777 No Stab Tank and 340/330 Trim tank
 

Hi All,

I hope this isn't too much of a pre-school question, but I can't seem to find a valid reason as to why Boeing opted not to include a Horizontal Stabilizer tank.Also, correct please me if I'm wrong but unlike the 747-400, it doesn't have a reserve tank either. Why would that be?

Furthermore, with regards to the Airbus I understand that the aircraft has an automatic trim tank. I've read that 75 minutes before arrival at it's destination, it will pump all the fuel to the center tank/left inner tank (Which tank?). Has it been designed to do this to improve longitudinal stability on the approach with a forward CoG, and would it be because it takes a long time to pump the fuel from the Horizontal Stabilizer (i.e the crew might need that fuel in a go around)?

Regards

I asked a Boeing test pilot the same question. I think he said it wasn't worth the cost/weight/effort/mx.

787 doesn't have a tail tank either.

The 777-200LR has an auxiliary fuel tank as an option.

Its not there for storage, but to provide a more aft CofG for the cruise. Then it comes back to the wing/centre tanks for landing, so its quickly available if needed. It transfers forward about 25000ft, if it hasn't auto-transferred as total fuel reduces, not 75min.

Boeing seem to have caught on with the 747-800, but they cant be used yet.

"Boeing seems to have caught up"? Er, hasn't the 747-400 has stab fuel tanks for years?

While an aficionado of the 'bus, I have wondered about the cost efficiency of the trim tank on the 330/340. AFAIR the MEL penalty for trim tank inop is 1.5%, I could look it up but it is nearly lunch time. In my experience with this MEL, we get the 1.5% but seem not to use it. I wonder if the cost/benefit analysis is there, especially in the 1-8 hour regime which is where we operate - I suspect not.
I guess the tanker guys will take the full 109 tonnes (ish) and be grateful for the extra 5 tonnes in the tail, or at least the F35 guys will be, on some 5000 mile deployment.

I normally get the economy passengers to move around during the flight to optimise my CofG. This reduces the fuel burn AND stops them getting DVT: win-win!

I have read that Airbus have given up on the trim tank on the 350. They can make it lighter and less complimicated, and are going to contol C of G by the eay fuel is stored in main tanks somehow.

Think 145 is about right.

The 350 will not be havin a tail tank at service entry they say, their data shows overall, not worth it at this time, but I guess it will always remain an option.

However research for Back Up Control's has looked at fuel location/trim along with all other various options in the future.

I will not be surprised if we start seeing new aircrafts with tail type fuel storage.

Don't forget the A330 came after the A340. The fuel savings using cofg control were probably calculated on the longhaul range of the A340. After all the design and developement work had been done it was cost effective to install the same system in the A330 for the sake of common systems benefit and all the regulatory approvals that go with it.

For the OP.

Not all A330s have a center tank so transfer is to/from the wings. Fuel is pumped/fed to the main tanks towards the end of the flight for many reasons. 1. You may need it! 2. You don't want the a/c sitting on it's ar5e on the roll out Hoskins.

B747-400 has a stab tank. A300 has a fin tank as does the VC-10 (I think). Of course the most iconic one is the Bent Nail's tail tank. Some are designed to use aft tanks purely for storage, others for trim.

A300-600R had stab, and not fin, tanks.


From Airbus' website -

The A300-600’s excellent range was further increased with the A300-600R variant, which incorporates an additional trim fuel tank in the horizontal tail.

Airbus guaranteed that the fuel could not be trapped in the stab tank of the A300-600R or A310-200 series. It had the same logic as the A340 in controlling it, and should have drained by gravity to the centre tank if there was a system failure. There were however documented failures where fuel was trapped in the stab. This results in aft CG landings and an emergency fuel state.

Quite simple really!! Boeing designed the 777 efficiently from the drawing board and didn't need to squeeze anymore performance out of it by moving the C OF G aft in cruise.

The A350 will use trailing edge Flaps to move the C OF P in flight to improve performance.

This is from the A350 pilots guide.

Flap Deployment in Cruise Function A small flap deployment in cruise function enables:

Wing camber control

Differential flap setting loads and drag control

Lateral trim. The function is active when the flaps lever is in position 0.

Not sure why the 777 would need a stab tank. It certainly doesnt need extra gas, it goes forever on what fits in the wings!

The Airbus A340-600 (the long one) had problems with gravity transfer from the tail (trim) tank under certain circumstances. As I recall, the two issues were the sharp corners of the trim tank outlet causing cavitation and the fuel in the pipe between the wing and tail degassing when the internal pressure went from sea level (valves at each end shut) to cruise altitude (valves open).

A full-size rig was built at Filton which replicated the connecting pipe in transparent material. The rig could be adjusted for angle and internal 'altitude'.

The waves that went down the length of the pipe would have been great fun for an inch high surfer who didn't mind JetA1!

juliet - with wing fuel it's an awesome trans Atlantic bird (8-9 hr). With center tank fuel it's an awesome 15 hr(+/-) bird.

After little bit extra research I came across the possibility that one of the reasons the 777 doesn't have a Stab Tank is that it's thrust line is very low.

In other words think of it as though CofG is the pole in a game of swing ball and the thrust line, the direction of the tennis ball. Yes it's bold statement, so could anyone confirm this or is it utter cr@p?

Almost every, if not all, high bypass wing mounted engines result in a low center of thrust.

The 777 (all variants) have no need for a stab/trim tanks for the same reason they don't have poofy turned up wingtips.

It was designed properly from the outset as a low drag design.

Any other tweaking is not worth the extra structure and design.

Maui;)