A descending DC10 flight crew misinterps their clearance and descends out of their last assigned alt...

Attributes
ACN	476637
Time
Date	200006
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : dtw.airport
State Reference	MI
Altitude	msl bound lower : 12000 msl bound upper : 17000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob.artcc tracon : d21.tracon tower : bwi.tower
Operator	common carrier : air carrier
Make Model Name	DC-10 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : vacating altitude
Route In Use	arrival : on vectors arrival star : polar
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 18000 flight time type : 6000
ASRS Report	476637
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : flight engineer pilot : commercial pilot : atp
Experience	flight time last 90 days : 250 flight time total : 9500 flight time type : 2500
ASRS Report	476640
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : company policies other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued advisory controller : issued new clearance
Consequence	Other
Supplementary
Problem Areas	ATC Human Performance Airspace Structure Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were descending into dtw on the polar STAR. We had been cleared to 17000 ft flying the STAR track. Polar intersection is normally crossed at 12000 ft (as published on the STAR). We were descending on profile (no power) for this crossing (with the clearance to 17000 ft). Descending through FL180 and about 18 mi from polar the controller cleared us direct to polar intersection and, I thought I heard, to cross at 12000 ft. There are 2 possibilities here: either the controller actually gave us the altitude, or I simply heard it because I expected it. In addition, having been cleared direct, cut off a couple of mi, and we had to keep descending if we had to cross polar at 12000 ft. As I descended through about 16000 ft the controller called us and said we were only cleared to 17000 ft. I immediately leveled off, and we responded that we thought he gave us direct polar and 12000 ft. The controller then came back and cleared us to 13000 ft and switched us over to approach, which immediately cleared us to 12000 ft. (As a side note: we never got the 'polar at 12000 ft' restr, and having leveled off to clarify the clearance we were too high to cross polar at 12000 ft.) we were in VMC, there was no conflicting traffic we could see, and our TCASII was on and showed no traffic at all. We continued to runway 21R for an uneventful approach and landing. Supplemental information from acn 476640: the captain was flying and I dialed 17000 ft into the altitude window on the autoplt window and armed it. We had been talking about the crossing at 12000 ft. A few seconds later, the captain said 12000 ft now at polar, and I wasn't sure if he said polar at 12000 ft, but instead of verifying that altitude with ZOB, I went with the captain. Since he seemed positive that's what we got. Instead of agreeing with the captain on the 12000 ft at polar, I should've questioned and verified the altitude with ZOB.

Original NASA ASRS Text

Title: A DSNDING DC10 FLC MISINTERPS THEIR CLRNC AND DSNDS OUT OF THEIR LAST ASSIGNED ALT TOWARDS THE XING ALT DEPICTED ON THE ARR STAR 50 MI NW OF DTW, MI.

Narrative: WE WERE DSNDING INTO DTW ON THE POLAR STAR. WE HAD BEEN CLRED TO 17000 FT FLYING THE STAR TRACK. POLAR INTXN IS NORMALLY CROSSED AT 12000 FT (AS PUBLISHED ON THE STAR). WE WERE DSNDING ON PROFILE (NO PWR) FOR THIS XING (WITH THE CLRNC TO 17000 FT). DSNDING THROUGH FL180 AND ABOUT 18 MI FROM POLAR THE CTLR CLRED US DIRECT TO POLAR INTXN AND, I THOUGHT I HEARD, TO CROSS AT 12000 FT. THERE ARE 2 POSSIBILITIES HERE: EITHER THE CTLR ACTUALLY GAVE US THE ALT, OR I SIMPLY HEARD IT BECAUSE I EXPECTED IT. IN ADDITION, HAVING BEEN CLRED DIRECT, CUT OFF A COUPLE OF MI, AND WE HAD TO KEEP DSNDING IF WE HAD TO CROSS POLAR AT 12000 FT. AS I DSNDED THROUGH ABOUT 16000 FT THE CTLR CALLED US AND SAID WE WERE ONLY CLRED TO 17000 FT. I IMMEDIATELY LEVELED OFF, AND WE RESPONDED THAT WE THOUGHT HE GAVE US DIRECT POLAR AND 12000 FT. THE CTLR THEN CAME BACK AND CLRED US TO 13000 FT AND SWITCHED US OVER TO APCH, WHICH IMMEDIATELY CLRED US TO 12000 FT. (AS A SIDE NOTE: WE NEVER GOT THE 'POLAR AT 12000 FT' RESTR, AND HAVING LEVELED OFF TO CLARIFY THE CLRNC WE WERE TOO HIGH TO CROSS POLAR AT 12000 FT.) WE WERE IN VMC, THERE WAS NO CONFLICTING TFC WE COULD SEE, AND OUR TCASII WAS ON AND SHOWED NO TFC AT ALL. WE CONTINUED TO RWY 21R FOR AN UNEVENTFUL APCH AND LNDG. SUPPLEMENTAL INFO FROM ACN 476640: THE CAPT WAS FLYING AND I DIALED 17000 FT INTO THE ALT WINDOW ON THE AUTOPLT WINDOW AND ARMED IT. WE HAD BEEN TALKING ABOUT THE XING AT 12000 FT. A FEW SECONDS LATER, THE CAPT SAID 12000 FT NOW AT POLAR, AND I WASN'T SURE IF HE SAID POLAR AT 12000 FT, BUT INSTEAD OF VERIFYING THAT ALT WITH ZOB, I WENT WITH THE CAPT. SINCE HE SEEMED POSITIVE THAT'S WHAT WE GOT. INSTEAD OF AGREEING WITH THE CAPT ON THE 12000 FT AT POLAR, I SHOULD'VE QUESTIONED AND VERIFIED THE ALT WITH ZOB.

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.

A descending DC10 flight crew misinterps their clearance and descends out of their last assigned altitude towards the crossing altitude depicted on the arrival STAR 50 mi northwest of dtw, mi.