The Vertical Speed Indicator Flashcards
Units used for VSIs
Feet per minute (fpm)
Metres per second (m/s)
200fpm = 1m/s
VSI operating principle
- capsule is connected to static pressure
- metering unit fees static pressure into case with controlled lag
- difference in pressure between the capsule (static) and case (static with delay) causes the capsule to expand or contract, moving the pointer on the face of the instrument
Lag
To function, the VSI must establish a pressure difference. Indications lag by one to two seconds
Inertial VSI
Are not used in light aircraft. Uses either blended air and inertial data or pure inertial data
IVSI operating principle
As per the VSI plus a system to overcome lag:
- A mass located in a cylinder which is open at each end connected to either side of a restrictor in the static line to the IVSI capsule. it acts as a piston
- The mass is held centrally in the cylinder by springs
- When the aircraft changes pitch rate, inertia moves the piston in the cylinder, causing a pressure rise or fall in the capsule, giving an instant indication of a climb or descent
- As the rate of climb stabilises, the piston returns to the middle of the cylinder. IVSI now operates as per normal VSI
VSI errors
1) Instrument error
2) Manoeuvre error
3) Position error
4) Static system error
5) Time lag (VI only)
Effect of a static system blockage
Pressure in the capsule and instrument case equalise, so the indication falls to zero
Effect of a static system leak
- Unpressurised aircraft: little effect
- Pressurised aircraft: erroneous indications depending on the relationship between actual ROC/ROD and cabin rate of climb/descent
IVSI errors
Turbulence: movement of the IVSI mass causes false climb/descent indications
Manoeuvre: movement of the mass under g causes false indications
Types of VSI display
- Logarithmic dial display: more accurate at smaller ROC/ROD
- linear dial display
- logarithmic electronic display on PFD
