710 MECHANICAL DESIGN AND PACKAGING 



environments. The elimination of moisture traps, wells, and pockets from 

 the design is also desirable. 



Fungus is closely associated with moisture. To prevent fungus from 

 damaging equipment, parts should be fabricated from materials which are 

 not nutrients for fungus — metals, ceramics, mica, teflon, nylon, rubber, 

 etc. Materials not resistant to fungus are cotton, linen, wood, leather, cork, 

 etc. These can be used when protected from a fungus environment — for 

 example, in hermetically sealed assemblies. 



13-10 STATIC ELECTRICITY AND EXPLOSION 



Static electricity is defined as electricity at rest — electric charges — as 

 compared with dynamic electricity, which is electricity in motion — electric 

 current. An electrically charged environment creates electrical noise which 

 may cause echo loss by the radar and malfunction of the radio or other 

 electronic equipment. Static electricity may arise from local thunderstorms, 

 which generate intermittent electrical impulses of high intensity. These can 

 damage the external surface of the radome. Distant thunderstorms can 

 generate continuous noise levels and cause radio interference. A voltage 

 differential incident to static electricity can be produced by rubbing 

 together dissimilar surfaces. Thus, during its flight an aircraft may build 

 up on its skin static charges from the action of snow, hail, dust and sand, 

 ice crystals, or smoke or exhaust particles from the power plant. Very high 

 voltages may be so generated. The charge leaks off in a corona discharge 

 which causes electromagnetic noise that may obscure the radar echoes. 

 The effect of static is less severe on equipment operating at the high 

 frequencies; it is increasingly critical at lower frequencies of operation. 



The adverse effects of static electricity may be offset to some extent by 

 the choice of bandwidth of the radar and by the design of the airframe. 

 In particular, it is helpful to insulate or to round off sharp points of the 

 airframe to prevent discharge from them. For low-speed aircraft, trailing 

 wicks give a relatively noiseless discharge. 



Ground operations, especially during fueling, can result in explosion 

 generated by static electricity. Proper grounding of the system will prevent 

 this. During flight, explosions can be caused by electrical equipment 

 capable of generating a spark — switches, motors, relays, etc. Hermetic 

 sealing or the use of pressurized containers will reduce the explosion hazard 

 for electrical components which in their normal operation could ignite an 

 explosive gaseous mixture. 



Proper protection from static electricity is necessary for the safety of 

 maintenance personnel as well as for successful completion of missions. 



