3MilesToWRO
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Shifting Load In Flight To Trim The Aircraft?

Tue Mar 30, 2010 8:59 pm

I was wondering recently - has there ever been a case when the crew tried to move the load (or maybe more likely - relocate passengers) to save the aircraft losing its trim in flight? Would it for example help with phugoid oscillations of flight 232?
On one hand you don't have people secured in their seats but possibly packed in front or end of cabin. It's of course unsafe. But on the other hand you might have more stable aircraft...
 
AutothrustBlue
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RE: Shifting Load In Flight To Trim The Aircraft?

Tue Mar 30, 2010 9:45 pm

I believe airliners have the ability to move fuel back and forth to maintain a CoG within safe limits (via gravity or pumps). Seems easier than relocating passengers.
 
lowrider
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RE: Shifting Load In Flight To Trim The Aircraft?

Tue Mar 30, 2010 10:03 pm

When weight and balance calculations are done, they ensure that the aircraft will be within an acceptable CG range throughtout the entire flight. I have not heard of any aircraft that, under normal operations, would have to move people or freight around just to stay with the acceptable envelope.

Quoting AutothrustBlue (Reply 1):
I believe airliners have the ability to move fuel back and forth to maintain a CoG within safe limits

I believe this was done on the Concorde, and it currently practiced on other types. Loss of this ability may lead to an emergency situation.
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JER757
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RE: Shifting Load In Flight To Trim The Aircraft?

Tue Mar 30, 2010 10:41 pm

Quoting lowrider (Reply 2):
I believe this was done on the Concorde, and it currently practiced on other types. Loss of this ability may lead to an emergency situation.

Really? Do you know what other types?

I know it was definitely done on Concorde, the wing pretty much stretched the entire rear 2/3 of the fuselage, giving a large range of possible fore/aft fuel movement. However on aircraft with a 'normal' wing, there is nowhere near the amount of fore/aft difference to play around with.

In a normal airliner the wing is attached near the centre of the fuselage (near the aircraft's centre of gravity (CoG)) where even large amounts of fore/aft weight transfer have minimum effect. This is again in contrast to Concorde where, as the wing stretched to very near the back (further away from the CoG), any movement of fuel would have a greater effect on trim. Remember that as you move a weight further away from the CoG (pivot), the greater the effect of the turning force (moment), and the greater effect on trim.

The trim limits of an aircraft on a commercial flight are much less than the 'safety critical' points. i.e. you can be pretty close to the edge of the box on the loadsheet, yet the aircraft is still well within its safety limits. I would therefore think that moving fuel fore and aft on a commercial airliner is a pretty pointless business, for trim reasons anyway.

However, as always, I may be completely wrong and am willing to be corrected!
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KELPkid
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RE: Shifting Load In Flight To Trim The Aircraft?

Tue Mar 30, 2010 10:43 pm

Quoting AutothrustBlue (Reply 1):

I believe airliners have the ability to move fuel back and forth to maintain a CoG within safe limits (via gravity or pumps). Seems easier than relocating passengers.

   As I recall, the MD-11 has tail tanks specifically for this purpose....
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Fly2HMO
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RE: Shifting Load In Flight To Trim The Aircraft?

Tue Mar 30, 2010 11:23 pm

Quoting AutothrustBlue (Reply 1):
I believe airliners have the ability to move fuel back and forth to maintain a CoG within safe limits (via gravity or pumps). Seems easier than relocating passengers.

Some. Not all. The few that do are almost always wide bodies, and a notable exception* as already mentioned, Concorde.

You can't actually transfer fuel in most narrow bodies, and even if you could, narrow bodies are not large enough to need a special trim tank*. The only real control you usually have in such planes is that you can select which tanks the engines will get fuel from.

I'd like to hear about more exceptions to the rule though.   

[Edited 2010-03-30 16:26:28]
 
avt007
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RE: Shifting Load In Flight To Trim The Aircraft?

Tue Mar 30, 2010 11:42 pm

There was a case where an F28 had a stab trim runaway in flight. The full nose up trim caused uncontrolled climbing, and several high speed stalls. The crew moved all of the pax as far forward as possible to help control the aircraft. Luckily it was not a full flight. They did manage an emergency landing in Portlan (PDX).
 
lowrider
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RE: Shifting Load In Flight To Trim The Aircraft?

Tue Mar 30, 2010 11:59 pm

Quoting JER757 (Reply 3):
Do you know what other types?

Aside from the MD11, I have been told that the A340-600 has this ability as well. I have only heard it second hand, so don't hold me to it. On the 747-400s that have stab tanks, the transfer of fuel from these tanks is delayed as long as practical to maintain the most advantageous CG for cruise. Those, however, only transfer 1 way. You cannot put fuel back into the stab tanks.
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tdscanuck
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 12:13 am

Quoting 3MilesToWRO (Thread starter):
Would it for example help with phugoid oscillations of flight 232?

It might have helped, but I don't think there's any way the crew could have known that.

Quoting AutothrustBlue (Reply 1):
I believe airliners have the ability to move fuel back and forth to maintain a CoG within safe limits (via gravity or pumps).

It should never be possible to put the CG outside safe limits using fuel transfer on a modern jet...that's just a failure mode waiting to happen. However, some Airbus and MD designs use this to improve cruise drag (they move the CG rather than incurr trim drag from the stabilizer). No legacy-Boeing airplane moves fuel for CG management in normal service, although (as mentioned previously) the 747-400 used to have a tail tank that was one-way transfer only.

Quoting JER757 (Reply 3):
I would therefore think that moving fuel fore and aft on a commercial airliner is a pretty pointless business, for trim reasons anyway.

On a conventionally configured jet, it's not necessary for trim. It may have economic advantages.

Tom.
 
AutothrustBlue
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 3:23 am

Quoting fly2hmo (Reply 5):
Some. Not all. The few that do are almost always wide bodies, and a notable exception* as already mentioned, Concorde.

I stand corrected.

Quoting tdscanuck (Reply 8):
However, some Airbus and MD designs use this to improve cruise drag
Quoting lowrider (Reply 7):
Aside from the MD11, I have been told that the A340-600 has this ability as well.

From what I read, most Airbii have this except the A32S and the ancient version of the A300 (A300B2 ?)
 
Tristarsteve
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 7:25 am

Quoting lowrider (Reply 7):
On the 747-400s that have stab tanks, the transfer of fuel from these tanks is delayed as long as practical to maintain the most advantageous CG for cruise.

Nearly all pax B744 have stab tanks. After flaps up, as soon as there is room in the centre tank fuel is transferred from the stab to the centre. At fuel loads below about 130 tons (about 11 hr flight), the stab tank is not used at all. It is not a trim tank.

On the A330/340 the stab trim tank is used at fuel loads over about 38 tons. Also during the climb more fuel is transferred into the trim tank. Fuel is only transferred back to the centre or wing tanks when CG calculations require it. Sometimes fuel will stay in the stab tank until the descent. The fuel will transfer fwd in small doses, and occasionally transfer aft again. It is all automatic.
 
744rules
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 8:24 am

Quoting Tristarsteve (Reply 10):
Quoting Tristarsteve (Reply 10):
Quoting lowrider (Reply 7):
On the 747-400s that have stab tanks, the transfer of fuel from these tanks is delayed as long as practical to maintain the most advantageous CG for cruise.

Nearly all pax B744 have stab tanks. After flaps up, as soon as there is room in the centre tank fuel is transferred from the stab to the centre. At fuel loads below about 130 tons (about 11 hr flight), the stab tank is not used at all. It is not a trim tank.

I read that the stab tanks are used as soon as possible. This is to prevent that when something goes wrong with the fuel transfer, the aircraft doesn't get out of trim. In case the a/c has problems transferring the fuel, the crew normally still has time enough to look for an alternate before the situation is critical. FYI, fuel load in 747-400 stab tank can be up to 10ton.
The -400 Freighters have no tail tank installed.
 
vc10
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 8:45 am

Going all the way back to the 1960s the Super VC-10 carried fuel in the fin and during flight it would be drained to a specfic figure for that flight so as to minimise drag in the cruise. prior to landing it would be drained to empty.

On Concorde the wing tanks played only a small part in shift the C of G. The tanks that had the greatest affect were the 3 fuselage tanks , with 9 & 10 being at the forward end and tank 11 at the rear.

littlevc10
 
3MilesToWRO
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 10:48 am

Quoting avt007 (Reply 6):
There was a case where an F28 had a stab trim runaway in flight. The full nose up trim caused uncontrolled climbing, and several high speed stalls. The crew moved all of the pax as far forward as possible to help control the aircraft.

Oh, so I'm not first with this concept   When did this happen? Any closer details?

Quoting lowrider (Reply 2):
I have not heard of any aircraft that, under normal operations, would have to move people

Oh, of course not under normal circumstances. I think about really hard time.

Quoting tdscanuck (Reply 8):
Quoting 3MilesToWRO (Thread starter):
Would it for example help with phugoid oscillations of flight 232?

It might have helped, but I don't think there's any way the crew could have known that.

Well, 232 was rather occupied, IIRC, so not much room for shifting unless people would be standing off the seats, so the example is even more theorethical. But I think there was quite a lot of time to simply try, provided someone had an idea.
Of course this does not mean that I consider not trying it a mistake.

Quoting vc10 (Reply 12):
Going all the way back to the 1960s the Super VC-10 carried fuel in the fin

Il-62M has this feature too.
 
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jetmech
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 11:37 am

Quoting AutothrustBlue (Reply 1):
I believe airliners have the ability to move fuel back and forth to maintain a CoG within safe limits
Quoting JER757 (Reply 3):
Really? Do you know what other types?
Quoting AutothrustBlue (Reply 9):
From what I read, most Airbii have this except the A32S and the ancient version of the A300 (A300B2 ?)

Airbus A330 / A340 use stab tank fuel to actively trim the aircraft.

Quoting lowrider (Reply 7):
On the 747-400s that have stab tanks, the transfer of fuel from these tanks is delayed as long as practical to maintain the most advantageous CG for cruise.
Quoting 744rules (Reply 11):
I read that the stab tanks are used as soon as possible. This is to prevent that when something goes wrong with the fuel transfer, the aircraft doesn't get out of trim. In case the a/c has problems transferring the fuel, the crew normally still has time enough to look for an alternate before the situation is critical.

I'm pretty sure 744 stab tank fuel is used up fairly early in the flight, and for the reasons given by 744rules. If a 744 is filled to capacity, fuel from the CWT is used first. When the CWT gets down to a certain level, the stab tank fuel is transferred forward to the CWT for use.

Once stab tank and CWT fuel is consumed, wing tank fuel will be used . The B744 does not use stab tank fuel to actively trim the aircraft. The stab tank gives additional capacity only.

Quoting tdscanuck (Reply 8):
No legacy-Boeing airplane moves fuel for CG management in normal service

Regards, JetMech
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tdscanuck
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 7:12 pm

Quoting Tristarsteve (Reply 10):

Nearly all pax B744 have stab tanks

True, but nearly all of them were deactivated in the wake of SFAR88 (the regulation that followed the TWA800 crash). They're still there, but rarely used.

Tom.
 
lowrider
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 7:59 pm

Quoting jetmech (Reply 14):
I'm pretty sure 744 stab tank fuel is used up fairly early in the flight, and for the reasons given by 744rules. If a 744 is filled to capacity, fuel from the CWT is used first. When the CWT gets down to a certain level, the stab tank fuel is transferred forward to the CWT for use.

If I remember correctly, the stab tanks are the last to be filled. And while you do burn the center tank first, the actual transfer of the stab tanks doesn't occur until the center tank is down to approx 30,000 lbs. Since you start with about 115,000 lbs in the center tank, you will be about 5 hours into the flight when the transfer occurs. Again, it is delayed as long as practical as opposed to as soon as volume permits.
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Tristarsteve
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RE: Shifting Load In Flight To Trim The Aircraft?

Wed Mar 31, 2010 9:31 pm

Quoting lowrider (Reply 16):
the actual transfer of the stab tanks doesn't occur until the center tank is down to approx 30,000 lbs.

Not quite.
At full tanks the Centre tank holds 51 tonnes of fuel, and is used first (except when the flaps are not up during take off when tanks 2 and 3 supply as well).
There are two point sensors in the centre tank that start stab transfer at 36 tonnes in the centre.
To get 36 tonnes in the centre you are looking at a fuel load of over 154 tonnes. There are not a lot of B744 flights that take that amount of fuel, its over 13 hours flying. With a fuel load of less than this, transfer will start in the air with flaps up.
 
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jetmech
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RE: Shifting Load In Flight To Trim The Aircraft?

Thu Apr 01, 2010 10:59 am

Quoting lowrider (Reply 16):
If I remember correctly, the stab tanks are the last to be filled.

It's been a while, but I seem to remember that beyond a certain fuel load in the CWT, one is meant to simultaneously fill the CWT and HST to the ratio of 5.2 to 1. Granted, the HST does fill fairly slowly.


Quoting Tristarsteve (Reply 17):
except when the flaps are not up during take off when tanks 2 and 3 supply as well

Interesting. According to the following diagram, Mains 2 and Mains 3 have override / jettison pumps as well as normal boost pumps. I always thought they only had override / jettison pumps. Do the override / jettison pumps in Mains 2 and 3 switch over to normal boost pumps when the flaps are fully retracted?

http://i94.photobucket.com/albums/l118/Jet-Mech/747fuel.jpg

Regards, JetMech
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Tristarsteve
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RE: Shifting Load In Flight To Trim The Aircraft?

Thu Apr 01, 2010 12:40 pm

Can't access yr picture at work.
But on a fully fuelled B744.
On departure all switches on the panel are latched in.
There are 10 pumps running, 2 in each wing tank and 2 in the centre tank.
The M2/3 O/J pumps are not running, neither are the stab tank pumps. These are controlled by the FSMC.
When you select flaps to take off, the 2 and 3 Xfeed valves close. C/T supplies 1 and 4. 2 and 3 supply their own engine.
When, after take off, the flaps are UP, the 2 and 3 Xfeed open and the Centre tank supplies all engines.
At 36T in the centre, stab transfer now starts. The pumps were already running.
When the stab tank is empty, the crew turn off these pumps (EICAS msg tells them to)
When the Centre tank pumps show low pressure, the M2 and M3 O/J pumps are started by the computor.
The crew turn off the Centre tank O/J pumps when commanded by EICAS.
When there is 18T in M2/M3 transfer starts from the 2R/3R tanks.
When total fuel is 56T, the crew get a Fuel Config EICAS message.
They now turn off the M2 and M3 O/J pumps, and close the 1 and 4 Xfeed valves.
Then it is tank to engine for rest of the flight.

Quoting jetmech (Reply 18):
Do the override / jettison pumps in Mains 2 and 3 switch over to normal boost pumps when the flaps are fully retracted?

No, the normal pumps run all the time, but only output when there is no high pressure O/J pump supplying.
The M2/3 O/J pumps are commanded on by the FSMC, and tyrned off by the crew.

It must be the most complicated fuel system on an aircraft without a flight engineer!
 
avt007
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RE: Shifting Load In Flight To Trim The Aircraft?

Fri Apr 02, 2010 12:14 am

Quoting 3MilesToWRO (Reply 13):
Quoting avt007 (Reply 6):
There was a case where an F28 had a stab trim runaway in flight. The full nose up trim caused uncontrolled climbing, and several high speed stalls. The crew moved all of the pax as far forward as possible to help control the aircraft.

Oh, so I'm not first with this concept When did this happen? Any closer details?

I've tried to find details online, but can't. Somewhere I have a video of the crew being interviewed. Basically, during the stab actuator (hydraulic, BTW) overhaul, due to a misunderstanding with the manual, a small piece was overtorqued, which caused it to later fail in flight. This failure mode caused the stab to go full nose up, and the standby trim system couldn't move it.
Thje AP kicked off,and the a/c climbed uncontrollably. The crew tried power settings, etc to control it, but had a very difficult time. They declared n emergency and managed to land in Portland. It took both pilots and a flight attendant to push forward on the yokes to keep it under control.

Stab trim failures are usually killers, but they were lucky that day.
 
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jetmech
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RE: Shifting Load In Flight To Trim The Aircraft?

Fri Apr 02, 2010 1:24 am

Quoting tristarsteve (Reply 19):
When, after take off, the flaps are UP, the 2 and 3 Xfeed open and the Centre tank supplies all engines.

G'day Steve,

That's what originally raised my question. I was under the erroneous assumption that all cross feeds where open, and all pumps in all tanks were running for takeoff, with fuel being preferentially fed from the CWT due to the higher output pressures of the CWT O/J pumps. I was unaware of the M2/3 O/J pumps and the actual takeoff fuel system configuration.

If one had a full load of fuel, and the need arose to jettison early in the flight what happens? Does the fuel system need to be reconfigured for direct tank to engine feed (M1 to #1, M2 to #2 , M3 to #3, M4 to #4) by shutting the four cross-feed valves?

Quoting tristarsteve (Reply 19):
It must be the most complicated fuel system on an aircraft without a flight engineer!

Indeed! Far more complicated compared with a 767 for example. I was unaware of the manual inputs required of the flight crew to reconfigure the fuel system. I always thought the fuel system computer took care of it all. Thanks for the info!

Regards, JetMech.

[Edited 2010-04-01 18:35:13]
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jetmech
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RE: Shifting Load In Flight To Trim The Aircraft?

Fri Apr 02, 2010 4:00 am

Quoting tristarsteve (Reply 19):
Can't access yr picture at work.

The original picture can be found here. I should have linked it before.

http://www.meriweather.com/747/over/fuel-layout.html

Quoting jetmech (Reply 18):
Mains 2 and Mains 3 have override / jettison pumps as well as normal boost pumps. I always thought they only had override / jettison pumps.
Quoting tristarsteve (Reply 19):
They now turn off the M2 and M3 O/J pumps, and close the 1 and 4 Xfeed valves.

I can see the M2/3 O/J pump buttons in this picture.

http://www.meriweather.com/747/over/fuel.html

Regards, JetMech
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Fabo
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RE: Shifting Load In Flight To Trim The Aircraft?

Fri Apr 09, 2010 4:15 pm

Quoting jetmech (Reply 21):
I always thought the fuel system computer took care of it all.

IIRC, the computers put up a note on EICAS when it is time to flip some switches.
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jetmech
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RE: Shifting Load In Flight To Trim The Aircraft?

Sat Apr 10, 2010 2:39 am

Quoting Fabo (Reply 23):
IIRC, the computers put up a note on EICAS when it is time to flip some switches.

Fair enough. I just imagined that the computer would command the valves of its own accord without pilot intervention.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .

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