747 flight controls

[fingpilot]

Originally posted by Tom Z.
This is almost certainly a floating flight spoiler.

SNIP.....

The term 'floating' is at stake here. If it was 'floating', it would not be spoiling lift. It would be freely floating, partly extended by flow of air lifting it from perfect stowed trail, but also perfectly opposed by the flow itself. FLOATING.

Different from extended (powered extended). How do we know? The autopilot had to counteract the powered extend (and the resulting spoiled lift) by compensating with aileron.


Since the spoiler panel is raised, a loss of lift occurs, which requires a counter input to keep the wing level.

You got it right that time, but did not use the term floating.

The amount of aileron deflection appears to be a lot, but remember that it's an inboard aileron without much roll-inducing horsepower. The opposite inboard aileron wouldn't show nearly this much deflection due to the assymetrical travel of ailerons... the downward aileron always travels further than the upward aileron.

The term adverse yaw. It means rigging ailerons so that the upward deflecting alieron ALWAYS deflects more than a downward deflecting one. Always. The drag from the downward one is so onerous (and causes adverse yaw), that it is much easier spoilng lift by deflecting the opposite aileron upwards instead. No adverse yaw that way. So how do we explain this pic? Hmmmmm. Somethings' not right.


Chances are that the upward-travelling aileron still appeared faired. This plane is going to burn a lot of extra fuel in this condition, especially on a long flight!

VERY true

This is almost certainly NOT an example of a misrigged flight control because of the onerous double and triple-checking and inspecting that accompanies rigging procedures.

SNIP

Hmmmmm. Dismissing the photographic fact that there clearly is something wrong by saying it didn't happen here in maintenance is frightening.

My $.02 worth from an Boeing-qualified airline maintenance instructor, although not a 747 instructor.

Nonetheless. I would bet .38 cents that it has been fixed correctly the first time back to the home base.

[Joeaksa]

Originally posted by swa911
Sure, but speaking for those of us on the ground with the wrenches in our hands it's just another know-it-all button pusher telling us how to do our jobs.

Ahmmm. We might cool our jets here in the pilots versus mechanics issue. Know that apologies have been made but might be nice to remember that not all pilots, nor mechanics are the same.

Have had an A&P license for 27 years and pilots license for 35 years. Ex-airline DOM (director of maintenance) and inspector for many, many years. Held an ATP (airline transport license) in 6 different countries. Flown for Boeing, McDonnell Douglas and Bombardier along the way as well.

Just trying to say that just because the old fart in the left seat in the pointy end of the plane is a pilot does not mean that he also is not extremely experienced in turning wrenches and troubleshooting maintenance issues.

As well I know mechanics who are just as good if not better a pilot than I am, so it works both ways. Anyone with a "us versus them" feeling is going to have a problem with it someday, so its better to settle that down right away IMHO.

Just my 3c worth...

Joe A

[Joeaksa]

Originally posted by fingpilot
Nonetheless. I would bet .38 cents that it has been fixed correctly the first time back to the home base.

The -400 has an EICAS system that would have sent an error message to the pilots when the surface did not stow properly, then their pulling up the flight controls page schematic on the EICAS would have shown the offending control surface not being stowed while in flight. It was probably written up even before they landed!

EICAS 1 & 2 are here in the center of the panel. They have various pages that can be pulled up to show all the systems on the bird.

[Tom Z.]

Fingpilot, you're correct about the aileron travel/adverse yaw(of course), sorry 'bout that.

The crux of this thread - as I read it is that you believe this is a misrigged spoiler, and all the maintenance types believe it's a floating spoiler panel. Furthermore, from your response to my first post, I think you don't believe in the aerodynamics of a floating spoiler panel killing the lift on that wing. I think we all agree that the aileron deflection is the autopilot's response to the spoiler.

I can speak for the maintenance folks when I say that any maintenance crew would see this amount of spoiler misrig, to say nothing of any flightcrew member performing a walk-around! If it was rigged to this position, it would power to this same position as soon as hydraulic power was applied - on the ground as well as in flight...therefore it's just too extreme an error not to be missed, if not by the maintenance crew you don't seem to have much faith in, then certainly by a flight crew member... hell, it'd be easily visible from the jetway!

Let's move on to floating spoiler panels. This is a phenomenon that's somewhat common... well, common enough so that most airline mechanics have dealt with this as a source of an uncommanded roll, or unexplained roll/yaw trim. Since I wouldn't be able to explain the aerodynamics of it to your satisfaction, let me share some examples:
From A.D. 85-01-03;
Part II
Applies to all DC-9 series aircraft, fuselage numbers 1 thru 1125, certificated in all categories, as indicated below:

To provide crews with operation information should spoiler float occur,: evidenced by abrupt roll, and to provide for a permanent change in the "Emergency Procedures" Section of the FAA approved Airplane Flight Manual (AFM) (and appropriate AFM sections of the operator's manual required by FAR 121.133 and 121.141), accomplish the following

From a paper on new flight recorders:
Flight spoiler extensions
A more serious incident concerns an operator’s report of difficulty retaining control of an aircraft following takeoff, in August 1998.
In this episode the inboard roll spoilers stopped tracking their respective ailerons shortly after takeoff. As the aircraft accelerated and was cleaned up, the inboard roll spoilers extended symmetrically—without being commanded by the pilots. The event lasted for about 12 seconds.
A second un-commanded extension occurred a few moments later, but this event was non-symmetrical. On the first occasion the spoiler extension resulted in the aircraft’s altitude increase by about 100 feet, while the second un-commanded extension resulted in an altitude increase of about 60 feet.
Both altitude excursions were the result of the pilot responding to the lift loss.
The pilots immediately recognized that they had a serious situation on their hands and elected to return to the departure airport. During the flap extension to 15 degrees for landing, the inboard roll spoilers extended, floating up to about eight degrees, over a period of 25 seconds. Following flap extension the spoilers operated appropriately in response to lateral control inputs. The pilots reported decreased aircraft controllability during this sequence of events, and we were
naturally very concerned, and decided to investigate the incident thoroughly—including analyzing data recovered from the aircraft’s flight data recorder.
Here again, the information from the flight data recorder provided a virtual reconstruction of the incident sequence.
First, I should note that at no time during this incident did the aircraft even approach an ‘uncontrollable’ state. Specifically, the lateral and directional control inputs indicated no controllability problems with the aircraft.
This is confirmed by the flight data recorder, which indicates that control deflections were minimal throughout the sequence, and that at no time were the crew attempting to overcome an engaged autopilot, which was initially postulated.
The Dash 8 aircraft has been extensively flight-tested and shown to be fully controllable with one engine inoperative, or with an inoperative hydraulic system—including spoiler float associated with the loss of spoiler hold-down pressure. The flight data recorder provided a more complete picture of the sequence of events, which—for purposes of this discussion—began with the pilots’ pre-flight preparations. As part of their pre-flight checks, the crew had pulled both “Pull Fuel Off” handles on the aircraft’s fire protection panel, which closes the fuel and hydraulic shutoff valves for each engine. The “Emergency Shut Off Valve” for the Number-1 hydraulic system had remained in
the closed position after the crew reset the “Pull Fuel Off” handle at the completion of this check. In turn, following engine start-up, the “Number-1 engine Hydraulic Pump” caution light remained illuminated.

Those were the two examples I found with a quick Google search. When I'm at work tomorrow, I'll see what I can find from Boeing.

[Tom Z.]

Here's an NTSB report of an uncommanded roll caused by a floating spoiler on a Comair (I guess a Bombardier CRJ):

NYC00IA124
On April 30, 2000, at 2142 Eastern Daylight Time, a Canadair CL-600-2B19, N924CA, operated by Comair as flight 5597, received minor damage when it experienced an uncommanded roll, while on approach to Westchester County Airport, White Plains, New York. The 2 certificated airline transport pilots, 1 flight attendant, and 50 passengers were not injured. Visual meteorological conditions prevailed for the scheduled passenger flight that originated from The Greater Cincinnati/Northern Kentucky Regional Airport, Covington, Kentucky. Flight 5597 was operated on an instrument flight rules (IFR) flight plan that was conducted under 14 CFR Part 121.

According to interviews with the pilots, the captain was the operating pilot. The pilot had been cleared for a visual approach to runway 34, and the flight was uneventful until the uncommanded roll. After turning final, the wing flaps were selected to move from 30 degrees to 45 degrees. As the wing flaps extended, the airplane performed an uncommanded roll to the right. In addition, the Master Caution light illuminated, and the engine indicating and caution advisory system (EICAS), displayed a flight spoiler caution alert.

The captain reported that he was already hand flying the airplane, when the roll occurred, and he applied correction with the rudder and aileron. In addition, he had to hold the correction, to maintain a wings level attitude. He also checked that the spoiler handle was stowed and full forward, and momentarily moved the ground spoiler ARM/DISARM switch from automatic, to manual, and back to automatic with no change in the situation. He then checked the flight control page of the EICAS display and remembered seeing a flight spoiler extension of about 10 degrees on the left wing and about 20 degrees on the right wing.

The captain reported that the airplane was fully controllable, and added additional power as the airplane continued toward the runway with about 10 extra knots of airspeed. After landing, the airplane was taxied to the gate, where the passengers were disembarked through the airstair door.

The airplane was parked at the gate, and electrical power remained applied. The captain then conducted a walk-around inspection of the airplane. All spoiler panels were found retracted except for the flight spoiler on the left wing, which was extended about 10 degrees.

The airplane was equipped with four spoiler panels on each wing. The outboard panels were referred to as spoilerons, and were located next to the ailerons. Their extension was tied to movement of the cockpit control yoke. The next inboard panel on each wing were the flight spoilers. A lever on the cockpit center pedestal controlled their extension and retraction. The two inboard panels were ground spoilers, and only extended when the airplane was in the ground mode. In addition, once the airplane had transitioned to the ground mode, the spoilerons and flight spoilers would also extend, if the ground spoilers were extended.

Each flight spoiler had two attach lugs, which were attached to independently operated hydraulic power control units (PCU). However, only one PCU was powered at a time. The system would switch over between PCUs after the ground spoilers were stowed, after landing.

Examination of the airplane by mechanics from Comair revealed that the outboard connecting lug between the right wing flight spoiler, and its associated PCU was fractured. Both halves of the failed lug were recovered, and forwarded to the Safety Board Materials Laboratory for testing. Functional testing of the other components from the spoiler system did not reveal any additional problems.

According to the Safety Board Materials Laboratory Factual Report;

"...Examination [of the fractured lug]...revealed that the fatigue originally initiated in the corner between the bore of the lug and the chamfered break edge on the outboard face...As the fracture progressed, secondary fatigue cracks initiated along the bore of the lug...The fatigue fracture extended approximately 0.16 inches toward the outer surface of the lug and approximately 0.31 inches inboard along the bore of the lug. The rest of the fracture face contained overstress fracture features stemming from the terminus of the fatigue fracture...The bore surfaces of all lugs from the accident spoiler rest on the outer race of spherical bearings that are inserted into the holes in the lugs. Examination of the bore surface in the fractured lug revealed fretting damage in the area located adjacent to the fatigue fracture...."

According to the METALS HANDBOOK, VOL 10, Failure Analysis and Prevention:

"FRETTING is a wear phenomenon that occurs between two mating surfaces; it is adhesive in nature, and vibration is its essential causative factor. Usually, fretting is accompanied by corrosion. In general fretting occurs between two tight-fitting surfaces that are subjected to a cyclic, relative motion of extremely small amplitude...Fretted regions are highly sensitive to fatigue cracking. Under fretting conditions fatigue cracks are initiated at very low stresses, well below the fatigue limit of nonfretted specimens....":

"...The relative motion required to produce fretting damage may be quite small. Displacements of 0.03...microns [1.15 X 10-6 inches] are sufficient to cause damage, but the amplitudes of displacement usually seen in service are of the order of a few thousandths of an inch...."

According to the Safety Board Flight Data Recorder Factual Report, the airplane was equipped with a digital flight data recorder (DFDR). The data from the recorder revealed the airplane was descending through 1,300 feet, with the wing flaps extending from 30 degrees to 45 degrees, when it suddenly rolled right about 11 degrees. The ailerons were immediately deflected about 10 degrees, the left spoileron extended about 4 degrees, and the airplane returned to a wings level attitude. With the exception of minor variations, the airplane remained in this configuration until touchdown, about 85 seconds later.

The failed lug was an integral part of the flight spoiler. According to Comair, the right wing flight spoiler was original equipment with the airplane. It had accumulated 16,194.7 flight hours, and 15,261 cycles.

The spoiler had last been inspected on November 10, 1998, during a 2C phase inspection. The inspection of the spoiler consisted of a visual, external inspection of the spoiler attach point, without disassembly. Since the inspection, the part had accumulated an additional 3,469.8 hours.

The right wing spoiler extension was not recorded on the DFDR. According to an engineering report from Bombardier:

"...The floating RH [right hand] FS [flight spoiler] surface event was not recorded on the FDR because it [the FDR] obtains its information from the active PCU, which remains retracted as commanded. It is conceivable that the flight crew obtained the surface information from the FLT/CTRL [flight control] Synoptic page (EICAS) on the Co-Pilot side, which receives its information from the floating standby PCU. The Pilot side FLT/CTRL Synoptic page would show the same information, for the spoilers, as is received by the FDR, and thus would not have shown a surface deployment...."

[Tom Z.]

Here's a european investigative report - their version of the NTSB, I guess about an Airbus A-320 which had several spoilers left in"maintenance"(hydraulic bypass) mode:

http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/ComAndRep/Gatwick/pm-excalibur.html

After the replacement of the right-hand outer flap (following damage from an earlier bird strike) the aircraft was handed over to the flight crew at 1500 hours UTC on 26 August 1993. The pilots carried out the usual pre-flight checks, during which they observed nothing amiss, and began the take-off roll at 15.30.

At the take-off rotation at 153 kt., an uncommanded roll to the right occurred. The co-pilot, who was the Pilot Flying (PF), at first attributed this to cross-wind, and applied left stick. When full left stick did not contain the roll, he assumed his sidestick was faulty and handed control to the commander, who immediately found that he too needed full left stick to keep the wings level.

As the aircraft passed 1,700 feet the Electronic Centralized Aircraft Monitoring (ECAM) system sounded a repetitive chime and displayed on its upper screen the messages F/CTL ALTN LAW and F/CTL SPLR FAULT, indicating a significant fault which had caused the Electrical Flight Control System (EFCS) to go into "alternate law" and that the fault involved the spoilers. Roll control improved slightly after flap retraction.

The co-pilot notified Gatwick Air Traffic Control (ATC) that they were returning, and was told to take up the holding pattern at Mayfield (10 nm SE of Gatwick) at 3,000 ft. This required several left turns. Unfortunately they could only turn right. ATC obliged with alternative directions.

The crew reviewed the warnings displayed on the ECAM screen and responded with the necessary actions. ECAM provides what is in effect a "help" facility to the pilots, and in this case instructed them to do a FLAPS 3 landing (22 degrees of slat and 20 degrees of flap) at 10 kt faster than normal reference air speed, and to allow for a 20% increase in landing distance. ATC guided them via right turns to intercept Runway 08 ILS centreline at about 8 nm.

They told the cabin crew to strap the passengers in and announced that they were returning to Gatwick as a result of a slight technical hitch.

As they prepared to land, they first selected FLAPS 1 (18 degrees of slat and 0 degrees of flap) and noticed no change in their (already poor) roll control. When they went to FLAPS 2 (slats 22 degrees, flaps 15 degrees) the commander found that once again he required full left stick to keep the wings level. Having judged landing to be unsafe in these conditions, they reverted to FLAPS 1 and went around.

The co-pilot looked at the Quick Reference Handbook (QRH) and at the Flight Crew Operating Manual, Volume 3 (FCOM 3), Section 2 "Abnormal and Emergency Procedures", for advice on a FLAPS 1 landing, but could not find the pages he wanted (containing the corrections to be made to the normal approach speed and required landing distance in various flight surface failure conditions). The commander then pulled from his flight bag a photocopy of the relevant section from an earlier version of FCOM 3, on which he had renumbered the pages according to the latest release. With the help of this the co-pilot was able to locate the correct manual page, and obtain the correction data for a FLAPS 1 landing (25% increase in approach speed, 30% increase in required landing distance).

They landed in FLAPS 1 configuration at 168 kt in "direct law" and came to a stop without difficulty. (Runway 08 was easily long enough to accommodate the increased landing distance required, and they left the runway at an exit 370m from the end.) The aircraft was towed to the stand, where the passengers disembarked normally.

During taxiing, it was observed that several spoilers were up, and inspection revealed that right-hand spoilers 2, 3, 4 and 5 were in "maintenance mode". These were returned to "operation mode", a duplicate inspection of the spoiler function was carried out, and the aircraft was returned immediately to service.

Following take-off, the pilots noticed a problem with roll control. Spoilers 2 to 5 on the right wing were floating, and were lifted by the airflow. However, only as they passed 1500 ft radio altitude did a warning sound. The Centralized Fault Display System (CFDS) detected the malfunction at 15.31, immediately after take-off (as shown by its automatic records, which were interrogated by the AAIB investigators).

[Tom Z.]

Yet another airworthiness directive on floating spoilers causing uncommanded roll, this time on a Learjet. Read the last sentence:

99-23-19 LEARJET, INC.: Amendment 39-11415. Docket 99-NM-15-AD. Issued October 28, 1999.
Applicability: Model 31 and 31A airplanes, serial numbers 31-033, 31-105, 31-114, 31-126, and 31-150 through 31-161 inclusive; Model 35 and 35A airplanes, serial numbers 35-065, 35-242, 35-300, 35-323, 35-447, 35-622,
and 35-670; and Model 60 airplanes, serial numbers 60-029, 60-050, 60-120 through 60-139 inclusive; certificated in any category.
NOTE 1: This AD applies to each airplane identified in the preceding applicability provision, regardless of whether it has been modified, altered, or repaired in the area subject to the requirements of this AD. For airplanes that have been modified, altered, or repaired so that the performance of the requirements of this AD is affected, the owner/operator must request approval for an alternative method of compliance in accordance with paragraph (b) of this AD. The request should include an assessment of the effect of the modification, alteration, or repair on the unsafe condition addressed by this AD; and, if the unsafe condition has not been eliminated, the request should include specific proposed actions to address it.
Compliance: Required as indicated, unless accomplished previously.
To prevent failure of the spoiler actuator, which could result in the spoiler panel floating and inducing an uncommanded roll of the airplane, accomplish the following:

[fingpilot]

That's all very nice. Lotsa data. Very nice. I'm sure there's a point in there, but everything I read above reinforced what I said about the aircraft pictured here in the beginning of the thread.

Thank you.

I will not ever make the mistake of doing this again.

[Tom Z.]

Hey fingpilot, don't go away mad...

Yes there's a point. Four different cases of "floating" spoilers destroying lift on the wing, requiring aileron input to maintain level flight."Floating", as in the spoiler's actuator unable to keep it retracted and, the low pressure on top of the wing pulling it open allowing it to destroy lift.

These cases were posted as anecdotal evidence because you wrote that you didn't think it worked that way when myself and at least 2 other posters theorized spoiler float and professed direct knowledge.

The purpose of these forums is to share experience, general knowledge (and P-car knowledge) and I feel that's what's happening here. No need to get upset or uptight.

Tom

[Eric Coffey]

Originally posted by Tom Z.
when myself and at least 2 other posters theorized spoiler float and professed direct knowledge.
Tom

Hey, don't forget first guy to thoeorize it with absolutely ZERO direct knowledge!

[Tom Z.]

Eric,

That's what's great here, no shortage of opinions and ideas!

Tom

[rick-l]

I am a real armature here but what are you guys arguing about?

Is it that aero forces won't lift the spoiler if its fail safe mode is damped free float?

[Tom Z.]

I am a real armature here but what are you guys arguing about?

Is it that aero forces won't lift the spoiler if its fail safe mode is damped free float?

Rick,
Spoilers shouldn't float, they should always be locked flush (on airliners, almost always with hydraulics), unless they are deployed as speedbrakes, to assist ailerons, or upon landing to dump lift (get weight on the wheels, for the brakes).

The photograph that began the post shows a spoiler and aileron fighting each other, which should never happen.

The basic argument was if the spoiler actuator loses its ability to hold the spoiler panel fair (flat), via internal bypass, will the low pressure atop the wing (the lift) raise the panel to destroy some of the lift and thus require the aileron input to correct... explaining the picture. That is my theory (and 2 other posters) and the NTSB reports and A.D.'s that I attached are examples of that same phenomenon happening on other aircraft.

Fingpilot theorized that the mechanics adjusted the spoiler cables incorrectly, resulting in the spoiler always hanging out. While a simpler explanation, this could be seen on the ground by everyone and would cause a constant correction during takeoff and climb. Most pilots would know something is wrong and not continue a flight (especially a long flight) with a plane that requires constant roll input. Kind of like taking a long trip in your Porsche with it pulling to the left and never checking out why it's pulling.

Tom