Aircraft depend for a great deal on measuring the correct outside air pressure for the indication of airspeed, altitude and vertical speed. This is a job for the pitot static system which senses air pressures around the aircraft for display in the cockpit and for input to air data computers and transponders.
In the cockpit we see instruments known as pressure instruments and they operate by sensing changes in the pressure of the air surrounding the aircraft. If the aircraft is equipped with an EFIS the pressure sensors are also connected to the air data computer (ADC) for processing.
These sensors, pitot tube and static port, are generally very reliable but in case of a problem you need to be able to recognize these errors before a minor problem becomes a serious issue.
During preflight the pitot tube cover must be removed and the pitot tube and all static ports carefully checked for blockage of any kind (especially after washing or painting of the aircraft; insects have been known to nest inside pitot tubes). Do not blow into the pitot tube and if heater is installed, turn it on briefly and touch carefully with your hand to see if it will warm up. On the center zero ammeter this will be indicated as a discharge from the battery. Indicated with the needle to the negative side (usually to the left).
During pre-start checks, inspect the instruments for obvious errors. The ASI should read zero (or indicate the headwind), the VSI must also read zero and the altimeter should indicate the aerodrome elevation. When obtaining the local QNH make sure that the altimeter is within +30 and -45 feet of aerodrome elevation or else it will need recalibration or even replacement.
During the initial takeoff roll check for airspeed alive (ASI) and increasing and shortly after climbing an increase in altitude and an indication of rate of climb on the VSI. In short: Airspeed Alive and Positive Rate.
If the pitot tube is blocked the ASI will be affected by reacting as an altimeter, should the pitot tube have a drain hole then the ASI will slowly indicate zero. Any blockage of the static port will trap the air in the static line and all three will be affected. The VSI will indicate zero and remain there and the altimeter will not indicate a change in altitude when climbing or descending and the ASI will either over- or under read.
The list below shows which of the pressure instruments is connected to what port. In case of a leak or blockage this information is very handy when troubleshooting the cause of the problem.
Below some indications when the pitot static system is blocked in one way or another.
|Pitot tube drain hole block (*)|
|Airspeed indicator||In climb, the indicator will show an increase in speed. In descent a decrease. In level flight and large power changes the indication will not change.|
|Pitot tube blockage (*)|
|Airspeed indicator||Indication will slowly return to zero, use pitot heat.|
|Static source blockage|
|Airspeed indicator||Will under read in a climb and over read in a descent.|
|Vertical speed indicator||Indicates zero in climb or descent.|
|Altimeter||Indicates a constant altitude in climb or descent.|
(*) Should both the pitot and its drain hole clog, the airspeed indicator will react like an altimeter.
When flying near clouds (visible moisture) and the outside air temperature is at or near zero, it is advisable to switch on pitot heat to clear any trace of ice as a precaution, should there be any. It can really save the day.