AIM
4/3/14
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.