Vertical Speed Indicators
This instrument shows to the pilot the rate of change in altitude in feet per minute (or m/s) and it operates due to the change in air pressure when the aircraft changes its altitude, height or flight level.
With this instrument the pilot can calculate and plan when he arrives at a certain altitude, very helpful when avoiding obstacles, mountains and when flying instrument approaches or just a basic VFR descent into an airport.
On this page we are going to take a look inside this vertical speed indicator and see how it operates and what limitations you may find here. Those of us flying with a retractable gear use this instrument to get the gear up when the indication shows a positive rate of climb.
Vertical Speed Indicator Operation
Shown on the image, you see an aneroid capsule connected to the static port. At the same connection there is a calibrated leak in the instrument housing. This leak creates a pressure difference inside the aneroid capsule which then slows the expansion or retraction of the aneroid capsule during altitude changes.
The aneroid capsule is mechanically linked to the pointer on the outside of the instrument and calibrated in feet/min. You will see VSIs from 0 to +/-2000 fpm up to even 0 to +/- 6000 fpm for the higher performance type of aircraft.
Due to this calibrated leak there is a lag of about six seconds before the instruments reacts to changes in pressure. Some manufacturers came up with a solution to accelerate the indication and they called it IVSI or Instantaneous Vertical Speed Indicator.
During preflight the needle should be the horizontal position and pointing to zero and it is serviceable if the indication is +/- 200 fpm between -20°C and +50°C.
During the actual flight the pilot can clock the time it took to climb from a certain altitude to a higher altitude and divide the difference, the VSI should have shown about the same result, of course within its limits mentioned above.
As with the other two pressure instruments the VSI is also subject to a number of errors the pilot must be intimately aware of:
Lag or instrument error
As mentioned above, this is the most common error and it shows up on every flight and altitude change. To get a reliable indication of vertical speed the aircraft must settle in the change (climb or descend) and after some six to nine seconds the instrument is stabilized. This is where an Instanteneous VSI (or IVSI) will help reduce this error to almost zero, but these are more expensive.
Until the instrument is stabilized you can interpret the indication as a trend of where things are going to be in a couple of seconds from now.
This is the same type of error as with the airspeed and altitude indicators caused by fluctuations in the static port or lines. Having two static ports, one on either side of the aircraft and connected together with tubing, takes care of most of this problem.
Other errors caused by turbulence, propeller slipstream and possibly flap changes along the fuselage are things a pilot can not do much about other than to learn to live with it.
Some ultralight or experimental aircraft do not have a static port in the fuselage, they measure static pressure inside of the cockpit. This also will introduce an error and any change in the cockpit, opening vents or windows, will show up on the instrument as a temporarily vertical speed change.
The instrument is compensated for by pressure and temperature errors so that the correct vertical speed is indicated at any density altitude you may encounter.