Rev. 1977 AC 43.13-2A
(2) Obstructed Visibility. Measure all solid angles of obstruction within the required field of coverage. For multiple light installations, coverage between the mounting levels is not necessary. When a multiple light installation is being repeated in each independent measurement (overlap) should be counted. Methods for determining the amount of obstructed visibility are given below; howerer, other methods can give acceptable results.
(a) Wall Shadows. This procedure is applicable to installations where shadows from light obstructions appear on a vertical surface such as a hangar wall. Validity is based on two facts: (1) that a vertical surface can approximate a sphere surface if the distance from the light is considerable, and the shadow is reasonably small, and (2) that sphere surface area can be converted to steradians by dividing by the radius squared.
1 Position the aircraft in a darkened hangar so that longitudinal axis is perpendicular to a hangar wall. Level as for weight and bal- The distance extend in each direction within at least 30° above and 30° below the horizontal plane of the aircraft, except that there may be solid angles of obstructed visibility totalling not more than the requirements of paragraph .1401(b) of the applicable airworthiness regulations.
5 For installations where shadows are restricted to directly aft and centered about the longitudinal axis, the following procedures apply:
a Establish a point on the wall which corresponds to a line parallel to the longitudinal axis and through the light associated with the shadow.
& Measure the distance from the light to the point and determine the area representing
0.15 steradians (A=0.15d'). The distance (d) should be at least 20 feet.
c Draw a circle, with the established point as the center, having an area equal to that found in b above.
d If the shadow falls within this circle, its position is acceptable. For multiple light installations, consider only the shadow above light level.
e If the shadow is partially out of the circle, the shape of the 0.15 steradian area may be varied, but the established point should remain at the center of the area (centroid).
(b) Ramp Shadows. This procedure is applicable to shadows which appear on a horizontal plane such as a flat level ramp and will be associated with a top mounted light and a
0.5 steradian limit. Area measurements as described in the wall shadow method should not be used. Some error is inherent, because horizontal angles are measured on a plane displaced from the light source. To compensate for these measinstallations, only shadows above the level of the urement errors, a table (fig. 4.8) is furnished top light should be considered. to convert from measured solid angles to true 3 Compute the solid angle obstruction solid angles. A term "square degrees" is used in steradians, by dividing each shadow area by to aid in the discussions of solid angle measurethe square of the distance from the center of the ments.
area to the light. 1 If no masking is required, remove the 4 Evaluate the results to determine if do mine te red cover and attach its clamp ring to the light the system consists of enough lights to illuminate the physical configuration and flight character- the vital areas around the aircraft, considering istics of the aircraft. The field of coverage must base. If masking is used, obtain a clear cover and install it with a duplicate mask. available dark ramp. A minimum of 50 feet 2 Center the aircraft on the largest Chap 4 28
Rev. 1977 AC 43.13-2A radius of clear ramp space will usually be needed.
If the installation is symmetrical, clear ramp space will be needed on one side only. Level the aircraft as for weight and balance check. Trim the flaps, rudder, elevator and ailerons. With jacks in place, raise the gear if the measurement results would otherwise be affected.
4 At night, turn on the anticollision light and chalk all shadow patterns (except jack shadows) which appear outside the circle.
the light rotates so as to cause the shadows to oscillate, lay twine along the outermost edge of the shadow, and chalk along the line.
5 Move the aircraft to facilitate area 3 Chalk the following marks on the measurements. Measure, sketch, and chart the ramp: ramp marks and other information as shown in a A reference point directly below Figure 4.6. Measure enough points along the the light. shadow patterns to accurately describe them.
b A circle centered on this reference Make enough sketches to include all shadows.
point having a radius equal to 1.732 times the 6 Convert the above measurements to a height of the light above the point. (The circle graph showing vertical degrees vs. horizontal represents area beyond the minus 30° vertical degrees as shown in Figure 4.7 (first quadrant).
The second quadrant will have to be measured tudinal centerline which passes through the ref- erence point. « A line parallel to the aircraft longi- also to obtain the total shadow area for one side. If the shadow pattern is symmetrical, no other measurements will be necessary. Use 1 degree for each space on the graph paper and count the square degrees of shadow. A total of 1,642 or IST QUADRANT less is within limits. A total of 1,872 or more is out of limits, and if possible, should be reduced O EXAMPLE POINT mask. If the count is between 1,642 and 1,872, by adding or moving a light, or by trimming a proceed as follows: For each 1 degree segment of vertical (pitch), convert the counted square degrees to true square degrees by use of the table of Figure 4.8. If the sum of the true square degrees from all segments exceeds 1,642 (0.5
- SHADOW steradian), the installation is out of limits.
17.3 FT. (c) Scale Drawings. Accurate scale draw- 1OFT. BELOW LIGHT POINT ON RAMP ings can be used to measure solid angles of obstruction. Such drawings should have sufficient size and accuracy to give dependable results. In some cases, actual measurements can be combined POINT O with small drawings as shown in Figure 4.9. For
• 20 the 6 points established on the left wing, a string can be used to connect the light successively to EXAMPLE POINT O A = 20 FT. each. A protractor can then be used to measure 8 = 20 FT. the vertical angle from level. The horizontal H = 10 FT. DISTANCE TO POINT I = 28.2 FT. angle for each point can be measured on the top HORIZONTAL ANGLE = 45° view (center). When both horizontal and verti- VERTICAL ANGLE = 19.4° cal angles for each point have been determined, they can be plotted on a graph as shown in Fig- FIGURE 4.6.-Ramp shadow pattern. ure 4.10. If a symmetrical condition exists, only the first and second quadrants need be measured.
d A line perpendicular to the above The first quadrant contains approximately 450 line, which also passes through the reference square degrees of obstruction. The other wing point. quadrant will double this to 900 square degrees.
Chap 4 29
Rev. 1977 AC 43.13-2A
• - EXAMPLE O:
DEGREES BELOW LIGHT (PITCH) HORIZONTAL ANGLE = 45° PITCH ANGLE = 19.4° TAN PITCH ANGLE = 10/28.2 o 20 - FUSELAGE WING SHADOW 28.2 SHADOW
LIGHT BASE SHADOW 30 20 30 50 60 70 80 90
HORIZONTAL DEGREES FROM NOSE FIGURE 4.7.-Conversion to vertical vs. horizontal degrees.
A measurement of the fin and rudder shadows adds approximately 100 for a total of 1,000 square degrees. Since a maximum of 1,642 square degrees is allowed (0.5 steradians), this installation is well within limits. Due to limitations of the method, results within 10% (0.05 steradians) of the limit are questionable. Many times a mask is required to prevent reflections into the cockpit.
In the example of figures 4.9 and 4.10, the installed mask blocks the light for +10 horizontal degrees and from -10 to -30 vertical degrees.
These 400 square degrees were measured at the
• light.
When larger drawings are used and no actual aircraft measures are made, vertical angles should not be taken directly from the drawing, but should be computed as follows:
1 On the aircraft side view, measure the vertical distance from a point to the light level.
2 On the top view, measure the distance from the point to the light.
3 Compute:
vertical distance Tangent of vertical angle= horizontal distance Pitch Segment (Degrees) Messured Degrees Square Factor Correction Square True Degrees (Degrees) Segment Pitch Measurco Square Degrees Correction Factor True Degrees Square 24-23 22-21 30-29 29-28 28-27 27-26 26-25 25-24 23-22 21-20 20-19 19-18 18-17 17-16 16-15 90 4826 90 • 87036 • 88701 • 87882 • 89493 • 90259 • 90996 • 91706 • 93667 • 94264 • 96363 • 95372 • 92388 • 93042 • 95882 , 94832 78.33 39.03 79.09 12-11 15-14 14-13 13-12 11-10 10-9 7-6 9-8 8-7 3-2 2-1 1-0 6-5 5-4 4-3 • 96815 • 98629 • 98325 • 98902 • 97237 • 97630 • 97992 • 99996 • 99813 • 99905 . 99144 • 99540 • 99966 • 99692 • 99357 TOTAL FIGURE 4.8.-Conversion to true square degrees-1st quadrant.
Chap 4 30
Rev. 1977 AC 43.13-2A
(d) Flashing Characteristic. Turn off any VERTICAL DISTANCE flashing supplementary lighting. Observe the POINT A flashing of the anticollision light system at a point where each light can be observed independently, and determine that each flashing rate is between 40 and 100 flashes per minute. For multiple light systems, observe at a point where overlap occurs, and determine that the combined flashing rate does not exceed 180 flashes per HORIZONTAL minute. Flashing outside the required field of
DISTANCE POINT A HORZ. = 10° coverage is not necessary.
-VERT. = 17.5° POINT POINTE H. = 35° DEGREES BELOW LIGHT 10
LEFT WING STRUT MASK GEAR
HORIZONTAL DEGREES FROM NOSE FIGURE 4.10.-Solid angle blockage1st quadrant.
FIGURE 4.9.- -Scale drawings. 57.-60. [RESERVED] Chap 4 31
AC 43.13-2A Rev. 1977 Chapter 5. SKI INSTALLATIONS Section 1. SELECTION OF SKIS
61. DETERMINING ELIGIBILITY OF AIRCRAFT. eligible for ski installations in accordance with this chapter. Eligibility can be determined by Only aircraft approved for operation on skis are referring to the Aircraft Specifications, Type mary of Supplemental Type Certificates, or by Certificate Data Sheets, Aircraft Listing, Sum- contacting the manufacturer. Also determine the to the aircraft to make it eligible for ski opera- need for the nature of any required alterations tion. 62. IDENTIFICATION OF APPROVED MODEL SKIS. Determining that the skis are an approved model plate or placard displayed on the skis. A Techni- can be done by referring to the identification cal Standard Order (TSO) number; Type Cer- ber, if the skis have been approved as a part tificate (TC) number; or an aircraft part num- of the aircraft, will be shown thereon if the skis 63. MAXIMUM LIMIT LOAD RATING. In order are approved models. for an approved ski to be installed on any given -2 5
- 9 10
6. Fabric removed to facilitate inspection
1. Fitting 7. Check Cable
2. Shock Cord 8. Clevis
3. Safety Cable 9. Ski Pedestal
4. Tape 10. Pedestal Height
5. Crust-cutter Cable FIGURE 5.1.-Typical ski installation.
33 Chap 5
AC 43.13-2A aircraft, determine that the maximum limit load rating (L) as specified on the ski identification plate or placard is at least equal to the maximum static load (S) times the limit landing load factor (n) previously determined from static drop tests of the airplane by the aircraft manufacturer.
In lieu of a value y determined from such drop tests, a value of y determined from the following formula may be used:
n =2.80 + W + 4000 9000 where "W" is the certificated gross weight of the airplane.
34 Rev. 1977 Skis approved for airplanes of greater gross weight than the airplane on which they are to be installed may be used provided the geometry of the ski is similar to that of a ski previously approved for the airplane (not more than 10 percent difference in width or length of contact surface). This limitation is to assure that the performance of the airplane will not be adversely affected by oversize skis.
64. LANDING GEAR MOMENT REACTIONS. In order to avoid excessive moment reactions on the landing gear and attachment structure, the ski pedestal height must not exceed 130 percent of the axle centerline height with the wheel and tire installed.
65. [RESERVED] Chap 5
Rev. 1977 AC 43.13-2A Section 2. CONVERSION AND INSTALLATION
66. HUB-AXLE CLEARANCE. The pedestal hub should fit the axle to provide a clearance of .005" minimum to .020" maximum. Hubs may be bushed to adjust for axle size, using any ferrous or nonferrous metal, hard rubber, or fiber. If rubber or fiber bushings are used, use retaining washers of sufficient size on each side to retain the hub if the bushing should slip or tail. (See fig. 5.2.)
67. CRUST-CUTTER CABLES. Crust-cutter cables are optional. However, when operating in severe crust conditions, it is advisable to have this cable installed to prevent the shock cord from being cut if the nose of the ski breaks through the crust while taxiing.
68. CABLE AND SHOCK CORD ATTACHMENT AND ATTACHMENT FITTINGS. Service reports indicate that failure of the ski itself is not a predominant factor in ski failures. Rigging (improper tension and terminal attachments) and cast-type pedestal material failures are predominant. Usually, failures of the safety cable and shock cord attachment fittings occur at the ski end and not at the fuselage end.
Do not attach tension cords and safety cables at the same point on the fuselage fittings. Provide separate means of attaching cables and shock cords at the forward and aft ends of the skis.
Usually, approved skis are supplied with cables, shock cord, and fittings; however, the following specifications may be used for their fabrication and installations:
retaining washers bushings pedestal hub axle FIoURE 5.2.-Typical hub installation.
Chap 5 35
AC 43.13-2A Rev. 1977 Minimum Cable and Shock Cord Sizes Ski Single Double Limit Single Double Crust- Crust- Single Double Load Safety Safety Cutting Cutting Shock Shock Rating Cable Cable Cable Cable Cord Cord 1500-3000 1/8" 1/8" 1/8' 1/8"' 1/2" 1/2" 3000-5000 1/8'' 1/8" 1/2" 5000-7000 5/32' 5/32'1 5/32" 3/4"' 3/4" 7000-9000 3/16" 5/32" 3/4"
a. Cables. Make the check cable, safety cable, and crust-cutting cable ends by the splice, swage, or nicopress methods; or if adjustable lengths are desired, use cable clamps. Use standard aircraft hardware only. (Hardware used to attach cables must be compatible with cable size.)
b. Shock cord ends may be fabricated by any of the following methods:
(1) Make a wrapped splice using a proper size rope thimble and No. 9 cotton cord or .035" (minimum) safety wire. Attach with clevis or spring steel snap fastener. (Do not use cast iron snaps.)
(2) Use approved spring-type shock cord end fasteners.
c. Fitting Specifications (see figure 5.3) and Installation:
(1) Fittings fabricated for 1/8-inch cable or 1/2-inch shock cord shall be at least .065" 1025 steel or its equivalent.
(2) Fittings fabricated for 5/32-inch cable or 3/4-inch shock cord shall be at least .080" 1025 steel or its equivalent.
FIGURE 5.3.-Typical fuselage fitting.
(3) An improperly installed fitting may impose excessive eccentric loads on the fitting and attach bolts and result in failure of the fitting or bolts.
69. PROVISIONS FOR INSPECTION. Aircraft using fabric-covered landing gear may have the lower 4 inches of fabric removed to facilitate inspection of the axle attachment area. (See fig. 5.1.)
70. [RESERVED] 36 Chap 5
Rev. 1977 AC 43.13-2A Section 3. RIGGING OF SKIS
71. LOCATION OF ATTACH FITTINGS ON FUSE- zero to 5-degree ski incidence angle (reference LAGE OR LANDING GEAR. Locate fittings so the figs. 5.4 and 5.6).
shock cord and cable angles are not less than b. Adjust length of safety cable to provide 20° when measured in the vertical plane with -20 to - 35-degree ski incidence angle (reference figs. 5.5 and 5.6).
the shock absorber in the fully extended position (see angle B, figs. 5.4 and 5.5). 73. TENSION REQUIRED IN MAIN SKI SHOCK CORDS. Apply sufficient shock cord tension to NoTE: Do not attach fittings to wing-brace struts, fore end of the skis to prevent flutter at various except by special approval (manufacturer or FAA).
72. MAIN SKI INCIDENCE ANGLES. (Aircraft leveled and shock absorbers fully extended.)
a. Adjust length of check cable to provide a airspeeds and attitudes. Because of the various angles used in attaching shock cord to the skis, shock cord tension cannot be specified. In most installations the downward force applied at the fore end of the ski, sufficient to cause the check FIGURE 5.4.-Main ski at maximum positive incidence (check cable tight).
Chap 5
AC 43.13-2A Rev. 1977 FIGURE 5.5.--Main ski at maximum negative incidence (safety cable tight).
cable to slacken, should be approximately as b. Where it is possible for the nose ski rigging listed below: to contact the propeller tips due to vibration, install a ¼-inch shock cord to hold the rigging Ski Limit Doronroard out of the propeller arc.
Load Force Rating (pounds) 75. TAIL SKI INSTALLATION.
a. Use tail skis that have been approved on 1500-3000 20-10 airplanes of approximately the same weight 3000-5000 40-60 (within 10 percent) or select as outlined in sec- 5000-7000 60-120 7000-9000 120-200
74. NOSE SKI INSTALLATION. The nose ski is installed in the same manner as the main skis (see fig. 57) except:
a. Adjust length of safety cable to provide
- 5- to - 15-degree ski incidence. FIGURE 5.6.-Main ski incidence angles.
38 Chap 5
Rev. 1977 tion 1. Depending upon the type of ski selected, the tail wheel may or may not have to be removed.
b. Adjust the length of the limiting cable (ref. fig. 5.8) to allow the ski to turn approximately 35 degrees either side of the straight- AC 43.13-2A forward position with the weight of the airplane resting on the ski.
c. The shock cord (ref. fig. 5.8) must be of a length that will hold the ski in the straightforward position during flight.
FIGUBE 57.-Typical nose ski installation.
Chap 5 39
AC 43.13-2A Rev. 1977 horiz.
ref.
LIMITING - SHOCK
CABLE CABLE FIGURE 5.8 Typical tail ski installation.
76.-78. [RESERVED] Chap 5 40