Instructions for checking the pressure tightness of the entire pressure system
The tightness of the pressure system is fundamental to the proper operation of all instruments. This should be checked in regular intervals. For the tests you will need the following:
- the air speed indicator
- a medical syringe, approx. 60ml; for a tight fit, lubricate the plunger with some Vaseline
- a piece of silicone tube, approx. 40cm
- a suitable T-connector
First check the ASI for pressure tightness
How it's done
Warning: All work should be done with care. Rapid pressure changes must be avoided, all pressure changes done with the syringe should be done slowly, otherwise the instruments may be damaged!
Connect the total pressure connector to the syringe using the piece of tubing and slowly increase the pressure until the ASI reads ~150 km/h (approx. 80 knots).
If the reading is maintained for at least 30sec then this side is OK. If the syringe is not 100% tight it may be necessary to kink the tube to seal it off.
Now connect the static port to the syringe and create an under pressure until the ASI reads ~150 km/h (approx. 80 kts).
If this is maintained for at least 30sec then the ASI can be considered air tight and be used for the remaining steps. If this is not the case, then either the instrument or the syringe is defective. If everything is okay then reconnect the total and static pressure to the ASI.
Checking the integrity of the total, static, and TE pressure systems including all connected instruments
How it's done
Warning: The fuselage ports need to be connected to ASI, altimeter, and transponder for total and static pressure. The static port of the vario is usually connected to a TE-probe, if a multi-probe is fitted then this should only be connected to the vario(s) and the flight computer, unless recommended otherwise by the airplane manufacturer.
The static pressure system is checked by under pressure: seal off the fuselage ports using soft tape and if necessary a silicone tube.
Connect the T-piece to the static system and its free part to the syringe. Carefully create under pressure by pulling on the syringe until the ASI reads ~150 km/h (approx. 80 kts).
If this reading is maintained for at least 30sec then the static system can be considered tight and the T-piece can be removed and everything be reconnected.
Now seal off the total pressure port on the fuselage and on the probe.
Repeat the previous procedure with a slight over pressure in the system. If the reading on the ASI is also maintained for at least 30sec then this system is sufficiently tight. If the syringe is not 100% tight it may be necessary to kink the tube to seal it off.
The TE pressure system is checked using under pressure also
How it's done
To do this, seal the TE port of the probe (ideal with some silicone tubing). Deconnect the TE line before the instrument split under the seat pan.
Since the split is usually carried out under the seat pan where an ASI may not be available, you can use your mouth for the hose from the tail fin and suck it vigorously.
If the tongue sticks to the hose for 30 seconds, the TE circle is most likely o.k. and the hose can be reconnected.
If it is possible to include the ASI as in the case of the measurement of the static, this method is preferred.
Should any leakage be detected it is paramount to isolate and fix it. Even small leaks, especially in the TE system, preclude a precise compensation. After prior consultation you can also send us a probe for pressure integrity checking.