Safely flying in any aircraft depends upon not only a pilot’s ability to interpret and operate flight instruments, but also to recognize when an instrument is malfunctioning. This week, we’ll take a look at common issues associated with pitot-static flight instruments: the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI). This content comes from the cornerstone FAA manual Pilot’s Handbook of Aeronautical Knowledge.
The pitot system can become blocked completely or only partially if the pitot tube drain hole remains open. If the pitot tube becomes blocked and its associated drain hole remains clear, ram air no longer is able to enter the pitot system. Air already in the system vents through the drain hole, and the remaining pressure drops to ambient (outside) air pressure. Under these circumstances, the ASI reading decreases to zero, because the ASI senses no difference between ram and static air pressure. The ASI no longer operates since dynamic pressure can not enter the pitot tube opening. Static pressure is able to equalize on both sides since the pitot drain hole is still open. The apparent loss of airspeed is not usually instantaneous but happens very quickly. [Figure 1]
If both the pitot tube opening and the drain hole should become clogged simultaneously, then the pressure in the pitot tube is trapped. No change is noted on the airspeed indication should the airspeed increase or decrease. If the static port is unblocked and the aircraft should change altitude, then a change is noted on the ASI. The change is not related to a change in airspeed but a change in static pressure. The total pressure in the pitot tube does not change due to the blockage; however, the static pressure will change.
Because airspeed indications rely upon both static and dynamic pressure together, the blockage of either of these systems affects the ASI reading. Remember that the ASI has a diaphragm in which dynamic air pressure is entered. Behind this diaphragm is a reference pressure called static pressure that comes from the static ports. The diaphragm pressurizes against this static pressure and as a result changes the airspeed indication via levers and indicators. [Figure 2]
For example, take an aircraft and slow it down to zero knots at given altitude. If the static port (providing static pressure) and the pitot tube (providing dynamic pressure) are both unobstructed, the following claims can be made:
1. The ASI would be zero.
2. There must be a relationship between both dynamic and static pressure. At zero speed, dynamic pressure and static pressure are the same: static air pressure.
3. Because both dynamic and static air pressure are equal at zero speed with increased speed, dynamic pressure must include two components: static pressure and dynamic pressure.
It can be inferred that airspeed indication must be based upon a relationship between these two pressures, and indeed it is. An ASI uses the static pressure as a reference pressure and as a result, the ASI’s case is kept at this pressure behind the diaphragm. On the other hand, the dynamic pressure through the pitot tube is connected to a highly sensitive diaphragm within the ASI case. Because an aircraft in zero motion (regardless of altitude) results in a zero airspeed, the pitot tube always provides static pressure in addition to dynamic pressure.
Therefore, the airspeed indication is the result of two pressures: the pitot tube static and dynamic pressure within the diaphragm as measured against the static pressure in case. What does this mean if the pitot tube is obstructed? If the aircraft were to descend, the pressure in the pitot system including the diaphragm would remain constant. It is clogged and the diaphragm is at a single pressure. But as the descent is made, the static pressure would increase against the diaphragm causing it to compress thereby resulting in an indication of decreased airspeed. Conversely, if the aircraft were to climb, the static pressure would decrease allowing the diaphragm to expand, thereby showing an indication of greater airspeed. [Figure 2]
The pitot tube may become blocked during flight due to visible moisture. Some aircraft may be equipped with pitot heat for flight in visible moisture. Consult the AFM/POH for specific procedures regarding the use of pitot heat.
We’ll continue with this theme on Thursday and, as always, there will be a test question!
