mhsieh
Topic Author
Posts: 327
Joined: Thu Jul 20, 2000 2:28 pm

777 Ailerons On Takeoff

Tue Oct 23, 2001 8:01 am

Just flew on 777 between HNL and LAX on United.
Noticed on all takeoffs that as the throttles are advanced initially, the inboard ailerons go down all the way and then return back up to the up position before settling into the normal drooped position.
Is this a function of the fly-by-wire system trying to "compensate" for pitch changes that occur with power change?
Thanks for any info
 
dragogoalie
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Joined: Sun Oct 14, 2001 3:58 pm

RE: 777 Ailerons On Takeoff

Tue Oct 23, 2001 8:45 am

I know that on small airplanes like the PA28 Cadet that I"m flying, when there is a crosswind, we have to adjust the ailerons accordingly to make sure that if a gust of wind came, the plane isn't overturned. I only see this being a possible problem for a 777 if it is at a high speed. I could be completely wrong on this though. I'm just taking a stab at it.

'--dragogoalie-#88--
Formerly known as Jap. Srsly. AUSTRALIA: 2 days!
 
mhsieh
Topic Author
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RE: 777 Ailerons On Takeoff

Tue Oct 23, 2001 10:23 am

I don't think it's pilot control input b/c only the inboard goes down and not outboard and the position is not maintained as you would if it's for a crosswind takeoff....
 
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fanoftristars
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RE: 777 Ailerons On Takeoff

Tue Oct 23, 2001 1:56 pm

Mhsieh,
Did you notice if both ailerons went down at the same time? I noticed this on a flight from DFW to MCO on a 763, I noticed that the one on my side went down, but the other side could have been up, maybe as a check, but then again, now that I think about it, I think the outboard ailerons may have mimicked the movement. I guess it stays in a drooped position as part of the flaps but you probably know that. Any 777 techs or Pilots know? Now I'm totally curious.
"FLY DELTA JETS"
 
UAL747
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RE: 777 Ailerons On Takeoff

Wed Oct 24, 2001 2:28 am

I know exactly what you are talking about. On all of the 777's I've been on, BA, AA, UA, they have all done this. The inboard and outboard ailerons pitch down with the flaps and act as flaperons. However, when you start a take off roll in a 777, and thrust is increased, the inboard aileron comes back up out of position, the drops back down again. It happens on both sides, so its not the pilot testing them. However, I don't know why it does this.

UAL747
"Bangkok Tower, United 890 Heavy. Bangkok Tower, United 890 Heavy.....Okay, fine, we'll just turn 190 and Visual Our Way
 
cdfmxtech
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RE: 777 Ailerons On Takeoff

Wed Oct 24, 2001 4:03 am

It's normal for the B777. The inboard ailerons (flaperons) have 2 modes of operation - Normal and Bypass.
Without getting too much into it, during the takeoff mode (at least one engine running at takeoff thrust and < than 85 knots), the flaperons will switch modes and they will droop because of their weight. Don't let this be confused with their regular droop. Up until about 100 knots the flaperons can and usually do flutter. When the aircraft speed reaches 100 knots, the PCU returns to it's Normal mode.
 
mhsieh
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RE: 777 Ailerons On Takeoff

Thu Oct 25, 2001 9:28 am

Thanks cdfmxtech...
To clarify....during the mode switch, is there a time period when there is no hydraulic pressure in the PCU to keep the aileron from drooping all the way down.
Can you explain the difference between normal and bypass modes? Is this unique to 777? Is it part of the FBW design?
In the Airbus, there is normal law and direct law. In the 777, Is the Bypass mode to allow the pilots direct control of the movement of control surfaces like in conventional aircraft?
Thanks
 
cdfmxtech
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RE: 777 Ailerons On Takeoff

Thu Oct 25, 2001 10:23 am

This mode is a bypass function of normal mode.

Here it is straight from the B777 Maintenance Training Manual:

Flaperon PCU Modes
The flaperon PCUs operate in two modes:

Rev 4 02/08/1997
- Normal
- Bypass.

The flaperon PCU is in the normal mode when there is no
failure in the PCU or its ACE. The ACE then energizes
the bypass solenoid.
The flaperon PCU is in the bypass mode when there is an electrical or hydraulic failure with the PCU or its
ACE. The ACE then de-energizes the bypass solenoid. If
both PCUs on a flaperon are in the bypass mode, the
flaperon can move freely. In flight, the aerodynamic
lift then causes the flaperon to move 10 degrees up
from the faired position.
When the airplane is on the ground with airspeed less
than 85 knots and at least one engine running near
takeoff thrust, the flaperon bypass logic of the PFC
normal mode operates. This function sends a command
signal to the ACEs to put all the flaperon PCUs in the
bypass mode. At low airspeed, the weight of the
flaperon causes the flaperons to droop. Also, the
flaperons can possibly flutter. As the airspeed
increases, the airloads gradually float the flaperons
to the neutral position.
When the airspeed is approximately 100 knots, the PFCs
send a command signal to the ACEs to put all the
flaperon PCUs in the normal mode. The PFCs then send
commands to the ACEs to gradually move the flaperons to
the drooped position.

Training Information Point
During an engine test run on the ground with thrust
near takeoff level, the flaperon behind the engine can
possibly flutter. This is a usual condition.
=========================================
This is unique to the B777. No other Boeing even has the PFCs (Primary Flight Computers) or the ACES (Actuator Control Electrics). Not familair with Airbus.
 
mhsieh
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RE: 777 Ailerons On Takeoff

Thu Oct 25, 2001 3:17 pm

What is the reason for having the bypass mode?
Specifically, what is the logic of switching modes during take off?
Thanks for the detailed info...
 
cdfmxtech
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Joined: Wed Jul 05, 2000 11:37 am

RE: 777 Ailerons On Takeoff

Thu Oct 25, 2001 10:21 pm

Having yet to be formally trained on the B777...the only thing that I could think of would be to assist the high-lift system (flaps) further during takeoff. I mean, the flaperons during this whole sequence go almost all the way down, fair somewhat, then droop, but they never go above the wing surface.
I would say they limit the authority of the flaperon during takeoff roll, but the pilot has NO business commanding roll at 85-100 knots anyhow...so that's not it.

When I find a definite answer (soon) I'll get back to you....unless someone else on this forumhas the answer.

 
mhsieh
Topic Author
Posts: 327
Joined: Thu Jul 20, 2000 2:28 pm

RE: 777 Ailerons On Takeoff

Fri Oct 26, 2001 2:16 am

My understanding of the design of FBW system is that the control stick input does not translate into a preset amount of control surface movement but a rather a G force or load.
Thus, there may be a different control mode when the plane is on the ground or near ground(ie to perform control check or during cross-wind landings the pilot may want direct control of the control surface movement)
I remember hearing about a switch in control law on the Airbus at 50 feet AGL from direct to normal law when I spoke to an A320 pilot....
 
cdfmxtech
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Joined: Wed Jul 05, 2000 11:37 am

RE: 777 Ailerons On Takeoff

Fri Oct 26, 2001 4:45 am

The control law that you are talking about with the Airbus is not a control law...it's called a flight phase.
For instance, The A320 has 3: Flight, Flare and ground. In ground phase, there is direct control over the ailerons and elevator. If the stick is moved for a full right turn, the ailerons and associated spoilers are going to command for a full right turn. There is no load alleviation or envelope protection. Why would you need any on the ground. Then there is flight phase (which operates .5 seconds after TO) and ground (.5 secs afetr LDG) which have their own rules and envelopes.

Well the B777 doesn't do that. It doesn't switch modes to that extent.

Both airplanes do share control modes in common somewhat. The B777 has normal, secondary and direct modes that are based on system health. (Normal inop, switches to secondary and so on).

The A320 has alternate modes for each Pitch and roll control:
Pitch - normal, alternate, and mechanical
Roll - normal, and yaw alternative
--------------------------------------------
Listen, just stick to 737s and everything will be much better!!!
Very simple and effective
 
mhsieh
Topic Author
Posts: 327
Joined: Thu Jul 20, 2000 2:28 pm

RE: 777 Ailerons On Takeoff

Sat Oct 27, 2001 2:10 am

On the ground, do you have direct control of the flight control surface movement on the 777? How does that differ in flight?
Thanks
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: 777 Ailerons On Takeoff

Sat Oct 27, 2001 7:55 am

The primary mode is always in affect (barring a failure), so no you don't have DIRECT control of the flight controls. Remember in the B777, direct would mean bypassing the Primary Flight Computers.

The Flight Controls mode doesn't change for those flaperons...the PCU mode does.
 
737LAME
Posts: 75
Joined: Sat Apr 07, 2001 3:14 pm

RE: 777 Ailerons On Takeoff

Tue Oct 30, 2001 2:16 am

I did also notice this motion by the inboard flaperons.

What I thought about,was the fact that they are straight back from the Engine`s exhaust thrust airflow.
This thrust would need to be going straight back in the beginning of the T/O roll. Would be a very large load on this small flaperon if it should be all way down.
After airborne or sometime later in the T/O roll the load on the whole flap is equal and then later in the climb the thrust is reduced to CLB. The engine`s on the 777 are powerfull.

Well this was just my thoughts, I made an comparason aginst the exhaust gates on the 737 classic flap. Have NO experience on the 777.

Please tell me if my theory is completely wrong.

Atle

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