The Airspeed Indicator (ASI) — Page 11, Lesson 9

I want to walk you through the Airspeed Indicator, starting with a critical vulnerability: the static head. The static head is more exposed to icing conditions than a static vent is, so it is more likely to become obstructed. That's an important distinction right at the start — the part that senses static pressure is physically more vulnerable to ice buildup. Now, what happens if the static source gets blocked during a descent? The 'old' static pressure — the higher altitude pressure that was trapped in the system — stays surrounding the capsule. That trapped pressure is lower than the actual static pressure at the lower altitude. If the pitot supply is normal, the capsule sees a larger pressure difference than it should, so the ASI will over-read. It will show a higher airspeed than the aircraft is actually doing. This is dangerous because the aircraft is nearer the stall than the ASI is indicating. You think you have more speed margin than you really do. Conversely, during a climb with a blocked static source and normal pitot air, the ASI will under-read. The trapped static pressure is higher than the actual pressure at the higher altitude, so the pressure difference across the capsule is smaller, and the indicated speed is lower than true. So the pattern is clear: with a blocked static source, the ASI over-reads in a descent and under-reads in a climb. If you select the alternative static source, an error may occur. That error is due to position error. Any dynamic or turbulence effects usually result in a higher static pressure being sensed, which produces an under-reading. This error is known and documented in the Flight Manual — it's not a surprise, it's a predictable, published value. For exams, there's a useful mnemonic: PUD so D. That stands for 'Pitot Blocked — Under-reads in Descent' and 'Static Blocked — Over-reads in Descent'. Notice the 'so D' part — both conditions have a rule for descent, and the first letter tells you the direction of the error. Now let's move to leaks. A leak in the pitot tube causes the ASI to under-read because you lose some of the dynamic pressure. Less pressure difference, lower indicated speed. Static leaks can occur either inside or outside the pressure cabin. In almost all unpressurized aircraft, the outside pressure is lower than the static pressure inside the tube, so a leak there causes the ASI to over-read — though probably not significantly. But with a pressurized cabin, if the leak occurs inside the pressure compartment, the cabin altitude pressure gets incorrectly sensed as static pressure. That usually renders the ASI useless — the reading becomes meaningless. Finally, let's look at the pre-flight serviceability checks you must perform on the ASI and its pressure supply system. First, the pressure head cover and static vent plug must be removed and stowed aboard the aircraft — you'd be surprised how often that gets forgotten. The pitot tube, the holes and slots in the static head, and the static vents should be checked free from obvious obstructions like insects. If a pitot head heater is fitted, check it's operative. The dial glass must be clean and undamaged. And the instrument should indicate airspeed in the correct sense shortly after starting the take-off run — that means you should see the needle move up as you accelerate, confirming the system is working.