Cessna 170, Service Manual

Cessna 170 Service Manual 
Electrical System 
 

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170.  Subsequent regulations increased candlepower requirements of aircraft lighting systems and aircraft 
designed to meet those regulations specify the No. 7512-12 lamp which is a 12/14 volt, 26 watt lamp of 
increased candlepower.  Either may be used in the 170, however the 7512-12 lamps will provide brighter 
light with only minimal increase (.66A) in load of the existing system. 
 
The rudder navigation lamp was originally Cessna PN 0433006-3, which has been superceded to a common 
GE-93.  This is a 12/14-volt, 15 candlepower lamp.  
  

PITOT AND STALL WARNING HEATER CIRCUITS

. All models 170 are capable of being provided a pitot 

heater and stall warning heater system.  The system originally designed for the aircraft is a Kollsman heated 
pitot unit in accordance with Cessna Dwg. 0511051.  This is a straight pitot similar to that used on C-
190/195 series aircraft.  Some aircraft have been modified with “L”-type units of the AN-5812 types found on 
later series Cessna aircraft.  The electrical circuits for both systems are identical.  Power is supplied via a 
current limiter and a switch usually located on the instrument panel, to the heater elements contained within 
the pitot tube and within the lift detector assembly.  Each heater element draws approximately 5 amperes.   
Both heated pitot types and lift detector assembly units heaters are field replaceable ceramic units 
containing resistance wire.  The only indication of failure of the heating element is loss of heat at the unit 
itself coupled with a slightly lower amperage indication on the ammeter.  This drop in electrical requirement 
is only slightly observable.  In rare instances fracturing of the heating units ceramic body may cause current 
limiter activation (blown fuse/activated circuit breaker.)  Pre-flight testing of the systems is imperative when 
freezing precipitation is anticipated in flight.  The system is tested during pre-flight by turning the Master 
switch ON, turning the pitot/static system heat switch ON for 30 seconds, and feeling the units by hand for 
generated heat.  This test period must be kept to a short duration to avoid overheating the units on the 
ground.  Caution:  The pitot/static heaters are capable of extremely high temperatures.  Caution must be 
taken to avoid serious burns and overheating of the units which will cause damage.  Avoid static testing of 
the units for periods exceeding 2 minutes. 
 

REMOVAL AND REPLACEMENT OF PITOT/STATIC HEATERS

.  The pitot tube has a nose-cone that is 

removable by unscrewing it from the main body, exposing the ceramic heating unit surrounding the inner 
pitot tube.  Remove the local inspection cover and disconnect the wiring terminals from the harness.   Slide 
the heater unit from the pitot tube.  Installation of the replacement unit is the reverse procedure. 
The stall warning lift detector heating unit is removed by first removing the lift detector from the leading edge 
of the wing.  First note the relative position of the lift detector on the leading edge of the wing so as to avoid 
the necessity of flight-testing the aircraft to correct an altered stall warning speed.  A grease pencil may be 
handy to mark the position of the lift detector prior to removal.  Remove the local inspection cover and the 
four PK screws mounting the lift detector to the wing.  Locate the tubular, ceramic heater element held by a 
small clamp inside the lift detector assembly, and remove it by loosening the clamp.  This will require only a 
screwdriver and a 5/16” wrench or socket.  Remove the two heater wire leads from the harness terminals.  
There is no polarity preference of this harness.  Installation of the replacement heater element unit is the 
reverse procedure.   
Caution:  Some pitot systems utilize poly tubing to carry air pressure to the cockpit instrumentation.  Be 
certain not to allow any resistance wiring to come into contact with the poly tubing or the tubing may melt 
giving erroneous airspeed indications to the pilot.  Use only the correct wiring terminal ends and isolate any 
poly tubing from wiring. 
 

STALL WARNING SYSTEM

.  All models 170 were capable of having a stall warning system installed in 

accordance with Cessna Dwg. 0511062.  It is required equipment on the 170 B-model.  The system consists 
of a current limiter (shared with the turn-and-bank indicator) a warning horn/lamp unit located on the 
instrument panel, a lift detector located on the left wing leading edge, and associated wiring.  Electrical 
power (12/14 volts DC) is supplied to the warning horn/light via the current limiter (fuse/circuit breaker) 
whenever the Master Switch is ON.  After the voltage passes through the warning unit it finds it’s ground out 
on the wing, by passing first through the Lift Detector switch (operated by relative wind against the lift 
detector vane on the leading edge) then to a nearby ground through a short wire terminating at a screw in 
the airframe, thereby completing the circuit.  When the lift detector completes the circuit, the warning horn 
sounds and the indicator lamp illuminates.  The lamp bulb is a GE-1816, 12/14volt .33 amp,  
T-3 ¼ miniature bayonet. 
   The lift detector position on the leading edge of the wing is adjustable and should be adjusted to give 
approximately 5 M.P.H. warning above a stall in level flight.