AC 20-30B

OC: tj.O/q / /: "·'/ j AC 20-308 DATE 7/20/81

ADVISORY CIRCULAR

DEPARTMENT OF TRANSPORTATION Federal Aviation Administration Washington, D.C.

FAR GUIDANCE MATERIAL Subject:AIRCAAFT POSITION LIGHI' AND ANTICOLLISION LIGHT INSTALLATIONS

1. PURPOSE. '!his circular sets forth acceptable ireans, but oot the ooly means, of showing compliance with the Federal Aviation Regulations (FAR) awlicable to installed IX)Sition lights and antioollision lights.

2. CAN::ELI.ATION. 'AC 20-30A dated April 18, 1968, is canceled.

3. REIATED FAR SF.cTIONS.

a. Sections •1385 through .1401 of FAR Parts 23, 25, 27, and 29.

b. Sections .33 and .73 of FAR Part 91.

4. REIATED READING MATERIAL.

a. Advisory Circular AC 20-74, Aircraft Position Lights and Antioollision Light Measurements.

b. Advisory Circular AC 43.13-2A, Acceptable Methods, Techniques, and Practices, Aircraft Alterations.

c. Technical Standard Order ('.ISO) C30b, Aircraft Position Lights.

5. BACKGROUND. Airworthiness regulations and Technical Standard Orders prescribe minimum intensities, light distribution, overlap limits, allowable obstructions to light visibility, and oolor for IX)Sition lights and antioollision lights.

6. IABORA'IORY MEASUREMENTS. Measurements of intensity, light distribution, and Iignt oolor are normally made under laboratory oonditions t,efore installation. Advisory Circular 20-74 oontains information ooncerning measurements of intensity, distribution, and oolor.

Initiated by: AWS-100

7. OOSITION LIGlIT SYSTEM INSTALLATION.

a. Location. In determining whether forward position lights have been "spaced laterally as far apart as practicable," and whether the rear position light has been ''mounted as far aft as practicable," as required by the FAR' s, each installation may be evaluated for special considerations.

Examples of special consideration are:

(1) Would the number of malfunctions be significantly increased due to increased vibration or other environmental conditions if the lights were spaced farther apart, or munted farther aft?

(2) Would accessibility for maintenance be significantly reduced if the lights ~re spaced farther apart or farther aft?

b. Rear Position Light Obstructions. A small light obstruction is per­ mitted within dihedral angle A (aft) described in§ .1387(d) of Parts 23, 25, 27, and 29. That obstruction is limited in size to O.04 steradian and in position to the 30° cone described in§ .1387(e) of Parts 23, 25, 27, and 29 and shown :in Figure 1. ~asurements to show compliance with the regulations can be made on actual aircraft or on appropriate scale draw:ings. The follow:ing procedure is one rreans of showing compliance with the regulations:

(1) On the side view drawing, draw a line through the light center perpendicular to the aircraft longitudinal axis. Draw a second line upward through the light center to the mst aft point en the vertical stabilizer.

The. angle z between the t¼O lines is limited by the airworthiness rules to 30°. Figure 1 shows an example of angle Z.

(2) On the rear view drawing, draw angle W which is fanned by two lines drawn upward from the light center to the maxinn.Jm right and left obstructions within angle z, When a protrusion causes a very small zone of obstruction it may be discounted unless total obstructions are near the regulatory limit.

When a rear view drawing is rot available, a combination of other drawings or measurements on the actual aircraft can be used to detemine angle w.

(3) Multiply angle z degrees by angle Wdegrees to obtain the amount of obstruction in square degrees. The rrethod is conservative, as obstructions as wide as angle Wmay rot exist throughout angle Z. Convert the rreasurement to steradians by dividing the square degree value by 3284. The number 3284 is a conversion factor to obtain steradians from square degrees.

8. ANTICOU.ISION LIGl:ITS

a. Airworthiness Requirements. Appendix 1, Tables 3 through 6., summarize the airworthiness requirements for anticollision lights and lists them according to the applicable amendments to the CAR's/FAR.'s. The airworthiness requirements for a specific aircraft can usually be determined by entering the applicable table with the latest amendment shown for the certification basis in the aircraft's type certificate data sheet.

Par 7 2

FIGURE 1. REAR POSITION LIGHT OBSTRUCTIONS

DIHEDRAL ANGLE

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REAR VIEW 3

AC2Q-30B 7/20/81

b. Operational Requirements. SOme airworthiness requirements have been made retroactive by .Amendment 91-90, which amended § 91.33(c)(3) of the F7\.R.

The tenn "initially installed" used in this section refers to new installations based on newly-approved design dnta or any installation \\bich includes a major change as defined in FAR 21.93(a) to the previously approved design data. In effect, new designs \\bich have had m previous FAA approval are considered initially installed systems. Anticollision light installations approved by Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance), FAA Fbnn 337, supplemental type certificate or amended type certificate prior to August 11, 1971, may be duplicated on like make and nod.el aircraft without being considered initially installed.

c. Cbstruction Measurements. When anticollision light oostructions are allowed within the required field of coverage, measurements on shadows, scale drawings or actual aircraft can be used to substantiate that solid angles of obstruction do mt exceed regulatory limits. When masking is used to prevent the iropainnent of crew vision, the mask becomes an additional light obstruction. 'Ihe arrount of obstruction caused by the mask depends mt only on the physical size of the mask, but also on the type and size of the light source. With rotating beacons, the mask obstruction may be slightly larger than indicated by the physical size of the mask. '!his condition results fran the lack of a sharp cutoff of light at the mask edges. As the reflector rotates, there is a gradual reduction of light near the mask edges due to the relatively large size of the light source. Accurate rceasurernent of mask obstruction can best be accomplished during the laooratory measurement of intensity and field of coverage. otherwise, total oostructions measured very near the regulatory limit may actually exceed that limit.

d. 'Ihe following procedures refer to the example shown in Figure 2.

Scale drawings and measurements from the light unit are used in substantiating compliance with the anticollision light requirements. 'Ihe procedure with variations can be applied to other aircraft. Variations include other shapes and vertical coverage requirements. 'Ihe procedure converts scale drawing obstructions to a plot of oorizontal versus vertical degrees \\here area units become s:J:Uare degrees. By counting the squares within c:bstructions and converting the sum t.o steradians, compliance can be shown. Scale drawings should be large enough t.o assure reasonable accuracy in the measurements. For the example, the vertical coverage requirement is +30°.

(1) Fuselage and Wings.

(i) Point selection. On the t.op view, establish enough p:>ints to adequately follow the shape of c:bstructions. In the example of Figure 2, seven p:>ints are established and numbered to represent the left half of the fuselage and the left wing. Because of sym:netry, measurements are limited to one side, and the measured c:bstructed area is doubled t.o account for the other side of the aircraft. Additional d:>structions, mt represented by the seven numbered p:>ints, include mask, wing struts, landing gear, and rudder.

These additional c:bstructions are considered separately.

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(ii) Horizontal ::!:!19les. On the top view of Figure 2, measure the horizontal angles between the aircraft centerline and lines connecting the anticollision light center to the numbered i;:oints.

(iii) Vertical at}9:les. On the side view of Figure 2, measure the vertical angles from the horizontal plane passing through the anticollision light center to lines connecting the light center to the numbered i;:oints.

(iv) Tabulation. Tabulate the measured horizontal and vertical angles as shown in Table 1.

NarE: In lieu of direct angle measurements, distance measurements may be used to calculate the angles using trigonometry relationships.

(2) Wiryg strut angles. On the top view of Figure 2, establish right and left i;:oints for the strut top and fore and aft i;:oints for the strut l::x:>ttom.

Horizontal angles for the strut l::x:>ttom points are approximately equal. Measure the horizontal and vertical angles to the four p:>ints and tabulate as shown in Table 1.

(3) Main 1 ear fairi~. On the top of Figure 2, establish four p:>ints for the main gearairing obstruction. '!he rear obstruction limit is the -30° vertical coverage requirement, and the forward obstruction extends to vertical angles of -27°. '!he effect of wheel fairing rounding is slight and is neglected. Measure the horizontal angles to the four p:>ints, and tabulate as shown in Table 1.

(4) Main gear strut. On the top view of Figure 2, establish four points for the main gear strut. Measure the horizontal and vertical angles to the four p:>ints, and tabulate as shown in Table 1.

(5) Rudder obstructions. On the side view of Figure 2, rudder d:>tstruc­ tions occur aft of the anticollision light ¼bile all other obstructions are forward. 'lherefore, it is practical to measure rudder obstructions independently.

On the side view of Figure 2, the rudder obstructs for 30 vertical degrees. Q1 the top view, the obstruction is 3 horizontal degrees on the left side. '!he left side rudder obstruction is 90 square degrees: i.e., 30° x 3° = 90 square degrees.

(6) Calculating obstructions. '!he gra:p1 of Figure 3 shows plotting of the collected data of Table 1 as vertical degrees versus horizontal degrees.

Each square is equal to one square degree so that obstructions can be measured by counting squares. Obstruction areas can be counted in zones rounded by vertical lines through the numbered p:>ints. Table 2 shows the counts including the rudder obstruction which was measured independent of the gra:p1. Square degrees are converted to steradians by dividing by 3284.

Director of Airworthiness Par 8 7

Appendix 1 -~ TABLE 1. OBSTRUCTION POINT ANGLES

OBSTRUCTION Wing/Fuselage Strut Gear Gear Strut '),, Mask (Positioned to eliminate reflections from prop) POINT 1 2 3 4 5 6 7 Top Right Top Left Bottom Upper Bottom Lower Front Right Front Left Rear Right Rear Left Top Front Top Rear Bottom Front Bottom Rear Top Centerline Top Left Bottom Centerline Bottom Left

HORIZONTAL VERTICAL

ANGLE ANGLE oo -7.5° 70 -7.0° 25° -5.5° 47° -7.5° 52° -12° 32° -13° ao -12° 22° -8.5° 24° -9.0° 70 -22° 70 -23.5° 12° -27°

15.5° -27° 13° -30°

16.5° -30° ao -24° so -25° 130 -2s.5° 13° -29.5° oo -10° 10° -10° oo -30° 10° -30° 1

Appendix 1 TABLE 2. OBSTRUCTION COUNT zam SQUARE DEGREES Point 1 to Point 2 159 Point 2 to Point 3 198 Point 3 to Point 4 132 Point 4 to Point 5 14 Total Grapi 503 Rudder 90 Total Left Side 593 TOI'AL AIRPIANE 1186 Steradians: (1186/3284) = 0.36 2

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TABLE :3. FAR 23/CAR 3 N - CXl -I-'

APPLICABLE INTENSITY VERTICAL COLOR FLASH LIGHT AMENDMENTS (CANDLES) COVERAGE SPECS. RATE OBSTRUCTION REMARKS

1. Before 3-1 No Anticollision (4/1/57) Light Requirements

2. Thru 3-1 100 +30 Aviation Red 40-100 0.03 STER. Position of Allowable (4/1/57) overlap 180 0.15 STER. Obstruction Limited

3. Thru 3-7 100 +30 Aviation Red 40-100 0.5 STER.

(5/3/62) overlap 180

4. Thru 23-11 400 +30 Aviation Red or 40-100 0.5 STER. New Aviation White (8/11/71) Aviation White overlap 180 Limits

5. Thru 23-20 400 +75 Aviation Red or 40-100 o.5 STER.

( 9/1/77) Aviation White overlap 180 NOTE: 91.33(c)(3) of the FAR also applies to anticollision light installations and allows the color to be either aviation red or aviation white.

I-' TABLE 4. FAR 25/CAR 4b

APPLICABLE INTENSITY VERTICAL COLOR FLASH LIGHT AMENDMENTS (CANDLES) COVERAGE SPECS. RATE OBSTRUCTION REMARKS

1. Before 4b-8 No Anticollision ( 5/16/53) Light Requirements

2. Thru 4b-8 If Used If Used If Used, On-Off ratio (5/16/53) Aviation Red 40-100 Not less than 1:75

3. Thru 4b-3 If Used If Used If Used, On-Off ratio ( 3/13/56) Aviation Red 40-100 Not less than 1:75 If Extra Light Installed, No Flash Rate Limit in overlaps

4. Thru 4b-4 100 +30 Aviation Red 40-100 0.03 STER. Position of Allowable (4/1/57) overlap 180 0.15 STER. Obstruction Limited .......

5. Thru 25-27 400 +30 Aviation Red or 40-100 0.03 STER. New Aviation White ( 8/11/71) Aviation White overlap 180 0.15 STER. Limits ­ (X) - N .....6. Thru 25-41 400 +75 Aviation Red or 40-100 0.03 STER.

( 9/1/77) Aviation White overlap 180 0.15 STER.

NOTE: 91.33(c)(3) of the FAR also applies to anticollision light installations and allows the color to be either aviation red or aviation white.

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-0 TABLE 5• FAR 27/CAR 6 ()) - N .....

APPLICABLE INTENSITY VERTICAL COLOR FLASH LIGHT

AMENDMENTS (CANDLES) COVERAGE SPECS RATE OBSTRUCTION REMARKS

1. Before Part 6 No Anticollision (4/1/57) Light Requirements

2. Thru 6-1 100 +30 Aviation Red 40-100 0.03 STER.

(4/1/57) overlaps 180

3. Thru 6-5 100 +30 Aviation Red 40-100 0.05 STER.

(5/3/62) overlaps 180

4. Thru 27-6 400 +30 Aviation Red or 40-100 0.5 STER New Aviation White (8/11/71) Aviation White overlaps 180 Limits

5. Thru 27-10 150 +30 Aviation Red 40-100 0.5 STER. Aviation White not (2/5/76) overlaps 180 acceptable NOTE: 91.33(c)(3) of the FAR also applies to anticolision light installations and allows the color to be either aviation red or aviation white.

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