Vertical Speed Indicator (VSI)

Wg Cdr Rajagopal

Vertical Speed Indicator

Index

Principle of Vertical Speed Indicator

  • VSI is also called Rate of climb or descent indicator (RCDI)
  • Indicates rate of climb or descent by rate of change of static pressure
  • Compares the present static pressure to that of 4 – 6 sec earlier

Construction of Vertical Speed Indicator

  • Air tight container with a metering unit retains past static pressure
  • Pressure sensing capsule senses the current static pressure
  • Static pressure differential between the capsule and container gives an indication of vertical speed

Metering Unit of VSI

  • Metering unit is a restrictive choke which allows delayed pressure transfer
  • Self compensates for the slow rate of static change at higher levels
  • Gives consistent indication at all altitudes and temperatures

VSI during Climb and Descent

  • During climb pressure in capsule would be lesser than the case
  • Capsule compresses showing climb on pointer
  • During descent pressure in capsule would be more than the case
  • Capsule expands showing descent on pointer

Instrument Error in VSI

  • Instrument errors are unavoidable due to manufacturing imperfections
  • Smaller pressure changes at higher altitude
  • Error increase with altitude

Position or Pressure Error in VSI

  • Position (Pressure) error is caused due to turbulence creating suction around static source
  • Error is reduced by using separate static vents
  • High speed aircraft use advanced pitot tubes to reduce error
  • Position error is noticeable while accelerating for take-off

Manoeuvre and Time Lag errors of VSI

  • Temporary fluctuations arise due to manoeuvres mainly changes in pitch or yaw
  • Leads to indication of false rate of climb or descent
  • Time lag error occurs due to slow capsule response to prolonged or rapid change of altitude
  • Error is obvious on leveling out

Static Blockage in VSI

  • Blockage in static system of VSI will make the VSI will read zero
  • If blocked during climb or descent will gradually return to zero as differential equalizes

Leaks in Static Vent (Outside Pressurised Cabin)

  • Leak (outside pressurized hull) in static line causes no error since the static will be sensed correctly

Leaks in Static Vent (Inside Pressurised Cabin)

  • Leak (inside pressurized hull) in static line will make the instrument sense rate of change of cabin altitude
  • Static change will be change in cabin altitude pressure & not actual static pressure

Display of VSI

  • Display of Vertical Speed Indicator could be of two types
  • Linear display with equal increments rate of climb irrespective of vertical speed in feet / minute
  • Logarithmic display with increased sensitivity at lower rate of climb or descent climb

Principle of an Instantaneous VSI

  • Instantaneous VSI has a Vertical Acceleration Pump or Dashpot in static line to the capsule
  • During climb or descent the piston is displaced causing immediate differential pressure
  • Once the instantaneous change changes to steady climb or descent the piston would return to its original position

Errors in IVSI

  • IVSI is a very sensitive instrument which senses vertical acceleration
  • Because of its sensitive nature, IVSI may give false climb or descent indications during turbulence or steep level turns
  • Sudden fluctuations have to be ignored during flight

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