background image

AIM

10/12/17

7−5−9

Potential Flight Hazards

7−5−11. Precipitation Static

a. Precipitation static is caused by aircraft in flight

coming in contact with uncharged particles. These

particles can be rain, snow, fog, sleet, hail, volcanic

ash, dust; any solid or liquid particles. When the

aircraft strikes these neutral particles the positive

element of the particle is reflected away from the

aircraft and the negative particle adheres to the skin

of the aircraft. In a very short period of time a

substantial negative charge will develop on the skin

of the aircraft. If the aircraft is not equipped with

static dischargers, or has an ineffective static

discharger system, when a sufficient negative voltage

level is reached, the aircraft may go into

“CORONA.” That is, it will discharge the static

electricity from the extremities of the aircraft, such as

the wing tips, horizontal stabilizer, vertical stabilizer,

antenna, propeller tips, etc. This discharge of static

electricity is what you will hear in your headphones

and is what we call P−static.

b. A review of pilot reports often shows different

symptoms with each problem that is encountered.

The following list of problems is a summary of many

pilot reports from many different aircraft. Each

problem was caused by P−static:

1. Complete loss of VHF communications.

2. Erroneous magnetic compass readings

(30 percent in error).

3. High pitched squeal on audio.

4. Motor boat sound on audio.

5. Loss of all avionics in clouds.

6. VLF navigation system inoperative most of

the time.

7. Erratic instrument readouts.

8. Weak transmissions and poor receptivity of

radios.

9. “St. Elmo’s Fire” on windshield.

c. Each of these symptoms is caused by one

general problem on the airframe. This problem is the

inability of the accumulated charge to flow easily to

the wing tips and tail of the airframe, and properly

discharge to the airstream.

d. Static dischargers work on the principal of

creating a relatively easy path for discharging

negative charges that develop on the aircraft by using

a discharger with fine metal points, carbon coated

rods, or carbon wicks rather than wait until a large

charge is developed and discharged off the trailing

edges of the aircraft that will interfere with avionics

equipment. This process offers approximately

50 decibels (dB) static noise reduction which is

adequate in most cases to be below the threshold of

noise that would cause interference in avionics

equipment.

e. It is important to remember that precipitation

static problems can only be corrected with the proper

number of quality static dischargers, properly

installed on a properly bonded aircraft. P−static is

indeed a problem in the all weather operation of the

aircraft, but there are effective ways to combat it. All

possible methods of reducing the effects of P−static

should be considered so as to provide the best

possible performance in the flight environment.

f. A wide variety of discharger designs is available

on the commercial market. The inclusion of

well−designed dischargers may be expected to

improve airframe noise in P−static conditions by as

much as 50 dB. Essentially, the discharger provides

a path by which accumulated charge may leave the

airframe quietly. This is generally accomplished by

providing a group of tiny corona points to permit

onset of corona−current flow at a low aircraft

potential. Additionally, aerodynamic design of

dischargers to permit corona to occur at the lowest

possible atmospheric pressure also lowers the corona

threshold. In addition to permitting a low−potential

discharge, the discharger will minimize the radiation

of radio frequency (RF) energy which accompanies

the corona discharge, in order to minimize effects of

RF components at communications and navigation

frequencies on avionics performance. These effects

are reduced through resistive attachment of the

corona point(s) to the airframe, preserving direct

current connection but attenuating the higher−fre-

quency components of the discharge.

g. Each manufacturer of static dischargers offers

information concerning appropriate discharger loca-

tion on specific airframes. Such locations emphasize

the trailing outboard surfaces of wings and horizontal

tail surfaces, plus the tip of the vertical stabilizer,

where charge tends to accumulate on the airframe.