Instrument Flying Handbook, Rate of Climb or Descent in Thousands of Feet Per Minute
Measures rate-of-pressure change
The air enters the aircraft through the static port
This air enters a diaphragm (aneroid) where it expands/retracts to indicate a climb/descent by measuring the ambient changes in pressure within the static system
Through a calibrated leak/orifice, the air leaves the instrument case slower than the aneroid and allows for a stabilized indication of pressure change on the face of the instrument
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
Vertical Speed Indicator Errors:
The vertical speed indicator inherently lags, but is more sensitive than an altimeter
This can result in a 6-9 second lag to stabilize which will inhibit accurate readings during turbulence or abrupt control movements
Alternate sources of static, when selected, will typically show a momentary climb
This is due to pressure differences in the cockpit than outside
A blocked static port will give a zero indication
Preflight Actions:
The VSI should read 0
If it indicates anything other than a zero feet per minute climb or descent on the ground, then the instrument can still be used, but that indication is the new "zero"
This, of course, is highly discouraged if the error is such that a mistake could create a serious hazard to flight, especially in weather
Inertial Reference Unit (IRU), Inertial Navigation System (INS), and Attitude Heading Reference System (AHRS)
IRUs are self-contained systems comprised of gyros and accelerometers that provide aircraft attitude (pitch, roll, and heading), position, and velocity information in response to signals resulting from inertial effects on system components
Once aligned with a known position, IRUs continuously calculate position and velocity. IRU position accuracy decays with time
This degradation is known as "drift"
INSs combine the components of an IRU with an internal navigation computer
By programming a series of waypoints, these systems will navigate along a predetermined track
AHRSs are electronic devices that provide attitude information to aircraft systems such as weather radar and autopilot, but do not directly compute position information
Aircraft equipped with slaved compass systems may be susceptible to heading errors caused by exposure to magnetic field disturbances (flux fields) found in materials that are commonly located on the surface or buried under taxiways and ramps
These materials generate a magnetic flux field that can be sensed by the aircraft's compass system flux detector or "gate", which can cause the aircraft's system to align with the material's magnetic field rather than the earth's natural magnetic field
The system's erroneous heading may not self-correct
Prior to take off pilots should be aware that a heading misalignment may have occurred during taxi
Pilots are encouraged to follow the manufacturer's or other appropriate procedures to correct possible heading misalignment before take off is commenced
