ATPL instrumentation course: 14 illustrated chapters

All air data instruments rely on two measurements:

The difference is the dynamic pressure: Pd = Pt − Ps = ½ ρ V² (incompressible flow). It increases with the square of the speed and in proportion to the air density.

The pitot tube is mounted on a mast that holds it away from the fuselage skin so that it sits outside the boundary layer. It is electrically heated and fitted with drains: purge holes at the low points let out the water that collects in the lines.

The static ports are flush openings, generally a symmetrical pair (left and right) connected together: in a slip, one port is in overpressure and the other in underpressure, and the average remains correct. If only one port is blocked, the indication is wrong only in asymmetric flight.

The airflow around the ports is never perfect: this is the position error (or installation error), which varies mainly with speed, Mach number, angle of attack and configuration.

The pitot-static system feeds four basic instruments:

A blocked pitot therefore affects only the airspeed indicator and the machmeter. A blocked static affects all four. The attitude indicator, the directional gyro and the suction gauge are not connected to the system.

On modern aircraft, electronic sensors (piezoresistive or piezoelectric) convert the pressures into signals for the air data computer (ADC). Located close to the probes (ADM modules), they shorten the lines: a saving in mass and almost no transmission delay.

Each sensor is chosen according to the pressure range to be measured.

Increasing range: capsule, bellows, Bourdon tube. Absolute pressure is measured from vacuum, gauge pressure from ambient pressure.

If the static source becomes blocked (ice, insect, forgotten tape), the trapped pressure remains that of the moment of blockage.

Typical symptom in a descent: constant altitude, vertical speed at 0 and indicated airspeed increasing.

Two very different cases:

Danger: in a climb, the airspeed indicator seems to accelerate, the crew pulls back on the stick to "hold" the speed, and the real speed collapses until the stall.

A leak in the Pt line into an unpressurised cabin causes the speed to be underread; if Pt becomes equal to Ps, the airspeed indicator reads zero.

If the static source is blocked, an alternate static port can be selected. On an unpressurised aircraft, it is often located in the cabin, where the pressure is slightly lower than the outside pressure (Venturi effect of the airflow).

Consequences: the altimeter overreads the altitude, the airspeed indicator slightly overreads the speed and the vertical speed indicator briefly shows a climb at the switchover, then returns towards zero with a small residual error.

As a last resort, on an unpressurised aircraft, you can break the glass of the vertical speed indicator: its case, connected to the static line, comes to cabin pressure and becomes a nearly correct static source again for the altimeter and airspeed indicator.

The static air temperature (SAT or OAT) is that of the undisturbed air; it is used for speed and altitude corrections and for calculating TAS and the speed of sound.

A moving sensor does not measure it directly: the air slowed down on the probe is compressed adiabatically (and rubs against it), so it heats up. The total air temperature (TAT) is that of the air brought to rest without heat exchange:

TAT = SAT (1 + 0.2 M²), temperatures in kelvins.

The difference (kinetic heating, or *ram rise*) depends on the Mach number and on the SAT, hence on the true airspeed: about 30 °C at Mach 0.8 in cruise. The TAT is always greater than or equal to the SAT.

A real probe recovers only a fraction Kr of the heating. It indicates:

Measured T = SAT (1 + 0.2 Kr M²)

Example: Mach 0.70, measured temperature −48 °C, Kr = 0.85. SAT = 225.15 / (1 + 0.2 × 0.85 × 0.49) = 225.15 / 1.083 = 207.8 K, i.e. −65 °C.

Another way to look at it: if Kr = 0.75, the probe shows only 75% of the heating. With 30 °C read for a SAT of 25 °C, the real heating is 5 / 0.75 = 6.7 °C.

The speed of sound depends only on the temperature of the air (not on pressure):

a (kt) = 38.94 √T with T in kelvins.

It therefore decreases in the climb up to the tropopause, then stays constant. To calculate it at a given level, you need the actual temperature: with an ISA deviation, start by finding the standard temperature of the level (15 − 2 °C per 1,000 ft).

Example: FL 290, ISA − 6. Standard temperature −43 °C, actual −49 °C = 224 K; a = 38.94 × 14.97 ≈ 583 kt.

Instruments are calibrated in the International Standard Atmosphere (ISA):

ISA deviation = actual temperature − standard temperature at the same level. Example: +25 °C at sea level gives ISA + 10.