Instruments & Avionic Systems, Module 11, Certify66

Notes

Main ideas

The PITOT-STATIC system feeds the pressure instruments: the STATIC port feeds the altimeter, airspeed indicator and vertical-speed indicator; the PITOT probe feeds only the ASI (dynamic + static). Air data computers now compute these electronically.
GYROSCOPIC instruments, attitude indicator, direction indicator and turn indicator, rely on gyroscopic rigidity/precession; modern aircraft use AHRS and laser/ring gyros feeding EFIS displays (PFD and ND).
EFIS/EICAS/ECAM present integrated flight, navigation and engine/system data on screens with alerting; air data and inertial data are shared over digital buses (ARINC 429/629).
AVIONIC systems cover communication (VHF/HF/SATCOM), navigation (VOR/DME/ILS/ADF/GPS/INS/FMS), autoflight (autopilot/flight director/autothrottle) and surveillance (transponder, TCAS, weather radar, GPWS/EGPWS).
Instrument errors (position, lag, blockage) matter: a blocked static or pitot line gives characteristic wrong readings.
⚠ Exam trap: the STATIC source feeds the altimeter, ASI and VSI; the PITOT feeds only the ASI, so a blocked STATIC port freezes the altimeter and makes the VSI read zero and the ASI misread, while a blocked PITOT affects only the ASI.

Solved examples

In the climb the altimeter sticks at a fixed reading and the VSI shows zero, but the ASI over-reads. Which system is at fault?
A blocked STATIC source. With the static pressure trapped, the altimeter cannot sense the falling pressure so it freezes, the VSI (which measures rate of static-pressure change) reads zero, and the ASI over-reads because the trapped static reference no longer falls as it should. A blocked pitot, by contrast, would affect only the ASI.

Instruments & Avionic Systems