Brazed or soldered parts should never be repaired by welding, since the brazing mixture or solder can penetrate the hot steel and weaken it.
Aircraft parts such as turnbuckle ends and aircraft bolts which have been heat treated to improve their mechanical properties should not be welded.
Repair of Tubular Members Welded steel tubing can usually be spliced or repaired at any joint along the length of the tube, but particular attention should be given to the proper fit and alignment to avoid distortion. Some of the many acceptable practices are outlined in the following paragraphs.
Dents at a steel tube cluster-joint can be repaired by welding a specially formed steel patch plate over the dented area and surrounding tubes, as shown in figure 6-38.
To prepare the patch plate, a section of steel sheet is cut from the same material and thickness as the heaviest tube damaged. The reinforcement plate is trimmed so that the fingers extend over the tubes a minimum of one and one-half times the respective tube diameters (figure 6-38). The reinforcement plate may be formed before any welding is attempted, or it may be cut and tack welded to one or areas of the original tube. The sleeve is welded along the length of the two sides, and both ends are welded to the damaged tube, as shown in figure
6-39.
Repair by Bolted Sleeve Bolted sleeve repairs on welded steel tubular structure are not recommended unless specifically authorized by the manufacturer or the Federal Aviation Administration. The material removed by drilling the boltholes in this type of repair may prove to weaken the tubular structure critically.
Welded-Patch Repair Dents or holes in tubing can be safely repaired by a welded patch of the same material but one gage thicker, as illustrated in figure 6-40, with the following exceptions:
(1) Do not use a welded patch to repair dents deeper than one-tenth of the tube diameter, dents that involve more than onefourth of the tube circumference, or those longer than the tube diameter.
(2) Use welded-patch repairs only if dents are free from cracks, abrasions, and sharp corners.
more of the tubes in the cluster-joint, then heated and formed around the joint to produce a smooth contour. Sufficient heat should be applied to the plate during the forming process so that no gap exists. If a gap exists it should not exceed 16 in.
from the contour of the joint to the plate. After the plate is formed and tack welded to the cluster-joint, all the reinforcement plate edges are welded to the cluster-joint.
Repair by Welded Sleeve
(3) Use welded-patch repairs only when the dented tubing can be substantially reformed without cracking before application of the patch.
(4) In the case of punctured tubing, use welded-patch repairs if the holes are not longer than the tube diameter and involve not more than one-fourth of the tube circumference.
This type of repair of a dented or bent tube is Splicing Tubing by Inner Sleeve Method illustrated in figure 6-39. The repair material se- If the damage to a structural tube is such that a lected should be a length of steel tube sleeving partial replacement of the tube is necessary, the having an inside diameter approximately equal to inner sleeve splice shown in figure 6-41 is recomthe outside diameter of the damaged tube and of the mended, especially where a smooth tube surface is same material and wall thickness. This sleeve rein- desired. A diagonal cut is made to remove the damforcement should be cut at a 30° angle on both aged portion of the tube, and the burrs are removed ends so that the minimum distance of the sleeve from the edges of the cut by filing or similar means.
from the edge of the crack or dent is not less than A replacement steel tube of the same material and one and one-half times the diameter of the damaged After the angle cuts have been made to the ends, the entire length of the reinforcement sleeve should be cut, separating the sleeve into half-sections (figure 6-39). The two sleeve sections are then clamped to the proper positions on the affected diameter, and at least the same wall thickness is then cut to match the length of the removed portion of the damaged tube. At each end of the replace.
ment tube a ⅛-in. gap should be allowed from the diagonal cuts to the stubs of the original tube.
A length of steel tubing should next be selected of at least the same wall thickness and of an outside 277
Longeron dented at a station.
Thickness of patch-plate same as longeron thickness.
Patch plate before forming and welding.
Patch plate formed and welded to tubes.
FIGURE 6-38. Repair of members dented at a cluster.
• Reinforcement tube split FIGURE 6-39
% Tube circum. •Not greater than "A" (мах) 2 A - 2A
- Damage FICURE 6-40. Welded-patch repair.
Rosette weld Inside sleeve tube Original tube ¼ A ¼½ А ½ А Replacement tube FIGURE 6-41. Splicing by inner-sleeve method.
structure must be maintained. This can be done by measuring the distance between points of corresponding members that have not been distorted, and by reference to the manufacturer's drawings.
If all members are out of alignment, the engine mount should be replaced by one supplied by the manufacturer or one built to conform to the manufacturer's drawings. The method of checking the alignment of the fuselage or nacelle points can be requested from the manufacturer.
Minor damage, such as a crack adjacent to an engine attachment lug, can be repaired by re-welding the ring and extending a gusset or a mounting lug past the damaged area. Engine mount rings which are extensively damaged must not be repaired, unless the method of repair is specifically approved by an authorized representative of the Federal Aviation Administration, or is accomplished using instructions furnished by the aircraft manufacturer.
Repair at Built-In Fuselage Fittings An example of a recommended repair at built-in fuselage fittings is illustrated in figure 6-42. There are several acceptable methods for effecting this type of repair. The method illustrated in figure 6-42 utilizes a tube (sleeve) of larger diameter than the original. This necessitates reaming the fitting holes in the longeron to a larger diameter. The forward splice is a 30° scarf splice. The rear longeron is cut approximately 4 in. from the center line of the joint, and a spacer 1 in. long is fitted over the longeron. The spacer and longeron are edgewelded. A tapered "V" cut approximately 2 in. long is made in the aft end of the outer sleeve, and the end of the outer sleeve is swaged to fit the longeron and then welded.
Spacer approximately Weld "A" long welded on inside tube FICURE 6-42. Repair at built-in fuselage fitting.
Landing Gear Repair Landing gear made of round tubing is generally repaired using repairs and splices illustrated in figures 6-39 and 6-42. One method of repairing landing gear wade of streamlined tubing is shown in figure 6-43.
Representative types of repairable and nonrepairable landing gear axle assemblies are shown in figure 6-44. The types shown in A, B, and C of this figure are formed from steel tubing and may be repaired by any of the methods described in this section. However, it will always be necessary to Gap %" Weld 1½ C - - 1½ C Form inside tube to fit At least At least 3C A - Slot width (original tube) B - Outside diameter (insert tube) C - Streamline tube length of major axis FIcURE 6-43. Streamlined tube splice on landing gear using round tube.
D FicuRE 6-44. Representative types of repairable and nonrepairable assemblies.
ascertain whether or not the members are heat treated. Assemblies originally heat treated must be re-heat treated after welding.
The axte assembly shown in D of figure 6-44 is, in general, of a nonrepairable type for the following
(1) The axle stub is usually made from a highly heat treated nickel alloy steel and carefully machined to close tolerances.
These stubs are usually replaced when damaged.
(2) The oleo portion of the structure is generally heat treated after welding and is perfectly machined to assure proper functioning of the shock absorber. These parts would be distorted by welding after the machining process.
A spring-steel leaf, shown in E of figure 6-44, supports each main landing gear wheel assembly on many light aircraft. These springs are, in general, nonrepairable and should be replaced when they become excessively sprung or are otherwise dam- Built-Up Tubular Wing or Tail Surface Spar Re- Built-up tubular wing or tail surface spars can be repaired by using any of the splices and methods outlined in the discussion on welding of aircraft steel structures, provided the spars are not heat treated. In the case of heat treated spars, the entire spar assembly must be re-heat treated to the manufacturer's specifications after completion of the Wing and Tail Brace Struts In general, it is advantageous to replace damaged wing-brace struts made either from rounded or streamlined tubing with new members purchased from the aircraft manufacturer. However, there is usually no objection from an airworthiness point of view to repairing such members properly. Members made of round tubes using a standard splice can be repaired as shown in figures 6-39 or 6-41.
Steel brace struts may be spliced at any point along the length of the strut, provided the splice does not overlap part of an end fitting. The jury strut attachment is not considered an end fitting;
therefore, a splice may be made at this point. The repair procedure and workmanship should be such as to minimize distortion due to welding and the necessity for subsequent straightening operations.
The repaired strut should be observed carefully during initial flights to ascertain that the vibration characteristics of the strut and attaching components are not adversely affected by the repair. A wide range of speed and engine power combinations must be covered during this check.
GENERAL Rain, snow, and ice are transportation's ancient enemies. Flying has added a new dimension, particularly with respect to ice. Under certain atmospheric conditions, ice can build rapidly on airfoils and air inlets.
The two types of ice encountered during fight are rime and glaze. Rime ice forms a rough surface on the aircraft leading edges. It is rough because the temperature of the air is very low and freezes the water before it has time to spread.
Glaze ice forms a smooth, thick coating over the leading edges of the aircraft. When the temperature is just slightly below freezing, the water has more time to flow before it freezes.
Ice may be expected to form whenever there is visible moisture in the air and the temperature is CHAPTER 7
ICE AND RAIN PROTECTION near or below freezing. An exception is carburetor icing which can occur during warm weather with no visible moisture present. If ice is allowed to accumulate on the wings and empennage leading edges, it destroys the lift characteristics of the airfoil. Ice or rain accumulations on the windshield interfere with vision.
Icing Effects Ice on an aircraft affects its performance and efficiency in many ways. Ice buildup increases drag and reduces lift. It causes destructive vibration, and hampers true instrument readings. Control surfaces become unbalanced or frozen. Fixed slots are filled and movable slots jammed. Radio reception is hampered and engine performance is affected (figure 7-1).
Effects of Icing are Cumulative
LIFT LESSENS
DRAG INCREASES THRUST FALLS OFF
WEIGHT GROWS Staling Speed Increases FIGURE 7-1. Effects of structural icing.
The methods used to prevent icing (anti-icing) or to eliminate ice that has formed (deicing) vary with the aircraft make and model. In this Chapter ice prevention and ice elimination using pneumatic pressure, application of heat, and the application of fluid will be discussed.
Ice Prevention Several means to prevent or control ice formation are used in aircraft today: (1) Heating surfaces using hot air, (2) heating by electrica.
elements, (3) breaking up ice formations, usually by inflatable boots, and (4) alcohol spray.
surface may be anti-iced either by keeping it dry by heating to a temperature that evaporates water upon impingement; or by heating the surface just enough to prevent freezing, maintaining it running wet; or the surface may be deiced by allowing ice to form and then removing it.
Ice prevention or elimination systems ensure safety of flight when icing conditions exist. Ice may be controlled on aircraft structure by the following methods.
Location of Ice Control
1. Leading edge of the wing Deicer tubes are inflated by an engine-driven air pump (vacuum pump), or by air bled from gas turbine engine compressors. The inflation sequence is controlled by either a centrally located distributor valve or by solenoid operated valves located adjacent to the deicer air inlets.
Deicers are installed in sections along the wing with the different sections operating alternately and symmetrically about the fuselage. This is done so that any disturbance to airflow caused by an inflated tube will be kept to a minimum by inflating only short sections on each wing at a time.
Deicing system not operating. Cells lie close to airfoil section. Ice is permitted to form.
Flexible hose
2. Leading edges of vertical and horizontal stabilizers
3. Windshields, windows, and radomes After deicer system has
4. Heater and Engine air inlets been put into operation, center cell inflates, cracking ice.
5. Stall warning transmitters
6. Pitot tubes
7. Flight controls edges
9. Carburetors Alcohol 10 When center cell deflates, outer cells inflate. This raises cracked ice causing it to PNEUMATIC DEICING SYSTEMS be blown off by air stream.
Pneumatic deicing systems use rubber deicers, called boots or shoes, attached to the leading edge of the wing and stabilizers. The deicers are composed of a series of inflatable tubes. During operation, the tubes are inflated with pressurized air, and deflated in an alternating cycle as shown in figure
7-2. This inflation and deflation causes the ice to crack and break off. The ice is then carried away by the airstream. FIcURE 7-2. Deicer boot inflation cycle.
DEICER BOOT CONSTRUCTION Deicer boots are made of soft, pliable rubber or rubberized fabric and contain tubular air cells. The outer ply of the deicer is of conductive neoprene to provide resistance to deterioration by the elements and many chemicals. The neoprene also provides a conductive surface to dissipate static electricity charges. These charges, if allowed to accumulate, would eventually discharge through the boot to the metal skin beneath, causing static interference with the radio equipment.
Deicer boots are attached to the leading edge of wing and tail surfaces with cement or fairing strips and screws, or a combination of both.
Deicer boots which are secured to the surface with fairing strips and screws or a combination of fairing strips, screws, and cement have a bead and bead wire on each lengthwise edge. On this type installation, screws pass through a fairing strip and the deicer boot just ahead of the bead wire and fit into Rivnuts located permanently in the skin of the aircraft.
The new type deicer boots (figure 7-3) are completely bonded to the surface with cement. The trailing edges of this type boot are tapered to provide a smooth airfoil. By eliminating the fairing strips and screws, this type installation cuts down on the weight of the deicer system.
- Wing skin
- Inflated tube Deflated tube FIGURE 7-3. Deicer boot cross section.
The deicer boot air cells are connected to system pressure and vacuum lines by nonkinking flexible hose.
In addition to the deicer boots, the major components of a representative pneumatic deicing system are a source of pressurized air, an oil separator, air pressure and suction relief valves, a pressure regulator and shutoff valve, an inflation timer, and a distributor valve or a control valve. A schematic of a typical system is shown in figure 7-4.
In this system, air pressure for system operation is supplied by air bled from the engine compressor.
The bleed air from the compressor is ducted to a pressure regulator. The regulator reduces the pressure of the turbine bleed air to the deicer system pressure. An ejector, located downstream of the regulator, provides the vacuum necessary to keep the boots deflated.
The air pressure and suction relief valves and regulators maintain the pneumatic system pressure and suction at the desired settings. The timer is essentially a series of switch circuits actuated successively by a solenoid-operated rotating step switch. The timer is energized when the deicing switch is placed in the "on" position.
When the system is operated, the deicer port in the distributor valve is closed to vacuum and system operating pressure is applied to the deicers connected to that port. At the end of the inflation period the deicer pressure port is shut off, and air in the deicer flows overboard through the exhaust port. When the air flowing from the deicers reaches a low pressure (approximately 1 p.s.i.), the exhaust port is closed. Vacuum is re-applied to exhaust the remaining air from the deicer.
This cycle is repeated as long as the system is operating. If the system is turned off, the system timer automatically returns to its starting position.
A pneumatic deicing system that uses an enginedriven air pump is shown in figure 7-5. The right hand side of the system is illustrated, however, the left hand side is identical. Notice that inflatable deicers are provided for the wing leading edges and the horizontal stabilizer leading edges. Included in the system are two engine-driven air (vacuum) pumps, two primary oil separators, two combination units, six distributor valves, an electronic timer, and the control switches on the deicing control panel.
To indicate system pressure, a suction indicator and a pressure indicator are included in the system.
Pneumatic System Operation As shown in figure 7-5, the deicer boots are arranged in sections. The right-hand wing boots include two sections: (1) An inboard (inner boot 287
Supply air Vertical stabilizer boots Vacuum Distributor valves - Pressure regulator and shutoff valve (numbers indicate inflation sequence Pressure relief valve Pressure switch (vacuum)
- Suction relief Exhaust port valve (typical) Pressure switch
- Ejector Outboard Outboard Center Inboard Inboard R.H. Horizontal stabilizer boots L.H. Horizontal stabilizer boots
• Cooling fins Off (On pneumatic system From wing Press suction Deicing control panel ports of all boots. Note: System not in operation. Vacuum supplied to all FIGURE 7-4. Schematic of a pneumatic deicing system.
Al and outer boot B2) section and (2) an outboard (inner boot A3 and outer boot B4) section.
The right-hand horizontal stabilizer has two boot sections (inner boot A5 and outer boot B6). A distributor valve serves each wing boot section and another distributor valve serves both horizontal stabilizer boot sections. Notice that each distributor valve has a pressure inlet port, a suction outlet port, a dump port, and two additional ports (A and B).
Distributor valve A and B ports are connected to related boot A and B ports. Pressure and suction can be alternated through A and B ports by the movement of a distributor valve solenoid servo valve. Note also that each distributor valve is connected to a common pressure manifold and a common suction manifold. When the pneumatic deicing system is on, pressure or suction is applied by either or both engine-driven air (vacuum) pumps. The suction side of each pump is connected to the suction manifold. The pressure side of each pump is connected through a pressure relief valve to the pressure manifold. The pressure relief valve maintains the pressure in the pressure manifold at 17 p.s.i. The pressurized air then passes to the primary oil separator. The oil separator removes any oil from the air. Oil-free air is then delivered to the combination unit. The combination unit directs, regulates to 15 p.s.i., and filters the air supply to the distributor valves.
When the pneumatic deicing system is off, air pump suction, regulated at 4 in. Hg by an adjustable suction relief valve, holds the deicing boots deflated. Air pump pressure is then directed overboard by the combination unit.
DEICING SYSTEM COMPONENTS Engine-Driven Air Pump The engine-driven air pump is of the rotary; four vane, positive displacement type and is mounted on 288
Right-hand wing deicers Inboard section Outboard section Inner SAI% Outer B2 20 Inner Outer B4 Suction indicator Solenoid Solenoid valve Distributor Dump Distributor Dump Pressure valve indicator Pressure Primary oil separator Combination unit manifold To opposite wing engine air supply, distributor valves, $ Ram air and boots inlet Fressure relief valve - Oil drain Suction Ram air manifold inlet Shroud Air overboard Electrical and electronic control Engine-driven air pump Motor-operated 'Oil cooler shutoff valve Suction relief valve Distributor valve Check valve (solenoid B shown energized, pressurizing boot B6) Horizontal stabilizer boots
• A5 Solenoid Inner ¡ Outer To boots on opposite side of horiz, stabilizer Pressure SE Suction — Electrical connection FicURE 7-5. A pneumatic deicing system using an engine-driven air pump.