Fuel I Ndicati Ng; Revision 4 - Cessna 172SPHUSR05 Pilot Operating Handbook

C ESSNA
MOD E L 1 72S
F U E L I N DI CATI N G
Fuel q uantity is measured by two float type fuel quantity
transmitters (one i n each tank) and i ndicated by an electrically
operated fuel quantity i ndicator on the left side of the i nstrument
panel. The gauges are marked in gal lons of fuel. An empty tank is
i ndicated by a red line and the n umber O. When an i ndicator shows
an empty tank, approxim ately 1 .5 gallons remain in each tank as
u nusable fuel . The i ndicators sho u ld not be rel ied upon for accu rate
readi ngs d u ring skids, slips, or u n usual attitudes .
Each fuel tank also incorporates warn ing circuits wh ich can
detect low fuel conditions and erroneous transmitter m essages.
Anytime fuel i n the tank drops below approximately 5 gallons (and
remains below this level for more than 60 seconds) , the amber LOW
F U EL message will flash on the annunciator panel for approxim ately
1 0 seconds and then remain steady amber. The annunciator can not
be tu rned off by th e pilot . If the l eft tank is low, the message wi ll
read L LOW FU EL. If the right tank is low, the message wil l read
LOW F U E L R. If both tanks are low, the message will read L LOW
FUEL R.
I n additi o n to low fuel annunciation , the warni ng circuitry is
designed to report fail u res with each transmitter caused by shorts,
opens or transmitter resistance which increases over ti me. If the
circuitry detects any one of these conditions , the fuel level i ndicator
needle wi ll go to the OFF position (below the 0 mark on the fuel
i ndicator) , and the amber annu nciator wi ll i l l u m inate. If the left tank
transmitter has fai led , the message wil l read L LOW FUEL. If the
right tank transmitter has fai led , the message wi ll read LOW F U EL
R. If both tanks transm itters h ave failed, th e message wi l l read L
LOW FUEL R.
Fuel pressu re is measu red by use of a transducer mounted near
the fuel man ifold. Th is transducer produces an electrical signal
wh i ch is translated for the cockpit-mou nted i ndicator in gallons-per­
hour.

Revision 4

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AI RPLANE SYSTEMS DESC R I PTION
SECTION
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7-25