Static Port Blockage - Airspeed

I've been working pretty hard lately on my next publication, Commercial Pilot License: Written Exam Preparation. The goal is to get it published in the first half of 2013. That's a pretty aggressive goal considering how much work is required, but I'm on it!

Today I was running through the book, reviewing and developing practice questions, and I stumbled across one that I expect to cause some heartache. I can see a long list of students and instructors contacting me to tell me that either the question is wrong or the answer key is wrong. So I think I'll just preempt that and look at the question here. The question is as follows:

While cruising at 3,500 feet ASL, your aircraft experiences a complete static source blockage. Upon commencing descent, you recognize the blockage due to the combined indications of the pitot-static instruments. The aircraft is not equipped with an alternate static source, so you continue your approach to your destination with the malfunctioning instruments. At your destination aerodrome, which has an elevation of 1,500 feet ASL, you decide to abort your approach and go around for another try. During the climbout from the go-around, your ASI will read:            a) zero.                     c) high.            b) accurately.            d) low.

This question is actually very straightforward (despite the wordiness!). It boils down to: "How does your ASI behave after a static port blockage?".

Why do I expect complaints about this questions? It comes back a a faulty mnemonic that gets used for this problem.

After a static blockage, the ASI will under-read in a climb and over-read in a descent. WRONG!

This statement is based on a faulty assumption, and can be misleading—for example in the present question.

Let's look at how the ASI works and how a static blockage affects indications. The ASI takes total pressure (which consists of both static and dynamic pressure) from the pitot tube, subtracts static pressure read from the static port, and gets dynamic pressure (the ½ρV² portion of the lift equation). In order for this to work, both the total pressure readings and the static pressure readings have to be accurate.

Static pressure varies with altitude—climb and static pressure drops, descend and static pressure increases. Under normal conditions, the static pressure reading from the static port cancels the static pressure component of the total pressure from the pitot tube. However, in the event of a blockage, these two pressures can become mismatched, resulting in erroneous readings.

If the static port is completely blocked, the static pressure reading will remain fixed. As long as we remain at the same altitude, this really doesn't matter. But if we change altitude, the static portion of the total pressure (sampled at the pitot tube) will change accordingly, but the static pressure sampled at the static port will not change. This mismatch between the two static pressures is what causes the problems with the ASI.

If we climb to an altitude above the blockage altitude, the static pressure portion of the total (pitot) pressure will be reduced. This means that the (fixed) static pressure in the static line will overbalance the actual static pressure and cancel out part of the dynamic pressure. The result: an under-reading ASI. The opposite happens if we descend to an altitude below the blockage altitude. The static component of the total pressure will be greater than the (fixed) static pressure in the static line. The excess static pressure in the pitot portion of the system will be interpreted by the ASI as more dynamic pressure. The result: an over-reading ASI.

Note that the over- or under-reading of the ASI is not influenced by whether we are climbing or descending. The (incorrect) statement above about climbing and descending is based on the assumption that the climb/descent starts from the altitude at which the blockage occurred. The question, however, contradicts that assumption. If we eliminate the assumption, the new rule for ASI errors after static a blockage is as follows:

After a static blockage, the ASI will under-read when above the blockage altitude and over-read when below the blockage altitude. RIGHT!

Using the first (incorrect) rule above, we can refer back to the question and note that the aircraft is climbing. Therefore we expect the answer to be (d) low. However, if we apply the second (correct) rule above, we expect the answer to be (c) high. This is indeed the correct answer.

As a final note, this misunderstanding is potentially dangerous if you ever face this situation. If we think the ASI is under-reading because we are climbing, but it is in fact over-reading because we are below the blockage altitude, any attempt to correct can quickly lead to a stall scenario.

Happy Flying!