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Old 30th Jan 2007, 02:43   #1 (permalink)
 
Join Date: Aug 2002
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747 Fuel Question

Why does the 747 pump fuel out of the horizontal stab trim tank 30 mins after takeoff?

Thanks,

Bigbob.
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Old 31st Jan 2007, 07:15   #2 (permalink)
 
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bigbob

The prime reason stabiliser fuel is burned fairly early in long range flight - before most of the centre wing tank fuel or virtually any of the wing tanks fuel is because of concerns about the aircraft CG following a failure of the stabiliser tank transfer/jettison pumps and a consequent inability to use any of the (now trapped) stabiliser fuel. Secondary reasons would include fuel temperature concerns and aircraft cruising efficiency.

On a fully fuelled B747-436, (in itself a very rare event) the CWT holds 52,167 kg, and the stabiliser tank holds 10,030 kg. Somewhere around 90 minutes after take-off, when the CWT fuel has decreased to 36,470 kg, the Fuel Management System Cards will automatically open the appropriate valves and activate the transfer/jettison pumps and transfer all the stabiliser fuel from the stabiliser tank into the CWT.

All the stabiliser tank fuel should be gone around two and a half to three hours into a 12+ hour flight, and this is done deliberately to avoid a rather nasty potential problem.

If we left the stabiliser tank fuel to the end of our long flight, when we have no fuel in the centre tank and little fuel in the wing tanks, and at that point we discovered that both the transfer/jettison pumps had failed leaving fuel trapped in the stabiliser tank, then, with little or no fuel in the centre or wing tanks, the CG would be far too far aft for the aircraft to land safely, and there would be very little we could do about it.

So, to guard against this problem, if there is to be a failure of both transfer/jettison pumps, we need to know about it early in the flight, so that we can land whilst we still have sufficient fuel in the centre and wing tanks to keep the CG forward of the aft limit for landing.

This scenario is generally covered on conversion courses, and new B744 pilots are often surprised at how awkward a problem this is. Very broadly, you will have to land within 6-7 hours, not the 12 -15 hours you'd planned!

Regards

Bellerophon
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Old 31st Jan 2007, 14:25   #3 (permalink)
 
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Very interesting - in a way not unlike the ETOPS scenario.

Wonder what are the operating statistics for the stab tank systems? Since it's a dual-pump system, what is the effect of one failing?

From a cruise optimization standpoint, you'd probably like to maintain an aft CG (min trim drag) as long as possible. Does the fuel transfer routine take you off this ideal profile?
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Old 1st Feb 2007, 05:31   #4 (permalink)
 
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The 744 tail tank is just somewhere to stuff extra fuel. It is not designed to be a trim tank as in the A330/340.

It's one tank, so if one pump fails the fuel will still transfer via the one operating pump. I don't ever recall hearing about a dual pump failure. It's usually the last tank to get filled on a high fuel load, so it only ever gets used on very long flights which reduces the incidences of this happening.

Certainly, the FUEL STAB TFR EICAS message will get your attention. But the C of G implications of such a failure mean you have to land with a minimum of 15.9 tonnnes in the inboard wing tanks and as this message will occur fairly early on in flight, you will have plenty of time to think about it.
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Old 1st Feb 2007, 07:36   #5 (permalink)
 
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Dan, shouldn't you be concentrating on your 'Airboos' fuel, rather than on your long lost love .... the 747?
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Old 1st Feb 2007, 11:20   #6 (permalink)
 
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Hi Mr B,

Yeah, you're right. The 744 was 3 types ago - it's amazing what you can recall when you can even find the house keys you put down 5 minutes ago. Old age!

Actually, I didn't love it that much. It flew too far and too long - thus accelerating the ageing process. The real love of my life had 4 Conways and a T-tail.

Last edited by Dan Winterland; 1st Feb 2007 at 11:40.
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Old 1st Feb 2007, 11:51   #7 (permalink)
 
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Quote:
Originally Posted by Dan Winterland View Post
The 744 tail tank is just somewhere to stuff extra fuel. It is not designed to be a trim tank as in the A330/340...
That's like saying "we just put the windshield on the plane to meet cert requirements - the pilots don't really need to see outside".

OF COURSE a stab tank will have CG trim implications as evidenced by the EICAS attention-getter. There must be some reason the stab tank is used only on extremely long flights; its ability to trim for optimum cruise CG seems to me to be an untapped ( ) economy measure.
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Old 1st Feb 2007, 14:16   #8 (permalink)
 
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barit1

...OF COURSE a stab tank will have CG trim implications ...

Yes, but I don't believe Dan Winterland is in any doubt about that.

What he said was:
Quote:
The 744 tail tank is just somewhere to stuff extra fuel. It is not designed to be a trim tank
a statement I completely agree with.


...There must be some reason the stab tank is used only on extremely long flights...

I thought I'd just given it, but let me try again.

The danger of getting fuel trapped in there - with the resultant AFT CG problems - means it is only used when necessary. It is only necessary with very high fuel loads. Very high fuel loads are only required on extremely long flights.


...its ability to trim for optimum cruise CG...

It can't actually trim, not as such, it can only move fuel forward.

I've flown an aircraft which used a rear fuel tank as a trim tank, but that was a completely different system to the B744, which just uses the stabiliser tank as a place to store fuel.

Whether Boeing missed something by not designing it as a trim tank is another question; however, on most of the ultra-long-range flights I've operated, the factor that has usually restricted our climb to sub-optimum cruise altitudes has not been a sub-optimum aircraft CG position, but a wretched Airbus, staggering along in pre-stall buffet, just above us.

Regards

Bellerophon
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Old 2nd Feb 2007, 01:47   #9 (permalink)
 
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OK, fair enough - I'm not quarreling; But it just seems unlike Boeing to overlook such an obvious cruise specific range optimizer, when the basic hardware (i.e. stab tank) is there.

But given the pump concerns, the ops routine (w/EICAS alert) is obviously correct.

Thanks for expanding on this.
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Old 8th Feb 2007, 10:18   #10 (permalink)
 
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Dan W,
Let the drooling commence!
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Old 8th Feb 2007, 13:52   #11 (permalink)
 
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Yes, I agree - a very nice piccie. But there was more than one type that had 4 Conways and a T-tail. Myself and Mr B flew them both!

And interestingly, the VC10 in your picture (K MK3 / Super VC10) had a tailfin tank which held 5.5 tonnes of fuel. It too was a place to stuff extra fuel rather than a trim tank. Although later in the VC10s life, fuel management was changed to adjust the C of G. But this was to conserve fatigue life rather than to save a bit of trim darg.
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Old 8th Feb 2007, 15:54   #12 (permalink)
 
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But there was more than one type that had 4 Conways and a T-tail


Did it win and start with a V?
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Old 8th Feb 2007, 16:03   #13 (permalink)

 
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Quote:
But there was more than one type that had 4 Conways and a T-tail
For the Non-Limeys who are still wondering.



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Old 8th Feb 2007, 19:04   #14 (permalink)
 
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Quote:
And interestingly, the VC10 in your picture (K MK3 / Super VC10) had a tailfin tank which held 5.5 tonnes of fuel. It too was a place to stuff extra fuel rather than a trim tank. Although later in the VC10s life, fuel management was changed to adjust the C of G. But this was to conserve fatigue life rather than to save a bit of trim darg.
Indeed, and if you've ever climbed through the small panel at the bottom of the fin and wriggled all the way up to the top (as I have done many times), you would have appreciated being at the sharp end even more. Quite tight in there.

Regards!

VC10 Rib22
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Old 9th Feb 2007, 02:09   #15 (permalink)

 
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Quote:
It flew too far and too long - thus accelerating the ageing process
So Einsteins theory that speed slows down time is bollocks then Dan Never mind, we understand what you are saying.
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