Falcon 20 air taxi flight crew has clearance confusion and descends below assigned altitude.

Attributes
ACN	343110
Time
Date	199607
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : rfd
State Reference	IL
Altitude	msl bound lower : 7000 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : rfd
Operator	common carrier : air taxi
Make Model Name	Falcon 20FJF/20C/20D/20E/20F
Operating Under FAR Part	Part 135
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 2800 flight time type : 363
ASRS Report	343110
Person 2
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While serving as the PF sic, we were descending into rfd. The captain was getting ATIS and I was conducting the descent. I thought that we were cleared down to 6000 ft, but I think that it had been amended. Anyway, I told the captain to set the altitude to 6000 ft as we were descending out of 7200 ft. He said 9000 ft was the correct altitude and to stop the descent and return to 9000 ft. So I initiated a climb back to 9000 ft. It was a 'pilot's discretion' descent. We then asked the controller for lower and she said continue down, verify passing 8700 ft which we responded 'affirmative.' we apologized for any confusion that we might have caused and she did not think there would be any problem. On the ground, the captain and I discussed the events and both vowed to never stray from the altitude in the alerter without first asking the other pilot. That's never happened before and hopefully won't again.

Original NASA ASRS Text

Title: FALCON 20 AIR TAXI FLC HAS CLRNC CONFUSION AND DSNDS BELOW ASSIGNED ALT.

Narrative: WHILE SERVING AS THE PF SIC, WE WERE DSNDING INTO RFD. THE CAPT WAS GETTING ATIS AND I WAS CONDUCTING THE DSCNT. I THOUGHT THAT WE WERE CLRED DOWN TO 6000 FT, BUT I THINK THAT IT HAD BEEN AMENDED. ANYWAY, I TOLD THE CAPT TO SET THE ALT TO 6000 FT AS WE WERE DSNDING OUT OF 7200 FT. HE SAID 9000 FT WAS THE CORRECT ALT AND TO STOP THE DSCNT AND RETURN TO 9000 FT. SO I INITIATED A CLB BACK TO 9000 FT. IT WAS A 'PLT'S DISCRETION' DSCNT. WE THEN ASKED THE CTLR FOR LOWER AND SHE SAID CONTINUE DOWN, VERIFY PASSING 8700 FT WHICH WE RESPONDED 'AFFIRMATIVE.' WE APOLOGIZED FOR ANY CONFUSION THAT WE MIGHT HAVE CAUSED AND SHE DID NOT THINK THERE WOULD BE ANY PROB. ON THE GND, THE CAPT AND I DISCUSSED THE EVENTS AND BOTH VOWED TO NEVER STRAY FROM THE ALT IN THE ALERTER WITHOUT FIRST ASKING THE OTHER PLT. THAT'S NEVER HAPPENED BEFORE AND HOPEFULLY WON'T AGAIN.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.