O  F  30.0  29.9  29.8  Figure 11-8. Vacuum failure. system’s vacuum/pressure gauge. This can be a hazardous  situation with the potential to lead the unsuspecting pilot into  a dangerous unusual attitude that would require a partial panel  recovery. It is important that pilots practice instrument flight  without reference to the attitude and heading indicators in  preparation for such a failure. A pitot or static system failure can also cause erratic and  unreliable instrument indications. When a static system  problem occurs, it affects the ASI, altimeter, and the VSI.  In most aircraft, provisions have been made for the pilot to  select an alternate static source. Check the POH/AFM for  the location and operation of the alternate static source. In  the absence of an alternate static source, in an unpressurized  aircraft, the pilot could break the glass on the VSI. The VSI  is not required for instrument flight, and breaking the glass  provides the altimeter and the ASI a source of static pressure.  This procedure could cause additional instrument errors. Avionics equipment has become very reliable, and the  likelihood of a complete communications failure is remote.  However, each IFR flight should be planned and executed in  anticipation of a two-way radio failure. At any given point  during a flight, the pilot must know exactly what route to fly,  what altitude to fly, and when to continue beyond a clearance  limit. Title 14 of the Code of Federal Regulations (14 CFR)  part 91 describes the procedures to be followed in case of a  two-way radio communications failure. If operating in VFR  conditions at the time of the failure, the pilot should continue  the flight under VFR and land as soon as practicable. If the  failure occurs in IFR conditions, or if VFR conditions cannot  be maintained, the pilot must continue the flight: The pilot should maintain the highest of the following  altitudes or flight levels for the route segment being flown: In addition to route and altitude, the pilot must also plan the  progress of the flight to leave the clearance limit. 1.  Along the route assigned in the last ATC clearance  received; 2.  If being radar vectored, by the direct route from the  point of radio failure to the fix, route, or airway specified  in the vector clearance; 3.  In the absence of an assigned route, by the route  that ATC has advised may be expected in a further  clearance; or 4.  In the absence of an assigned route or a route that ATC  has advised may be expected in a further clearance,  by the route filed in the flight plan. 1.  The altitude or flight level assigned in the last ATC  clearance received; 2.  The minimum altitude (converted, if appropriate, to  minimum flight level as prescribed in 14 CFR, part  91 for IFR operations); or 3.  The altitude or flight level ATC has advised may be  expected in a further clearance.  Pitot/Static System Failure Communication/Navigation System  Malfunction 11-8