12.7 Helicopter instruments and avionic systems

Scope

• 12.7.1 Instrument systems (ATA 31) — A 1, B1.3 2: pitot-static instruments, gyroscopic instruments, compasses, vibration indicating systems/HUMS, glass cockpit, indications of other systems.
• 12.7.2 Avionic systems — A 1, B1.3 1: fundamentals of autoflight (ATA 22), communications (ATA 23) and navigation (ATA 34), and general avionic test equipment. At level 1 you need to know what each system does and roughly how — detailed theory is a B2 (Module 13) subject.

Pitot-static instruments

Instrument	Senses	Principle
Altimeter	Static pressure	Aneroid (evacuated) capsule expands as static pressure falls; subscale set to QNH/QFE/1013.25 hPa
Airspeed indicator	Pitot (total) minus static = dynamic pressure	Diaphragm with pitot pressure inside, static outside
Vertical speed indicator	Rate of change of static pressure	Capsule with a calibrated leak (metering orifice); lag inherent unless instantaneous (IVSI) type with accelerometer dashpots

Helicopter specifics: airspeed indications are unreliable at low speed and in the hover because of rotor downwash; static ports must be placed to minimise downwash effects. Blockages: a blocked static source freezes the altimeter and VSI and makes the ASI over-read in a descent/under-read in a climb; a blocked pitot (drain open) makes the ASI read zero, a fully blocked pitot makes the ASI act like an altimeter.

Gyroscopic instruments

Gyro properties: rigidity in space and precession.

Instrument	Gyro type	Displays
Artificial horizon (attitude indicator)	Earth (vertical) gyro, spin axis vertical	Pitch and roll
Attitude director indicator (ADI)	Electronic/remote vertical gyro + flight director command bars	Attitude plus steering commands
Direction indicator (DI / DG)	Horizontal spin axis, free in azimuth; must be aligned periodically with the compass (drift/apparent wander)	Heading
Horizontal situation indicator (HSI)	Slaved heading plus nav deviation	Heading, course, VOR/ILS deviation
Turn and slip indicator	Rate gyro — one gimbal, spin axis horizontal athwartships	Rate of turn (yaw); ball shows slip/skid
Turn coordinator	Rate gyro with gimbal canted about 30°	Rate of roll and rate of turn

Gyros are driven by air (vacuum/pressure) or electrically. In modern aircraft they are replaced by AHRS (attitude and heading reference system) using solid-state rate sensors and accelerometers.