Vertical Speed Indicator
The VSI is a differential pressure instrument powered by static air pressure. The instrument consists of a diaphragm inside the VSI case, to which static air pressure is routed. The static air pressure is connected directly to the inside of the diaphragm, allowing the pressure inside the diaphragm to change immediately with changing static pressures.
The VSI case is vented to static pressure through a calibrated leak. As a result, the pressure in the VSI case does not immediately match the static pressure, but lags behind whenever the static pressure changes. This is because it takes time for air to enter or escape the case through the calibrated leak.
This results in a difference in pressures between the inside and outside of the diaphragm anything static pressure changes, which it does anytime the airplane is in a climb or descent.
A mechanical linkage translates this pressure differential into an indication of rate of climb or descent.
The VSI is not required by regulations to be on board the aircraft.
Should ice or debris block the pitot tube or static port, the pitot-static instruments will provide the pilot erroneous indications.
Blocked Pitot Tube with an Open Drain Hole / Open Static Port
If the pitot tube becomes blocked, the airspeed will drop to zero, while the altimeter and VSI will work normally.
Blocked Pitot Tube, Blocked Drain Hole / Open Static Port
Should the forward facing port and drain hole of the pitot tube both become blocked, the airspeed indicator would no longer respond to changes in airspeed. Instead, the airspeed would increase with increasing altitude. In a descent, it would indicate the aircraft is slowing.
This is because the pressure would be trapped inside the pressure line from the pitot tube to the airspeed indicator. Since that trapped pressure does not change in response to changing airspeed, the airspeed indicator would show the difference between this unchanging pitot pressure and varying static pressure. In a climb, the static pressure decreases, resulting in a greater differential between pitot and static pressures, which is indicated as higher airspeed. In a descent, pitot pressure increases, which decreases the pressure difference between pitot and static pressure.
The altimeter and VSI would still work normally, since the static ports are unrestricted. This situation could occur if the entire pitot tube became iced over, for example.
Open Pitot Tube / Blocked Static Port
If the static port became clogged with ice or debris, while the pitot tube remained unrestricted, the altimeter would freeze in place. The VSI would return to its zero position. Since the static pressure in the static lines is trapped, neither of these instruments would respond when aircraft altitude changes. This situation might occur in icing conditions, where pitot heat prevents ice formation on the pitot tube.
The airspeed indicator still shows airspeed variations, since the pitot tube is open. With fixed static pressure, however, it will be inaccurate at any altitude other than the altitude at which the blockage occurred. At altitudes higher than the blockage, the airspeed displays an erroneously low indication. At altitudes lower than the blockage altitude, the airspeed indicator reads high.
This is because the pitot tube senses total pressure, which is the sum of dynamic and static pressure. Under normal circumstances, the static pressure from the static port cancels the static pressure portion of the total pressure registered by the pitot tube, leaving dynamic pressure only to be displayed by the airspeed indicator. In this situation, however, the two static pressures do not completely cancel each other out, since they are not the same.
At altitudes higher than the blockage altitude, the actual ambient pressure is lower than the trapped pressure in the static line. This means the total air pressure is lower than it should be, and the pressure difference registered by the airspeed indicator is lower than it should be. At altitudes below the blockage, the static portion received by the pitot tube is greater than the trapped pressure in the static lines. So, the pressure differential displayed by the airspeed indicator is greater than it should be, resulting in the indication of an erroneously high airspeed.
Alternate Static Source
Some aircraft are equipped with an alternate static source, which is normally a valve in the cockpit that vents the static lines to the air in the cockpit. Should a static blockage occur, the pilot can simply open the valve and regain almost normal operation of the pitot-static instruments. Pilots should ensure this valve is closed for normal operations before every flight.
When operating on the alternate static source, the pitot-static instruments will not be entirely accurate. This is due to the venturi effect created by air flowing around the aircraft cabin. Since airplanes are wider in the front and narrow in the aft portions, this venturi effect is created as the airplane flies. The result is cabin pressure that is slightly lower than ambient pressure. This means when using an alternate static source, the pressure in the static lines is slightly lower than actual ambient pressure.
As a result, the altimeter reads slightly high, and the airspeed indicator displays a greater than actual airspeed. The VSI will momentarily show a climb when the alternate static source is opened, then will operate normally.
On aircraft without an alternate static source, it is sometimes mentioned that cabin air could be introduced into the static lines by breaking the glass of the VSI. The VSI is chosen because it is not required for IFR flight and considered the least important flight instrument, if you had to lose one.