By Default systems are initialized by the Aircraft/generic/generic-system.xml
This initializes the following:
- The generic electrical system
- 1 pitot system, index [0]
- 1 static system index [0]
- 2 vacuum systems [0] and [1], depending on engine rpm of engine[0] and
  engine[1] respectfully

If you want to define more systems, copy the generic-system file to your
aircraft-name/Systems folder and rename it systems.xml

In your aircraft -set file add the path to the system.xml file:
<sim>
    ....
    <systems>
        ....
        <path>Aircraft/aircraft-name/Systems/systems.xml</path>
        ....
    </systems>
    ....
</sim>


** Adding a second pitot system.
In your systems.xml, you should already have

  <pitot>
    <name>pitot</name>
    <number>0</number>
    <stall-deg>60</stall-deg>           # optionnal, default to 60 degrees
  </pitot>

and you need to add for a pitot system with index 1:

  <pitot>
    <name>pitot</name>
    <number>1</number>
    <stall-deg>60</stall-deg>  #optionnal
  </pitot>

For the any pitot system except for the first (with index 0)
add in the aircraft -set file (below for index 1):

<systems>
      <pitot n="1">
                <serviceable>1</serviceable>
      </pitot>
</systems>

Of course you can add a third or fourth etc.


** Adding a second static system
Absolutely analog with the pitot system. So add in systems.xml:

  <static>
    <name>static</name>
    <number>1</number>
    <tau>1</tau>
    <type>0</type>                          #optionnal: 0,1 or 2 default is 0
    <error-factor>0.5</error-factor>        #optionnal see below default = 0
  </static>

and in the aircraft -set file:

<systems>
      <static n="1">
                <serviceable>1</serviceable>
      </static>
</systems>

Now you can source your instrumentation relying on static and pitot
pressure (airspeed, altimeter, vertical speed indicator) from different
and independent systems


** The PITOT System

The pitot system measures impact pressure and is basically a tube pointing forward.
Small aircraft have one, small IFR aircraft have one or two (of which at least
one is heated) and larger commercial aircraft have three or even more. In those large
aircraft the left pitot serves the pilot instruments, the right the co-pilot and
the third system the back-up instruments. This might be different for each type
of aircraft of course.
In Flightgear the pitot system outputs the total pressure to the following property:
/systems/pitot[n]/total-pressure-inhg and
/systems/pitot[n]/measured-total-pressure-inhg
which are the same except at supersonic speeds. For supersonic aircraft use the "measured"
property. See also the README.airspeed-indicator.
However it is advised for every aircraft to use the measured property. In future
this will be the property where all the measurement faults are reflected.
the following "measurement failures" are currently applied:
1) decrease of total and measured pressure due to side-slip and angle of attack
2) at 60 deg the pitot tube will stall and the value will fall back to static pressure
3) for the "measured" property only: at Mach>1, a shock wave is assumed in front of the
  pitot tube, decreasing the total pressure.

The stall angle may be (optionally) set to any angle between 0 and 90 deg
(default = 60 deg) like so:
  <pitot>
    <name>pitot</name>
    <number>0</number>
    <stall-deg>45</stall-deg>
  </pitot>

Both the decrease of the pitot pressure and the default stall angle are based on a measurement
on an AN5812 pitot tube.


** The STATIC system

The static system measures the static pressure. So all influences of airspeed are eliminated.
In real life this is however not always easy. Effects from angle of attack, side-slip, flap defection,
gear extension, engine power setting and airspeed are present and for the aircraft
designer it is not alway easy to find a good position for the static port.
Usually the number of static systems are equal to the number of pitot systems.
In Flightgear there are 3 types of static systems modelled.
Type 0 (default): the perfect sensor. No measurement failures.
Type 1: Dual static ports on the fuselage sides. Side-slip angle influence only. this model the whole pair.
Type 2: Static port on the pitot tube. Both angle of attack and side-slip influence.
If you want to use type 1 or 2:
  <static>
    <name>static</name>
    <number>0</number>
    <tau>0.1</tau>
    <type>1</type>
    <error-factor>0.5</error-factor>
  </static>
The output property /systems/static[n]/pressure-inhg is filtered. Therefore, if you want to see
the effect of the measurement failure, "tau" should be 0.1 or smaller.
The "error-factor" should be between 0.2 and 0.7. Setting it to 0 equals a "perfect sensor".
A setting of 1 means the whole (projected on static port face) impact pressure is applied.
This is not realistic as usually there are more than one static pick-up points
and so the pressure increase gets "flattened".