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737NG centre tank

Old 12th Nov 2009, 11:10
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737NG centre tank

A new thread to give Yury a break on his 767 crossfeed thread

For skerry the ref is AD2002-19-52 for the unmodified pumps.

I have not had time to dig out any other posts on the topic - it may well be we can merge the threads.

I fully understand the 'system' of the procedure leder and PM describe, but not the logic. If we are dealing with modified pumps (I assume we are), the queries I have are:-

What is 'wrong' with allowing one pump to run 'dry' while waiting for the second? There was no procedural restriction on the Classic although STRICT interpretation of the QRH (which was never resolved) might suggest some 'action'.

Regarding the 'unuseable fuel', it has been suggested that a nuisance warning during a g/a might be a problem under the 'old' procedure - surely crews would be aware of the fuel state and the centre tanks pumps would probably not be switched on until the g/a is complete anyway? In any case the original AD allowed usage of tank fuel down to dry pumps if required.

I have (thanks to the prompts from contributors) now found a Smart Cockpit paper at www.smartcockpit.com/data/.../B737-Center_Tank_Fuel_Pumps.pdf which is a little confusing since the AMOC seems to run on into X-ray inspection, the automatic shut off and MC Logic change - is it fact conditional on all those?

Personally if I am flying an a/c with 'unmodified' fuel system I would prefer to operate under the old system and handle a fuel caption after any g/a if necessary.

So, thanks to the previous posters I am now aware of the AMOC
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Old 12th Nov 2009, 11:51
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Boeing think that running one pump dry for an extended period can occur if you leave both center tank pumps on if an NG has not been modified, due to the master caution logic. So we are talking about a procedure that can be used on unmodified aircraft.

As most people know there are some fundamental differences in the way the the classic and NG fuel system works. The pumps are in other places etc. Therefore there is a difference in procedures.

You are free to use whatever procedure you are comfortable with. NGs were not exactly dropping out of the air over the years before this system was introduced and to the best of my knowledge the only problems that did occur were on the ground at high ambient temperatures in South East Asia. That saga got Boeing taking a closer look at the system and recognising that there might be a potential risk.

So far that risk appears to be lower than the documented instances of gross mismanagement of the fuel system. The BA incident fuel balancing over the alps followed by aerobatics and the Qantas 'lets see what happens when the main tanks run dry' are but two examples (both classics). I would also be interested to hear the highest imbalance anyone has heard off, obviously happened to someone else!
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Old 12th Nov 2009, 13:34
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Thanks for the references guys. Although I didn't fit into the most people category, I do now, and having read around the subject a little I think I understand what you are talking about.

I was confused because the BKK 737 was a Classic, and the ADs you refer to are specific to the NG. As Leder says, I guess Boeing took a close look at all fuel pumps after BKK, and found particular problems with the NG.

Good stuff
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Old 12th Nov 2009, 14:36
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You are free to use whatever procedure you are comfortable with.
- actually no! One is required to use your operator's procedures unless you consider them dangerous.

I'm not convinced that the Thai event was due to the c/t pumps, incidentally, due to the pump location. I believe the trigger for all this fuss was the TWA 747 'explosion'? (and there is an opening for the conspiracy theorists).

The simple answer is the auto shut-off and MC mod. Cheap and cheerful. Job done.

As an aside, it was good that Boeing changed the pick-up points for the c/t pumps with the NG so you do not get the Jurassic/Classic built-in climb imbalance.
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Old 12th Nov 2009, 15:40
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You are of course right BOAC and our OM offers alternative methods of compliance (AMOC). If you do not like the crossfeed open you can still use the original procedure.

The TWA 747 is an interesting one, who knows? There are similar Boeing systems issues and it is the only aircraft that actually exploded in the air. You are right in that they spent a lot of time looking at the fuel system. But I think the Thai and PAL incidents focussed them on the 737.

In general the NG stays much better balanced. The classic can be a nightmare and every aircraft behaves differently.
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