FIGURE 6-36. Typical lap joint fillet weld.
large globules of molten metal will drop from the electrode and the base metal. The transfer of metal is aided by first shortening and then lengthening the arc. However, care should be taken not to carry too large a pool of molten metal in the weld. The procedures for making bead, groove, and fillet welds in the overhead position are discussed in the following paragraphs.
Bead Welds For bead welds, the electrode should be held at an angle of 90° to the base metal. In some cases, however, where it is desirable to observe the arc and the crater of the weld, the electrode may be held at an angle of 15° in the direction of welding.
Weave beads can be made by using the weaving motion. A rather rapid motion is necessary at the end of each semicircular weave to control the molten metal deposit. Care should be taken to avoid excessive weaving. This will cause overheating of the weld deposit and form a large pool of metal, which is hard to control.
Groove Welds (Butt Joints) Improved overhead groove welds can be made by using a backup strip. The plates should be prepared in a manner similar to preparing plates for welding butt joints in the flat position. If no backup strip is used and the plates are beveled with a featheredge, the weld will burn through repeatedly, unless the operator is extremely careful.
Fillet Welds When making fillet welds on overhead tee or lap joints, a short are should be held, and there should be no weaving of the electrode, The electrode should be held at an angle of about 30° to the vertical plate, and moved uniformly in the direction of welding.
The arc motion should be controlled to secure good penetration to the root of the weld and good fusion with the sidewalls of the vertical and horizontal plates. If the molten metal becomes too fluid and tends to sag, the electrode should be whipped away quickly from the crater ahead of the weld to lengthen the arc and allow the metal to solidify. The electrode should then be returned immediately to the crater of the weld and the welding continued.
Welding on heavy plates requires several passes to make the joint. The first pass is a string bead with no weaving motion of the electrode. The second, third, and fourth passes are made with a slight circular motion of the end of the electrode, while the top of the electrode is held tilted at an angle of about 15°.
Vertical Position Welding The vertical position, like the overhead position just discussed, is also more difficult than welding in the flat position. Because of the force of gravity, the molten metal will always have a tendency to run down. To control the flow of molten metal, a short arc is necessary, as well as careful are voltage and welding current adjustments.
In metallic arc welding, current settings for welds made in the vertical position should be less than those used for the same electrode size and type on welds made in the flat position. The currents used for welding upward on vertical plate are slightly lower than those used for welding downward on vertical plate. The procedure for making bead, groove, and fillet welds in the vertical position are discussed in the following paragraphs.
Bead Welds When making vertical bead welds, it is necessary to maintain the proper angle between the electrode and the base metal to deposit a good bead. In welding upward, the electrode should be held at an angle of 90° to the vertical. When weaving is necessary, the electrode should be oscillated with a "whipping up" motion. In welding downward, bead welds should be made by holding the top end of the electrode at an angle of about 15° below the hori.
zontal to the plate with the arc pointed upward toward the oncoming molten metal. When a weave bead is necessary, in welding downward, a slight semicircular movement of the electrode is necessary.
In depositing a bead weld in the horizontal plane on a vertical plate, the electrode should be held at 275
right angles to the vertical. The top of the electrode should be tilted at an angle at about 15° toward the direction of welding to obtain a better view of the arc and crater. The welding currents used should be slightly less than those required for the same type and size of electrode in flat position welding.
Groove Welds (Butt Joints) Butt joints in the vertical position are "groove welded" in a manner similar to the welding of butt joints in the flat position. To obtain good fusion with no undercutting, a short arc should be held, and the motion of the electrode should be carefully controlled.
Butt joints on beveled plates ¼ in. in thickness can be groove welded by using a triangular weaving motion, In groove welding butt joints in the horizonta. position on identical plates, a short arc is necessary at all times. The first pass is made from left to right or right to left, with the electrode held at an angle of 90° to the vertical plate. The second, third, and, if required, any additional passes are made in alternate steps, with the electrode held approximately parallel to the beveled edge opposite to the one being welded.
Fillet Welds When making fillet welds in either tee or lap joints in the vertical position the electrode should be held at an angle of 90° to the plates or at an angle of up to 15° below the horizontal, for better control of the molten puddle. The arc should also be held short to obtain good penetration, fusion, and molten metal control.
In welding tee joints in the vertical position, the electrode should be moved in a triangular weaving motion. The joint should be started at the bottom and welded upwards. A slight hesitation in the weave, as shown in figure 6-37, will improve sidewall penetration and allow good fusion at the root of the joint. If the weld metal overheats, the electrode should be lifted away quickly at short rapid intervals without breaking the arc. This will allow the molten metal to solidify without running down. The electrode should be returned immediately to the crater of the weld to maintain the desired size of the weld.
When more than one layer of metal is needed to make a vertical tee weld, different weaving motions may be used. A slight hesitation at the end of the weave will result in good fusion without undercutting the plate at the edges of the weld. When welding lap welds in the vertical position, the same pro- Hesitate Direction of welding For 1/4", 3/8", and 1/2" size fillets FICURE 6-37. Vertical tee joint fillet weld.
cedure is followed as that outlined for welding vertical tee joints, except that the electrode is directed more toward the one vertical plate. Care should be taken not to undercut either plate, or to allow the molten metal to overlap the edges of the weave. On heavy plate, lap joints require more than one layer of metal.
WELDING OF AIRCRAFT STEEL STRUCTURES Oxyacetylene or electric arc welding may be utilized for repair of some aircraft structures, since most aircraft structures are fabricated from one of the weldable alloys; however, careful consideration should be given to the alloy being welded since all alloys are not readily weldable. Also, certain structural parts may be heat treated and therefore could require special handling. In general, the more responsive an alloy steel is to heat treatment, the less suitable it is for welding because of its/tendency to become brittle and lose its ductility in the welded area. The following steels are readily weldable: (1) Plain carbon of the 1000 series, (2) nickel steel of the SAE 2300 series, (3) chrome/nickel alloys of the SAE 3100 series, (4) chrome/molybdenum steels of the SAE 4100 series, and (5) low-chrome/ molybdenum steel of the SAE 8600 series.
Aircraft Steel Parts Not To Be Welded Welding repairs should not be performed on aircraft parts whose proper function depends on strength properties developed by cold working, such as streamlined wires and cables.
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