747-400 Horizontal Stab. Tank

[blue_side_up]

Did a quick search, couldn't find a link. So, anyone know the answer or can direct me to one much appreciated. If not...

A tech question I've heard CX like to ask regarding the 747-400 Horizontal Stabilizer Tank:

Why does the 747-400 (Pax, and converted pax -400BCF) have a HST, yet the 747-400F and -400ERF do not? Any ideas?

[TopBunk]

BSU

Try the www.boeing.com website

AFAIK, it would appear that Pax aircraft are designed for range. Pax+bags +limited cargo results in a lower ZFW, so with a given Max TOW, the fuel and range is greater
.

The converted aircraft (pax->cargo) have the plmbing already in place for the extra fuel tanks.

Cargo aircraft, however, are all about maximising payload at the expecnse of range, hence they don't need the range that the extra tanks give them - they much prefer payload.

Some numbers from the Boeing site, suggest the originally designed freighters have a range of 4-5000nm vs pax variants of 7-8000nm. Specifically, the 747-400ER and 747-400ERF give ranges of 7670 vs 4970nm for the same max TOW. The -ERF can load about 6500kgs less fuel. Ths translates into much more payload, probably about 30 tonnes or so.

HTH

[jettison valve]

blue_side_up,

May I interject that not all pax B744 have an HST installed...! :=
It seems as if the HST was an option, at least on the passenger airplanes. :confused:

Regards,
J.V.

[skiesfull]

The HST when full, only extends range by approximately 45 mins in cruise i.e. increases range by up to 450 nms. Although the freighter is not fitted with the HST, I am not aware that the PAX version was available without it. Perhaps a Cathay pilot with 747-400BCF experience can verify whether or not the HST is usable on this version? It is my guess that range for the freighter is not as important as trim and the possible restrictive effect that a full HST may have on loading.

[gas path]

May I interject that not all pax B744 have an HST installed...!


Quite so! BA were daft enough to order some 744 (Lites) sans the HST:rolleyes:

[744rules]

It is my guess that range for the freighter is not as important as trim and the possible restrictive effect that a full HST may have on loading.
Note that the balance effect of a full HST on a pax a/c is almost zero.
On the other hand, a 747F with high fuel loads (+ 120T) needs to be loaded almost at the aft ZFW limit to allow fwd TOW limit to be within trim limits.
Tipping might be a problem during the turnround of a 747F with fuel in the HST (if one installed)

[jtr]

The HST when full, only extends range by approximately 45 mins in cruise i.e. increases range by up to 450 nms.

You sure about those numbers?

[skiesfull]

I did say approximately! What figures can you come up with?

[52049er]

IIRC BA also had some -200's with HST fitted but not available/used.

[jtr]

Well firstly 45min = 450nm is not going to happen, lets pick .85 as a typical CRZ m no. which is around 490kts, or at your given 45min about 370nm. UNLESS you cruize around at 10nm/min, in which case your numbers are right.

The other point that I am NOT SURE about is the 45min itself. Your mob may have a different fit, but from memory the -400's I have flown have about 5.2t capacity, which is not 45min worth of fuel.



<Edited re. Spanners post - Thx - looks like memory doesn't serve after all. I must have been thinking of the 5.2 ratio of HST to CWT... or maybe just too long on the crack pipe last night>

[Spanner Turner]

Your mob may have a different fit, but from memory the -400's I have flown have about 5.2t capacity, which is not 45min worth of fuel.

For reference to aid in your discussion.
Horizontal Stabiliser Tank capacity of the 747-400 is as follows.
(SG approx .802)

Gallons:- 3,300
LBS:- 22,110
Litres:- 12,492
Kilograms:- 10,029

:ok:

[Spanner Turner]

<Edited re. Spanners post - Thx - looks like memory doesn't serve after all. I must have been thinking of the 5.2 ratio of HST to CWT... or maybe just too long on the crack pipe last night>
Jtr, maybe you haven't been on the pipe long enough! I find the correct info eventually comes to mind. You are on the ball with the 5.2 ratio. :D
However at the risk of being a complete pedant, the ratio is the other way around. It's 5.2 tons in the CWT to every ton in the HST whilst fuelling!
I'll go and get my pipe out now. :ok: :ok:

[SMOC]

IIRC BA also had some -200's with HST fitted but not available/used.

Just to add, every 747 including the classics, after line number 703 has the so called -400 stab, except the classics like the -400Fs have no plumbing for fuel and blank plates fitted where required, they also have the so called -400 wing to body fairings.

[skiesfull]

jtr
At max. t/o weight, achievable cruise level is FL310 (east) FL320(west). At these levels and at crz power, the fuel flow is in the order of 3,300kgs per hour/ per engine i.e 13,200 kgs/hr-which is where my approx 45mins comes from. It is not beyond the realms of possibility that at M0.86, the g/s with a good wind "up the chuff" could be 600kts-hence the 450nms. But if you prefer to consider the extra 10,000kgs from the HST to be usable at the end of the cruise, say FL400 and M0.845 when the fuel flow would be 8,000kgs/hr, then that 'extra' 10,000kgs gives about 1 hour of fuel depending on headwinds/tailwinds etc. -which is why I used approximately in my earlier posting.
However, the question originally asked why 747-400F did not use a stab tank (whether fitted or not) and I stick to my original guess, that loading 10 tonnes of fuel in the stab. may make trimming the a/c impossible without restricting the load and it's distribution.
By the way, a recent Boeing modified software programme prevents HST fuel from being used until total fuel reaches about 136,000kgs, when an eicas prompt is displayed to switch on the stab pumps.
Hope this clarifies the situation!

[jtr]

Well bless your little cotton socks skiesfull, looks like you do get 10 miles a minute after all. Hope that 100kt tailwind follows you around the globe?

Just for future ref. <and this is entirely my opinion only, so you might want to check> when discussing fuel burns in a general sense, I think the majority tend to talk still air TAS figures.

[skiesfull]

jtr
(1) Thanks but they're not pure cotton
(2) always - I get home quicker that way
(3) will try to remember that in future - whilest on the subject have just found a book describing the differences for the 400, which includes "a new stabiliser tank holding 3,300 US gallons, which should give 350nms extra range". I guess my definition of approximate may be a little bit loose, but then aircraft manufacturers ranges are notoriously approximate themselves!
(4) I stick by my answer to the original question - it's trim/load versus range for the freighter and good luck for the interview.

[blue_side_up]

Skiesfull, topbunk, and others,

Many thanks for all your quick and thoughtful responses to my question. It makes sense to me now, the idea of range vs payload on the pax vs freighter a/c. I hope that's the same answer Cathay are thinking!

Cheers!