In ground school I missed two questions in the same neighborhood: one on pitot-static blockage and one on compass acceleration error. They looked like different topics. They are not. Both are about instruments that keep giving you a confident-looking answer after something has gone wrong, and both stop being confusing once you know what each instrument is actually measuring.
This post combines the two. It is a study aid built from the FAA's Pilot's Handbook of Aeronautical Knowledge, Chapter 8 (Flight Instruments), not a substitute for your airplane's POH or your instructor. Where the two disagree, the POH and your CFI win.
Start With What Each Instrument Needs
Skip the memorized table for a minute and ask one question per instrument: what pressure does it need?
- Airspeed indicator (ASI): needs both pitot (ram) pressure and static pressure. It shows the difference between them.
- Altimeter: needs static pressure only.
- Vertical speed indicator (VSI): needs static pressure only. It reads how fast static pressure is changing.
Everything else follows from that. If the pitot side is blocked, only the instrument that uses pitot pressure can be wrong, and that is the ASI. If the static side is blocked, all three instruments lose their reference, so all three are affected.
The Pitot-Static Blockage Matrix
Pitot tubes can block with ice, insects, or debris, and the static port can block with ice or debris too. Many pitot tubes also have a small drain hole so moisture can escape, and whether that hole stays open changes the symptoms. The PHAK describes the behaviors below.
| What is blocked | Airspeed indicator | Altimeter | VSI |
|---|---|---|---|
| Pitot tube blocked, drain hole open | Falls toward zero as the trapped pressure bleeds out the drain | Normal | Normal |
| Pitot tube and drain hole blocked, static open | Acts like an altimeter: reads higher in a climb, lower in a descent, and does not respond to speed changes in level flight | Normal | Normal |
| Static port blocked, pitot open | Works, but wrong: reads low when you are above the blockage altitude and high when you are below it | Frozen at the altitude where the blockage happened | Stuck at zero |
| Pitot and static both blocked | Frozen | Frozen | Stuck at zero |
Why the pitot-blocked case reads like an altimeter
If the pitot tube and its drain hole are both blocked, the pressure inside the ASI's diaphragm gets trapped at whatever it was. Static pressure outside the diaphragm still changes with altitude. Climb, static pressure drops, the trapped pressure now looks bigger by comparison, and the ASI reads faster. Descend and the opposite happens. That is the textbook "ASI acts like an altimeter" result, and it is the exact scenario I missed: pitot and drain blocked, static clear, so only the ASI misbehaves.
Why the static-blocked case is the reverse
With the static port blocked, the ASI is stuck with the old static pressure while the pitot side keeps working. Climb above the altitude where the blockage happened and the ASI reads lower than your real airspeed. Descend below it and it reads higher. The PHAK also notes that the altimeter freezes and the VSI shows zero. A common memory hook is that blocked static makes the ASI a reverse altimeter, which is the mirror image of the pitot-blocked case.
What you can do about it
- Prevent it on the ground. The pitot cover comes off and the pitot tube and static ports get checked during the walkaround. Our preflight checklist post covers the broader walkaround.
- Pitot heat. Many airplanes have it for flight in visible moisture. Use the procedure in your POH, and verify the heater works before you need it.
- Alternate static source. Some airplanes have one in the cabin. Cabin pressure is usually a little lower than outside static pressure, so expect the altimeter to read slightly high, the ASI slightly fast, and a momentary climb indication on the VSI. Your POH lists any correction.
- Cross-check. The attitude indicator, power setting, and known pitch attitudes do not depend on the pitot-static system. Learn the pitch and power combinations for your trainer from your instructor so a lying ASI does not control the airplane.
The Magnetic Compass Is Simple, and Still Wrong Often
The compass has no power source, no gyro, and no pitot-static plumbing, which makes it the backup for everything else. It is also only accurate in straight, level, unaccelerated flight. The PHAK covers it as two kinds of error: the ones built into the Earth and the airplane, and the ones you cause with maneuvering.
| Error | Where it shows up | What happens | Memory hook |
|---|---|---|---|
| Variation | Depends on your location | Magnetic north differs from true north. Colorado is roughly 7 to 9 degrees east. | East is least, west is best |
| Deviation | Depends on the airplane and the heading | The airplane's own magnetic fields pull on the compass. The correction card shows the fix. | Read the card in the window |
| Acceleration error | Easterly and westerly headings only | Accelerating shows a turn toward north. Decelerating shows a turn toward south. | ANDS: Accelerate North, Decelerate South |
| Turning error | Turns toward northerly and southerly headings | Roll out early on north, roll out late on south. | UNOS: Undershoot North, Overshoot South |
Variation and deviation are not the same thing
Variation is geography. It is the angle between true north and magnetic north, and the isogonic lines printed on your chart show it for your area. Convert from true to magnetic by subtracting easterly variation and adding westerly variation, which is all that "east is least, west is best" means. In Colorado, a true course of 090 with about 8 degrees of east variation becomes a magnetic course near 082. You can check the current value for any location with the NOAA magnetic declination tools.
Deviation is the airplane. Electrical current, magnetized metal, and equipment in the cockpit all tug on the compass, and the effect changes with heading. That is why the compass has a correction card, and why the PHAK warns about equipment that causes magnetic interference, naming radios, deicing equipment, and pitot heat among the examples. Yes, the same pitot heat that protects your ASI can nudge your compass.
ANDS: why acceleration only matters on east and west
The compass float is built to stay horizontal, and it only reads correctly while the card is level. The catch is the pendulum-style mounting. Per the PHAK, the aft end of the card tilts upward when you accelerate and downward when you decelerate, and on an easterly or westerly heading that shows up as a false turn. Accelerate and the compass indicates a turn toward north. Decelerate and it indicates a turn toward south. If the compass seems to wander during a power change on an easterly or westerly heading, that is why.
UNOS: turning errors and the 15-plus-half-latitude rule
Turns tilt that same float. The PHAK says the float leads in a turn toward north, so you stop the turn before the compass reaches the heading. In a turn toward south it lags, so you let the airplane pass the heading before you stop. The rule of thumb for the lead or lag is 15 degrees plus half your latitude. Colorado's Front Range sits near 40 degrees north, which is the same 40-degree example the PHAK uses: stop 35 degrees early (15 plus 20) when turning to a northerly heading. Nobody should be rolling out steep turns off the compass when the heading indicator works. This matters when the heading indicator is not available, or when you are setting it.
Where the Compass Meets the Heading Indicator
The heading indicator is a gyro, and gyros drift. The PHAK recommends checking it against the magnetic compass about every 15 minutes in straight, level, unaccelerated flight and resetting it if it has drifted. Doing that in a turn or while climbing hard means you are resetting a good instrument to a bad reading. In rough air, the compass can swing back and forth, so use the average of the swings rather than a single moment.
Which Systems Fail Together
The practical payoff of all this is knowing what survives a given failure. The PHAK says the gyroscopic instruments are typically powered by a vacuum or pressure system, and a normal vacuum range is about 4.5 to 5.5 inches of mercury. In many trainers, vacuum runs the attitude indicator and heading indicator, and electrical power runs the turn coordinator.
- Pitot-static blockage: the ASI, altimeter, and VSI are affected. The attitude indicator, heading indicator, turn coordinator, and compass are not.
- Vacuum failure: the attitude and heading indicators go bad. The pitot-static instruments and the compass keep working.
- Electrical failure: the turn coordinator can go dark, and the ball still works because it only responds to gravity.
- Compass error: independent of all three, which is why you cross-check it against the others.
When one instrument disagrees with the rest, the question is not which one to believe. It is which system that instrument lives in, and what the other systems say.
Six-Question Self-Test
Cover the answers and run these before your next quiz.
- Pitot tube and drain hole are blocked, static is clear, and you descend. What does the ASI do? Answer: it reads lower, and the altimeter and VSI are normal.
- The pitot tube is blocked but the drain hole is open. Which instrument is affected? Answer: only the ASI, which falls toward zero.
- The static port is blocked and you climb above that altitude. What do the ASI, altimeter, and VSI show? Answer: the ASI reads lower than actual, the altimeter stays frozen, and the VSI shows zero.
- You are on a heading of 270 and accelerate. What does the compass show? Answer: a turn toward north.
- At about 40 degrees north latitude, you are turning to 360 by the compass alone. How early do you roll out? Answer: about 35 degrees early, which is 15 plus half of 40.
- Your true course is 090 in Colorado with roughly 8 degrees of east variation and no deviation. What is the magnetic course? Answer: about 082.
Bottom Line
Do not memorize the blockage table as four unrelated rows. Remember that the ASI is the only instrument that needs pitot pressure, so a pitot problem can only hurt the ASI, while a static problem hits all three. For the compass, remember that it only tells the truth in straight, level, unaccelerated flight, and that variation, deviation, acceleration, and turning each add their own predictable error.
The point is not to out-think a broken instrument in the middle of a flight. It is to recognize the pattern fast, work out which system just failed, and fly the airplane with the instruments that are still telling the truth. Check the details against your POH, the PHAK, and your instructor before you rely on any of this in the airplane.