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

I want to walk you through the Airspeed Indicator now, and we're going to focus on what happens when things go wrong with its pressure supplies. We've already covered how the ASI works with pitot and static pressure, so now let's look at the failure modes, because that's what really matters for you as a pilot. First, let's talk about the static head versus the static vent. The static head is more exposed to icing conditions, and that means it's more likely to become obstructed than a static vent is. That's a key difference to remember — the static head is the vulnerable one. Now, here's the critical scenario. If the static source becomes blocked during a descent, the 'old' static pressure — the higher altitude pressure that's trapped around the capsule — will be lower than it should be. So if the pitot supply is normal, the ASI will over-read. And here's why that's dangerous: the aircraft is actually nearer the stall than the ASI is indicating. You think you have more speed than you really do, and that's a trap. Now contrast that with a climb. If you have a blocked static source but normal pitot air during a climb, the ASI will under-read. So the rule is simple: with a blocked static source, the ASI over-reads in a descent and under-reads in a climb. There's also the alternative static source. If you select it, an error may occur, and that error is due to position error. Any dynamic or turbulence effects would usually result in a higher static pressure, and that higher pressure produces an under-reading. This error is known and documented in the Flight Manual, so you'd know what to expect. Now, there's a mnemonic for exams that I want you to remember: PUDSOD. It stands for 'Pitot Blocked: Under-reads in Descent, Static Blocked: Over-reads in Descent.' So P-U-D-S-O-D — Pitot blocked under-reads in descent, Static blocked over-reads in descent. That's your quick memory aid. Let's move on to leaks. A leak in the pitot tube causes the ASI to under-read, because you're losing dynamic pressure. That makes sense — less dynamic pressure means less indicated speed. Static leaks are a bit more nuanced. 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 happens 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 problem. Finally, let's go through the serviceability checks you should make before flight. These are the checks of the ASI and the pressure supply system. First, the pressure head cover(s) and static vent plug(s) should be removed and stowed aboard the aircraft. Second, the pitot tube(s), the holes or slots in the static head(s), and/or the 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. So to tie it all together: the ASI is only as good as its pressure supplies. A blocked static source gives you the over-read in descent, under-read in climb. A pitot leak under-reads. A static leak can over-read, or in a pressurized cabin, render the instrument useless. And before every flight, you run through those checks to make sure the system is clean, unobstructed, and working.