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Learning hub · Instrumentation · Lesson 1

Pitot-static and gyro instruments

Three of the main instruments are driven by nothing more than two pressure sources. When one of them blocks, the instruments lie in a predictable way, and you can learn to recognise it.

About 14 minutes, plus the quiz. Part of Instrumentation. It builds on Pressure, QNH and the altimeter.

1. Two pressures

  • The pitot tube points into the airflow and collects the total pressure: the static pressure plus the dynamic pressure from moving through the air.
  • The static port is a small opening on the side of the fuselage which senses the static pressure of the air around you.
  • The airspeed indicator uses both. It shows the difference between them, which grows with the square of the speed.
  • The altimeter uses only static pressure, and turns it into a height.
  • The vertical speed indicator uses only static pressure too, and shows how quickly it is changing. It lags behind by a few seconds.

2. What happens when something blocks

Pitot tube blocked (ice, insects, a cover left on): the pressure trapped in the instrument stays the same while the static pressure changes. The airspeed indicator then works like an altimeter. It reads more in a climb and less in a descent, and does not change if you change the speed in level flight. This is the case when the front opening and the small drain hole are both blocked. If only the front opening is blocked and the drain hole is open, the trapped pressure leaks away and the reading falls slowly towards zero.

Static source blocked: the pressure trapped in the system stays the same as when it blocked. The altimeter freezes at that altitude, the vertical speed indicator shows zero, and the airspeed indicator reads too low in a climb and too high in a descent. Many aeroplanes have an alternate static source: use it, according to the manual.

The best protection is the pre-flight check: remove the pitot cover, look for dirt and insects, check that the static ports are clear, and use the pitot heat in visible moisture or when icing is possible.

3. Gyroscopic instruments

A spinning gyroscope has two helpful habits. It stays pointing in the same direction in space (rigidity), and when you push it, it reacts at 90 degrees to the push (precession).

  • The attitude indicator uses a gyro to show pitch and bank.
  • The heading indicator uses a gyro to give a steady heading, but it slowly drifts (friction and the rotation of the Earth both cause this). Check it against the compass regularly, about every 15 minutes, in straight and level flight.
  • The turn coordinator shows the rate of the turn and, with the ball, whether it is balanced.

4. The magnetic compass and its errors

The compass is simple and reliable, but it is only accurate in straight and level, unaccelerated flight. Two errors catch students out, in the northern hemisphere:

  • Turning error: UNOS, which stands for Undershoot North, Overshoot South. When you turn onto a northerly heading, roll out before the compass reads north. When you turn onto a southerly heading, roll out after the compass reads south.
  • Acceleration error: ANDS, which stands for Accelerate North, Decelerate South. On an east or west heading, speeding up makes the compass show a turn towards north, and slowing down shows a turn towards south.

When one instrument disagrees with the others, believe the ones that agree, then work out why. Use the manual for the procedures that apply to your aeroplane.

5. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

  1. You climb through 3,000 ft and the static source blocks. You carry on and climb to 4,500 ft. What do the altimeter, the vertical speed indicator and the airspeed indicator show? What can you do about it?
    Show the answer

    The pressure trapped in the static system stays the same as when it blocked. So the altimeter freezes at about 3,000 ft, the vertical speed indicator shows zero, and the airspeed indicator reads too low in the climb (it would read too high in a descent).

    Use the alternate static source, if the aeroplane has one, according to the manual. Cross-check with the attitude indicator, the engine power and the nose attitude, and believe the instruments that agree with each other.

  2. You are in level cruise at 100 kt at 4,000 ft when ice blocks both the front opening of the pitot tube and its drain hole. The static port is clear. What does the airspeed indicator do (a) when you climb to 5,000 ft at the same true speed, (b) when you descend again, and (c) when you add power in level flight?
    Show the answer

    With both the opening and the drain hole blocked, the trapped pressure stays constant while the static pressure changes, so the airspeed indicator works like an altimeter.

    (a) In the climb the static pressure falls, so the indicator reads more than the true speed. (b) In the descent it reads less. (c) If you speed up in level flight, the indicator does not change.

    The best protection is the pre-flight check and the use of pitot heat in visible moisture or when icing is possible. If only the front opening were blocked and the drain hole open, the trapped pressure would leak away and the reading would fall slowly towards zero.

  3. Northern hemisphere, magnetic compass. (a) You turn right from an easterly heading onto north (360°). Do you roll out before, at, or after the compass reads 360? (b) You turn onto south (180°). (c) You fly on 090° and open the throttle to accelerate. What does the compass appear to show?
    Show the answer

    The two memory aids are UNOS (Undershoot North, Overshoot South) and ANDS (Accelerate North, Decelerate South).

    (a) Turning onto north, the compass undershoots, so roll out before it reads 360. (b) Turning onto south, it overshoots, so roll out after it reads 180.

    (c) On an east or west heading, accelerating makes the compass show a turn towards north. On 090°, north is to the left, so the reading drops below 090 although you have not turned. Slowing down would show a turn towards south. The compass is only accurate in straight and level, unaccelerated flight.

6. Watch

An animated look at how airspeed, altitude and vertical speed come from the pitot and static pressures.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A short video on how a pitot or static blockage affects the altimeter, the airspeed indicator and the vertical speed indicator, which goes with section 2 of this lesson.

Open on YouTube if the player does not load. Not made by VectorEFB.

7. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

1. Which instruments use only the static pressure?

The altimeter and the vertical speed indicator use static pressure alone. The airspeed indicator uses the difference between the pitot and static pressures.

2. The static source blocks during a climb. What does the altimeter do?

The pressure trapped in the system does not change, so the altimeter freezes and the vertical speed indicator shows zero.

3. The pitot tube and its drain hole are both blocked. In a climb at a steady airspeed, the airspeed indicator:

The trapped pressure is constant while the static pressure falls as you climb, so the indicator acts like an altimeter and the reading increases.

4. How often should you reset the heading indicator?

It drifts slowly. Check it against the compass regularly in straight and level flight.

5. What does ANDS stand for?

In the northern hemisphere, on an east or west heading, acceleration makes the compass show a turn towards north, and deceleration a turn towards south.

Back to the learning hub.

For training and situational awareness only. Not certified for navigation. Always use official charts, the AIP, NOTAMs and a proper weather briefing for real flights.