Pitot and Static Sources

1. Introduction — Static, Dynamic & Pitot Pressure

An aircraft at rest in still air is subject to static pressure — ambient atmospheric pressure bearing equally on all parts. In flight it remains subject to static pressure but the leading edges also encounter an additional pressure due to the air resisting the aircraft's motion. This is dynamic pressure, whose value depends on the aircraft's speed through the air and on air density. The leading edges therefore feel a total (pitot) pressure = static + dynamic.

Dynamic pressure cannot be measured directly because it cannot be separated from the associated static pressure. Instruments therefore measure total (pitot) and static pressure, and subtract static from pitot internally to derive dynamic pressure.

Which instrument needs which pressure

flowchart LR
  P[Pitot source
total pressure] --> ASI[ASI]
  P --> M[Machmeter]
  S[Static source
ambient pressure] --> ASI
  S --> M
  S --> ALT[Altimeter]
  S --> VSI[VSI]

2. Pitot / Static Heads

Because internal pressure/temperature differ from outside, pitot and static pressures must be sensed by devices mounted on the outside of the aircraft. An open-ended tube parallel to the longitudinal axis senses total pressure — a pitot tube in a pitot head. The open end faces the airstream; the moving air is brought to rest in the tube, generating the dynamic pressure which, with the static pressure already present, gives total (pitot) pressure.

A static head is a tube with its forward end sealed but with holes or slots in the sides. The slots do not face the airflow, so in theory they sense only static pressure (in practice a slight suction makes the sensed value a little low when moving). The static and pitot sources may be combined in one pressure head, the static tube surrounding the pitot tube.

3. Requirements of a Pitot Tube

• Must be positioned outside the boundary layer — hence a head on a strut, or a nose tube ahead of the fuselage.
• Opening must be parallel to the airflow in the normal flight attitude.
• Air is either brought to rest against a stagnation wall then piped to the ASI/Machmeter, or passed directly up the pitot pipelines (more usual in elementary aircraft).

4. Requirements of a Static Source

The static source — whether a simple hole or a combined probe — should have its opening at right angles to the airflow, so only static pressure is sensed with no dynamic component. Some static sensors (especially in combined probes) are electrically heated.

5. Position Error & the Static Vent

With forward motion the sensed static pressure is slightly low due to suction; as speed increases, turbulent airflow near the heads makes the error grow. This is Position Error (also called pressure error). At large angles of attack (lower airspeeds) the head sits at an angle to the airstream, so position error is usually greater. Flight manuals may list different position-error values for different flap settings.

Turbulence from the head itself affects the static reading more than the pitot reading, because the turbulence is downstream of the pitot opening.

The static vent

To reduce this, the static vent was introduced as the static source (pitot pressure then sensed by a simple pitot head). A flat metal plate with a small circular hole is fitted where true (or nearly true) static pressure exists across the whole speed range. A matching vent on the opposite side is interconnected so errors from yawing are largely eliminated.

6. High Speed Probes

Shock waves at high Mach numbers can cause significant errors in pressure sensed by a static vent. High-speed aircraft therefore use a more sophisticated combined pitot/static pressure head to keep position error within acceptable limits. Typical locations: ahead of a wing tip, under a wing, ahead of the vertical stabiliser tip, at the side of the fuselage nose, and ahead of the fuselage nose.

7. Manoeuvre-induced Error

Manoeuvre-induced error is caused by short-term pressure fluctuations at the static vents and delays in the pipelines transmitting changes to the instruments. Even servo altimeters and air-data computers suffer it, as they use the same static vents. Prime causes: changes in angle of attack and turbulence from lowering/raising flaps and landing gear.

• Most commonly appears as a marked lag in pressure-instrument indications.
• More significant during pitch changes than yaw/roll — worst at start of climb/descent and on levelling out.
• Go-around (overshoot) and flight in rough air are particularly vulnerable.
• Errors are unpredictable in size and sense — pressure instruments cannot be relied on for accurate instantaneous values or rates during manoeuvres.

8. Full Pitot/Static System

Transmission uses pipelines in older systems and electrical wires in modern aircraft. Both pitot and static pipelines have built-in water traps. Modern systems use electronic pressure transducers at the sources with built-in error correction; the analogue measurement is converted by A/D interface units (A/D IFUs) to digital form, often carried onward by data digital buses — usually to the air data computer.

Alternate / emergency static source

9. Covers, Heaters & Preflight Checks

Pitot/static openings are highly sensitive — dirt, dust, sand or insects can drastically distort readings, so they are covered when the aircraft is not in use. Pitot covers are canvas/rubber tubes over the probe; static plugs are rubber, cork-shaped. Both carry a conspicuous streamer up to a metre long so they are not overlooked.

Practice Questions & Detailed Answers

Master Reference Tables

All numerical values in this chapter

Mnemonics & memory aids

Answer key