Capt.Fantastic From United States, joined Aug 1999, 598 posts, RR: 0 Posted (3 years 4 months 3 days 19 hours ago) and read 2621 times:
I had a few questions regarding the Boeing 777
Each time I fly on a 777 I sit behind the wing – 2 questions:
1) Just as we spool up for take-off, I notice that the inboard ailerons droop down for a few seconds and than comes back up to neutral – Why does this happen?
2) When landing, just as we pass over the threshold, I hear what sounds like a hydraulic actuator that hums until touchdown – What is happening here?
Starlionblue From Greenland, joined Feb 2004, 13564 posts, RR: 68 Reply 1, posted (3 years 4 months 3 days 18 hours ago) and read 2591 times:
Quoting Capt.Fantastic (Thread starter):
1) Just as we spool up for take-off, I notice that the inboard ailerons droop down for a few seconds and than comes back up to neutral � Why does this happen?
Pilot trying the surfaces?
Quoting Capt.Fantastic (Thread starter):
2) When landing, just as we pass over the threshold, I hear what sounds like a hydraulic actuator that hums until touchdown � What is happening here?
It's a hydraulic system My guess is that it's getting ready to lift the spoilers. But I don't know.
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
N8076U From United States, joined Jun 2006, 425 posts, RR: 13 Reply 2, posted (3 years 4 months 3 days 18 hours ago) and read 2578 times:
Quoting Capt.Fantastic (Thread starter): 1) Just as we spool up for take-off, I notice that the inboard ailerons droop down for a few seconds and than comes back up to neutral – Why does this happen?
There is another thread floating around which answers this to greater detail. In simple terms, the flight control computer that controls these ailerons on the 777 does odd things when high thrust settings are applied. They also droop with the flaps at higher flap settings, and are actually called flaperons. The computer may think it is compensating for something when it is in fact not doing anything but fascinating any A.netters looking out the windows.
777236ER From United Kingdom, joined Aug 2001, 12706 posts, RR: 57 Reply 3, posted (3 years 4 months 3 days 17 hours ago) and read 2571 times:
I'm just guessing, but I bet the highspeed flaperons are raised when take off thrust is set until airspeed has built up a bit to prevent FOD damage. The flaperons on 777 also go to neutral when the spoilers raise.
777236ER From United Kingdom, joined Aug 2001, 12706 posts, RR: 57 Reply 5, posted (3 years 4 months 3 days 17 hours ago) and read 2558 times:
Quoting Bri2k1 (Reply 4):
The problem is, they go down, not up.
Oh, OK. I don't know then!
Quoting Capt.Fantastic (Thread starter): 1) Just as we spool up for take-off, I notice that the inboard ailerons droop down for a few seconds and than comes back up to neutral � Why does this happen?
The flaperon should be angled down with takeoff flap set. Do they go full down then back to the original position?
Bri2k1 From United States, joined Dec 2004, 798 posts, RR: 4 Reply 6, posted (3 years 4 months 3 days 16 hours ago) and read 2537 times:
I've watched it in person enough times to believe it goes down as thrust is increased, not up. I think the video shows that, too. The outboard flap sections aren't moving, and you hear the whine as the engine spools up, that seems to be when the thrust is applied.
777236ER From United Kingdom, joined Aug 2001, 12706 posts, RR: 57 Reply 7, posted (3 years 4 months 3 days 16 hours ago) and read 2533 times:
Quoting Bri2k1 (Reply 6): The outboard flap sections aren't moving, and you hear the whine as the engine spools up, that seems to be when the thrust is applied.
Are you talking about the outboard lowspeed flaperons, or the inboard allspeed flaperons? Refering to the second video, at about 18-20 seconds, takeoff thrust is reached, and the inboard flaperon slowly moves up to the neutral position. At 40 seconds it definately and suddenly moves back down to its takeoff position.
Bri2k1 From United States, joined Dec 2004, 798 posts, RR: 4 Reply 8, posted (3 years 4 months 3 days 15 hours ago) and read 2521 times:
Maybe I was talking about something other than the OP. The question I've asked before, and would like to have answered, relates to the 777-200 that UA flies, the only one I've flown on, and involves the inboard flaperon. I've seen it fully downward while parked at the gate, then rapidly jump up when hydraulics are turned on, but the strangest to me are the wild fluctuations it makes during small thrust changes on the ground during taxi. I'm not sure if either of these videos shows it, but in all my experiences, the inboard flaperon (the section directly behind the engine) drops all the way down again as soon as takeoff thrust is selected, but just as the a/c begins to roll, it goes back to its previous setting and then makes normal movements seemingly to assist the ailerons.
Matt72033 From United Kingdom (England), joined May 2005, 1617 posts, RR: 6 Reply 9, posted (3 years 4 months 3 days 15 hours ago) and read 2516 times:
Quoting Capt.Fantastic (Thread starter): 2) When landing, just as we pass over the threshold, I hear what sounds like a hydraulic actuator that hums until touchdown – What is happening here?
Flare? could be a hydraulic demmand pump running, if the pilots using a bit of aileron, the electric demmand pump may kick in to build pressure back up when he begins the flare with the elevators?
N8076U From United States, joined Jun 2006, 425 posts, RR: 13 Reply 10, posted (3 years 4 months 3 days 13 hours ago) and read 2465 times:
Quoting Bri2k1 (Reply 8): relates to the 777-200 that UA flies
I was a mechanic for UA, and worked on their 777s for years on the line. I know exactly what you are talking about, and have seen what you've said on flights I have been on. Although I have no actual specific answer in writing from Boeing engineering, I can give my own observations, and what I have been told and what I have read, and give you my own summary of all that information.
Quoting Bri2k1 (Reply 8): I've seen it fully downward while parked at the gate, then rapidly jump up when hydraulics are turned on
With no hydraulic pressure, their own weight makes them sag, as they are not counterweighted. You can also go up to them on a lift and move them up and down by hand, as long as the hydraulics are off. When the hydraulic pressure comes up, they center, as that's where they should be at that time.
Quoting Bri2k1 (Reply 8): the strangest to me are the wild fluctuations it makes during small thrust changes on the ground during taxi
The thing is, they are designed to droop with the flaps. But their primary purpose is to be ailerons, and that overrides their flap function. And they also are designed to go to different positions depending on the thrust setting. Generally speaking, landing thrust makes them go to the flap position, takeoff thrust makes them center. On top of that, they are fly-by-wire controlled and the computer is trying to take everything into account and position them correctly. At low speeds or sitting still, the computer gets confused, for lack of a better word. So they act as if they are doing stupid things when the aircraft is on the ground, but the computer is only trying to do what it was programmed to do. During takeoff, consider them drooping for a second as a hiccup, as for that second, the parameters given to the computer require them to droop, and a second later, the parameters require a neutral position. Sorry, if I can't explain it any clearer.
Not sure what your "hydraulic actuator" sound is during landing, but there are hydraulic pumps right behind the wing in the fuselage directly under the floor, so this seems like one plausible explanation for that sound.
Starlionblue From Greenland, joined Feb 2004, 13564 posts, RR: 68 Reply 11, posted (3 years 4 months 3 days 13 hours ago) and read 2458 times:
Quoting N8076U (Reply 10):
Not sure what your "hydraulic actuator" sound is during landing, but there are hydraulic pumps right behind the wing in the fuselage directly under the floor, so this seems like one plausible explanation for that sound.
Having flown a large number of times in the overwing exit row on the 772, I can confirm this noise is present pretty much every time just before landing, without any related flap/spoiler movement. Of course the ailerons are moving.
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
777236ER From United Kingdom, joined Aug 2001, 12706 posts, RR: 57 Reply 12, posted (3 years 4 months 3 days 13 hours ago) and read 2450 times:
Quoting N8076U (Reply 10): During takeoff, consider them drooping for a second as a hiccup, as for that second, the parameters given to the computer require them to droop, and a second later, the parameters require a neutral position. Sorry, if I can't explain it any clearer.
It does it everytime. It's not the flaperons acting as ailerons because the lowspeed ailerons don't move, the spoilerons don't move and both ailerons go in the same direction! The inboard flaperons definately both move to neutral when takeoff thrust is set, and move down when the airspeed increases.
I was hoping to get an explanation from a mechanic, thanks. It seems like you've observed the same things I have, so I know I'm not hallucinating at least. I'm not sure I can believe they're not intentional movements though. There are some pretty advanced FBW systems on that plane, and for it to not be sure how to position the flight control surfaces for specific regimes of operation just seems like a major flaw. The FOD explanation given by 777236ER doesn't add up in my mind either; first because they're behind the engines, and second because I don't think they move out of the way but rather move more into the exhaust stream when thrust is increased. There are so many systems on the 777 that I couldn't even begin to guess what the reason could be though.
CX flyboy From Hong Kong SAR, PRC, joined Dec 1999, 5572 posts, RR: 61 Reply 14, posted (3 years 4 months 3 days 10 hours ago) and read 2407 times:
I would be interested to know as well!! These days pilots training is on a need to know basis only it seems and those two questions you have asked are things which really are inconsequential to the pilots, hence we are not told!
I've not sat in the cabin for many 777 landings, especially not behind the wing so I;ve never noticed myself!
N8076U From United States, joined Jun 2006, 425 posts, RR: 13 Reply 15, posted (3 years 4 months 3 days 10 hours ago) and read 2407 times:
I myself was never able to get a real clear cut answer from Boeing, any maintenance manual or other more knowledgeable people, other than it is an idiosyncrasy of that aircraft, and that it is "the computer's fault" and "they all do that". I was hoping someone here could give a more specific reason from an engineering point of view.
But, I will say that the movements themselves are very intentional. The inputs that are used for calculations may merely be inadequate or conflicting at times, especially during any transition periods like when the throttles are advanced at a slow or no airspeed, so the logic merely moves the flaperons where it believes it should be for the given parameters.
If the ADIRU switch on the overhead (Air Data Inertial Reference Unit) is off during taxi or during a high-power engine run up, the flaperons do nothing out of the ordinary, they stay solid as a rock. They only "act funny" when this switch is on, so this has something to do with it. The ADIRU provides the flight computers with the various data about the aircraft's position, airspeed, attitude, etc.
Quoting Bri2k1 (Reply 13): The FOD explanation given by 777236ER doesn't add up in my mind either; first because they're behind the engines, and second because I don't think they move out of the way but rather move more into the exhaust stream when thrust is increased.
There was actually an AD for checking for cracking on the flaperon support structure on some 777 aircraft, due to high engine thrust, so they move out of the way primarily to avoid being subjected to the exhaust blast at high thust settings, although a secondary benefit would be being that much more out of the way of any FOD as well.
I did a quick search and found this regarding the airworthiness directive:
Bri2k1 From United States, joined Dec 2004, 798 posts, RR: 4 Reply 16, posted (3 years 4 months 3 days 10 hours ago) and read 2385 times:
I think the link might have been malformed when it was posted. I can't get it or a logical variant to work.
I see what you mean about the benefits of moving out of the way when thrust is high. I'm just saying, I don't think that's what I see. I can't know the throttle position from row 20, but as I hear the engines spool up, I see those flaperons move downwards for a while, then back to their normal position.
Quoting Bri2k1 (Reply 16): I don't think that's what I see. I can't know the throttle position from row 20, but as I hear the engines spool up, I see those flaperons move downwards for a while, then back to their normal position.
Well, I agree, as I've seen the same thing myself. In general, they're supposed to move out of the way at high thrust settings, but apparently, sometimes this does not happen, at least on the ground, as several of us have personally seen.
N8076U From United States, joined Jun 2006, 425 posts, RR: 13 Reply 18, posted (3 years 4 months 3 days 9 hours ago) and read 2371 times:
Perhaps someone with a 777 flight manual can confirm this, but it seems if hydraulic pressure is low, OR the aircraft "anticipates" the need for it (like during landing), then hydraulic demand pump(s) automatically come on, if the associated pump switch is in the auto position. I called a friend of mine I used to work with, and this is what he thinks is happening, as he remembers the 777 having a feature along these lines, but I have no idea which (one, two, all three???) of the three hydraulic systems this may be true for, if at all. If it is true, this may explain the hydraulic pump noise right before landing, which is probably triggered by ground proximity. But I'll keep digging as I am curious about several things now...
This website states the following for the left/right electric and center air demand hydraulic pumps:
OFF: Pump is off and cannot operate automatically.
AUTO: Pump operates when primary pump pressure is low or when system logic anticipates high demand. (Right demand pump runs continuously on the ground).
On: Pump runs continuously.
Of particular interest is what happens in the "auto" position, the normal flight position.
Apparently this may be the answer to the noise before touchdown.
Starlionblue From Greenland, joined Feb 2004, 13564 posts, RR: 68 Reply 20, posted (3 years 4 months 2 days 21 hours ago) and read 2252 times:
Quoting N8076U (Reply 19): AUTO: Pump operates when primary pump pressure is low or when system logic anticipates high demand. (Right demand pump runs continuously on the ground).
As N8076U states, this is the interesting part. Obviously the system logic will anticipate high demand prior to spoiler activation.
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
TristarSteve From Sweden, joined Nov 2005, 2806 posts, RR: 22 Reply 21, posted (3 years 4 months 2 days 20 hours ago) and read 2241 times:
Quoting N8076U (Reply 19): Of particular interest is what happens in the "auto" position, the normal flight position.
The B777 has two Air Driven Pumps (ADP) in the centre system. The normal pressurisation is by two Electrical driven hyd pumps which run all the time and the ADPs come on as required. They usually only run for short periods.
One ADP will run when:-
Landing gear extension
Flap/slat operation
Low hyd pressure (2400psi)
30 ft Rad alt before Touchdown
And in the air with flaps not up and one ADP turned OFF or inop.
The touchdown mode is to supply pressure to operat the autospeedbrakes.
Both ADPs will operate:-
Autoslat operation
Landing gear retraction
Take off roll on the ground over 80kts flaps in t/o posn. This is to ensure that one of them will operate iuf an engine fails to provide hyd press for the rudder.
There are other reasons but they are to do with different failures.
TristarSteve From Sweden, joined Nov 2005, 2806 posts, RR: 22 Reply 22, posted (3 years 4 months 2 days 20 hours ago) and read 2236 times:
Quoting Capt.Fantastic (Thread starter): 1) Just as we spool up for take-off, I notice that the inboard ailerons droop down for a few seconds and than comes back up to neutral – Why does this happen?
The reason is to decrease the flaperon actuator fatigue cycles caused by engine exhaust on the flaperons.
What happens is With one engine running at near take off power, on the ground with airspeed less than 85kts the flaperon actuators are commanded to bypass mode. This disconnects them from the control inputs and lets them flap. At low airspeed the weight of the flaperon will allow it to droop down to the PCU stops, The exhaust flow from the engine will act on it to push it back up again and the flaperons may flutter. (In fact Boeing says that fluttering flaperons are normal on static engine runs.) As the airspeed increases during take off roll the airloads will move the flaperons back to faired. At about 100kts the flaperons are then put back into normal mode and will be driven back to their commanded position.
This is inhibited during a touch and go. And there is more to it.
All this from the B777 AMM Pt 1.
NW727251ADV From , joined Dec 1969, posts, RR: Reply 24, posted (3 years 4 months 1 day 19 hours ago) and read 2008 times:
This question about the drooping flaperon has been asked and answered before. This isn't an answer but here's a video of the Continental "Peter Max" 777-200ER taking off and you can clearly see the flaperon on the port wing in the drooped position and it leveling off at the beginning of the takeoff roll.
N8076U From United States, joined Jun 2006, 425 posts, RR: 13 Reply 25, posted (3 years 4 months 1 day 18 hours ago) and read 1981 times:
Quoting N8076U (Reply 15): an idiosyncrasy of that aircraft, and that it is "the computer's fault" and "they all do that"
Apparently, what I was told is correct, but I misunderstood the underlying meaning, and I was missing a couple other tidbits of information as well. Yes, they all do that. Yes, it is an idiosyncrasy of the 777, and it is in fact the computer's fault the flaperons do what they do. But rather than commanding the wild movements (as I surmised), the computer is merely allowing the flaperons to go limp, becoming "tissues in the wind" so to speak. And turning off the ADIRU makes the aircraft go into a mode where the computer is in full control all the time and won't put the flaperons into the disconnected state, hence the lack of any movements at that time.
Chris
Don't blame me, I don't work here...
26 Bri2k1: That makes a lot of sense, thanks. I just knew there was an explanation for it that I could believe. As usual, is of limited value as it does not answ
27 N8076U: Glad I know the whole story now as well. If things were like that, this site would only have a list of FAQ's, rather than a discussion forum. No fun
28 Jetlagged: If the ADIRU is switched off, the aircraft is not about to go flying, as the attitude reference will be invalid. So what the ailerons do on takeoff w
29 N8076U: True, the aircraft cannot legally fly without an operational ADIRU and it is a non-deferrable item, but the aircraft physically could fly and would h
30 TristarSteve: OK I will look it up tommorrow when I am at work. Remember that on the B777 there is only one ADIRU. But it contains six laser gyros and a host of ac
31 TristarSteve: Ok The B777 Flight Control System has three modes Normal secondary and Direct. I have never seem an aircraft in anything but normal mode. When the sy
32 N8076U: Thanks for looking into that. One thing I miss about being out of the industry is my lack of access to any info or manuals, so all this information yo
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My guess is that it's getting ready to lift the spoilers. But I don't know.
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