has some effect on a petroleum base fire by preventing oxygen from getting to the fire.
2. Loaded stream contains an antifreeze as well as a fame retardant.
B. Dry Chemical.
Four types of chemicals are used:,
1. Sodium bicarbonate (Formula H). For ordinary risk class B and C fires.
2. Ammonium phosphate (Multipurpose). For multiple risk class B and C fires.
3. Potassium bicarbonate (Purple K). For high risk class B and C fires
4. Multipurpose dry chemical (ABC). For use on Class A, B, and C fires. The dry chemicals extinguish a fire by smothering it, cutting off oxygen, and the blanket of dry chemicals prevents reflash fires. It also affords the operator some protection from the heat.
dry chemicals are nonconductors of electricity.
C. Gas.
1. Carbon dioxide (COz) has a toxicity rating (Underwriter's Laboratory) of 5A especially recommended for use on class B and C fires.
Extinguishes flames by dissipating oxygen in the immediate area.
2. Halogenated hydrocarbons (commonly called freon by the industry), are numbered according to chemical formulas with Halon numbers.
Carbon tetrachloride (Halon. 104). Chemical formula CCL. UL toxicity rating of 3. It is poisonous and toxic. Hydrochloric acid vapor, chlorine and phosgene gas are produced whenever carbon tetrachloride is used on ordinary fires. The amount of phosgene gas is increased whenever carbon tetrachloride is brought in direct contact with hot metal, certain chemicals, or continuing electri- It is no longer approved for any fire extinguishing use.
Methyl bromide (Halon 1001). Chemical formula CH Br-a liquified gas, UL toxicity rating of 2. Effective but very toxic and also is corrosive to aluminum alloys, magnesium and zinc. Not recommended for aircraft use.
Chlorobromomethane (Halon 1011). Chemical formula CH¿CIBr-a liquified gas, UL toxicity rating is 3. Not recommended for aircraft use.
Dibromodifluoromethane (Halon 1202). Chemical formula CBrzF2. UL toxicity rating of 4. Not recommended for aircraft use.
Bromochlorodifluoromethane (Halon 1211). Formula CBrCIF2-a liquited gas with a toxicity rating of 5. It is colorless, noncorrosive and evaporates rapidly leaving no residue whatever.
It does not freeze or cause cold burns and will not harm fabrics, metals, or other materials it contacts.
Halon 1211 acts rapidly on fires by producing a heavy blanketing mist that eliminates air from the fire source, but more importantly interferes chemically with the combustion process. It has outstanding properties in preventing reflash after the fire has been extinguished.
Group Definition Examples rases or vapors which in concentrations up to at least 20% by volume fo Bromotrifluoromethane (least lurations of exposure of the order of 2 hours do not appear to produce (Halon 1301) toxic injury.
5a Gases or vapors much less toxic than Group 4 but more toxic than Group Carbon dioxide 4 Gases or vapors which in concentrations of the order of 2 o 2% for Dibromodifluoromethane durations of exposure of the order of 2 hours are lethal or produce serious (Halon 1202) injury.
3 Gases or vapors which in concentrations of the order of 2 to 2:2% for dura- Bromochloromethane tions of exposure of the order of 1 hour are lethal or produce serious injury. (Halon 1011), Carbon tetrachloride (Halon 104) 2 Gases or vapors which in concentrations of the order of 1/2 to 1% for dura- Methyl bromide tions of exposure of the order of 1/2 hour are lethal or produce serious (Halon 1001) injury.
FICURE 11-10. Toxicity table.
Classes of Self-generating Self-expelling Cartridge of N, cylinder Stored oressure fire Pump Hand Extinguishing Materials B Water and antifreeze Soda-acid (water) Wetting agent Foam Loaded stream X+ Multipurpose dry chemical X+ Carbon dioxide X + Dry chemical Bromotrifluoromethane - Halon 1301 Bromochlorodifluoromethane - Halon 1211 Dry powder (metal fires) + Smaller sizes of these extinguishers are not recognized for use on these classes of fires.
FIGURE 11-11. Extinguisher operation and methods of expelling.
Bromotrifluoromethane (Halon 1301). Chemical formula CF Br is also a liquified gas with a UL toxicity rating of 6. It has all the characteristics of Halon 1211. The significant difference between the two is: Halon 1211 forms a spray similar to CO2, while Halon 1301 has a vapor spray that is more difficult to direct.
D. Powder.
Dry powder for metal fires. Fires in metal require special handling. If water is used on a magnesium fire the burning is accelerated. Special dry powders are available for use wherever metal fires are possibilities. These are normally applied by scoop or shovel. Multipurpose (ABC) dry chemicals have a limited use on metal fires such as fires in wheel brakes or in magnesium fires.
(See figures 11-10 and 11-11).
RECOMMENDED MARKINGS TO INDICATE EX-
TINGUISHER SUITABILITY (From NFPA Standard #10) The following recommendations are given as a guide in marking extinguishers, and/or extinguisher locations, to indicate the suitability of the extinguisher for a particular class of fire.
Markings should be applied by decalcomanias, painting or similar methods having at least equivalent legibility and durability.
Where markings are applied to the extinguisher, they should be located on the front of the shell above or below the extinguisher nameplate. Markings should be of a size and form to give easy legibility at a distance of 3 feet.
Where markings are applied to wall panels, etc., in the vicinity of extinguishers, they should be of a size and form to give easy legibility at a distance of 25 feet. (See figures 11-12 and 11-13).
AIRCRAFT FIRE EXTINGUISHERS Fire is one of the most dangerous threats to aircraft- either in flight or on the ground. Airborne fixed, powerplant and airframe, detection and extinguishing systems are designed and installed by the manufacturer in compliance with applicable FAR's. The requirement for portable fire extinguishers installed in the crew and passenger compartment says, the extinguisher must be approved, must be appropriate for the kind of fire likely to occur and must minimize the hazard of toxic gases.
ORDINARY FLAMMABLE 3. Methyl bromide is more toxic than CO, and cannot be used in confined areas. It is also very B corrosive to aluminum alloy, magnesium and zinc. 4. Chlorobromomethane, although an effective extinguishing agent, is toxic.
COMBUSTIBLES LIQUIDS
TYPICAL EXTINGUISHER MARKINGS
ELECTRICAL COMBUSTIBLE
ORDINARY
COMBUSTIBLES
1. Water
EQUIPMENT METALS
FLAMMABLE ELECTRICAL FIGURE 11-12. Identification of fire extinguisher type location.
LIQUIDS EQUIPMENT The National Fire Protection Association tinguishers, suggests the type, capacity, location and quantity of hand fire extinguishers for the (NFPA) Standard #408, Aircraft Hand Fire Ex- 2. Carbon Dioxide, Dry Chemica! Bromotrifluoromethane Bromochlorodifluoromethane and protection of aircraft compartments occupied by passengers and crew. This standard suggests car- ORDINARY FLAMMABLE ELECTRICAL bon dioxide and water (water solution) as the extinguishing media for hand type extinguishers.
Both type extinguishers are suggested for use in the passenger compartment, the number of units COMBUSTIBLES LIQUIDS EQUIPMENT being regulated by the number of passengers carried. Carbon dioxide is suggested for the crew 3. Multipurpose Dry Chemical compartment. A halogenated hydrocarbon extinguishing agent (Halon 1211 or Halon 1301) having FLAMMABLE ELECTRICAL an Underwriter's Laboratory toxicity rating of 5 or higher may be substituted for the carbon dioxide, C if there is sufficient free air volume in the immediate area to prevent serious irritating effects on LIQUIDS EQUIPMENT CAPABILITY the occupants.
The following extinguishing agents are not rec- 4. Multipurpose Dry Chemical (Insutricient Agent tor ommended for aircraft use: "A" Rating)
1. Dry chemical extinguishers are very effective COMBUSTIBLE in Class B and C fires but they leave a residual dust or powder. This obstructs vision, is difficult to clean up and causes damage to electronic equipment.
2. Carbon tetrachloride is no longer approved as METALS gas (phosgene) when in contact with hot metals. a fire extinguishing agent. It produces a poisonous 5. Dry Powder Soda acid and foam, these are toxic to a degree and can be corrosive to adjacent materials. FIGURE 11-13. Typical extinguisher markings.
Extinguishers The common aerosol can type extinguishers are definitely not acceptable as airborne hand type extinguishers. In one instance, an aerosol type foam extinguisher located in the pilot's seat back pocket exploded and tore the upholstery from the seat.
The interior of the aircraft was damaged by the foam. This occurred when the aircraft was on the ground and the outside temperature was 90° F. In addition to the danger from explosion, the size is inadequate to combat even the smallest fire.
another instance, a dry chemical extinguisher was mounted near a heater vent on the floor. For an unknown reason, the position of the unit was reversed. This placed the extinguisher directly in front of the heater vent. During flight, with the heater in operation, the extinguisher became overheated and exploded filling the compartment with dry chemical powder. The proximity of heater vents should be considered when selecting a location for a hand fire extinguisher.
Additional information relative to airborne hand fire extinguishers may be obtained from the local FAA District Office and from the National Fire Protection Association, 470 Atlantic Ave., Boston, MA 0221 Ground Type Extinguishers-Hand Type The selection of a fire extinguisher for ground installation, shop, fueling station, etc., is not restricted as it is for airborne installations. The range of selection for agent and type extinguishers is shown in figure 11-11. (See figures 11-16 and 11-17).
Methods of Extinguisher Operation. The methods of operation of extinguishers are most conveniently arranged by grouping extinguishers according to their expelling means. Six methods are in common use.
Self-generating-actuation causes gases to be generated that provide expellent energy.
Self-expelling-the agents have sufficient vapor pressure at normal operating temperatures to expel themselves.
Gas cartridge or cylinder-expellent gas is confined in a separate pressure vessel until an operator releases it to pressurize the extinguisher shell.
Stored pressure-the extinguishing material and expellent gas are kept in a single container.
Mechanically pumped-the operator provides expelling energy by means of a pump and the vessel containing the agent is not pressurized.
Hand propelled--the material is applied with scoop, pail, or bucket.
Several different extinguishing materials are handled by each of these expelling means.
Discontinued Fire Extinguishers (see figures 11-14 and 11-15) There are still in use today several million fire extinguishers of a design no longer manufactured.
These are the 1½, 2½ and 5 gallon "invert to use" liquid extinguishers. The last of this type were manufactured in the 2½ gallon size. The agents used in these extinguishers are:
1. Soda-acid.
2. Foam.
3. Water cartridge.
4. Loaded stream cartridge.
The reasons which influenced the decision to discontinue manufacturing these extinguishers are:
1. Invert to use-difficult and unorthodox method of activation.
2. Limited to the types of fires for which they are suitable. Mostly Class "A" fires, very limited application of foam on Class "B' fires.
3. None approved for electrical fires.
4. Effective on only minimal size fires.
5. The container does not meet current pressure vessel standards. This is the most significant of all.
Comparison of Safety Factors Discontinued types of extinguishers use a shell rated at either 350 or 500 p.s.i. However, when these extinguishers are inverted to operate, the pressures generated often are unpredictable, totally unlike the pressures in other extinguisher designs.
Pressures may range from 100 to 300 p.s.i for sodaacid extinguishers and from 100 to 350 p.s.i. for the foam extinguishers.
SERVICING AIRCRAFT WITH OIL Aircraft oil tanks are normally checked at the the t the same time the fuel tanks are filled. There are a few exceptions to this general rule, since some manufacturers recommend that the oil level in certain jet engines be checked within a specified time after engine shutdown. In all cases, the manufacturer's instructions should be followed for the specific aircraft not only for servicing procedures but also for type and grade of oil used.
Aircraft oil tanks should never be filled to capacity or above the labeled full mark on the gage or dipstick. This is because oil expands when it becomes hot, and at high altitude it bubbles and expands. The extra space in oil tanks allows for expansion and prevents overflowing. The aircraft's oil requirements should be checked, and no substitutions should be made for the type of oil to be used unless substitute oils have been approved 505
PARROL Soda-acid fire extinguisher. Cartridge operated water fire extinguisher.
FIGURE 11-14. Discontinued types of fire extinguishers #1.
CARBON TETRACHIORIDE Foam fire extinguisher. Vaporizing liquid fire extinguisher.
FIcURE 11-15. Discontinued types of fire extinguishers #2.
FIGURE 11-16. CO2 fire extinguisher.
for use. When pouring oil into the tanks, be sure that cleaning rags or pieces of rag or other foreign substances do not get into the tanks. Foreign material in the oil system restricts the flow of oil and can cause engine failure.
Lubricating oil is nonexplosive, very difficult to ignite in bulk, and is not normally capable of spontaneous combustion. However, if oil is ignited, a hotter fire results than that from gasoline. The vapor of the oil, however, is explosive when mixed with air in certain proportions. Vapors of many petroleum products are highly toxic when inhaled or ingested. It is therefore necessary to take all precautions when handling lubricating oil.
MAINTENANCE SAFETY Good housekeeping in hangars, shops, and on the flight line is essential to safety and efficient maintenance. The highest standards of orderly work arrangements and cleanliness should be observed during the maintenance of aircraft. Where continuous work shifts are established, the outgoing shift should remove and properly store personal tools, rollaway boxes, all workstands, maintenance stands, hoses, electrical cords, hoists, crates, and boxes that are superfluous to the work to be accomplished.
Safety Lanes Pedestrian walkways or fire lanes should be painted around the perimeter inside the hangars.
This should be done as a safety measure to prevent accidents and to keep pedestrian traffic out of work areas.
Power Cords I. Power cords should be heavy industrial type which are able to resist abrasion and impact.
2. Power cords should not be run over by any equipment.
3. Lights should be explosion proof.
4. Connections should be locking type to prevent accidental disconnection.
5. All lights or equipment should be switched "off"', , to prevent arching before connecting or disconnecting.
6. Power cords should be straightened, coiled, and properly stored when not in use.
Disregard of the above suggestions may result in explosions and fires with damage into millions of dollars and loss of life.
Compressed Air System Compressed air is like electricity-an excellent tool as long as it is under control.
1. Air hoses should be inspected frequently for breaks and worn spots. Unsafe hose should be replaced immediately.
2. All connections should be kept in a ino leak condition".
3. In line oilers, if installed, should be maintained in operating condition.
4. The system should have water sumps installed FIGURE 11-17. Dry chemical extinguisher. tervals. and they should be drained at regular in- 507
5. Air used for paint spraying should be filtered to remove oil and water.
6. Never use compressed air to clean hands or clothing. Pressure can force debris into the flesh leading to infection.
7. Never use compressed air for "horse play".
8. Air hoses should be straightened, coiled, and properly stored when not in use.
Spilled Oil and Grease Oil, grease, and other substances spilled on hangar or shop floors should be immediately cleaned or covered with an absorbent material to prevent fire or personal injury. Drip pans should be placed beneath engines and engine parts wherever dripping exists. Waste oil and dirty cleaning fluid should be stored in containers for future salvage. Under no circumstances should oil or cleaning fluid be emptied into floor drains. Fumes from this type "disposal" may be ignited and cause severe property damage.
Aircraft Tire Mounting To prevent possible personal injury, tire dollies and other appropriate lifting and mounting devices should be used in mounting or removing heavy aircraft tires. When inflating tires on wheels equipped with locking rings, tire cage guards should always be used.
should be inflated in tire cage guards. Becaus of possible personal injury, extreme caution is required to avoid overinflation of high-pressure tires.
Pressure regulators should be used on high pressure air bottles to eliminate the possibility of overinflation of tires.
Tire cages need not be used when adjusting pressure in tires installed on aircraft.
Welding should not be performed except in designated areas. Any part to be welded should be removed from the aircraft, if possible. Repair would then be accomplished in the welding shop under controlled environment. A welding shop should be equipped with proper tables, ventilation, tool storage, and fire prevention and extinguishing equipment.
Welding on an aircraft should be performed outside if possible. If welding in the hangar is necessary, these precautions should be observed:
1. No open fuel tanks or work on fuel systems should be in progress.
2. No painting in progress.
3. No aircraft within 35 feet.
4. Immaculate housekeeping should prevail around the welding area.
5. Only qualifed welders should be permitted to do the work.
6. The area should be roped off and placarded.
7. Fire extinguishing equipment of a minimum rating of 20B should be in the immediate area with 80B rated equipment as a backup.
8. There should be trained fire watches in attendance at the above equipment.
9. Aircraft should be in towable condition, with a tug attached, aircraft brakes off, and a qualified operator on the tug with mechanics available to assist in the towing operation. Hangar doors should be opened
SERVICING AIRCRAFT OXYGEN SYSTEMS Before servicing any aircraft, consult the specific aircraft maintenance manual to determine the proper type of servicing equipment to be used Two persons are required to service an aircraft with gaseous oxygen. One man should be stationed at the control valves of the servicing equipment and one man stationed where he can observe the pressure in the aircraft system. Communication between the two men is required in case of an emergency. Aircraft should not be serviced with oxygen during fueling, defueling, or other main tenance work which could provide a source of ignition. Oxygen servicing of aircraft should be accomplished outside hangars.
Oxygen Hazards Gaseous oxygen is chemically stable and is nonflammable; however, combustible materials ignite more rapidly and burn with greater intensity in an oxygen-rich atmosphere. In addition, oxygen combines with oil, grease, or bituminous material to form a highly explosive mixture which is sensitive to impact. Physical damage to, or tailure oi, oxygen containers, valves, or plumbing can result in explosive rupture, with danger to life and property.
It is imperative that the highest standard of housekeeping be observed in handling oxygen and that only authorized persons be permitted to service aircraft.
In addition to aggravating the fire hazard, liquid oxygen will cause severe "burns" (frostbite) if it comes in contact with the skin because of its low temperature. (It boils at - 297° F.)
Only oxygen marked ¿Aviators Breathing Oxygen" which meets Federal Specification BB-0-925a.
Grade A or equivalent may be used in aircraft breathing oxygen systems.
AIRCRAFT TIEDOWN Aircraft tiedown is a very important part of aircraft ground handling. The type of tiedown will 508
be determined by the prevailing weather conditions. In normal weather a limited or normal tiedown procedure is used; but when storm conditions are anticipated, a heavy weather or storm condition tiedown procedure should be employed.
Normal Tiedown Procedure Small aircraft should be tied down after each flight to preclude damage from sudden storms. The direction in which aircraft are to be parked and tied down will be determined by prevailing or forecast wind direction.
Aircraft should be headed, as nearly as possible, into the wind, depending on the locations of the fixed, parking area tiedown points. Spacing of tiedowns should allow for ample wingtip clearance (figure 11-18). After the aircraft is properly located, lock the nosewheel or the tailwheel in the fore-and-aft position.
Tiedown Anchors All aircraft parking areas should be equipped for three-point tiedowns. This is facilitated at most airports by use of tiedown anchors installed in concrete parking areas. Tiedown anchors, sometimes called "pad eyes," are ringlike fittings installed when the parking area is poured. They are normally set flush with the surface of the concrete or no more than one inch above it. There are several types of tiedown anchors in use. The type selected is usually determined by the material used in aircraft parking areas, since it may be a Major axis +
- 10 FIGURE 11-18. Diagram of tiedown dimensions.
concrete paved surface, a bituminous paved surface, or an unpaved turf area.
Location of tiedowns is usually indicated by some means such as white or yellow paint markings or by surrounding the tiedown anchor with crushed stone.
Tiedown anchors for small single-engine aircraft should provide a minimum holding power (strength) of approximately 3,000 pounds each.
Although this minimum can be achieved when stake-driven tiedowns are used in dry or turfed areas, such stakes will almost invariably pull out Manila Nylon Dacron Yellow Polypropylene Size (Twist) (Braid) (Twist) (Braid) (in.) Minimum Minimum Minimum Minimum Minimum Minimum tensile tensile tensile tensile tensile tensile strength strength strength strength strength strength (lbs.) (Ibs.) (Ibs.) (Ibs.) (Ibs.) (Ibs.)
• 3/16 960 850 730 800 600 1/4 600 1,500 1,440 980 1,300 1,100 5/16 1,000 2,400 2,200 1,650 1,900 1,375 3/8 1,350 3,400 3,120 2,300 2,750 2, 025 7/16 1,750 4,800 4,500 2,900 1/2 2,650 6,200 5,500 3,800 4,200 3,800 5/8 4,400 10,000 3/4 5,400 9,000 FIGURE 11-19. Comparison of common tiedown ropes.
when the ground becomes soaked from torrential rains which accompany hurricanes and some thunderstorms.
Tiedown Ropes Tiedown ropes capable of resisting a pull of approximately 3,000 pounds should be used to Release lever Lock Unlock FIGURE 11-21. A multipurpose tiedown chain.
secure light aircraft. Cable or chain tiedown is usually preferred for tying down large aircraft.
Manila ropes should be inspected periodically for mildew and rot. Nylon or Dacron tiedown ropes are preferable to manila rope. The objection to manila rope is that it shrinks when wet, is subject to mildew or rot, and has considerably less tensile strength than either nylon or Dacron. Various types of commonly used tiedown rope are compared in figure 11-19.
Tiedown Cable Starwheel Tiedown cables are often used to secure aircraft, especially in the case of large aircraft. Most cable-type tiedowns are accomplished with some form of tiedown reel designed for rapid and reliable securing of all types of aircraft. Figure Locking handle 11-20 illustrates the operation of a typical cable tiedown reel.
Locking cam In A of figure 11-20 the cable is released by D depressing the release lever to provide cable slack.
FIGURE 11-20. Operation of a cable tiedown reel. One end of the cable is then attached to the 510
Up through Back down This one must be under and around back through (A) Tying a bowline knot.
Over Ov
- Under Under (B) Tying a square knot.
FIGURE 11-22. Knots commonly used for aircraft tiedown.
aircraft tiedown ring and the other end to a SECURING LIGHT AIRCRAFT tiedown anchor. The starwheel on the reel (B of figure 11-20) is then turned clockwise to remove excess slack from the cable. The locking handle is then secured to the bar when the cable has been adjusted for the desired tautness (C of figure 11-20). Finally, as shown in D of figure 11-20, the locking cam is secured to complete the tiedown procedure.
Tiedown Chains The chain-type tiedown sometimes is used as a better and stronger tiedown to secure the heaviest aircraft. This tiedown assembly is composed of an all metal quick-release mechanism, a tensioning device, and a length of chain with hooks (figure 11-21).
Light aircraft are most often secured with ropes tied only at the aircraft tiedown rings provided for securing purposes. Rope should never be tied to a lift strut, since this practice can bend a strut if the rope slips to a point where there is no slack.
Manila rope shrinks when wet; about 1 inch of slack should be provided for movement. Too much slack will allow the aircraft to jerk against the ropes. Tight tiedown ropes put inverted flight stresses on the aircraft, many of which are not designed to take such loads.
A tiedown rope holds no better than the knot.
Antislip knots such as the bowline or square knots are quickly tied and are easy to untie (figure 11-22). Aircraft not equipped with tiedown fittings 511
should be secured in accordance with the manufacturer's instructions. Ropes should be tied to outer ends of struts on high-wing monoplanes, and suitable rings should be provided where structural conditions permit, if the manufacturer has not already provided them.
4. Attach tiedown reels to airplane tiedown loops and to tiedown anchors or tiedown stakes. Use tiedown stakes for temporary tiedown only. If tiedown reels are not available, 4-inch wire cable or 1½-inch manila line may be used.
SECURING HEAVY AIRCRAFT The normal tiedown procedure for heavy aircraft can be accomplished with rope or cable tiedown. The number of such tiedowns should be governed by anticipated weather conditions.
Most heavy aircraft are equipped with surface control locks which should be engaged or installed when the aircraft is secured. Since the method of locking controls will vary on different type aircraft, check the manufacturer's instructions for proper installation or engaging procedures. In case high winds are anticipated, which may damage the control surfaces or locking devices, control surface battens can also be installed to prevent damage.
Figure 11-23 illustrates four common tiedown points on heavy aircraft.
In general, the normal tiedown procedure for heavy aircraft should include the following:
1. Head airplane into prevailing wind whenever possible.
2. Install control locks, all covers and guards.
3. Chock all wheels fore and aft.
Aft fuselage Nose landing gear tiedown ring tiedown loop Underside of wing tiedown Toop Main gear wheel tiedown loop FIGURE 11-23. Common tiedown points.
AIRCRAFT TIEDOWN FOR STORM CONDITIONS Each year many aircraft are needlessly damaged by windstorms because of negligence and improper tiedown procedures. A storm can turn a local airport into a junkyard in a matter of minutes. If an aircraft is damaged during a windstorm, the chances are it was improperly secured or was not tied down at all.
Most windstorm damage occurs during the early summer months, but can continue with lessening frequency throughout the year. As a general rule, the large and very severe windstorms cause less damage than the small local ones. This is because there is usually sufficient advance warning for the former, but the latter build up quickly and give little warning of their coming.
According to available weather records, Tampa, Florida, is the storm center of this country, with an average of 94 thunderstorms a year. Santa Fe, New Mexico, is second with a yearly average of 73.
Other cities are less frequently visited by such storms. The Pacific Coast States average only one to four such storms a year. Generally speaking, thunderstorms and tornadoes are accompanied by high surface winds which account for most of the damage to aircraft on the ground.
Although most storms are generated in the daytime, many sections of the United States, including the Southwest, the lower Michigan Peninsula, and an extensive area centered in eastern Nebraska, are plagued with night storms during the summer months. Thunderstorms are bothersome in the Central States during the months of July, August, and September. On the other hand, in the winter the greatest storm activity takes place in the lower Mississippi Valley.
The map in figure 11-24 shows the yearly average number of days with thunderstorms based on observations from all U. S. Weather Bureau first-order stations in the United States. A thunderstorm day is considered any day during which one or more thunderstorms occur. It should be realized, however, that there probably are variations which do not show on this map because 512
20 50 1O 20 80 80 FIGURE 11-24. Map showing average number of thunderstorm days per year.
of the local nature of thunderstorms and the sparsity of observations from some areas. July and August are the months with the greatest number of thunderstorms over most sections of the United States, while December and January have the least number.
Thunderstorms are not the only concern of aircraft owners, fixed-base operators, airport service crews, etc., from a tiedown sense. There are also hurricanes and tornadoes. Figure 11-25 is a map showing the tornado frequency in the various states during a recent 10-year period.
A map showing the principal storm belts in the continental U.S.A. is presented in figure 11-26.
Precautions Against Windstorm Damage The best protection against windstorm damage is, of course, to fly the aircraft out of the impending storm area when there is sufficient 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. When securing aircraft, fasten all doors and windows properly to minimize damage inside the aircraft. Engine openings (intake and exhaust) for both reciprocating and gasturbine type should be covered to prevent entry of foreign matter. Pitot-static tubes should also be covered to prevent damage.
Be prepared for the worst storm conditions; for example, pouring rain, and gusty winds with intermittent sheets of water flowing across the runways, ramps, and parking areas, with perhaps no hangar facilities available. With such conditions in mind, responsible service crews should plan in advance by becoming familiar with their aircraft manufacturer's instructions for the following: (1) Tiedown ropes; (2) installation of tiedown rings for attachment of tiedown ropes; (3) securing nosewheel type aircraft vs. tailwheel type aircraft;
and (4) aircraft weights and relative wind velocities that would make varied tiedown procedures necessary for pending weather emergencies.
The following suggestions will materially reduce aircraft damage from windstorms:
1. Partially disassembled aircraft which are outdoors (particularly light aircraft with 513