AC 20-35C

Advisory U.S. Department of Transportation Circular Federal Aviation Administration Subject: Date: 7/12/83 AC No: 20-35C TIEDOWN SENSE Initiated by: AWS-340 Change:

1. PURPOSE. This advisory circular provides updated information of general use for aircraft tiedown techniques and procedures.

2. CANCELLATION. AC 20-35B, TIE-DOWN SENSE, dated April 19, 1971, is canceled.

windstorms because of inattention to weather forecasts, negligence, or improper BACKGROUND. Each year numerous aircraft are needlessly damaged by tiedown procedures. Windstorms may be broadly classified as cyclonic storms or low pressure systems, regional or localized terrain induced winds, thunderstorms or tornado induced winds and hurricanes.

4. STORMS.

Cyclones. Cyclones are the lows of the weather maps. In the United States the name does not suggest any degree of intensity in the purest meteorological sense and is applied to moderate as well as intense storms. Winter storms are atmospheric disturbances that may become intense low pressure systems churning over tens of thousands of square miles. In our northland the severity of these storms makes them seasonal threats. These intense low pressure systems combine winds sometimes as strong as 90 mph with snow and cold temperatures.

The National Weather Service (NWS) issues timely watches and warnings against the hazards of winter weather so that persons in the alerted area may take precautionary measures.

b. Regional or Localized Terrain Induced Winds. Geography is a factor in producing local and sometimes strong winds due to peculiar or unique terrain effects. The Santa Ana winds of Southern California, or the winds in Boulder, Colorado, are examples of infrequent localized winds that are generally forecast far enough in advance to minimize the potentially damaging effects upon parked aircraft.

c. Thunderstorms. Individual thunderstorms may measure from less than 5 miles to more than 30 miles in diameter, and may occur both in isolation and as a part of larger weather systems. In some places and seasons thunderstorms recur almost daily at almost the same time. In other places they occur rarely or irregularly. Some last a few minutes and on other occasions a succession of related events may last most of the day. The significant features of thunderstorms are high wind velocity, lightning, intense precipitation and hall. All these are variable features that appear in many combinations. Strong straightline winds accompany thunderstorms more often than tornadoes and may be as damaging to persons and property as small tornadoes. Strong and shitting winds along thunderstorm gust fronts have been associated with tragic accidents to commercial aircraft.

Average number of Thunderstorm days per year

HUNDERSTORM DAYS Puerto Rico and Virgin Islands 50 Alaska and Hawaii are less than 10 The map above shows the yearly average number of days with thunderstorms based on observations in the U.S. A thunderstorm day is considered any day during which one or more thunderstorms occur. It should be realized, however, that there are local variations which do not show on this map because of the sparsity of observations from some areas. July and August are the months with the greatest number of thunderstorms over most sections of the U.S. while December and January have the least number.

2 Par 4

d. Hurricanes. Compared to the great cyclonic storms of the Temperate Zone, hurricanes are of moderate size and their worst winds do not approach tornado velocities. Their winds exceed 70 mph and may reach 200 mph, and their lifespan is measured in days or weeks, not minutes or hours. No other atmospheric disturbance combines duration, size, and violence more destructively.

Hurricanes are a threat to the Gulf and East Coast during the hurricane season from June through November. Decaying tropical storms may on rare occasions move inland and dump flash-flood producing rainfall over California and the desert southwest. Timely detection of and warning against hurricanes has been the task of the NWs for nearly a century.

(1) Hurricane Camile slammed into the Northern Gulf Coast near Christian, Mississippi, on the night of August 17, 1969. Camile's 200 mph winds produced a devastating tidal storm surge of nearly 25 feet just east of where the onter made landfall. Camile ranked as one of the most destructive storms ever to strike the U.S. Total damage was about 1.4 billion dollars with 256 deaths credited to the hurricane along the Gulf Coast. The remnants of the dying storm produced killer flash floods in Virginia.

(2) Hurricane Frederic in September 1979 brought 160 mph winds and 12-foot tidal storm surges to a densely populated area in and near Mobile, Alabama. Destruction was widespread and damage was estimated at more than 2 billion dollars. Precise forecasts and warnings, together with prompt local action, permitted about a quarter of a million people to evacuate to safe areas.

The loss of life was extremely low -- five people died. President Carter credited the low death toll of Hurricane Frederic to accurate and timely warnings coupled with orderly and extensive evacuation.

e. Tornadoes. Tornadoes are fearsome storms that usually move from the southwest. The more severe tornadoes move at forward speeds of about 60 mph accompanied by winds of 200 mph or more. Since the tornado core is rarely larger than .6 mile in diameter, destructive winds generally last at one place for less than a minute but practically all structures and much of the natural environment can be destroyed by severe tornadoes in just a few seconds. More intense than the worst hurricanes, tornadoes command much attention because of their sudden and violent onslaught and occasional sharp alteration in path.

Storm forecasting and warning operations (including weather advisories to the aviation public) help reduce the harmful effects of storms.

AVERAGE NUMBER OF TORNADOES AND TORNADO DEATHS BY STATE 1953 - 1980

TORMADO DEATHE Par 4 3

TORNADOES, DEATHS, AND TORNADO DAYS 1953-1980 1200

TORNADOE DEATHS 1856 868 700 - 550 YEAR 1953 | 1954 | 1955 |1956 DAYS / 135 TORNADO INCIDENCE BY MONTH 1953-1980 160 155 140 -

TORNADOES 120- 107, 100 - 81 80 60

TORNADO DAYS 49 40 38 38 26 25 20 16 15 18 6 DEATHS 13

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Par 4

AC 20-350 7/12/83 Average Annual Tornado Incidence Per 10,000 Square Miles 1953 - 1980

MONTHS OF PEAK TORNADO ACTIVITY Par 4

STORM BELTS A Primary hurricane danger zone.

/// Fringe area.

Tornado belt.

HURRICANE DAMAGE 6 Par 4

7/12/83 AC. 20-35C is, of course, to fly the aircraft out of the impending storm area provided you have sufficient warning time. The next best protective measure is to secure the aircraft in a stormproof hangar or other suitable shelter. The remaining alternative is to assure that the aircraft is tied down securely.

securing your aircraft, it is considered good practice to fasten all doors and windows properly, thereby minimizing damage inside the aircraft. Engine openings (intake and exhaust) for both reciprocating and gas turbines should be covered to prevent entry of foreign matter. Pitot-static tubes should also be covered to prevent damage or entry of foreign matter. Make sure your neighbor's aircraft is also tied down.

6. ADVANCED PLANNING. It is the mission of the NWS to help mitigate the threat to life and property from natural hazards through the issuance of tornado and severe thunderstorm watches and warnings. NWS meteorologists at the National Severe Storms Forecast Center (NSSFC) monitor atmospheric conditions utilizing information from many sources and locations. When hazardous conditions are anticipated or detected, watches or warnings are issued.

Watches are issued by the NSSFC to indicate when and where severe thunderstorms and/or tornadoes are most likely to occur. Watches are usually issued for areas about 140 miles wide, 200 miles long and generally 2 to 4 hours in advance of severe weather. Listen to the National Oceanic and Atmospheric Administration (NOAA) weather radio (162.400-162.550MHZ) continuous broadcasts for the latest weather information directly from NWS offices, and use commercial radio or television for further information.

b. Warnings are issued by local NWS offices when severe thunderstorms or tornadoes are indicated by weather radar, weather observers or trained spotters.

A warning describes an imminent risk from a tornado or severe thunderstorm in a relatively small area such as one or several counties. The key to damage avoidance or reduction is to be routinely weather conscious.

c. Be prepared for the worst conceivable windstorm conditions: pouring rain, gusty winds ranging from 30 mph and up, for example intermittent sheets of water blowing across the runways, ramps, and parking areas, and lack of hangar facilities. With such conditions in mind, aircraft owners and operators should plan in advance by learning their aircraft manufacturer's instructions for tiedown; location and/or installation of tiedown rings for attachment of tiedown ropes; any special instructions for securing nosewheel type aircraft vs.

tailwheel type aircraft; and manufacturer's charts and graphs denoting aircraft weights and relative wind velocities that would make varied tiedown procedures necessary for pending weather emergencies.

7. TIEDOWN FACILITIES. Any aircraft parking area should be equipped for three-point tiedowns. Aircraft should be tied down at the end of each flight to preclude damage from sudden storms. The direction in which the aircraft are to be parked and tied down will be determined by prevailing or forecast wind direction.

Par 5 7

a. Aircraft should be headed into the wind, or as nearly as possible, depending upon the locations of the fixed parking area mooring points.

MAJOR AXIS

b. Spacing of tiedowns should allow 10 for ample wingtip clearance. Spacing should be equal to the major axis (wingspan or fuselage length) of the largest aircraft usually operated plus 10 feet.

WINGSPAN

c. After the aircraft is properly located, lock the nosewheel or the tailwheel in the fore-and-aft position.

(1) Tiedown anchors for single-engine aircraft should provide a minimum holding power (strength) of approximately 3,000 pounds each. The type of anchors in use varies depending upon the type of parking area- whether for a concrete paved surface, a bituminous paved surface, or an unpaved turf area.

Location of tiedowns are usually indicated by some suitable means, either white or yellow paint, or a painted tire which has been fastened into the ground, or surrounding the tiedown anchor with crushed stone. The tiedown anchor eye should not protrude more than 1 inch above ground.

TIEDOWN ANCHORS FOR CONCRETE PAVED AREAS 1½ MIn. 4" TO 5" - TROWELED DEPRESSION „CONCRETE

• ROD 2'-0

CASTING -

MALLEABLE IRON CONCRETE AVEMENT 4" TO 5 / TROWELED SSION

• ROD

• Dial 10" Dia

• ROD

SECTION A - A Par 7

TIEDOWN ANCHOR FOR

BITUMINOUS PAVED AREAS 1½ WEARING SURFACE 3" BITUMINOUS BASE- 1-6" TO 2'-0"

LY BASE SUB- DO CONCRET SUBGRADE 3" Ø ROD APPROX. 2-6" LONG

TIEDOWN ANCHORS FOR TURFED AREAS 1½" MIN. EYE 1%" MIN. EYE

• GROUND LINE GROUND LINE S REINF. STEEL ROD APPROX %%" DIA. APPROX. ~5% DIA. ROD 2'-0" SQUARE CONCRETE BLOCK < DISG BLADE DISCARDED FARM

BOLTED THROUGH DISC

ROD AND BLOCK ANCHOR MUSHROOM TYPE ANCHOR Par 7

DON'T depend on wooden stakes

(2) Stake-driven tiedowns such as depicted above will almost invariably pull out when the ground becomes soaked from torrential rains which accompany hurricanes and some thunderstorms.

(3) Tiedown ropes capable of resisting a pull of approximately 3,000 pounds should be used. Manila ropes should be inspected periodically for mildew and rot. Nylon or dacron tiedown ropes are preferred over manila ropes. The objection to manila rope is that it shrinks when wet, is subject to mildew and rot, and has considerably less tensile strength than either nylon or dacron.

10 Par 7

COMPARISON OF TIEDOWN ROPES

FIBER CORDAGE - TYPICAL WEIGHTS AND

MINIMUM BREAKING STRENGTHS (POUNDS) NOMINAL SIZE SANILI (HIgh Tonscity ALT.) WYLON • M.T. POLYESTER BU PONT DACRON POLYOLEFINS (19.7.) and /or Pelyethylese) (Polypropylese -OLE AT POLYESTER/POL TOLEFI SOUBLE BRAIS Dia. Circ. (Jaches) 100° Net Ft. per •. Strength Breaking 100° Net wr. Ft. strangti breaking 100 Breaking Strength • Breaking Strength 100 Breaki trenz Brooke trend

1.47 450 100 1,000 1.3 1,000 137 750 .75 | 133 900 Ha 3% 1.96 600 1.5 66.6 1,700 2.1 47.5 1,700 124 80 1,250 1.66 60.3 2,100 1.7 60.2 1,700 * 1 2.84 35 1,000 2.5 40 2,650 3.3 30 2,550 1.88 53 1,850 2.78 36 3,500 26 38.4 2,600 % 1% 4.02 25 1,350 3.6 28 3,650 4.7 21.3 3,500 2.9 34.5 2,600 3.33 30 4,200 3.5 28.5 3,500 Ke 1% 5.15 19.4 1,750 5 20 5,100 6.3 15.9 4,800 3.9 25.5 3,400 5.0 20 6,000 5.1 20 5,100 ½. 14½ 7.35 13.8 2,650 6.6 15 6,650 82 12.2 6,100 4.9 20.4 4,150 6.67 14.9 7,500 6.8 15 6,800 Ke 1% 10.2 9.8 3,450 8.4 11.9 8,500 10.2 9.8 7,700 6.2 16 4,900 8.33 12 9,500 NA NA * 2 13.1 7.6 4,400 10.5 9.5 10,300 13.2 7.6 9,500 12.8 5,900 12,000 11,000 % 2% 16.3 6.1 5,400 14.5 6.9 14,600 5.6 13,200 9 7,900 15.0 6.7 17,000 15 6.7 15,000 Y% 2% 22 4.55 7,700 20 5 19,600 24.9 17,500 15.4 6.5 11,000 20.8 4.8 23,700 20 5 20,000

26.5 3.77 9,000 25 3.84 25,000 130.4 3.3 22,000 18.6 5.4 13,000 25.0 28,500 3.6 28,000 1% 34½ 35.2 2.84 12,000 34 2.94 33,250 40.5 2.5 26,500 2422 17,500 35.0 2.8 39,000 35 2.8 35,000 1% 3% 40.8 2.45 13,500 39 2.56 37,800 46.2 2.16 30,500 27.5 3.6 20,000 40.0 2.5 44,000 40 25 40,000 1X6 4 46.9 2.13 15,000 45 2.22 44,500 53.4 1.87 34,500 31.3 3.2 23,000 45.0 2.2 49,500 45 22 45,000 1½ 4% 58.8 1.7 18,500 55 1.8 55,000 67 1.5 43,000 ￾ 39.5 2.5 29,000 60.0 1.6 65,000 60 1.6 60,000 1 inch = 2.54 cm. 1 foot = 0.3048 m. 1 pound = 0.4536 kg.

NOTE: - The figures on synthetics, above, are an average of those available from four large cordage manufacturers. Those for the sope you buy should be availabe at your dealers. Check them carefully. Also check the rope. In general a soft, sleazy rope may be somewhat stronger and easier to splice but it will not wear as well and is more apt to hockle or unlay than a firm, well "locked-up" rope. Blended ropes, part polyolefins and part other fibers, may be found. Multiflament (fine filament) polypropylene looks like nylon-don't expect it to be as strong or do the job of nylon. (If floats, nylon doesn't.) Spun, or stapled, nylon and Dacron are not as strong as ropes made from continuous filaments but are *Du Pont registered trademark.

(4) Securing aircraft. Tie only at the tiedown rings provided for that purpose. Never tie to a strut itself. The practice of tying to lift struts has in itself caused frequent damage. Ropes slip to a point when even slight pressure may bend the struts.

DON'T tie ropes on struts so that it is possible for them to slip down.

Allow for about 1 inch of movement, and remember that manila rope shrinks when it gets wet. Too much slack will allow the aircraft to jerk against the ropes. Avoid tightening the ropes too much. Tight tiedown ropes actually put inverted flight stresses on the aircraft, and many of them are not designed to take such loads. A tiedown rope holds no better than the knot. Antislip knots such as a bowline or a square knot are quickly tied, and easy to untie.

Ho0. 81 81 Par 7 11

(See illustrations at the bottom of this page for tying knots.) Aircraft not equipped with tiedown fittings should have them installed in accordance with manufacturers' instructions.

DO tie ropes to outer ends of struts on high wing monoplanes.

Provide suitable rings, where structural conditions permit, if manufacturer has not already provided them.

TYING A BOWLINE

BE UNDER THIS ONE MUST UP THROUGH AND AROUND BACK BACK DOWN THROUGH

TYING A SQUARE KNOT OVER UNDER 12 Par 7