Narrative:

We were being vectored for ILS runway 18 at clt. We were on a right downwind descending to 4600 ft MSL on approximately a 030 degree heading. I was listening to the controller talking to an aircraft ahead of us. He asked that aircraft to fly at 190 KTS to match our speed (we were following him). The pilot responded that they were unable to maintain 190 KTS. So the controller told us to maintain 5600 ft MSL. I read back '5600.' the first officer (who was the PF) set 5600 ft in the altitude alerter window, and I pointed to it and verified it. A few seconds later the altitude alerter chime went off and the first officer responded '6600 ft for 5600 ft.' I was watching the traffic on TCASII and noticed that we were still closing on him and descending down on him. This confused me because I thought that at 5600 ft MSL we would be 1000 ft above him. It was then that I double checked my altimeter, it read approximately 5000 ft and descending. I realized then that the first officer had misread our altitude and I had failed to catch the mistake. I said we are too low climb back up to 5600 ft. So the first officer added power and caught our altitude and said, 'just maintain your current altitude and turn left to 010 degrees.' there was no actual conflict with the other aircraft. I believe that this error was a direct result of misreading the altimeter. The altimeter in our aircraft is the old barrel type with a single hundreds foot pointer. The problem is you really have to think about what thousands you are on and if you are above or below it. The newer type with the entire altitude read out in the barrel (thousands, hundreds, tens) eliminates this problem. Callback conversation with reporter revealed the following information: both sides of the cockpit have this type of altimeter. In a quick glance, it's confusing to read. This is one of those times that the captain (reporter and PNF) failed to recognize the deviation because of the focus that reading this particular altimeter requires and the conflicting demands for his attention. The captain was concerned with the traffic and spacing and relying heavily upon the first officer to fly the aircraft. Supplemental information from acn 328357: factors that contributed to the problem were pilot fatigue (short overnight) and a captain under high workload. I was thinking descending, not climbing to an altitude.

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Original NASA ASRS Text

Title: ALT OVERSHOT BY DC9 FLC DURING DSCNT WHILE ON DOWNWIND VECTOR FOR APCH. WHEN THE ALT ALERTER SOUNDED, THE PF MISTAKENLY CALLED 6600 FT FOR 5600 FT WHEN THE ACFT WAS ACTUALLY DSNDING THROUGH THE ASSIGNED ALT OF 5600 FT. THE PNF WAS WATCHING TFC AHEAD WHICH WAS UNABLE TO MAINTAIN MATCHING SPD AND DISTANCE. CLRNC TO MAINTAIN 5600 FT WAS ASSIGNED FOR SEQUENCING.

Narrative: WE WERE BEING VECTORED FOR ILS RWY 18 AT CLT. WE WERE ON A R DOWNWIND DSNDING TO 4600 FT MSL ON APPROX A 030 DEG HDG. I WAS LISTENING TO THE CTLR TALKING TO AN ACFT AHEAD OF US. HE ASKED THAT ACFT TO FLY AT 190 KTS TO MATCH OUR SPD (WE WERE FOLLOWING HIM). THE PLT RESPONDED THAT THEY WERE UNABLE TO MAINTAIN 190 KTS. SO THE CTLR TOLD US TO MAINTAIN 5600 FT MSL. I READ BACK '5600.' THE FO (WHO WAS THE PF) SET 5600 FT IN THE ALT ALERTER WINDOW, AND I POINTED TO IT AND VERIFIED IT. A FEW SECONDS LATER THE ALT ALERTER CHIME WENT OFF AND THE FO RESPONDED '6600 FT FOR 5600 FT.' I WAS WATCHING THE TFC ON TCASII AND NOTICED THAT WE WERE STILL CLOSING ON HIM AND DSNDING DOWN ON HIM. THIS CONFUSED ME BECAUSE I THOUGHT THAT AT 5600 FT MSL WE WOULD BE 1000 FT ABOVE HIM. IT WAS THEN THAT I DOUBLE CHKED MY ALTIMETER, IT READ APPROX 5000 FT AND DSNDING. I REALIZED THEN THAT THE FO HAD MISREAD OUR ALT AND I HAD FAILED TO CATCH THE MISTAKE. I SAID WE ARE TOO LOW CLB BACK UP TO 5600 FT. SO THE FO ADDED PWR AND CAUGHT OUR ALT AND SAID, 'JUST MAINTAIN YOUR CURRENT ALT AND TURN L TO 010 DEGS.' THERE WAS NO ACTUAL CONFLICT WITH THE OTHER ACFT. I BELIEVE THAT THIS ERROR WAS A DIRECT RESULT OF MISREADING THE ALTIMETER. THE ALTIMETER IN OUR ACFT IS THE OLD BARREL TYPE WITH A SINGLE HUNDREDS FOOT POINTER. THE PROB IS YOU REALLY HAVE TO THINK ABOUT WHAT THOUSANDS YOU ARE ON AND IF YOU ARE ABOVE OR BELOW IT. THE NEWER TYPE WITH THE ENTIRE ALT READ OUT IN THE BARREL (THOUSANDS, HUNDREDS, TENS) ELIMINATES THIS PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: BOTH SIDES OF THE COCKPIT HAVE THIS TYPE OF ALTIMETER. IN A QUICK GLANCE, IT'S CONFUSING TO READ. THIS IS ONE OF THOSE TIMES THAT THE CAPT (RPTR AND PNF) FAILED TO RECOGNIZE THE DEV BECAUSE OF THE FOCUS THAT READING THIS PARTICULAR ALTIMETER REQUIRES AND THE CONFLICTING DEMANDS FOR HIS ATTN. THE CAPT WAS CONCERNED WITH THE TFC AND SPACING AND RELYING HEAVILY UPON THE FO TO FLY THE ACFT. SUPPLEMENTAL INFO FROM ACN 328357: FACTORS THAT CONTRIBUTED TO THE PROB WERE PLT FATIGUE (SHORT OVERNIGHT) AND A CAPT UNDER HIGH WORKLOAD. I WAS THINKING DSNDING, NOT CLBING TO AN ALT.

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.