Instrument flying
Review
Flight-instrument systems
- There are two flight-instrument systems, producing two types of flight instruments:
- Pitot-static instruments
- Gyro instruments.
Pitot-Static instruments
- The pitot-static system records static and dynamic air pressure, providing the pilot with information related to airspeed and altitude.
- The airspeed indicator (ASI).
- The vertical speed indicator (VSI).
- The altimeter (ALT).
- Only the ASI is connected to both pressure sources.
- Static port:
- Measures pressure that is static (not moving), the ambient air.
- Pitot tube (dynamic tube):
- Measures impact, or dynamic, pressure (ram air).
Aispeed Indicator
- Measure the speed at which the aircraft is moving through the air.
- Connected to both the static and pitot (dynamic) ports.
- ASI records the pressure difference between the two ports.
Altimeter
- Report the height above sea level.
- The ALT is an instrument case connected only to the static port.
- As an aircraft increases in altitude the aneroid capsules expand due to decreasing ambient air pressure, this expansion is registered on the instrument face.
Vertical speed Indicator
- Indicates the vertical speed of the aircraft (climb or descent).
- Connected to static port only.
- Has a few second lag.
- As pressure drops, the aneroid compresses, indicating a climb.
- As the pressure increases, the aneroid expands, indicating a descent.
- As the aircraft levels off, pressure no longer changes and the pointer returns to its zero position.
Static port blocked
- Airspeed Indicator:
- Under-read in climb.
- Over read in descent.
- Vercical speed indicator:
- Constant zero indication.
- Altimeter:
- Frozen.
Pitot static blocked
- Affects airspeed indicator only.
- ASI act like an altimeter.
- Over-read in climb.
- Under-read in descent.
- Pitot tube partially blocked:
- Airspeed Indicator:
- Decrases to Zero.
Alternate static source
- Can be selected if the outside sources become clogged with ice.
- The instrument relying on the static pressure may operate slightly differently.
- Altimeter:
- May indicate a higher-than-actual altitude.
- VSI:
- Will momentarily indicate a climb.
- ASI:
- Will indicate greater-than-normal speed.
Gyroscopic instruments
- Heading indicator (HI)
- Attitude indicator (AI)
- Turn and slip indicator or turn co-ordinator.
- May be powered by:
- Vacuum/ pressure system.
- Electrically.
Gyroscope properties:
- Rigidity:
- Regardless of the position of its base, a gyro remains rigid in space, with its axis of rotation pointed in a constant direction.
- Precession:
- If an external force is applied to the gyroscope to change the direction of the rotor axis, the gyro resists the angular movement and moves in a plane at a right angle to that of the applied force, the resulting movement being called precession.
Turn coordinator Vs Turn and Bank Indicator
- The purpose of these two intruments is to indicate:
- Rate of turn.
- Quality of turn (coordinated or not).
- Turn and bank indicator (turn and slip Indicator) can only identifying:
- Yaw.
- Turn coordinator will identify both:
- Yawing motion.
- Rolling motion.
- Inclinometer:
- Shows slip and skid.
- Shows quality of a turn.
- In a standard rate turn the aircraft moves through 3° every second. It takes 2 minutes to turn 360°.
Review questions
- What is the main difference between a turn coordinator and a turn and bank indicator?
- Which flight instruments are part of the pitot-static system?
- Which instruments are driven by a vacuum pump?
- How do the gyroscopic instruments work?
- The pitot system provides impact pressure for which instrument?
