OPERATION OF MASS BALANCE

Typically checks involve verifying that the static weight condition of component balances within a specified range. If the control surface has either too much weight ahead of or behind the hinge line, the control surface may flutter during flight. In extreme cases, flutter causes oscillations that progressively increase in amplitude, which ultimately can cause the pilot to lose control of the aircraft or could cause a catastrophic structural failure.
The balance check is done with the control surface remove from the aircraft and assembled as specified in the manufacturer’s instructions. Generally all the mounting hardware and components contained within the control surface should be installed in their relative positions.
Once assembled, the control surface is placed on balancing madrels within a draft-free room. The mandrels are positioned under the hinge points in a way that allows the control surface to pivot freely without dragging or binding. A balance beam or jig is then installed to determine the moment arm required to balance the control surface.
The beam consists a graduated measuring scale in which weight can be moved to bring the control surface to horizontally level position or other position specified by the manufacturer. Once in the proper position, the distance that the weight is located ahead of or behind the hinge line is recorded and checked against the manufacturer’s specification. If the moment arm exceeds the manufacturer’s specification, weight is added or removed from the control surface so that the balnce beam weight falls within the manufacturer’s specified range.
The manufacturer usually provides an area on the control surface specifically designed to hold balance weight and prescribes actions to take to correct an out-of-balance condition. For example, some manufacturers require the application of solder or the installation or the removal of lead weightto a part of control surface. Regardless of what technique is used, the technician must make sure any weight that is applied will not become lose or fall off during flight.

As example, we choose aircraft Boeing 737-400. For operation of mass balance of primary flight control as follow:

Rudder 1) The rudder control system is operated by two sets of pedals, one for each pilot. Pedal movement is transmitted to forward quadrants and jackshafts by rods. The two forward quadrants are interconnected by a bus rod. Cables transmit forward quadrant motion to the aft quadrant located in the vertical fin. Rotation of the aft quadrant in turn causes rotation of the rudder control torque tube. 2) The rudder control torque tube, when rotated, provides simultaneous input to the rudder power control unit, the standby rudder actuator, and to the rudder feel and centering mechanism. In normal operation, the power control unit is powered by hydraulic systems A and B. Control input to the power unit occurs when a torque tube-mounted crank rotates. The crank is connected by a rod to the input linkage of the power unit. As the input linkage moves, the control valve opens and ports hydraulic fluid to the actuator cylinder. The actuating piston then moves and causes rudder deflection. Nominal rudder travel is 26 degrees in both directions. 3) If hydraulic systems A or B are not available to power the rudder, standby power may be turned on, and the rudder operated with the standby actuator. Rudder pedal input positions a servovalve in the actuator which ports fluid to the actuating piston. Movement of the piston closes off the ports when desired rudder travel is reached. The rudder will also operate automatically on standby power when all of these conditions occur: system A or B hydraulic pressure drops below 1200 psi, the flaps are extended, and the wheel speed is greater than 60 knots or the airplane is in the air. 4) The STBY RUD ON light will illuminate any time the standby PCU is commanded to be powered. 5) In addition to operating the rudder power unit and the standby actuator, the torque tube moves the centering cam in the feel and centering mechanism. When the cam shaft is rotated, the spring-loaded cam follower is displaced. 6) If rudder trim is adjusted with the power control unit depressurized, the drag of the unpowered rudder system will allow the system to remain out of center. This will force the rudder feel and centering mechanism cam follower out of the cam detent. Then when hydraulic power is applied, the rudder pedals and rudder will rapidly move to the newly trimmed position.

Aileron
(1) Remove the balance tab
(a) Remove the three screws that attach the forward part of the tab fairing to the tab.
(b) Remove the fairing.
(c) Remove the two nuts, bolts, and washers that attach the tabcontrol rods to the tab mast.
(d) Collect and tag the bolts, nuts, and washers.
(e) Turn the balance tab until you can see the screws that attach the tab hinge covers to the nose cap.
(f) Loosen the screws.
(g) Move the tab hinge covers into the nose caps to get access to the tab hinge bolts.
(h) Disconnect the bonding jumpers that are adjacent to the inboard and outboard hinges.
(i) Remove the nuts from the hinge bolts at four hinge locations.
(j) Hold the balance tab and remove the four hinge bolts.
(k) Carefully remove the balance tab from the hinge assemblies.
(l) Remove the balance tab from the aileron.
(2)Install the balance tab:
(a) Put the balance tab in its position.
(b) Install the bonding jumpers to the forward spar of the tab assembly with the applicable screws, washers and nuts.
(c) Install the four hinge fittings on the tab with the fittings on the aileron.
(d) Install the four hinge bolts with two thin washers below each bolt head and one thick washer below the nut.
(e) Tighten the nuts to 7-17 pound-inches.
(f) Install the four cotter pins.
(g) Clean surface to give electrical bonding.
(h) Install the tab bonding jumpers with applicable screw, nut, and washers to the aft spar of the aileron assembly.
(i) Fillet seal the nut and bonding jumper end with sealant.
(j) Apply primer plus enamel to exposed bare metal not covered by faying surfaces.
(k) Make sure the total (metal to metal) resistance across the bond is no more than 0.001 ohm.
(l) Move the hinge covers to close the bolt access areas.
(m) Tighten the hinge cover screws.
(n) Install the bolts, washers, and nuts that connect the tab control rods to the tab mast.
(o) Tighten the nuts to 50-70 pound-inches.
(p) Install the forward part of the tab fairing.
1) If it is necessary, adjust the forward part of the tab fairing:
a) Loosen the three screws.
b) Move the fairing position to give a clearance with the aileron fairing at all the tab positions.

(3) Inspect or check balance tab (a) Remove the parts to be measured.
(b) Measure the dimensions of these parts.
(c) Compare the part dimensions to the limit dimensions.
(d) If the part dimension is out of tolerance, replace the part.
(e) Install all the parts removed to do the inspection.

Elevator

(1) Remove the elevator tab :
(a) If the same tab will be re-installed then do the following:
1) Make sure that the elevator tab meets the freeplay limits.
2) While you move the elevator trailing edge down, align the projected center of the elevator trailing edge with the index mark within +/- 0.04 inch or less.
3) Measure and record the elevator tab position, using single point method.
4) If elevator tab rods are fabricated from aluminum, ambient temperature must be recorded.
(b) If same tab is not to be re-installed then do the following:
1) Do this task to measure the elevator tab position and the ambient temperature.
(c) Remove the aft mounting bolts to disconnect the elevator tab pushrods from the elevator tab.
(d) Disconnect the bonding jumpers from the elevator tab at the inboard and outboard hinge fittings.
(e) Remove the hinge bearing bolts from the tab hinge half.
(f) Remove the elevator tab from the elevator.

(2) Install the elevator tab:
1) Lubricate the hinge fitting.
2) Temporarily install the hinge fitting and the shims in the tab rear spar.
3) Drill the holes in shims and the hinge fitting to align with the locations in the tab.
4) Remove the elevator tab and disassemble the shims and fittings.
5) Install the nutplates on the hinge fittings.
6) Apply parting agent to the tab spar in the fitting area.
7) Install the shims and the fitting with sealant on the mating surfaces.
8) Install the retaining screws and the washers.
(3)Elevator Tabs Inspectio
Elevator Tab trailing edge play limits will take precedence over above wear limits.
Examine the parts for worn areas:
(a) Use a micrometer or a vernier caliper to measure the parts for the worn areas.
(b) Compare the dimensions you measured with the permitted dimensions
(c) Replace the parts that are not in the tolerance.
(d) Install the parts.

TYPE OF MASS BALANCE

There are two type of mass balance: 1) Static balance 2) Dynamic balance

1. Dynamic balance
The portion of a control surface on an airplane extending ahead of the hinge line. Air striking this portion of the control surface produces an aerodynamic force that aids the movement of the control.
A means of reducing the hinge movement and thereby the physical effort needed to control an aircraft. If control surfaces were hinged at their leading edge and allowed to trail from this position in flight, the forces required to change the angle on all except light and slow aircraft would be prohibitive. Some form of aerodynamic balancing is required to assist the pilot to move controls easily in the absence of power-assisted controls. The most common forms of aerodynamic balancing are inset hinges, horn balances, internal balances, and tab balances.

2.1 Tab Balance
A small, movable control hinged to the trailing edge of one of an airplane's primary flight control surfaces. Tabs can be used to help the pilot move the primary control surface, or they can be used to produce an aerodynamic load on the primary surface that trims the aircraft for a hands-off flight condition. See antiservo tab, balance tab, servo tab, spring tab, trim tab. * Antiservo Tab
A tab installed on the trailing edge of a stabilator. The tab automatically moves in the same direction as the stabilator to produce a stabilizing aerodynamic force that tries to bring the surface back to a streamline position. An antiservo tab is also called an antibalance tab.

* Servo Tab
A small movable tab built into the trailing edge of an aircraft primary control surface. A servo tab is actuated by the cockpit control, and it moves in the direction opposite to the direction the primary surface is to move. The servo tab produces an aerodynamic force that moves the surface on which it is mounted.

* Spring Tab
A small auxiliary tab, set into the trailing edge of a primary control surface on a high-speed airplane. The control surface is attached to the control horn through a torsion rod. Under normal flight loads, the spring tab remains fixed to the control surface and serves no purpose. But, when the air loads are high and a large amount of force is needed to move the control surface, the torsion rod twists, and the control rod moves the spring tab in a direction opposite to that of the surface on which it is mounted. It then acts as a servo tab and aids the pilot in moving the control surface. See servo tab.

* Balance Tab
A small adjustable tab on a primary control surface of an airplane. The tab is automatically actuated in such a way it moves in the direction opposite to the direction the control surface on which it is mounted moves. Air flowing over the balance tab produces a force on the control surface that reduces the amount of force needed by the pilot to move the surface.

* Trim Tab
A small movable control mounted on a primary control surface. The trim tab is adjustable in flight to change the aerodynamic load on the control surface. The aerodynamic load produced by the trim tab deflects the control surface enough for it to trim the aircraft for hands-off flight at any normal speed.

2.2 Horn Balance
An aerodynamic balance for the ailerons, elevators, and rudders in which the area ahead of the hinge is concentrated on one part of the surface in the form of a horn. This horn produces a balancing moment, thus reducing the amount of force required to move the controls or the control’s CG (center of gravity) forward of the hinge to reduce the likelihood of control flutter.

2.3 Inset Hinge
If distance (d) is reduced the hinge moment will be reduced. Smaller the hinge moment, smaller the stick force. Aerodynamic force should never be forward of the hinge. If it does then it will be an "overbalance" condition in which at firs there is a reduction then reversal of force. Overbalance is the static balance condition of a control surface in which the area behind the fulcrum is lighter than the area ahead of he fulcrum. The trailing edge rises above the fulcrum.

2.4 Internal Balance
Aerodynamic balance area is inside the wing. Movement of the control causes pressure changes on the aerofoil, and these pressure changes are felt on the balance area. The pressure difference on the balance therefore gives a hinge moment which is the opposite to the hinge moment on the main control surface, and the overall hinge moment is reduced.

2. Static Balance
A condition of balance of an aircraft control surface in which the surface balances about its hinge line. Lead weights are often installed in parts of the surface ahead of the hinge line to balance the surface. Controls are statically balanced to prevent control-surface flutter.

The exact distribution of weight on a control surface is very important. For this reason, when a control surface is repainted, repaired or component parts replaced, it is essential to check for proper balance and have it rebalanced if necessary. To do this, the control surface is removed, placed in a jig and the position of the centre of gravity checked against the manufacturer's specifications. Without any airflow over the control surface, it must balance about its specified C.G. This is known as static balance.

Method Of Attachment
Concentrating a single externally mounted mass balance weight in one location to balance the control surface may not be ideal but because of limited space available inside the structure, it is usually more convenient to do so. However, whenever possible, distribute the weight uniformly along the span of the control surface. If the weights must be separate and attached in two or more locations along the span, they should be positioned, if possible, on either side of hinges to reduce flexing and torsional stresses on the structure.
A method for obtaining good distribution of balance is through the installation of a solid steel rod along the entire length of the leading edge or perhaps you could install a steel tube instead. Although the steel tube would be lighter than a steel rod, its weight could be increased to exceed that of the solid rod by pouring in molten lead to obtain whatever additional weight is needed. You may have to pre-heat the tube to achieve this objective.
Not only must balance weights be attached solidly, they must also be capable of withstanding high G loads. How high's high? Well, in a yesteryear study conducted at the NASA Langley Research Center by Arthur A. Regier (Flutter of Control Surfaces and Tabs), it was determined that the balance weights should be capable of withstanding 36 G's normal to the surface. However, more recently (1979), the FAA, in its AC23.629-1 "Means of Compliance with FAR 23.629, Flutter," states that all balance weight supporting structure should be designed for a limit static load of 24 G's normal to a plane containing the hinge and the weight and 12 G's within that plane parallel with the hinge. FAA also points out that proof of these criteria can be accomplished by simple static tests of the control surface mounted in a jig. That's really not too much as a 2 lb. weight need only be static tested to 48 lbs. to equal the requirement imposed on store bought aircraft.
On aircraft boeing 737-400 the mass balance of aileron is attach into the aileron system with the tab adjust weights. These weights are attached to the lower surface of the aileron balance panel with bolts. The correct tab weight and number of the tab adjust weights necessary are on the data plate. The data plate is attached to the outboard end of the tab.

REFERENCES 1) Maintenance Manual Boeing 737 2) Jeppesen A&P Technician Airframe Textbook 3) http://www.theairlinepilots.com 4) http://www.atpforum.eu 5) http://members.eaa.org 6) http://www.pilotfriend.com 7) http://www.answers.com