Aircraft Instruments

Being able to interpret what the airplane is doing is crucial to safe flying.  Understanding which system powers which instruments is also comforting when you know which instruments may be relied on in a failure.

Pitot-Static Instruments

The pitot-static system consists of the airspeed indicator, the altimeter, and the vertical speed indicator (VSI).

The pitot tube measures Dynamic Pressure for the Airspeed Indicator.  It is an indication of how much air is flowing over the aircraft - this could be from moving through still air or experiencing strong headwinds.

The back of the pitot tube allows moisture to exit.

In order to show accurate readings when altitude changes, a static chamber must also be vented to the instrument.

The Altimeter and VSI both use just the static port and its connected lines for readings.

Altimeter

Operation:  A stack of aneroid wafers is free to expand and contract with changes in pressure.  Higher pressure presses down.  Mechanical linkages connect wafer to needles on indicator face.

The instrument would only be accurate, however, when temperature is standard and pressure is standard.  Thus, there is a knob on the instrument that allows us to manipulate changes in barometric pressure.  We can select the new barometric pressure using the Kollsman Window.

This does not, however, compensate for changes in temperature.

Types of Altitude:
Indicated = Read off the altimeter
True Altitude = Vertical distance above sea level
Absolute = Vertical distance above ground
Pressure Altitude = Alt. indicated when scale adjusted to 29.92
Density Altitude = Pressure Alt. corrected for variations in temperature

VSI

Operation:  Differential Pressure between diaphragm and that of the instrument case measures rate of change.  Inside of diaphragm connected directly to static lines.  The casing is also connected to the static lines, but through a calibrated leak.  Diaphragm air is unrestricted, but case receives a metered leak.  Diaphragm thus changes pressure first.  When altitude is level, the pressure is equalized and the instrument indicates 0 fpm.  There is a slight delay, so a trend indicates a direction of movement prior to stabilizing.  Some VSIs incorporate accelerometers to compensate for this lag.

Airspeed Indicator

Operation:  Measures difference between impact/dynamic pressure and static pressure.  Dynamic pressure from pitot tube expands or contracts one side of diaphragm moving linkages to indicate speed.

Types of airspeed:
Indicated = read off of instrument
Calibrated = IAS corrected for installation error/ instrument error.
True = Calibrated corrected for nonstandard temperature and pressure
Groundspeed = actual speed of airplane over the ground.  True airspeed adjusted for wind.

Markings:

White Arc = Flap Range
Lower Limit White Arc = Vso
Green Arc = Normal operating range
Lower limit Green Arc = Vs1
Upper limit Green Arc = Max. Structural Cruising Speed
Yelow Arc = Caution range.
Red Line = Vne

Other Airspeeds are not shown, such as . . .
Va, Vlo, Vle, Vx, Vy

Troubleshooting Pitot-Static System
Blockage of Pitot System results in inaccurate readings of ASI.
•If opening in pitot tube is blocked, ASI reads 0
•If drain hole also blocked at same time, ASI will read airspeed when blockage occurred
•Pitot Heat (electric heating) may clear the block

A Static Port block will affect all 3 instruments.
•Altimeter will indicate altitude where blockage occurred
•Alternate Static source may be used, but lower pressure inside cockpit will result in higher altimeter indication, temporary climb shown on VSI, and faster than normal airspeed indication

Gyroscopic Instruments
Gyroscopic Instruments include the Heading Indicator, Attitude Indicator, and Turn Coordinator

Principles of Operation:  Gyroscopes operate based off of rigidity in space.  Gimbal rings are tilted, twisted, or moved, but gyro remains fixed.  Gyros also operate on principle of precession.  When a force is acted upon the gyro, this force does not occur until 90º ahead in rotation.

Sources of Power:  The Heading and Attitude Indicator are Vacuum powered.  The turn coordinator is electrically powered.

Vacuum system

The vacuum pump is driven by the engine, which generates a stream of air over the rotor vanes of the gyros.  Air goes through a filter then spins gyros and is expelled overboard or used in other systems like pneumatic deicing boots.

Attitude Indicator - mounted horizontally.  The horizon bar remains fixed and is attached to gyro.

Heading Indicator - mounted vertically.  Aircraft rotates around the gyro.  Compass card may begin to drift over time requiring proper calibration.  It is also measuring changes in earth's rotation (15º per hour) so requires periodic adjustment.

Turn Coordinator - Electronic.  The gimbal is canted to sense both rate of roll and rate of turn.  Used to conduct standard-rate turns. (3º/sec).  Indicates rate and direction.

Inclinometer - Depicts aircraft yaw.  Force of gravity causes ball to rest at lowest part of tube.

Compass
Remote Indicating Compass consist of pictorial navigation indicator and slaving control and compensator unit.  Pictorial part is called HSI.  Slaving control and compensator has a push button that provides a way of selecting either "slaved gyro" or "free gyro" mode.  Slaving meter indicates difference between displayed heading and magnetic heading.  Slaving transmitter (usually in wingtip) contains flux valve with is direction-sensing.  Signal operates torque motor that processes the gyro until it is aligned with transmitter signal.

RMI is a combination instrument to reduce amount of scanning necessary.  Two pointers driven by ADF, VOR while compass card driven by flux valve signals.

Magnetic Compass - Possibly the simplest instrument on the plane, and least likely to fail.  It does however have its own errors based on its design.  The compass consists of a float with a compass card wrapped around it with magnets.  It floats inside compass fluid (similar to kerosene).  It has a pivot point it rotates around.  The fluid dampens oscillations, but banks over 18º present problems.

A compensator assembly above or below the compass allows a maintenance technician (AMT) to create a magnetic field inside the housing that cancels the influence of local outside magnetic fields.
•Variation is the difference between true north and magnetic north
•Deviation is caused by interference from other metals or fields that draw the compass.  These errors are indicated on the compass correction card.

•Dip - when turning from a northerly heading, compass indication lags behind turn.  When turning from south, compass leads turn.  This is a result of vertical components of earth's magnetic field.  UNOS

•Acceleration Error - The correction weight is heavier than the magnet.  However, a change in inertia moves this weight, thus causing an erroneous indication.  ANDS
•Oscillation Error - combination of previous errors .  Could be from turbulence.