The airspeed indicator, vertical speed indicator and altimeter source their data from a pitot probe located under the left wing. This pitot probe has a cover fitted by default with a red 'remove before flight tag' and thus, must be removed prior to flight. If this cover is not removed, the aforenoted instruments will not function correctly. In this situation, the instruments can be brought back to life by setting the alternate static source switch, located on the lower left side wall, to the 'alternate' position. 

These instruments can also be effected if the aircraft flies in icing conditions and the pitot is covered with ice. The pitot heat switch on the pedestal should be used if icing conditions are suspected.