The Airspeed Indicator (ASI) — Page 49, Lesson 57

Right, let's pick this up with the Airspeed Indicator. We've already covered the basic pressure system, so now we're looking at what happens when that system fails, and how we check it before flight. First, a key distinction: the static vent versus the static head. The static vent is a small flush port. The static head is a more exposed probe. Because it sticks out, it's more exposed to icing conditions, and therefore more likely to become obstructed than a static vent. That's the first thing to remember. Now, the critical failure cases. Let's think about a blocked static source during a descent. The static line is blocked, so the capsule inside the ASI is still surrounded by the 'old' static pressure—the higher altitude pressure from when the blockage occurred. That old pressure is lower than it should be for the current, lower altitude. If the pitot supply is normal, the differential pressure is too high, and the ASI will over-read. It will show a higher airspeed than you actually have. That's dangerous because the aircraft is nearer the stall than the ASI is indicating. You think you have more margin than you do. Now the opposite case: a climb with a blocked static source and normal pitot air. The old static pressure is now higher than the actual ambient pressure at the higher altitude. That reduces the differential, so the ASI under-reads. So the rule is: with a blocked static source, the ASI over-reads in a descent and under-reads in a climb. There's a mnemonic for exams: PUDSOD. It stands for 'Pitot Blocked: Under-reads in Descent; Static Blocked: Over-reads in Descent.' That's the quick way to remember the dangerous case. Now, what about the alternative static source? If you select it, an error may occur. That error is due to position error. The alternative source is in a different position on the aircraft, so it senses a slightly different pressure. Any dynamic, or turbulence, effects would usually result in a higher static pressure, and that higher pressure would produce an under-reading. This error is known and would be documented in the Flight Manual. So you know the magnitude and sense of it before you need it. Next, leaks. A leak in the pitot tube causes the ASI to under-read, because you're losing dynamic pressure. That's straightforward. Static leaks are more subtle. They can occur either inside or outside the pressure cabin. If the leak is in the static tube where the outside pressure is lower than static—which is the case with almost all unpressurized aircraft—the ASI will over-read, but probably not significantly. However, with a pressurized cabin, if the leak occurs within the pressure compartment, the cabin altitude will be incorrectly sensed as static pressure. That usually renders the ASI useless. So a static leak inside a pressurized cabin is a much more serious failure. Finally, the serviceability checks before flight. These are the checks of the ASI and the pressure supply system. First, pressure head cover(s) and static vent plug(s) must be removed and stowed aboard the aircraft. Second, the pitot tube(s), holes/slots in static head(s) and/or static vent(s) should be checked free from obvious obstructions such as insects. Third, the pitot head heater should be operative, if fitted. Fourth, the dial glass should be clean and undamaged. And fifth, the instrument should indicate airspeed in the correct sense shortly after starting the take-off run. That last one is your final confirmation that the whole system is working before you commit to the take-off. So to tie it together: the ASI is only as good as its pressure supply. Blocked static, pitot leaks, static leaks—each has a distinct effect on the reading, and the pre-flight checks are designed to catch the preventable ones before they become a problem in the air.