First of all, aircraft use pitot static information to measure speed and altitude.
Aircraft have 2 main air data systems wich uses this information and 1 standby air data system for if the main systems become unreliable or when there is a total electrical failure (standby instruments are completely self sufficient, usually connected straight onto air hoses and mechanical by nature or with a dedicated battery pack).
The air data systems have a pitot probe and static ports on the left and right hand side of the aircraft so any yaw effect (turning the nose) has no effect on the measurements. On older aircraft the static ports are on the pitot probes themselves.
So how does the air data system use the outside air to measure altitude and airspeed?
The pitot probe measures the rammed air pressure (this is the ambient pressure plus the force of the air due to the speed of the aircraft), let's call it Pt (total pressure). The static port is flush with the fuselage and measures only the Ps (static pressure) so the difference between the two is the ram air pressure, this ram air pressure can be calculated back into airspeed.
Some aircraft also have airspeed corrections on the Angle of Attack sensor due to the changing pitch of the aircraft but for now we will ignore this.
The altitude is taken from the static port, we know the barometric pressure is a function of temperature and altitude, for example: at sealevel on a standard day (around 18 degrees celsius) the ambient pressure is 1013.2 hPa. With around 8.2 meters altitude increase the pressure will decrease 1 hPa. The temperature is measured with the TAT probe and the barometric pressure is set to QNH before departure to the correct value so the altitude can now be calculated.
These calculations are made in the Air Data Computers or ADC's. These ADC's are only installed in the 2 main air data systems because the standby air data system is completely autonomous and must still function when all power is lost.
This is why the standby indicator allways shows TAS (True Airspeed, as measured from the probe) and not CAS (callibrated Airspeed, callibrated by ADC).
Here's a pathetic attempt of a drawing of a general idea about a pitot static system.
Looking at this drawing and taking the right hand aft pitot tube, it only has 2 static connections on it and no pitot hose. U can see that the left hand forward pitot is connected to the ADC #1 and the right hand forward to the ADC #2 and the left hand aft to the standby instruments, leaving this the only probe with no pitot connection at all.
Naturally, this is important to know if u are going to connect the testset and only want to test ADC #1 for example.
The small connector on the right is the power for the probes anti ice heating system.
Now then. Let's talk about the penny and giles testset that we have.
The testset has a display with a control panel on it, several pitot tube adaptors, flush static ports adaptors, 2 hoses wich can connect to a valve station wich can connect and disconnect parts of the system and a whole bunch of hoses. Preferably, I don't use the valve station because everything that we put into the measurement loop is another potential leakrate in the test result.
We start with connecting the pitot probe adapters to the probes.
Then we connect the 2 main hoses to the testset, red for pitot, blue for static, the connections are 'fool proof' (even though it is impossible to make anything fool-proof because fools are so ingenious).
If u need to connect three pitot probes on the pitot line u can use a junction.
I also connect the standby static port (on this aircraft type the standby is the only flush-with-the-fuselage type).
Note that the hose must be kept up by the vacuum clamp to keep it from hanging on the static port adapter causing a leak in the static system.
We connect this adapter on one side only and tape the other side off. If we connected 2 adapters we would increase the leakrate with more testequipment.
The testset takes it's power from a receptacle in the aircraft making it easy to use out on the platform or somewhere in the middle of nowhere.
Having connected all the hoses and the adapters I can take the display with the control panel with me to the cockpit.
This is a drawing of the air data system as it is installed in this aircraft.
And we've connected the testset to it like this.
We select the leak check option.
Then when we select a leak test altitude and speed, we can see the resulst on the connected systems.
When u are done with the leak check, don't forget to vent the system to ground slowly before taking any of the couplings off.
This testset can create a vacuum or a pressure depending on the situation. Normally a pitot is positive pressure and static is a underpressure.
Remember, higher altitude means lower pressure. Pitot pressure is the altitude pressure plus the airspeed pressure so if the (lower than ambient) altitude pressure is not compensated by airspeed pressure (lower airspeed selected on the testset), the pitot line will have a underpressure relative to ambient.
U can see that both the red pitot line and the blue static line have a underpressure.
Inversely, the static line can also create a overpressure to ambient.
This is because the testset is calibrated to a certain 0 feet pressure wich is 1013.2 mb (standard day at sealevel).
If the pressure is lower than that, at 0 feet selected, the testset creates a overpressure in the static line relative to ambient.
There are different pitot static testsets, we also have a older type wich we pump manually and regulate the pressures with manual air valves.
We also have a new pitot static testcart with every pitot static testtool included.