CPR LTT HAS LESS THAN STANDARD SEPARATION WITH ANOTHER ACFT.

Narrative

THE FLT IN DISCUSSION WAS A PAX FLT FROM SARANAC LAKE; ADIRONDACK ARPT; NY TO TETERBORO ARPT IN NJ. THE PIC; BEING A DESIGNATED PPE FOR THE ACFT TYPE; WAS GIVING A BIENNIAL FLT CHK TO THE OTHER CAPT. THE PIC WAS OCCUPYING THE RIGHT SEAT AT THE TIME OF THE OCCURRENCE. AFTER A NORMAL TKOF AND AREA DEP THE ACFT WAS CLBING TO AN ATC CLRNC ALT OF 14000'. THE PF WAS USING THE AUTOPLT AND DURING THE LAST 1000' OF CLB HE WAS USING THE PITCH CTL OF THE AUTOPLT TO REDUCE THE RATE OF CLB FOR A SMOOTH LEVEL OFF. AT APPROX 13800' THE AUTOPLT ALT PRESELECT MODE WENT FROM AN AMBER ARMED INDICATION TO A GREEN CAPTIVE ANNUNCIATION. THIS OCCURS AT 20% OF THE VERT SPD RATE AT WHICH THE ACFT IS CLBING. WE WERE CLBING AT 1000 FPM THUS THE AUTOPLT WAS CORRECT IN ITS OPERATION. HOWEVER; IF THE PLT INPUTS A PITCH TRIM COMMAND TO THE AUTOPLT WHEN IT IS BEGINNING THE ALT CAPTURE; IN THIS CASE 13800' TO 14000'; THIS WILL DISENGAGE THE ALT CAPTURE FEATURE OF THE AUTOPLT. TO CONTINUE; THE PNF PIC NOTICED THE CAPTURE LIGHT EXTINGUISH AT APPROX 13800' AND STATED; 'NO ALT CAPTURE; GO MANUAL.' THE PF THEN DISENGAGED THE AUTOPLT AND PROCEEDED TO LEVEL THE ACFT MANUALLY. IN THE PROCESS THE ACFT DRIFTED TO AN ALT OF 14200'. THE PNF PIC STATED; 'GET IT DOWN; THERE'S TFC AT 15000'.' AT THAT TIME ATC ADVISED THAT THE COMPUTER SYS ALARM HAD TRIGGERED AND ASKED US TO CONFIRM OUR ALT. ATC FURTHER ADVISED THAT THEY INDICATED OUR ALT WAS 14500'. AT THE SAME TIME I HAD VISUAL CONTACT WITH THE 15000' TFC AT THE 12 O'CLOCK POS EBND. WHEN ATC ASKED OUR ALT WE WERE DSNDING FROM 14100' TO 14000'. IN DISCUSSING THE SITUATION WITH ATC THEY WERE STILL READING OUR ALT AS 14100' WHEN WE WERE IN FACT AT 13900' DUE TO THE SLIGHT OVER-CORRECTION TO RE-ESTABLISH THE 14000' ALT. LATER; IN CONVERSATION WITH THE NEXT ATC SECTOR; WE WERE ASKED OUR ALT AND WE WERE INDICATING 16000' IN LEVEL FLT. HOWEVER; THEIR RADAR READ 16300'. ANALYZING THE SITUATION IN RETROSPECT; WE COULD HAVE CTLED THE ACFT SO AS TO HAVE NOT EXCEEDED 14000' ALT; BUT WE HAD TO CONSIDER THE WELFARE OF THE PAX ALSO; AND A MORE ABRUPT MANEUVER WOULD NOT HAVE BEEN IN THEIR INTERESTS. ANOTHER FACTOR FOR CONSIDERATION IS ALTIMETER BAROMETRIC PRESSURE SETTINGS. THE UNCONTROLLED ARPT FROM WHICH THE FLT DEPARTED HAD AN ALTIMETER SETTING OF 29.85 AND THE SETTING THAT ATC WAS GIVING WAS 30.04. THIS DIFFERENCE WOULD INDICATE THE ACFT TO BE 190' HIGHER ON THE RADAR READOUT THAN THE CREWS' INDICATIONS ON THE COCKPIT ALTIMETERS. THIS VARIATION; COUPLED WITH THE 200' OVERSHOOT PLUS THE MODE C CALIBRATION FOR THIS SPECIFIC ACFT WOULD EQUAL 430' OF ALT. ALTHOUGH NOT PRIVILEGED TO THE ATC CALIBRATION ERROR; I WOULD HAVE TO ASSUME THEIR EQUIP TO BE IN THE AREA OF +50-+70'; THUS THE TOTAL WOULD EQUAL 500'; WHICH IS SUFFICIENT TO TRIGGER THE WARNING SYS. A DEPT PLT MEETING WILL BE HELD TO DISCUSS THE HAPPENSTANCE AND TO CREATE AN AWARENESS OF THE VARYING FACTORS THAT CAN CREATE DIFFERENCES IN READOUTS AND THE NECESSITY FOR CLOSELY MONITORING AUTOPLT OPERATIONS.