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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. 

## Pitot/Static System 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. 

## Communication/Navigation System Malfunction 

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: 

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. 

The pilot should maintain the highest of the following altitudes or flight levels for the route segment being flown: 

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. 

In addition to route and altitude, the pilot must also plan the progress of the flight to leave the clearance limit. 

