Extra Info for Experts; Not Needed for this Course
Precipitation static (P-static)
Quoted from FAA AIM section 7-5-11.
- 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
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
- P−static can affect aircreaft the following ways:
- 1. Complete loss of VHF (radio) communications.
- 2. Erroneous magnetic compass readings (30 percent in error).
- High pitched squeal on audio.
- Motor boat sound on audio.
- Loss of all avionics (navigation and communication instruments) in
- Very Low Frequency (VLF) navigation system inoperative most of the
- Erratic instrument readouts.
- Weak transmissions and poor receptivity of radios.
- “St. Elmo’s Fire” on windshield.
- 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.
- 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
- 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.
- 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−frequency components of the
- Each manufacturer of static dischargers offers information
concerning appropriate discharger location 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. Sufficient
dischargers must be provided to allow for current−carrying capacity
which will maintain airframe potential below the corona threshold of
the trailing edges.
- In order to achieve full performance of avionic equipment, the
static discharge system will require periodic maintenance. A pilot
knowledgeable of P−static causes and effects is an important element in
assuring optimum performance by early recognition of these types of