The removal and installation of stabilizers are similar, in most cases, to that of wings and wing panels. On many aircraft the horizontal stabilizer is a movable airfoil, controllable from the cockpit. On some of these aircraft, it is used in conjunction with the elevators to maintain longitudinal control at sonic speeds where the elevators have a tendency to lose their effectiveness. On other aircraft the movable horizontal stabilizer serves the dual purpose of elevators and stabilizers and, in many instances, is referred to as a stabilator.

Some aircraft have an empennage or tail group that consists of all-movable horizontal stabilizers and a single all-movable vertical stabilizer. These aircraft do not have elevators or a rudder.

The removal and installation of stabilizers, like that of the wing, are major jobs and must be accomplished with care and close supervision. Step-by-step instructions of the removal and installation of stabilizers are also included in the "Airframes" section of the applicable MIM. Many of the general precautions listed under "Removal and Installation of Wings" also apply to stabilizer removal and installation.

It is sometimes necessary to remove control surfaces from aircraft to repair or replace them. The instructions presented in the following paragraphs are general instructions, applicable to several types of aircraft. For specific instructions and precautions, you should always consult the MIM before removing a control surface from any aircraft.

Removal of a control surface should not be attempted until the aircraft is placed in a hangar or an area protected from the wind. Before any control surface is removed from the aircraft, it should be tagged with the bureau number of the aircraft and the location of the control surface on the aircraft.

The first step is to remove the access covers and fairings. To prevent the loss of these parts, they should be left attached to the aircraft by one screw or by a piece of safety wire. The other screws should be put in a container to prevent them from being lost.

Disconnect bonding wires, electrical con-nectors, and control linkage. Before disconnecting cable linkage, you should relieve the tension at the most convenient turnbuckle. Next, support the entire control surface, either manually or with mechanical supports, in such a manner as to remove all the load from the hinges. Remove the hinge bolts by using a mallet and brass pin. The control surface should be supported and all the hinges kept in alignment until the last hinge bolt has been removed. On long controI surfaces, it may be necessary to replace the hinge bolts with drift pins to keep the hinges aligned while removing the remaining hinge bolts.

Control surfaces are sometimes attached with piano wire hinges. Removal of the piano wire can be accomplished by removing the ends, securing one end of the wire in the chuck of a hand drill, and rotating the wire with the drill while withdrawing it.

Excessive spinning will have a wearing effect on the hinge material and should be avoided. The reuse of piano hinge wire is not safe; therefore, any wire removed should be discarded.

After all the hinges are disconnected, remove the control surface from the aircraft and support it carefully to prevent damage to the hinge brackets and adjoining surfaces. Before installing a the hinges to prevent them from being lost or damaged. Replace the hinge bolts in control surface, check the identification tag to determine its proper location on the aircraft. Place the surface in position carefully. You should ensure that all the hinge holes are properly aligned. Drift pins may be used to align the holes. With the control surface correctly supported, install the hinge bolts. For a surface attached by piano hinge wire, a new wire should be used. After a control surface is installed, connect the control linkage and check the rigging of the system.

Some flight control surfaces are balanced at the time of manufacture by adding counterweights to the inside of the leading edge of the control surface. This balance must be maintained (within certain tolerances) throughout the service life of the control surface because flutter or dynamic oscillation of these surfaces in flight is not desirable. Balance tolerances are always specified in the aircraft structural repair manual.

Alignment of the airframe structure means checking the position relationship of each major componentthe wing group, tail group, and fuselage group-to the other. The alignment of the airframe is important since it is directly related to the aerodynamic performance of the aircraft. Misalignment may affect the flight characteristics of the aircraft, and consequently, the efficiency of the pilot-aircraft combination.

For this reason and for purposes of determining if any hidden structural failures exist, an alignment check should be performed when an aircraft has encountered excessive gs in flight, when a hard landing has been experienced, or when the aircraft has been subjected to extensive damage.

The need for an alignment check after extensive damage is rather apparent; however, this is not necessarily so in situations where the aircraft exceeds the g design limit or where a hard landing has been experienced. The alignment check under these conditions may expose damage that might otherwise go unnoticed.