When I was studying for my instrument rating, I did what most pilots do with pitot-static blockages. I found the table, memorized what each instrument does under each failure scenario, and moved on.
Then I sat down for my mock oral. The examiner asked me what happens to the airspeed indicator with a ram blockage versus a static blockage. Under pressure, I mixed them up.
Not because I hadn't studied. Because I had memorized the outcome without understanding the mechanism. When the stress hit, I had nothing to fall back on except a table I couldn't quite recall.
If I had understood why the instruments behave the way they do under each failure, I would have been able to reason my way to the right answer even under pressure. That's the difference this post is about.
## How the System Actually WorksThe pitot-static system feeds three instruments: the airspeed indicator, the altimeter, and the vertical speed indicator. Understanding what each instrument needs to function tells you everything about what happens when something blocks.
The pitot tube faces into the oncoming air and captures ram air pressure — the pressure created by the aircraft moving through the air. Only the airspeed indicator uses pitot pressure.
The static ports are flush with the fuselage and capture ambient static pressure — the atmospheric pressure at your current altitude. The altimeter and VSI use only static pressure. The airspeed indicator uses static pressure as a reference to compare against ram pressure.
The airspeed indicator works by measuring the difference between ram pressure (from the pitot tube) and static pressure (from the static ports). The greater the difference, the higher the indicated airspeed.
That one sentence explains everything about blockage behavior.
What Actually Happens When Something Blocks
Pitot tube blocked, drain hole open: Ram pressure is lost. Static pressure still reaches the airspeed indicator normally. As you climb, static pressure decreases — so the reference pressure drops while the trapped ram pressure stays constant. The difference between them appears to grow. The airspeed indicator reads higher than actual airspeed as you climb, and lower as you descend.
Think about it from the physics: the instrument is comparing a fixed trapped pressure against a changing static reference. As static pressure drops with altitude, the gap widens. The indicator sees a bigger difference and reports higher airspeed.
Pitot tube blocked, drain hole also blocked: Both ram and static pressure are now trapped at whatever they were when the blockage occurred. The airspeed indicator freezes. It reads the same airspeed regardless of what you do — climbs, descents, power changes, nothing moves it. A frozen airspeed indicator is the signature of a completely blocked pitot with a blocked drain hole.
Static port blocked: This one catches more pilots off guard. The altimeter and VSI both lose their pressure reference. The altimeter freezes at the altitude where the blockage occurred — it has no way to detect that you've changed altitude because it's comparing against trapped pressure. The VSI drops to zero and stays there.
The airspeed indicator is also affected but differently. It still receives ram pressure from the pitot tube, but now its static reference is trapped. As you climb above the blockage altitude, actual static pressure decreases but your reference doesn't — so the airspeed indicator reads higher than actual. As you descend below it, the opposite occurs.
This is why static port blockage is particularly dangerous in IMC. You lose your altimeter and VSI entirely, and your airspeed becomes unreliable in a way that's less obvious than a frozen needle.
Why This Matters More Than the Table
When I mixed up ram and static blockages in my mock oral, the problem was that I was trying to recall a stored answer rather than reason through the physics.
A pilot who genuinely understands the system doesn't need to memorize the table. They can reconstruct it from first principles. If the pitot is blocked with the drain open, I know ram pressure is being lost while static still works — so I know the airspeed indicator is comparing a trapped reference against changing atmospheric pressure, and I can reason out what that means for the reading at different altitudes.
That reasoning process holds up under pressure. Memorized tables don't always.
What to Actually Do in the Airplane
Before flight: Check that your pitot cover is off — this one is embarrassing and more common than it should be. Verify pitot heat is functional if you're flying in IMC or potential icing conditions. Check static port covers if your aircraft uses them.
Pitot heat: Use it proactively in visible moisture at temperatures near freezing, not reactively after you notice airspeed behaving strangely. Ice accumulates fast and a partially blocked pitot gives you unreliable rather than obviously wrong information — which is worse.
Alternate static source: Know where it is and how to activate it before you need it. In most aircraft the alternate static source is inside the cockpit, which means the pressure it draws is slightly lower than outside static pressure — your altimeter will read slightly high and airspeed will read slightly high when using it. The POH has the exact corrections for your aircraft.
Cross-check: This is why you have multiple instruments. A suspicious airspeed reading should immediately prompt you to cross-check with power setting, attitude, and GPS groundspeed. No single instrument failure should catch you completely off guard if you're actively cross-checking.
The Oral Question You'll Get
Your examiner will give you a scenario. Aircraft is in cruise flight. Airspeed begins climbing steadily but you haven't changed power or attitude. What happened and what do you do?
Steady climb in indicated airspeed with no input changes is the pitot blocked with drain hole open. You're climbing, static pressure is dropping, trapped ram pressure stays constant, the difference grows, the indicator reads high.
Frozen airspeed with no response to any input is the completely blocked pitot — both tube and drain hole.
Altimeter frozen and VSI at zero is a static port blockage. Your airspeed is also unreliable. Activate alternate static if available.
You don't need to memorize those answers. Understand the system and you can reason your way there every time.
Pitot-static and aircraft systems are covered in depth in my ground school sessions. If your oral is coming up and you want to work through systems knowledge with real scenarios, $25 per session at andrewserrazina.com/ground-school — first session free with code FIRSTFREE.