Last Friday I flew back from Moscow on SQ361 and we hit significant turbulence just above the bay of Bengal. Once out of the most significant turbulence, you could hear the hydraulic actuators in the wing of what sounded like the flaps retracting. FlightRadar24 shows the following speed and altitude numbers:

18:37:52 480kt 41000ft
18:38:22 486kt 40925ft
18:39:03 469kt 40700ft
18:39:28 454kt 40525ft
18:40:15 422kt 40400ft
18:40:45 406kt 40425ft
18:41:36 420kt 39650ft
18:43:07 438kt 39125ft
18:43:58 456kt 39000ft

If was quite a wild ride with lightning outside the plane and unbelievable wind noise and vibrations going through the plane (an A350). But after that, the hydraulic noise of what sounded like the flaps retracting was strange - it did not last very long and it sounded like the flaps were just extended by a little bit. Or was this sound something else?

The 350 partially extends the flaps in order to fine tune the center of lift for higher efficiency in the cruise. They don't move a lot but it is quite normal for them to do so. It is all automatic and we don't see an indication of extension in the cockpit.

The old system, as seen on the 330, was fine tuning the center of gravity by transferring fuel to and from the trim tank.

You are correct Starlionblue!
As A350 is not fitted with Trim tanks so it deploys flap to maintain the Center of Gravity within the limits. I think it is some thing like Aileron droop that happens during the flap extension and this small droop is not indicated in the displays.

Probably the sound of removing the newspapers that stopped then seeing outside and the WX radar

787 has a similar flap extension feature which has nothing to do with fuel, other than the fact that computers know where the CG is at any given moment.

skywalker92 wrote:

You are correct Starlionblue!
As A350 is not fitted with Trim tanks so it deploys flap to maintain the Center of Gravity within the limits. I think it is some thing like Aileron droop that happens during the flap extension and this small droop is not indicated in the displays.

IIRC aileron droop during flap extension can be seen if you look closely at the flight control page. However automatic flap extension during cruise is not shown.

The cruise flap extension system increases efficiency. It is not there to keep things within limits.

AngMoh wrote:

But after that, the hydraulic noise of what sounded like the flaps retracting was strange - it did not last very long and it sounded like the flaps were just extended by a little bit.

Page 147 and 174 of the following PDF talks briefly about the flap functions mentioned by Starlionblue;

http://www.smartcockpit.com/docs/a350-9 ... pilots.pdf

Regards, JetMech

skywalker92 wrote:

so it deploys flap to maintain the Center of Gravity within the limits.

How can flaps alter the CoG?

hivue wrote:

skywalker92 wrote:

so it deploys flap to maintain the Center of Gravity within the limits.

How can flaps alter the CoG?

The whole point is to optimize the relative position of CoG vs the Centre of Pressure (i.e. where Lift force is located on the wing).

A330 does that by pumping fuel back and forth to move the CoG

In A350 CoG is constant, whereas the flaps modify position of the CoP.

Isn't there also gust aleviation - what control surfaces do that?

Starlionblue wrote:

skywalker92 wrote:

You are correct Starlionblue!
As A350 is not fitted with Trim tanks so it deploys flap to maintain the Center of Gravity within the limits. I think it is some thing like Aileron droop that happens during the flap extension and this small droop is not indicated in the displays.

IIRC aileron droop during flap extension can be seen if you look closely at the flight control page. However automatic flap extension during cruise is not shown.

The cruise flap extension system increases efficiency. It is not there to keep things within limits.

If the flap extension during cruise is not to keep the CG within limits, how does it cater with the shifting CG without Trim tanks?

skywalker92 wrote:

Starlionblue wrote:

skywalker92 wrote:

You are correct Starlionblue!
As A350 is not fitted with Trim tanks so it deploys flap to maintain the Center of Gravity within the limits. I think it is some thing like Aileron droop that happens during the flap extension and this small droop is not indicated in the displays.

IIRC aileron droop during flap extension can be seen if you look closely at the flight control page. However automatic flap extension during cruise is not shown.

The cruise flap extension system increases efficiency. It is not there to keep things within limits.

If the flap extension during cruise is not to keep the CG within limits, how does it cater with the shifting CG without Trim tanks?

Maybe we're arguing semantics, but for me "keep within limits" implies that it risks going out of limits. The A350 CG will be within safe limits if left as is. The fuel is all in the wings and the center tank, nicely located close to the CoG. Fuel burn won't affect the CoG very much. The cruise flap extension logic shifts the Center of Pressure forward, closer to the CoG. This means the stabiliser must produce less downward force, and thus fuel burn is decreased. However the stabiliser easily has enough authority to keep the CoP where it needs to be for safe operation

The 330, on the other hand, has a trim tank. Here also, the trim tank is not there to keep the CoG within safe limits. The CoG will be safe as is. At 25500 feet, the system starts transferring fuel to the trim tank. This shifts the CoG backwards, closer to the CoP. The effect is the same. The stabilser must produce less downward force, and thus fuel burn is decreased. The fact that the Aft Transfer Valve is an MEL item shows that the trim tank is not essential to CoG control.

kurtverbose wrote:

Isn't there also gust aleviation - what control surfaces do that?

A330: The outer spoilers and the ailerons deflect upwards to under certain conditions.
A350: Spoilers deflect upwards and ailerons deflect upwards or downwards under certain conditions.

I was not arguing with you Starlionblue, I'm not that much genius
Thanks for the crystal clear explanation and I thought that Trim tanks are solely to cater the CG shifts and never thought about improving efficiency.

skywalker92 wrote:

I was not arguing with you Starlionblue, I'm not that much genius
Thanks for the crystal clear explanation and I thought that Trim tanks are solely to cater the CG shifts and never thought about improving efficiency.

Concorde did in fact use tank trimming to cater for center of lift shifts because of the very large speed range. According to ConcordeSST.com, the CoP could shift six feet**. Also the tanks in Concorde were widely distributed along the length. On a subsonic airliner, while the center of lift does shift in the transonic range, the magnitude is not great, and besides most of the fuel is close to the middle.

The efficiency gain from tank trimming on the 330 is about 1%. The trim tank has the additional advantage of providing extra tankage and a simple means of CoG control at take-off. If we take on more than about 37000kg, some of the fuel goes in the trim tank.

I'm not a genius. I just have access to manuals and trainers keep "encouraging" me to study by asking questions I don't know the answer to.


** http://www.concordesst.com/fuelsys.html

Starlionblue wrote:

skywalker92 wrote:

I was not arguing with you Starlionblue, I'm not that much genius
Thanks for the crystal clear explanation and I thought that Trim tanks are solely to cater the CG shifts and never thought about improving efficiency.

Concorde did in fact use tank trimming to cater for center of lift shifts because of the very large speed range. According to ConcordeSST.com, the CoP could shift six feet**. Also the tanks in Concorde were widely distributed along the length. On a subsonic airliner, while the center of lift does shift in the transonic range, the magnitude is not great, and besides most of the fuel is close to the middle.

The efficiency gain from tank trimming on the 330 is about 1%. The trim tank has the additional advantage of providing extra tankage and a simple means of CoG control at take-off. If we take on more than about 37000kg, some of the fuel goes in the trim tank.

I'm not a genius. I just have access to manuals and trainers keep "encouraging" me to study by asking questions I don't know the answer to.


** http://www.concordesst.com/fuelsys.html

So is the fact that it was clearly noticeable a result of the turbulence? For example are the flaps fully retracted during the serious turbulence to reduce loads and then reset to the optimum position after exiting turbulence or settings affected by turbulence and then reset to normal. The turbulence was serious - by far the worst I have experienced in 30 years of regular flying. Also, I have never heard the effects of the trimming on the A350 before even though the hydraulic system is very loud.

AngMoh wrote:

Starlionblue wrote:

skywalker92 wrote:

I was not arguing with you Starlionblue, I'm not that much genius
Thanks for the crystal clear explanation and I thought that Trim tanks are solely to cater the CG shifts and never thought about improving efficiency.

Concorde did in fact use tank trimming to cater for center of lift shifts because of the very large speed range. According to ConcordeSST.com, the CoP could shift six feet**. Also the tanks in Concorde were widely distributed along the length. On a subsonic airliner, while the center of lift does shift in the transonic range, the magnitude is not great, and besides most of the fuel is close to the middle.

The efficiency gain from tank trimming on the 330 is about 1%. The trim tank has the additional advantage of providing extra tankage and a simple means of CoG control at take-off. If we take on more than about 37000kg, some of the fuel goes in the trim tank.

I'm not a genius. I just have access to manuals and trainers keep "encouraging" me to study by asking questions I don't know the answer to.


** http://www.concordesst.com/fuelsys.html

So is the fact that it was clearly noticeable a result of the turbulence? For example are the flaps fully retracted during the serious turbulence to reduce loads and then reset to the optimum position after exiting turbulence or settings affected by turbulence and then reset to normal. The turbulence was serious - by far the worst I have experienced in 30 years of regular flying. Also, I have never heard the effects of the trimming on the A350 before even though the hydraulic system is very loud.

I don't know. The flaps aren't part of the load relief logic and the cruise flap system is only a couple of degrees at most in any case.

Trust Airbus to use such advanced aerodynamics.

I recall when flying the A310-300 almost thirty years ago, that if the CGCC (the computer controlling the trim tank operation) was MELed inop, then fuel burn on an Atlantic crossing increased by about 1000 lbs. I was amazed then and using flaps today for the same function amazes me now!

It was fun explaining to people that moving 5000 lbs of fuel to the tail reduces the effective weight of the aircraft. (I won a lot of beers that way!)

However .... I was always mistrustful of the system, as the "what if" in me wondered about what would happen if the fuel ever became trapped back there. And it did happen on one of our flights flying YVR-AMS. They had to land in YFB with the aft MAC close to the limit. Personally, I would have preferred a hand control to open the valve manually to let the fuel trickle back into the centre tank. Some panel under row 30 or so, open it up, turn the handle and "thunk" fuel is released.

longhauler wrote:

Trust Airbus to use such advanced aerodynamics.

I recall when flying the A310-300 almost thirty years ago, that if the CGCC (the computer controlling the trim tank operation) was MELed inop, then fuel burn on an Atlantic crossing increased by about 1000 lbs. I was amazed then and using flaps today for the same function amazes me now!

It was fun explaining to people that moving 5000 lbs of fuel to the tail reduces the effective weight of the aircraft. (I won a lot of beers that way!)

However .... I was always mistrustful of the system, as the "what if" in me wondered about what would happen if the fuel ever became trapped back there. And it did happen on one of our flights flying YVR-AMS. They had to land in YFB with the aft MAC close to the limit. Personally, I would have preferred a hand control to open the valve manually to let the fuel trickle back into the centre tank. Some panel under row 30 or so, open it up, turn the handle and "thunk" fuel is released.

I see what you mean but the worst case scenario in the 330 is trim tank forward transfer pump failing, as well as trim tank non return valve failing. (If the forward transfer pump fails, forward transfer is by gravity through the valve). Even if that happens, and fuel distribution is also worst case, maximum flight time from that point is four hours of flight remaining. EDTO/ETOPS is more limiting than four hours so you should always have a designated alternate to divert to.

I did my initial A310 course in Toulouse, as we had just bought 12 new A310s and I got to fly a new one home.

I had the pleasure of training with the very pilots that did the initial certification of the aircraft. (A310s as well as the new A300-600). It was very interesting to talk with them between training. When I brought up the "fuel trapped in the trim tank" scenario, everyone's answer was ... "It will never happen". (say it in a thick French accent ... it will nevair appen).

Six months later, on my first reccurrent simulator, I told them about the YFB incident. Their answer .... "Yes, we heard about that, but It will never happen ... again".

You are right about the ETOPS/EDTO points, but think about this ... on the YVR-AMS flight, the first ETOPS diversion alternate wasn't until 7 1/2 hours into the flight, (it was KEF), as the first 6 hours of the flight was over land. They were lucky that YFB was available as they wouldn't have made it to KEF. They landed with about 40% MAC, and very little pitch authority!

I'd have to dig into my old manuals, but if I recall the caution was at 39% and the warning was at 41%. I remember talking with the Captain of that flight, He said his last trick in his toolbox was to move passengers to sit on the floor in Business Class until the MAC moved forward enough to land. (Yikes!)

Hi Longhauler, that’s an interesting incident.

Across the Atlantic there should be an airport available within 120min from pretty much anywhere - how long did the A/C fly with fuel stuck in the TT and how problematic has it become?

Is there any report available for this incident?

thepinkmachine wrote:

Across the Atlantic there should be an airport available within 120min from pretty much anywhere - how long did the A/C fly with fuel stuck in the TT and how problematic has it become?

Is there any report available for this incident?

That is a bit of a "gotcha" with respect to long, domestic flights. In theory an enroute alternate need not be available nor above minima. Take, for example, a flight from St. John's, Newfoundland to Vancouver, British Columbia. It is scheduled at around 7 hours and the aircraft flying it need not be ETOPS certified!

This flight had already spent over 5 1/2 hours over land and was southeast of YFB when the ECAM MAC caution sounded. YFB was the nearest suitable landing site, but YYR, KEF and SNN were the ETOPS alternates.

There was an internal report and our SOPs changed as a result of the this incident. At every flight plan waypoint, not only did Fuel on Board need to be recorded, but current MAC as well. I'd have to dig around to find a report in the public domain. As a brand new and junior A310 Captain, I flew the "rescue" flight ... YYZ-YFB-AMS

Starlionblue wrote:

Maybe we're arguing semantics, but for me "keep within limits" implies that it risks going out of limits. The A350 CG will be within safe limits if left as is. The fuel is all in the wings and the center tank, nicely located close to the CoG. Fuel burn won't affect the CoG very much. The cruise flap extension logic shifts the Center of Pressure forward, closer to the CoG. This means the stabiliser must produce less downward force, and thus fuel burn is decreased. However the stabiliser easily has enough authority to keep the CoP where it needs to be for safe operation

The 330, on the other hand, has a trim tank. Here also, the trim tank is not there to keep the CoG within safe limits. The CoG will be safe as is. At 25500 feet, the system starts transferring fuel to the trim tank. This shifts the CoG backwards, closer to the CoP. The effect is the same. The stabilser must produce less downward force, and thus fuel burn is decreased. The fact that the Aft Transfer Valve is an MEL item shows that the trim tank is not essential to CoG control.

I haven't heard this explanation before. The explanation I heard is that the A350s cruise flaps can alter the lift distribution span-wise across the wing to optimize it towards the ideal elliptical distribution, which decreases lift-induced drag. I've always thought of the flaps as shifting the center of lift aftward a bit by extending the area of low pressure above the wing during flight towards the trailing edge.

Can you clarify a bit? I'm not challenging you (I'm not that foolish! ), but I am interested in making sure I have my information right.

longhauler wrote:

That is a bit of a "gotcha" with respect to long, domestic flights. In theory an enroute alternate need not be available nor above minima. Take, for example, a flight from St. John's, Newfoundland to Vancouver, British Columbia. It is scheduled at around 7 hours and the aircraft flying it need not be ETOPS certified!

Ah, the old 'adequate' vs 'suitable' discussion!

This flight had already spent over 5 1/2 hours over land and was southeast of YFB when the ECAM MAC caution sounded. YFB was the nearest suitable landing site, but YYR, KEF and SNN were the ETOPS alternates.

There was an internal report and our SOPs changed as a result of the this incident. At every flight plan waypoint, not only did Fuel on Board need to be recorded, but current MAC as well. I'd have to dig around to find a report in the public domain. As a brand new and junior A310 Captain, I flew the "rescue" flight ... YYZ-YFB-AMS

Did the A310 have an ECAM caution for fuel XFR fault, or was it transparent for the crew?

DocLightning wrote:

Starlionblue wrote:

Maybe we're arguing semantics, but for me "keep within limits" implies that it risks going out of limits. The A350 CG will be within safe limits if left as is. The fuel is all in the wings and the center tank, nicely located close to the CoG. Fuel burn won't affect the CoG very much. The cruise flap extension logic shifts the Center of Pressure forward, closer to the CoG. This means the stabiliser must produce less downward force, and thus fuel burn is decreased. However the stabiliser easily has enough authority to keep the CoP where it needs to be for safe operation

The 330, on the other hand, has a trim tank. Here also, the trim tank is not there to keep the CoG within safe limits. The CoG will be safe as is. At 25500 feet, the system starts transferring fuel to the trim tank. This shifts the CoG backwards, closer to the CoP. The effect is the same. The stabilser must produce less downward force, and thus fuel burn is decreased. The fact that the Aft Transfer Valve is an MEL item shows that the trim tank is not essential to CoG control.

I haven't heard this explanation before. The explanation I heard is that the A350s cruise flaps can alter the lift distribution span-wise across the wing to optimize it towards the ideal elliptical distribution, which decreases lift-induced drag. I've always thought of the flaps as shifting the center of lift aftward a bit by extending the area of low pressure above the wing during flight towards the trailing edge.

Can you clarify a bit? I'm not challenging you (I'm not that foolish! ), but I am interested in making sure I have my information right.

We are both right.

The 350 has several fancy flap functions. The ones we are discussing are grouped under "Differential flap setting" and do two things:
- Flap Cruise Deployment Function - Symmetrical small flap deflection and aileron droop during cruise in order to manage the center of lift of the wing and adjust the camber or the wing in order to reduce drag and slightly improve performance.
- Differential flaps setting. Modifies the spanwise lift distrubution to reduce wing bending moment at high weights by deflecting inner flaps down a maximum of 2 degrees.

Please continue to challenge me. I'm hardly a training captain.

thepinkmachine wrote:

longhauler wrote:

That is a bit of a "gotcha" with respect to long, domestic flights. In theory an enroute alternate need not be available nor above minima. Take, for example, a flight from St. John's, Newfoundland to Vancouver, British Columbia. It is scheduled at around 7 hours and the aircraft flying it need not be ETOPS certified!

Ah, the old 'adequate' vs 'suitable' discussion!

We do the same. We'll typically fly past quite a few viable diversion airports but for planning most long haul flights are EDTO. That way the weather only needs to be above EDTO alternate minima at the designated alternates. Makes planning simpler I think. Of course if we have a serious problem we won't just fly past a suitable and available airport just because it isn't one of the designated alternates!

longhauler wrote:

That is a bit of a "gotcha" with respect to long, domestic flights. In theory an enroute alternate need not be available nor above minima. Take, for example, a flight from St. John's, Newfoundland to Vancouver, British Columbia. It is scheduled at around 7 hours and the aircraft flying it need not be ETOPS certified!

Are these ancient rules or specific canadian domestic rules?
Either we need a STOPS alternate available at any point along the planned route within 60 minutes flight time with a required minimum weather to allow an OEI approach. If we can't fullfill this requirement we have to opperate under ETOPS with its specific fuel and weather requirements. All this irrespective of beeing over land or water or domestic (well domestic in Switzerland isn't so long anyway ) or international.
So yes, the aircraft need not be ETOPS certified for such a long flight, but in this case we have an alternate every 60 minutes.

glen wrote:

Are these ancient rules or specific canadian domestic rules?

There is no where in CARS that states that an overland flight must be within 60 minutes of a legal alternate at all times.

longhauler wrote:

There is no where in CARS that states that an overland flight must be within 60 minutes of a legal alternate at all times.

O.k. interesting, thanks for the information.

BravoOne wrote:

787 has a similar flap extension feature which has nothing to do with fuel, other than the fact that computers know where the CG is at any given moment.

Bravo, Bravo One!

The topics explanation was CoG vs CoL shift.
On the A330 this is done by changing the CoG via fuel transfer.
On the A350 this is done by changing CoL via changing camber.
ON the 787 .... computers know everything at any given moment.
( though I get the impression that on the 787 everything works by flapping PR shiny4colors.)

zeke wrote:

Probably the sound of removing the newspapers that stopped them from seeing outside and the WX radar.

Biggest chuckle I've had this week. Thanks.

And, t.net at its best... the CoG vs. CoL discussion. Learned alot. Thanks StarlionBlue. Your trainers must think "He's coming in for recurrent. Time to conjure up something new."

WPvsMW wrote:

zeke wrote:

Probably the sound of removing the newspapers that stopped them from seeing outside and the WX radar.

Biggest chuckle I've had this week. Thanks.

And, t.net at its best... the CoG vs. CoL discussion. Learned alot. Thanks StarlionBlue. Your trainers must think "He's coming in for recurrent. Time to conjure up something new."

You are so correct. The "computers" even know if the toilet seats are up or down. Can the Bus do that?

The computer needs to monitor the lav deck and send the FAs a message... "Your attention in needed in Lav 1R. Bring extra paper towels." This system is not needed on KE, JL, NH, CX, or SG.

WPvsMW wrote:

zeke wrote:

Probably the sound of removing the newspapers that stopped them from seeing outside and the WX radar.

Biggest chuckle I've had this week. Thanks.

And, t.net at its best... the CoG vs. CoL discussion. Learned alot. Thanks StarlionBlue. Your trainers must think "He's coming in for recurrent. Time to conjure up something new."

Haha! If only that were the case. I'm nowhere near that level. Learning everyday and simply trying to retain it all.

Besides, in the sim you don't really have the time to look up technical stuff like that. The pucker factor has a tendency to make your brain decide to arbitrarily discard stuff that you should have down cold. On my last recurrent I was kicking myself during the debrief for a few things. But making errors is how knowledge eventually sticks.