Electrostatic Discharge Sensitive (ESDS) Devices

Many electronic Line Replaceable Units (LRU's) on aircraft contain printed circuit boards containing components that are susceptible to damage from electrostatic discharges.  Such components are referred to as electrostatic discharge sensitive (ESDS) devices.  Decals installed on ESDS LRU's, indicate that special handling is required  

Static Electricity & Electrostatic Discharge

The most common conception of static electricity and its accompanying discharge, is the miniature lighting shock you receive when you touch a metal door handle having walked across a nylon carpet.  If the door handle is touched with a key first, the discharge will be seen but not felt.

The discharge occurs because different materials receive different levels of charge as materials are rubbed together or pulled apart.  The different charge levels create potential differences between the different materials, and when materials of different electrical potential are brought into close proximity with each other, a discharge occurs as the potentials equalise.

The different levels of charge with respect to cotton (the reference material) are shown on the following page, in what is known as the Triboelectric Series.

The further up or down, the greater the charge and hence the greater the discharge when the two materials are brought together.

Triboelectric Series

                Material

Air	Increasingly Positive
Human Hands
Asbestos
Rabbit Fur
Glass
Mica
Human Hands
Nylon
Wool
Fur
Lead
Silk
Aluminium
Paper
Cotton-----------------------------------------------	Increasingly Negative
Steel
Wood
Amber
Sealing Wax
Hard Rubber
Nickel Copper
Brass Silver
Gold Platinum
Sulphur
Acetate Rayon
Polyester
Celluloid
Orion
Saran
Polyurethane
Polyethylene
Polypropylene
PVC (vinyl)
Kelf (ctfe)
Silicon
Teflon

Electronic Instrument Systems EIS

Modern technology has enabled some significant changes in the layout of flight instrumentation on most aircraft currently in service.  The biggest change has been the introduction of Electronic Instrument systems.  These systems have meant that many complex Electro-mechanical instruments have now been replaced by TV type colour displays.  These systems also allow the exchange of images between display units in the case of display failures.

There are many different Electronic Instrument Systems, including:

i.    Electronic Flight Instrument System (EFIS).

ii.   Engine Indicating & Crew Alerting System (EICAS).

iii.  Electronic Centralised Aircraft Monitoring (ECAM).   

The Electronic Instrument System (EIS) also allows the flight crew to configure the instrument layout by allowing manual transfer of the Primary Flight Display (PFD) with the Navigation Display (ND) and the secondary Electronic Centralised Aircraft Monitoring (ECAM) display with the ND.

As well as a manual transfer, the system will automatically transfer displays when either the PFD or the primary ECAM display fails.  The PFD is automatically transferred onto the corresponding ND, with the ECAM secondary display used for the primary ECAM display.

The system will also automatically transfer the primary ECAM information onto the ND if a double failure of the ECAM display system occurs.

Slab Desynn

If the voltage at the 3 tappings of the transmitter of a basic Desynn are measured as the wiper arms are rotated 360°, it will be seen that they produce a sawtooth waveform as opposed to a sinewave. 
This results in the pointer of the indicator not following the transmitter exactly.  In most instances the difference is insignificant, however their may be certain circumstances where it cannot be overlooked.

The solution is to use a modified Desynn transmitter called a 'slab Desynn'.  In a slab Desynn, the resistor is wound on a slab former and has the power supply connected to it, whilst the wiper arms now provide the output to the receiver, there being 3 wiper arms each displaced from the next by 120°.  The output from this device is a sinewave.  It can be connected to the same type of indicator and operates in the same way as the basic Desynn.