessel I.e the cargo bays.

Quote from Mach Stall:
"In my question I was excluding center section fuel tanks contained within the wing's pass-thru structural center section."

Thanks for the clarification. Sorry I can't help on regulations.

Quote from DaveReidUK:
"The other side of the coin is that many (most?) current narrow-body airliners have ACT provision, but only at the expense of baggage/cargo capacity."

In terms of weight (mass), there is a trade-off that applies to any transport aircraft, for the usual structural reasons. In terms of volume, are you suggesting that on narrow-body airliners the centre-section void occupied by the centre tank could otherwise contain baggage/cargo?

No, I'm saying that a centre tank, typically between the wheel wells, is the only space that couldn't otherwise (easily) be used for baggage/cargo.

But when additional centre tankage is fitted (727, Airbuses, etc), the space that it occupies reduces baggage/cargo volume.

Certain Airbus's fill the horizontal stab with fuel and also use it for centre of gravity trimming. Also if I remember correctly I think some BAE146s have pannier tanks in the wing to fuse fairings just behind the wing.

IIRC the DC-10-30 had a lower aux fuel tank which I think was in the centre section below the centre wing box. I've been searching the web for details but can't find a diagram.

Quote from dixi188:
"IIRC the DC-10-30 had a lower aux fuel tank which I think was in the centre section below the centre wing box."

Yes, the "compartmented center auxiliary tank" was located in the wing centre-section, and had upper and lower compartments. The larger, upper part had a capacity of about 40 tonnes. The lower part, consisting of a bladder cell, carried about 5 tonnes.

To put that 45 tonnes of centre-tank fuel in context, the total fuel capacity of the DC-10-30 was about 111 tonnes.

The DC-10-10 also had an option for a Center Wing Auxiliary tank, albeit smaller (about a third of the size) than the one on the -30.

Hi Dave,

In which case I wonder if the optional auxiliary tank on the Dash-10 also had the lower section with the bladder. The structure of the wing centre-section on the Dash-10 was presumably not as beefed-up as the Series 30 and 40, which might have left more space?

A cutaway drawing would have been useful, but is (predictably) absent from my course notes and the FCOM. And one has to pay for one from Flight Global, which is fair enough I guess.

On the Dash-30, the remaining 65 tonnes capacity was allegedly in the wings only, shared between 3 tanks (one per engine, although any combination of crossfeeding was available). Presumably these 3 tanks were similar to the Dash-10's. The number 2 tank (primarily for the centre engine) was bigger, and somehow split between the left and right wings. Whether its two sides were connected merely by a pipe or something more substantial is unclear from the fuel schematic.

IIRC, the 737BBJ has options for fuel tanks in the fwd and/or afd lower cargo holds.

Yes, up to nine additional tanks in the case of the BBJ, seven for the BBJ2.

It's the only NG variant apart from the -900ER that offers that option, although all the original and Classic 737s did.