91-43 DATE 0 0 6/26/75
ADVISORY
CIFCULAR
DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION SUBJECT: UNRELIABLE AIRSPEED INDICATIONS 1 l 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 l 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 oorrective 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 0 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.
Initiated by: ~~~-223
AC 91-43 6,/q/75 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 0 Check position of pitot heat switches and associated circuit breakers.
c. If conditions during or shortly after takeoff are conducive to pitot system icing, p itot heat should be on prior to flight.
I’ # i;)#;.~ &+i/&+y,yiy,yi . R. . SKULLY, Director Flight Standards Service Page’ 2