Narrative:

Put together an all night flight on the back side of the clock, when the pilots would normally be sleeping. An over zealous mechanic, an almost empty B747 freighter, a call from center -- and you have the ingredients for an altitude deviation. The flight originated at sfo as cargo flight xx of aug/xx/97, with the captain flying the first leg. Departure was at XA19Z with the landing in ord at XE18Z. The flight was normal in all respects, except the captain's standby altimeter was sticking and needed to be tapped to get it to indicate correctly when level. This was written up going into ord as 'altimeter sticks.' the mechanic there felt that this write-up rendered the altimeter inoperative, and because the MEL allowed no relief for this, it would have to be changed. This was done by swapping a standby altimeter from a B747 that had been purchased recently from abc air carrier. The flight then departed ord for msp with the first officer flying. Departure was at XF56Z. Departure and climb to altitude were normal with the autoplt, autothrottles, and pms engaged for the climb. I made the 2000 ft and 1000 ft calls before reaching our clearance altitude of FL390. After making the last call, I think I made a radio frequency change to center. When I next looked up, the airplane was climbing smartly through FL390. I think I said (loudly) something like 'this thing isn't stopping' and started to push on the control column to override the autoplt. The first officer then clicked off the autoplt and descended us back down to FL390. Because the aircraft was so light we could not stop it until it had reached FL394. Normally these freighters creep up on FL390 at the weights we operate them. The next thing we noticed was that the pms still thought it was in a climb and would not capture cruise mode. A check of the pms altitude showed why. It thought it was still some 1500 ft below the actual altitude of the aircraft. For that reason, when we were at FL390, the pms thought it had another 1500 ft to go. Yes, the pms gets its altitude input from the standby altimeter -- the one changed at ord. With over 7 yrs on B747's, I have never seen this error before. We had no hint that the pms was getting anything other than a correct altitude input because all the altimeters indicated within a few ft of each other at all times. The darned thing continued to think itself low by the same amount for the remainder of the flight, which is very strange. I really don't know what I would have done differently. I had warned the first officer and so that we were all tired and needed to be extra vigilant. When I would normally be awake, I had tried to sleep all day in sfo before the flight. How much rest I really got is anyone's guess.

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Original NASA ASRS Text

Title: A B747-200 EXCEEDS ITS ASSIGNED ALT BY 400 FT BECAUSE OF ALTIMETER PROB THAT INDUCES AN ERROR INTO THE AUTO FLT SYS. ALL IN ZAU AIRSPACE.

Narrative: PUT TOGETHER AN ALL NIGHT FLT ON THE BACK SIDE OF THE CLOCK, WHEN THE PLTS WOULD NORMALLY BE SLEEPING. AN OVER ZEALOUS MECH, AN ALMOST EMPTY B747 FREIGHTER, A CALL FROM CTR -- AND YOU HAVE THE INGREDIENTS FOR AN ALTDEV. THE FLT ORIGINATED AT SFO AS CARGO FLT XX OF AUG/XX/97, WITH THE CAPT FLYING THE FIRST LEG. DEP WAS AT XA19Z WITH THE LNDG IN ORD AT XE18Z. THE FLT WAS NORMAL IN ALL RESPECTS, EXCEPT THE CAPT'S STANDBY ALTIMETER WAS STICKING AND NEEDED TO BE TAPPED TO GET IT TO INDICATE CORRECTLY WHEN LEVEL. THIS WAS WRITTEN UP GOING INTO ORD AS 'ALTIMETER STICKS.' THE MECH THERE FELT THAT THIS WRITE-UP RENDERED THE ALTIMETER INOP, AND BECAUSE THE MEL ALLOWED NO RELIEF FOR THIS, IT WOULD HAVE TO BE CHANGED. THIS WAS DONE BY SWAPPING A STANDBY ALTIMETER FROM A B747 THAT HAD BEEN PURCHASED RECENTLY FROM ABC ACR. THE FLT THEN DEPARTED ORD FOR MSP WITH THE FO FLYING. DEP WAS AT XF56Z. DEP AND CLB TO ALT WERE NORMAL WITH THE AUTOPLT, AUTOTHROTTLES, AND PMS ENGAGED FOR THE CLB. I MADE THE 2000 FT AND 1000 FT CALLS BEFORE REACHING OUR CLRNC ALT OF FL390. AFTER MAKING THE LAST CALL, I THINK I MADE A RADIO FREQ CHANGE TO CTR. WHEN I NEXT LOOKED UP, THE AIRPLANE WAS CLBING SMARTLY THROUGH FL390. I THINK I SAID (LOUDLY) SOMETHING LIKE 'THIS THING ISN'T STOPPING' AND STARTED TO PUSH ON THE CTL COLUMN TO OVERRIDE THE AUTOPLT. THE FO THEN CLICKED OFF THE AUTOPLT AND DSNDED US BACK DOWN TO FL390. BECAUSE THE ACFT WAS SO LIGHT WE COULD NOT STOP IT UNTIL IT HAD REACHED FL394. NORMALLY THESE FREIGHTERS CREEP UP ON FL390 AT THE WTS WE OPERATE THEM. THE NEXT THING WE NOTICED WAS THAT THE PMS STILL THOUGHT IT WAS IN A CLB AND WOULD NOT CAPTURE CRUISE MODE. A CHK OF THE PMS ALT SHOWED WHY. IT THOUGHT IT WAS STILL SOME 1500 FT BELOW THE ACTUAL ALT OF THE ACFT. FOR THAT REASON, WHEN WE WERE AT FL390, THE PMS THOUGHT IT HAD ANOTHER 1500 FT TO GO. YES, THE PMS GETS ITS ALT INPUT FROM THE STANDBY ALTIMETER -- THE ONE CHANGED AT ORD. WITH OVER 7 YRS ON B747'S, I HAVE NEVER SEEN THIS ERROR BEFORE. WE HAD NO HINT THAT THE PMS WAS GETTING ANYTHING OTHER THAN A CORRECT ALT INPUT BECAUSE ALL THE ALTIMETERS INDICATED WITHIN A FEW FT OF EACH OTHER AT ALL TIMES. THE DARNED THING CONTINUED TO THINK ITSELF LOW BY THE SAME AMOUNT FOR THE REMAINDER OF THE FLT, WHICH IS VERY STRANGE. I REALLY DON'T KNOW WHAT I WOULD HAVE DONE DIFFERENTLY. I HAD WARNED THE FO AND SO THAT WE WERE ALL TIRED AND NEEDED TO BE EXTRA VIGILANT. WHEN I WOULD NORMALLY BE AWAKE, I HAD TRIED TO SLEEP ALL DAY IN SFO BEFORE THE FLT. HOW MUCH REST I REALLY GOT IS ANYONE'S GUESS.

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.