A pitot/static tube is a sensor for the air data instruments of the aircraft, the airspeed and the altitude are taken from this information.
On this aircraft both the pitot pressure (ram air pressure) and the static pressure (ambient pressure) are both measured on the same probe. The 'auxilliary' or 'standby' system takes it's static pressure from a seperate static source on both sides of the fuselage. On several other aircraft also the main systems take static pressure from static source's on the sides of the fuselage. With this information two air data computers or 'ADC's' calculate the so called 'calculated airspeed' 'CAS' and altitude. The standby systems are directly linked to the air source and display only true airspeed TAS and altitude. The airspeed that comes from the ADC, the CAS is corrected for angle of attack inputs, the angle of attack sences the angle between the neutral pitch and the actual pitch of the aircraft, changing this angle will obviously alter the airspeed indication because the ram air pressure angle is changed, the TAS does not have this correction. A cabin pressure indicator is also in the system but only uses static pressure and compares that to the pressure in the pressure cabin of the aircraft. A metric altimeter is also installed in our aircraft but this is optional.
Today, we'll replace three pitot/static tubes.
This is one of the pitot tubes that we need to replace, I have allready removed the screws from the base.
I pull the pitot sensor towards me so I can start to disconnect the air lines and the electrical connector for the heating of the probe.
I disconnect the pitot tube and I attach locking wire to the connectors to keep them from falling inside the fuselage, if that happens I will have to gain access to them in the cockpit and I would like to avoid doing that.
Note that on this particular pitot tube, there is no pitot pressure connector, that is because on this pitot tube this pitot pressure is not used.
There are 2 main air data systems and one auxilliary system, therefore there are 2 air data computers (the standby instruments are connected straight to the sensors). Both these computers need an input from one pitot pressure and one static pressure.
The pitot pressure from one probe is very accurate but the static pressure from one probe is affected by crosswinds and yawing motions so therefore a single static source is cross connected to the other side of the fuselage allowing any pressure bumbs in the line to be dissipated through the other static opening. As a example of a air data system here's a rough sketch of one:
I hope u can understand it, as u can see I haven't used CAD to draw this.
ofcourse I need to clean off the remaining sealant.
note that we are not allowed to use the old sealant scratchers anymore, cuting away sealant with a knife of another metal or sharp carbon scratcher has been proven to be the cause of the aloha disaster back in 1988. We now use these plastic scratchers.
This is one of the shims that was used to level the base of the pitot tube with the aircraft fuselage. The surface of the fuselage and the surface of the base of the pitot tube must be very aerodynamically smooth because the air sensing must obviously not have turbulent areas of air.
This is especially important due to the RVSM requirement of the aircraft.
(I removed some corrosion with a piece of scotchbrite)
The new pitot tubes straight from the box.
Here's the old pitot tube on the left and the new pitot tube on the right.
In the meantime my colleague is removing the pitot tubes from the other side of the aircraft, while I do some paperwork and chat to my mates that are walking around the hangar.
When we have the three pitot tubes off, we install the new ones with two screws to see if the base plate needs adjusting.
We measure the base of the pitot tube with a accurate and callibrated tool depth meter.
and find that it is ok.
We put masking tape around the gaps between the pitot tube and the fuselage so that we can seal it with aerodynamic sealant without getting sealant all over the fuselage.
We are short on time and decide to seal the pitot tubes first and test the system afterwards so that the sealant can dry while we test the system.
I get some sealant and we are ready to go.
There, that's the sealing done.
This is a pitot static testset, it simulates the airpressures on the sensors. This is a very accurate and extremely expensive tool to test the air data system.
With this testset we want to simulate an altitude of 10.000 feet and an airspeed of 300 knots and make sure that the system hasn't got any leaks.
We connect the pitot tube adapters to the tube's.
Now that we have everything connected we want to start the test but we are halted by the fact that another colleague of ours has just removed the aircraft battery for replacement. With this battery removed the aircraft's power is down and the testcart takes it's power from a receptacle on the aircraft.
Therefore, we have to wait...
The new battery get's connected, we now have power for our testset and we get ready to do the test. We have to pull circuit breakers because other systems would be affected by fooling the aircraft's speeds and altitudes. Systems like the flap load relief, on this aircraft the flap load relief would retract the flaps to 30 when the airspeed gets too high.
We are now ready to start the test. We will create a vacuum to fool the aircraft that it thinks it's on 10.000 feet and flies at 300 knots.
Just click play to view our test:
And then..... disaster strikes. A 300 feet per minute leak.
We test the adapters with test tubes from the testset, they are leaktight. We test the A.D.C. 1 system and the A.D.C. 2 system seperately and find that the leak is in the A.D.C. 1 system.
We remove one of the pitot tubes from that system again and blank off the connector openings to test one of the two new tubes that are suspect.
At this time, a record 6 avionic colleague's come in to take over the job. We are going home a bit dissapointed that the system doesn't work.
The next day we came in, turns out that the metric altimeter had an internal leak. The leak was small enough not to have a altitude split between the two systems (because the static line is connected to ambient air pressure a minor leak can have no indication at all) but when u leak check the system, a leak like that will show.