Vertical Speed Indicator

The Vertical Speed Indicator (VSI) is an instrument that displays the rate of climb and descent to the pilot.


Introduction

Introduction

Pitot-Static System

Pitot-Static System
  • Pitot Static systems measure and compare air pressures from the Pitot tube and static port
  • These measurements display indications for the Airspeed Indicator (ASI), Altimeter, and Vertical Speed Indicators (VSI)
  • The system consists of two components:

  • Pitot Tube:

    • Pitot Tube
      Pitot Tube
    • Invented by Henri Pitot in 1732, the Pitot tube measures fluid flow (air) velocity.
    • The device measures ram (forced) air through a small hole in its front.
      • The device's placement is often on the front of the aircraft or an airfoil to capture smooth (undisturbed) air. [Figure 1]
    • The ram air captured produces an airspeed reading on the airspeed indicator (ASI).

    • Pitot Heat:

      • Pitot Tubes are often electrically heated, which can prevent and remove ice accumulation.
      • Note that these devices should only be utilized during ground operations when necessary.
        • Leaving them on unnecessarily can cause heat to the point of damage/malfunction.
        • When airborne, the airflow otherwise cools the device.

  • Static Port:

    • Static Port
      Static Port
    • Static ports measure ambient still-air atmospheric pressure.
    • Typically, ports are flush mounted on the side of the aircraft where air is undisturbed. [Figure 2]
    • Some aircraft may utilize heated static ports to mitigate ice.
    • Dual ports remove errors due to slips and skids.
    • Responsible for Airspeed Indicator, Altimeter, and Vertical Speed Indicators.
    • The POH/AFM contains any corrections to the airspeed for the various flaps and landing gear configurations.

    • Alternate Static Source:

      • An alternate static air source valve is available for emergencies on some aircraft.
      • If the alternate source ports inside the airplane, where static pressure is usually lower than outside, selection may result in the following erroneous instrument indications:
        1. The altimeter reads higher than normal.
        2. Indicated airspeed (IAS) reads greater than normal.
        3. VSI momentarily shows a climb.
      • Many POHs provide a correction table and aircraft-specific instructions.
      • The alternate static source is not corrected for non-standard pressure (as with an altimeter's Kollsman window).
      • Using alternate static sources may impact other instruments that rely on static pressure (i.e., autopilots, TCAS, transponder, etc.).
      • Using alternate static sources can also decrease the accuracy beyond the 75 feet recommendation outlined in the Aeronautical Information Manual.
    • Instrument Flying Handbook. Figure 3-1, A Typical Electrically Heated Pitot-Static Tube
      Instrument Flying Handbook, A Typical Electrically Heated Pitot-Static Head
    • Static Port
      Static Port

Vertical Speed Indicator Design and Function

Vertical Speed Indicator Design and Function
  • Vertical Speed Indicator
    Instrument Flying Handbook, Rate of Climb or Descent in Thousands of Feet Per Minute
  • Vertical Speed Indicator (VSIs) measure rate-of-pressure change to provide a rate of climb or descent indication.
  • Air is provided through the static ports where it enters a diaphragm (aneroid).
  • The diaphragm expands/retracts to indicate a climb/descent by measuring the ambient changes in pressure within the static system.
    • As pressure drops, the aneroid compresses, indicating a climb
    • As the pressure increases, the aneroid expands, indicating a descent
  • 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 the aircraft levels off, pressure no longer changes and the pointer returns to its zero position
  • Vertical Speed Indicator
    Instrument Flying Handbook, Rate of Climb or Descent in Thousands of Feet Per Minute

Vertical Speed Indications

Vertical Speed Indications
  • Vertical speed indicator start with a "0" indication at the 90 degree position and have notches to indicate the rate of climb both up and down that circle the dial
  • Indicators may be calibrated differently depending on the country and its specified indication, which can be:
    • Feet Per Minute (FPM)
    • Meters Per Second (MPS)
    • Nautical Miles Per Hour (Knots)

Vertical Speed Indicator Regulations

Vertical Speed Indicator Regulations
  • There are no regulations that require a vertical speed indicator by federal aviation regulations.
    • The vertical speed indicator is not required to meet a calibration standard or even be functional for flight to be legal.
  • Always check the Pilot Operating Handbook to learn the specifics of it's operation.

Instrument Errors

Instrument Errors
  • The vertical speed indicator inherently lags, but is more sensitive than an altimeter.
  • This lag can result in a 6-9 second lag to stabilize which will inhibit accurate readings during turbulence or abrupt control movements.
    • Instantaneous VSIs (IVSIs) use two accelerometer actuated pumps to give you a lag free indication
  • 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.

Vertical Speed Indicator Preflight Actions

Vertical Speed Indicator Preflight Actions
  • 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)

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.

Vertical Speed Indicator Anomalies and Malfunctions

Vertical Speed Indicator Anomalies and Malfunctions

  • Static Port Blockage:

    • If in a real emergency, malfunctions can be corrected with alternate air or breaking the glass on a Pitot static instrument (VSI).
    • The trapped pressure in the static system causes the altimeter to remain at the altitude where the blockage occurred.
    • The VSI remains at zero.

Private Pilot (Airplane) Operation of Aircraft Systems Airman Certification Standards

Private Pilot (Airplane) Operation of Aircraft Systems Airman Certification Standards

Private Pilot (Airplane) Operation of Aircraft Systems Knowledge:

The applicant demonstrates an understanding of:

Private Pilot (Airplane) Operation of Aircraft Systems Risk Management:

The applicant is able to identify, assess, and mitigate risk associated with:

Private Pilot (Airplane) Operation of Aircraft Systems Skills:

The applicant exhibits the skills to:

  • PA.I.G.S1:

    Operate at least three of the systems listed in K1a through K1l appropriately.

  • PA.I.G.S2:

    Complete the appropriate checklist(s).

Private Pilot (Airplane) Systems and Equipment Malfunctions Airman Certification Standards

Private Pilot (Airplane) Systems and Equipment Malfunctions Airman Certification Standards

Private Pilot (Airplane) Systems and Equipment Malfunctions Knowledge:

The applicant demonstrates an understanding of:

  • PA.IX.C.K1:

    Causes of partial or complete power loss related to the specific type of powerplant(s).
    • PA.IX.C.K1a:
      [Archived].
    • PA.IX.C.K1b:
      [Archived].
    • PA.IX.C.K1c:
      [Archived].
    • PA.IX.C.K1d:
      [Archived].

  • PA.IX.C.K2:

    System and equipment malfunctions specific to the aircraft, including:

  • PA.IX.C.K3:

    Causes and remedies for smoke or fire onboard the aircraft.

  • PA.IX.C.K4:

    Any other system specific to the airplane (e.g., supplemental oxygen, deicing).

  • PA.IX.C.K5:

    Inadvertent door or window opening.

Private Pilot (Airplane) Systems and Equipment Malfunctions Risk Management:

The applicant is able to identify, assess, and mitigate risk associated with:

  • PA.IX.C.R1:

    Checklist usage for a system or equipment malfunction.

  • PA.IX.C.R2:

    Distractions, task prioritization, loss of situational awareness, or disorientation.

  • PA.IX.C.R3:

    Undesired aircraft state.

  • PA.IX.C.R4:

    Startle response.

Private Pilot (Airplane) Systems and Equipment Malfunctions Skills:

The applicant exhibits the skills to:

  • PA.IX.C.S1:

    Describe appropriate action for simulated emergencies specified by the evaluator, from at least three of the elements or sub-elements listed in K1 through K5 above.

  • PA.IX.C.S2:

    Complete the appropriate checklist(s).

Common Training Aircraft Pitot-Static System Characteristics

Common Training Aircraft Pitot-Static System Characteristics

  • Piper Arrow:

    • Composed of a heated Pitot tube on the lower left wing
    • Two static ports are located on each side of the fuselage
    • Alternate static air (below the pilot control yoke) provides static pressure from inside the cabin

  • Cessna 172:

    • Composed of a heated Pitot tube on the lower surface of the left wing
    • An external static port is located on the lower left side of the forward fuselage
    • Pitot Tube consists of a heating element, a 5-amp switch/breaker, and associated wiring
    • Alternate static (below throttle) provides pressure from inside the cabin

Conclusion

Conclusion
  • The vertical speed indicator can go by many different names to include:
    • Variometer
    • Rate of Climb Indicator
    • Vertical Velocity Indicator
    • Vertical Speed Indicator (VSI) is however, most common
  • Always keep in mind the effects of parallax error
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References

References