A DH8-200 IN CRUISE AT FL230 DECLARES AN EMER AND DIVERTS DUE TO LOW #1 HYD QUANTITY CAUSED BY ENVIRONMENTAL FLUID HEATING AND COOLING.

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AN EMER SIT OCCURRED ENRTE DURING A SCHEDULED ACR FLT FROM PHX TO ASE. THE #1 HYD SYS ON THE DHC-8 ACFT PROGRESSIVELY LOST FLUID DURING CRUISE FLT. THE QUANTITY INDICATION DROPPED FROM OVER 1.5 QUARTS TO JUST UNDER 1.0 QUART; WHICH IS LESS THAN THE REQUIRED MINIMUM FOR THIS SYS. THE #1 SYS OPERATES THE FLAPS; INBOARD ROLL SPOILERS; #1 RUDDER ACTUATOR; AND NORMAL BRAKING INCLUDING THE ANTISKID. PRIOR TO TKOF FROM PHX; AN EXCESSIVELY LONG TAXI OF 15-20 MINS TO THE RWY WAS ENCOUNTERED. THE OUTSIDE AIR TEMP WAS OVER 100 DEGS F. A QUICK ATTEMPT TO XFER FLUID FROM THE #1 SYS TO THE #2 SYS PRIOR TO DEP WAS UNSUCCESSFUL. BOTH SYS WERE ABOVE DEP MINIMUMS; BUT THE #1 SYS QUANTITY WAS SHOWING CLOSE TO THE MINIMUM REQUIREMENT. AFTER RETRACTING THE FLAPS ON CLBOUT; THE #1 SYS QUANTITY DROPPED SLIGHTLY. AS THE FLT PROGRESSED; THE #1 SYS CONTINUED TO SHOW A DECREASE IN QUANTITY. WITH LITTLE OVER 1 QUART INDICATION ON THE FLT DECK GAUGE; THE DECISION WAS MADE TO CONTACT THE COMPANY DISPATCHER AND MAINT CTLR FOR ADDITIONAL OPTIONS. AN EMER WAS DECLARED WITH ZDV WITH A REQUEST TO HAVE EMER EQUIP STANDING BY. A DSCNT FOR THE VISUAL APCH WAS STARTED; BUT AS THE PWR WAS REDUCED; THE #1 ENG FUEL PRESSURE CAUTION LIGHT ILLUMINATED. THE FUEL PUMP IS ENG DRIVEN AND IS UNRELATED TO THE HYD SYS. THE FUEL PRESSURE ABNORMAL CHKLIST WAS COMPLETED; AND THE AUX FUEL PUMP TURNED ON EXTINGUISHING THE CAUTION LIGHT. THE #1 HYD SYS ABNORMAL CHKLISTS; INCLUDING THE IN-RANGE AND LNDG; WERE COMPLETED. IN AN ATTEMPT TO DETERMINE IF THE #1 HYD SYS SVCS WERE OPERATIONAL; THE FLAPS WERE EXTENDED NORMALLY WITHOUT ANY INDICATION OF ADDITIONAL FLUID LOSS. A NORMAL APCH AND LNDG WERE MADE. AFTER LNDG; THE ENGS WERE SHUT DOWN AND THE ACFT WAS TOWED TO THE GATE TO PREVENT THE RISK OF NORMAL BRAKE FAILURE. THE FOLLOW-UP WITH MAINT ON THE FUEL PRESSURE CAUTION LIGHT POINTED TO A BAD SENSOR; WHICH WAS REPLACED. THE #1 HYD SYS SHOWED NO SIGNS OF LEAKS; BUT THE RESERVOIR QUANTITY IN THE WHEEL WELL INDICATED APPROX .75 QUART REMAINING. MAINT'S CONCLUSION WAS: FROM AN EXCESSIVELY LONG TAXI IN THE HIGH HEAT AT PHX; THE HYD FLUID HEATED AND EXPANDED SHOWING A NORMAL QUANTITY FOR DEP; BUT AFTER THE ACFT WAS AIRBORNE AND THE FLUID WAS ALLOWED TO COOL AND CONTRACT; THE FLUID SHOWED A STEADY DECREASE IN QUANTITY. THE ACFT WAS SVCED; RUN UP AND TESTED FOR LEAKS. NOTHING WAS FOUND.