Module 11B.5–11B.6 — Piston aeroplanes: instruments, avionics overview and electrical power

Levels

Sub-module	A2	B1.2
11.5.1 Instrument systems	1	2
11.5.2 Avionics systems	1	1
11.6 Electrical power	1	3

Electrical power is a level 3 topic for B1.2 — expect troubleshooting-style questions on alternators, regulators, master switches and ammeters.

11.5.1 Instrument systems on piston aeroplanes

The "six-pack" and its power sources

Instrument	Power source (classic installation)
Airspeed indicator	Pitot + static
Attitude indicator	Vacuum (air-driven gyro)
Altimeter	Static
Turn coordinator	Electric gyro (gives redundancy if the vacuum fails)
Heading (direction) indicator	Vacuum
Vertical speed indicator	Static

Plus a direct-reading magnetic compass and engine instruments.

Pitot-static specifics

• Heated pitot probe; drain holes in the pitot head; static ports on the fuselage sides.
• Alternate static source inside the cabin: cabin pressure is usually slightly lower than outside static (venturi effect), so with alternate static the altimeter reads high, the ASI reads high, and the VSI shows a momentary climb.
• Blockages: pitot blocked with drain open → ASI drops to zero; pitot and drain blocked → ASI behaves like an altimeter; static blocked → altimeter frozen, VSI zero, ASI errors.
• Leak test with a pitot-static tester after disturbance; altimeter/transponder (Mode C) correlation checks at required intervals.

Gyroscopic instruments

• Vacuum-driven gyros: air drawn through a filter, across the rotor buckets, and out to the vacuum pump; suction gauge shows about 4.5–5.5 inHg. Low suction → gyros spin slowly → erroneous indications.
• Attitude indicator (vertical gyro with pendulous vanes for erection), heading indicator (horizontal gyro, must be reset to the compass about every 15 minutes), turn coordinator (canted rate gyro), slip ball.
• Gyro properties: rigidity and precession; apparent drift of DI.

Compass

Direct-reading magnetic compass: deviation (compass swing, deviation card), variation, turning and acceleration errors due to dip. Avoid magnetic tools near the compass; compass swing after installing electrical equipment near it.

Stall warning and AoA

• Reed-type stall warning horn (pneumatic — suction at the leading-edge inlet sounds a reed) or electrical vane switch on the leading edge that closes as the stagnation point moves down at high AoA → horn/light.
• Some aircraft have AoA indicators (retrofit kits).

Glass cockpit in GA

Integrated flight decks (e.g. PFD/MFD units) with AHRS (solid-state gyros and magnetometer) and air data computers; standby instruments (or standby electronic instrument) with backup battery; synthetic vision; engine indication system (EIS) on the MFD. Software and database updates are maintenance tasks.

11.5.2 Avionics systems — fundamentals (level 1)

System	Light-aircraft examples
Autoflight (ATA 22)	Single- or two-axis autopilots (wing leveller, heading, altitude hold), electric trim
Communications (ATA 23)	VHF COM (118–136.975 MHz, 8.33 kHz), intercom/audio panel, ELT (406 MHz + 121.5 MHz homing), HF rare
Navigation (ATA 34)	VOR/ILS receivers, ADF, DME, GNSS (GPS navigators, RNAV), Mode S transponder (1030/1090 MHz) with ADS-B out, TCAS/TAS traffic systems, weather (datalink/stormscope)
Recorders	Usually not fitted on light aircraft