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jetmech
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Airbus And Boeing Trim System Differences.

Fri Sep 29, 2006 12:37 pm

Most Airbus (A330/A340) aircraft have additional fuel storage in the horizontal stabiliser that can be used to actively trim the aircraft. The Boeing 744 can also carry additional fuel in the stabiliser, but this cannot be used to actively trim the aircraft.

My question is, does the pilot of the 744 have to manually trim the aircraft when fuel is moved out of the stabiliser tank? When you refuel a 744 to high fuel loads, you must be careful to maintain a 5.2:1 ratio of fuel weight between the Centre Wing Tank (CWT) and Horizontal Stabilizer Tank (HST). That is, for every 5.2 kg of fuel in the CWT, there must not be more than 1 kg of fuel in the HST. This prevents the aircraft tipping up, and also maintains the C of G within limits. I dunno what the ratio is with Airbus as the fuelling is automatic.

IIRC, when fuel is consumed from the CWT and HST of the 744, this ratio is not maintained, I seem to remember that fuel is used down to a certain level in the CWT, then all the fuel is transferred from the HST. This indicates to me that there is a trim change.

I think that Airbus aircraft automatically adjust the incidence of the stabiliser when fuel is transferred out of the HST, but does this occur with the Boeing?

I apologise if this has been asked before, but a search did not yield any similar threads. I though it was about time to get some juicy, in-depth discussion going in Tech/Ops   !

[Edited 2006-09-29 05:38:10]

[Edited 2006-09-29 05:38:41]
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jetmech
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RE: Airbus And Boeing Trim System Differences.

Sun Oct 01, 2006 1:12 pm

Erm, I guess my original question was not too clear and written in a confusing manner, so I will simplify it.

During flight operations of a B744 where there is fuel in the horizontal stabiliser tank (HST), does the pilot need to manually adjust the longitudinal trim (tailplane incidence) when fuel is transferred out of the HST? I guess B744 pilots would probably be the best ones to answer this.

[Edited 2006-10-01 06:22:59]
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Shenzhen
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RE: Airbus And Boeing Trim System Differences.

Mon Oct 02, 2006 6:42 am

I'll take a stab... Since the fuel is transfered to the center tank, once x amount has been used out of the center, one would think that at this point in time the airplane is being flown on autopilot, therefore it (trim) should be automatic.

Horizontal should trim to keep the elevators neutral.


Cheers
 
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jetmech
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RE: Airbus And Boeing Trim System Differences.

Mon Oct 02, 2006 1:52 pm

Cheers Shenzhen  bigthumbsup , thanks for the answer, it certainly sounds good! In fact, your answer leads me to my real question, which I wrote in the thread starter.

IIRC, Boeing has stated that the fuel in the HST of the 744 cannot be used to actively trim the aircraft, whereas with the Airbus it can. But your answer, which is very likely to be correct seems to indicate that the B744 can automatically adjust for the trim change resulting from the forward transfer of fuel from the HST.

Come to think of it Shenzhen, you may have provided the answer. I guess what Boeing means is that the B744 can automatically maintain a preset C of G location, whereas the Airbus can automatically and actively adjust the C of G for the optimum balance between stability and fuel efficiency.
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