Pitot static instruments are a favourite topic in the DGCA Technical General paper because one idea explains a whole set of questions. If you understand which pressure goes where, you can work out every pitot static system blockage question without memorising tables. This guide explains the three instruments, how each one works, and exactly what they show when something blocks.
What the pitot static system measures
The pitot static system measures two air pressures and feeds them to the flight instruments.
- Static pressure is the pressure of the still air around the aircraft. It is sensed by static ports, small holes on the side of the fuselage placed where the airflow disturbs the reading least.
- Total pressure (also called pitot pressure) is static pressure plus the extra pressure caused by the aircraft's motion. It is sensed by the pitot tube, which faces forward into the airflow.
- Dynamic pressure is the difference between the two: total pressure minus static pressure. It depends on air density and speed, so it is what an airspeed indicator really measures.
Many pitot tubes have a small drain hole to let water out and an electric heater (pitot heat) to stop ice from blocking the inlet.
The three pitot static instruments
Three basic instruments use the system. Only one of them needs the pitot tube.
- Airspeed indicator (ASI): uses pitot and static.
- Altimeter: uses static only.
- Vertical speed indicator (VSI): uses static only.
The Machmeter, found on faster aircraft, also uses both pitot and static pressure, but most DGCA questions focus on the three above.
Airspeed indicator (ASI)
The ASI has a thin capsule fed with pitot (total) pressure, sitting inside a sealed case fed with static pressure. Static pressure acts on both sides, so it cancels, and the capsule moves only with dynamic pressure. The pointer shows indicated airspeed (IAS).
IAS corrected for instrument and position error gives calibrated airspeed (CAS). Corrected further for compressibility it becomes equivalent airspeed (EAS), and corrected for density it becomes true airspeed (TAS). At height the air is thinner, so TAS is higher than IAS for the same dynamic pressure.
Altimeter
The altimeter contains a sealed, partly evacuated aneroid capsule inside a case fed with static pressure. As the aircraft climbs, static pressure falls, the capsule expands, and the linkage drives the pointers to a higher reading. The subscale lets you set the reference pressure, such as QNH, so the reading means something useful.
Vertical speed indicator (VSI)
The VSI shows rate of climb or descent in feet per minute. A capsule is fed directly with static pressure, while the case gets static pressure through a narrow calibrated leak (metering unit). When you climb, the pressure in the capsule falls straight away, but the case pressure lags behind. The pressure difference while this happens moves the pointer. Because of this design, a basic VSI has a short lag of a few seconds. An instantaneous VSI (IVSI) adds small accelerometer-driven pumps to reduce the lag.
Pitot static system blockage: what each instrument shows
The key to every pitot static system blockage question is simple: the trapped pressure stays constant while the real pressure changes. Work out which pressure is trapped, then think through each instrument.
Static port blocked
Static pressure is trapped at the value where the blockage happened.
- Altimeter: freezes at the height where the blockage occurred.
- VSI: returns to zero and stays there, whatever the aircraft does.
- ASI: under-reads in a climb and over-reads in a descent. In a climb the real static pressure falls, but the trapped static in the case stays high, so the instrument "sees" less dynamic pressure than there really is.
Pitot tube blocked
If only the pitot inlet blocks and the drain hole stays open, the pressure in the pitot line leaks away to static. The ASI reading falls towards zero.
If both the pitot inlet and the drain hole block, total pressure is trapped. The ASI then behaves like an altimeter: it over-reads in a climb and under-reads in a descent, because the static pressure in the case falls as you climb while the trapped pitot pressure stays the same. Speed changes in level flight no longer show. This is dangerous, because a pilot climbing with a falling real airspeed may see a rising IAS.
The altimeter and VSI are not affected by a pitot blockage, because they use static pressure only.
Worked example: static blockage in a climb
Aircraft levels at 3,000 ft. The static ports ice over.
The pilot then climbs to 5,000 ft at a steady speed.
Altimeter: shows 3,000 ft (frozen)
VSI: shows 0 ft/min
ASI: under-reads (trapped static is higher than the
real static at 5,000 ft, so the difference the
ASI sees is smaller than the real dynamic pressure)
Reverse the climb into a descent and the ASI over-reads instead. The altimeter and VSI behave exactly the same way.
Alternate static source
Many light aircraft have an alternate static source, a valve that takes static pressure from inside the cabin if the external ports block. In an unpressurised aircraft the cabin pressure is usually slightly lower than outside, because air flowing past the fuselage lowers the pressure inside.
When the alternate static source is selected, typically:
- the altimeter reads slightly higher than the true altitude,
- the ASI reads slightly higher than normal,
- the VSI shows a brief climb before settling.
The exact errors for a type are given in its flight manual, so always use those figures in practice.
Common mistakes in DGCA technical general instruments questions
- Saying the altimeter uses pitot pressure. It uses static only, as does the VSI.
- Mixing the two pitot blockage cases. Drain hole open: ASI falls to zero. Drain hole also blocked: ASI acts like an altimeter.
- Reversing the static blockage ASI error. Static blocked: under-reads in the climb. Pitot and drain blocked: over-reads in the climb.
- Forgetting the VSI lag when a question asks why the VSI shows a delayed reading after a pitch change.
Find the trapped pressure first; every pitot static instrument error follows from that one step.
Frequently asked questions
Which instruments use the pitot static system?
The three basic instruments are the airspeed indicator, the altimeter and the vertical speed indicator. The airspeed indicator uses both pitot and static pressure. The altimeter and vertical speed indicator use static pressure only. On faster aircraft the Machmeter also uses both pitot and static pressure.
What happens if the static port is blocked?
Static pressure is trapped. The altimeter freezes at the height where the blockage happened, and the vertical speed indicator shows zero. The airspeed indicator under-reads in a climb and over-reads in a descent, because the trapped static pressure no longer matches the real outside pressure as height changes.
What happens if the pitot tube is blocked?
It depends on the drain hole. If only the pitot inlet blocks, pressure leaks out through the drain and the airspeed indicator drops towards zero. If the drain hole blocks as well, total pressure is trapped and the airspeed indicator acts like an altimeter, over-reading in a climb and under-reading in a descent.
What does the alternate static source do?
It supplies static pressure from inside the cabin when the external static ports are blocked. In most unpressurised aircraft the cabin pressure is slightly lower than outside, so the altimeter and airspeed indicator read a little high and the vertical speed indicator briefly shows a climb when the source is selected.
Why does the VSI have a lag?
The VSI compares the static pressure inside its capsule with the pressure in its case, which changes slowly through a calibrated leak. That leak creates the pressure difference that moves the pointer, but it also means the reading takes a few seconds to settle. An instantaneous VSI uses accelerometer pumps to reduce this lag.
Next steps for pitot static instruments
Pitot static instruments carry easy marks once the trapped pressure idea is clear, so test yourself until each blockage answer feels automatic. Try the free DGCA practice questions and watch the free DGCA video lessons for diagrams of each instrument. Fly with Sudharshan Aviation Academy (FSAA) covers flight instruments, systems and principles of flight in the DGCA Technical General course.