Date:  6/26/75
Initiated By:  AFS-223
      AC No.:  91- 43


     Subject:  UNRELIABLE AIRSPEED INDICATIONS


1. PURPOSE.  The purpose of this Advisory Circular is to alert pilots to the
possibility of erroneous airspeed/Mach indications that may be caused by
blocking or freezing of the pitot system, and corrective action to be taken.

2. BACKGROUND.  Unreliable airspeed/Mach indications may have contributed to
one recent serious incident and a fatal accident.  A frozen or blocked pitot
system may have caused the unreliable indications.  These erroneous readings
can be so subtle, that a pilot may not detect the problem until it is too late
for adequate corrective action.  Many pilots may associate a frozen pitot head
with symptoms of a rapidly decreasing or zero airspeed indication.  This is
not always the case, especially when operating high performance aircraft at
high altitudes.

3. DESCRIPTION.  When blocking or freezing of the pitot system occurs, two
situations can develop as follows:

     a.  If the ram air input to the pitot head is blocked, the indicated
	  airspeed may drop to zero; and

     b.  If the ram air input plus the drain hole is blocked, the pressure is
	  trapped in the system and the airspeed indicator may react as an
	  altimeter; e.g.:

	  (1)  During level flight, airspeed indication will not change even
		when actual airspeed is varied by large power changes;

	  (2)   During climb, airspeed indication will increase; and

	  (3)   During descent, airspeed indication will decrease.

4. RECOMMENDATIONS.  Due to the critical nature of this in-flight problem,
pilots should be aware of indications symptomatic of a frozen or blocked pitot
system and take the following corrective action:

     a.  Emphasis should be on attitude flying when these symptoms are
	  recognized.

     b.  Check position of pitot heat switches and associated circuit
	  breakers.

     c.  If conditions during or shortly after takeoff are conducive to
	  pitot system icing, pitot heat should be on prior to flight.



R.  P. SKULLY, Director
Flight Standards Service