Narrative:

We were ferrying the aircraft from phoenix to denver for a charter. Climbing through transition and resetting altimeters to 29.92 inches; the first officer and I crosschecked the altimeters to find they were within tolerance. We continued the climb to FL330 using the 'a' autoplt which derives its information from the captain's side equipment. Leveling at FL330; we again crosschecked the altimeters which my altimeter read FL330; first officer's read FL331.6; and the standby altimeter read FL331.4. The first officer's altimeter was reading slightly higher (it normally read about 100 ft higher than the captain's altimeter) than normal; but the standby altimeter was reading 140 ft higher than the captain's altimeter; which is what it normally read. Since the altimeters were within tolerance; there was no cause for alarm. Since the captain's altimeter and standby altimeters were reading their 'normal' split; the first officer and I agreed that the captain's altimeter was most likely the closest to correct information; so I kept autoplt 'a' engaged. After about 15 mins at that altitude; I crosschecked the altimeters again. This time the captain's and the standby altimeters were unchanged; but the first officer's altimeter was now reading FL332. Because we have an onboard mechanic for charter operations; I summoned him to the cockpit to see if he may have seen a reason for the split. He deduced the same as me and the first officer; that since the captain's and standby altimeters showed their normal split; that the information was most likely the correct information; so we continued on autoplt 'a' and the captain's altimeter continuing to read FL330. As we continued; the first officer's altimeter continued to slowly creep to FL332.4. We were then advised by the controller that he was showing our aircraft at FL333. The aircraft is equipped with an altitude alert light and tone that alerts the pilots when they have wandered 300 ft from their chosen altitude and neither the light nor tone were engaged to alert us. There was no other aircraft in the area and there were no traffic conflicts. We immediately knew then that the captain's altimeter was the culprit and descended to FL330 using the first officer's altimeter. We continued the flight to denver with the altitude based on the first officer's altimeter without incident. On the ground in denver; the mechanic tested the air data computers and found both to be operating normally. He then cleaned all the cannon plugs in the altimetry system and the altimeters then agreed identically. From denver to los angeles; the altimeters agreed with less than 10 ft difference. The 'normal' split between the captain's altimeter and the standby altimeter has never been explained and it continues to this day. Since they derive their information from different sources; the split between the two should not have stayed constant or nearly constant. The captain's altimeter was slowly descending and the autoplt; in response; was ever so slowly climbing in response to maintain FL330 and we didn't catch it. Looking back; I should have immediately queried the center controller as to what he was showing on his screen and now that drvsm has begun; rest assure that talking to the controller will be the first step should this or similar problem with altitude holding occur.

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Original NASA ASRS Text

Title: FLT CREW OF B737-400 ENCOUNTER PITOT STATIC ERRORS IN ALTIMETRY WHICH RESULT IN ALTDEV ALERT ISSUED BY RADAR CTLR.

Narrative: WE WERE FERRYING THE ACFT FROM PHOENIX TO DENVER FOR A CHARTER. CLBING THROUGH TRANSITION AND RESETTING ALTIMETERS TO 29.92 INCHES; THE FO AND I XCHKED THE ALTIMETERS TO FIND THEY WERE WITHIN TOLERANCE. WE CONTINUED THE CLB TO FL330 USING THE 'A' AUTOPLT WHICH DERIVES ITS INFO FROM THE CAPT'S SIDE EQUIP. LEVELING AT FL330; WE AGAIN XCHKED THE ALTIMETERS WHICH MY ALTIMETER READ FL330; FO'S READ FL331.6; AND THE STANDBY ALTIMETER READ FL331.4. THE FO'S ALTIMETER WAS READING SLIGHTLY HIGHER (IT NORMALLY READ ABOUT 100 FT HIGHER THAN THE CAPT'S ALTIMETER) THAN NORMAL; BUT THE STANDBY ALTIMETER WAS READING 140 FT HIGHER THAN THE CAPT'S ALTIMETER; WHICH IS WHAT IT NORMALLY READ. SINCE THE ALTIMETERS WERE WITHIN TOLERANCE; THERE WAS NO CAUSE FOR ALARM. SINCE THE CAPT'S ALTIMETER AND STANDBY ALTIMETERS WERE READING THEIR 'NORMAL' SPLIT; THE FO AND I AGREED THAT THE CAPT'S ALTIMETER WAS MOST LIKELY THE CLOSEST TO CORRECT INFO; SO I KEPT AUTOPLT 'A' ENGAGED. AFTER ABOUT 15 MINS AT THAT ALT; I XCHKED THE ALTIMETERS AGAIN. THIS TIME THE CAPT'S AND THE STANDBY ALTIMETERS WERE UNCHANGED; BUT THE FO'S ALTIMETER WAS NOW READING FL332. BECAUSE WE HAVE AN ONBOARD MECH FOR CHARTER OPS; I SUMMONED HIM TO THE COCKPIT TO SEE IF HE MAY HAVE SEEN A REASON FOR THE SPLIT. HE DEDUCED THE SAME AS ME AND THE FO; THAT SINCE THE CAPT'S AND STANDBY ALTIMETERS SHOWED THEIR NORMAL SPLIT; THAT THE INFO WAS MOST LIKELY THE CORRECT INFO; SO WE CONTINUED ON AUTOPLT 'A' AND THE CAPT'S ALTIMETER CONTINUING TO READ FL330. AS WE CONTINUED; THE FO'S ALTIMETER CONTINUED TO SLOWLY CREEP TO FL332.4. WE WERE THEN ADVISED BY THE CTLR THAT HE WAS SHOWING OUR ACFT AT FL333. THE ACFT IS EQUIPPED WITH AN ALT ALERT LIGHT AND TONE THAT ALERTS THE PLTS WHEN THEY HAVE WANDERED 300 FT FROM THEIR CHOSEN ALT AND NEITHER THE LIGHT NOR TONE WERE ENGAGED TO ALERT US. THERE WAS NO OTHER ACFT IN THE AREA AND THERE WERE NO TFC CONFLICTS. WE IMMEDIATELY KNEW THEN THAT THE CAPT'S ALTIMETER WAS THE CULPRIT AND DSNDED TO FL330 USING THE FO'S ALTIMETER. WE CONTINUED THE FLT TO DENVER WITH THE ALT BASED ON THE FO'S ALTIMETER WITHOUT INCIDENT. ON THE GND IN DENVER; THE MECH TESTED THE AIR DATA COMPUTERS AND FOUND BOTH TO BE OPERATING NORMALLY. HE THEN CLEANED ALL THE CANNON PLUGS IN THE ALTIMETRY SYS AND THE ALTIMETERS THEN AGREED IDENTICALLY. FROM DENVER TO LOS ANGELES; THE ALTIMETERS AGREED WITH LESS THAN 10 FT DIFFERENCE. THE 'NORMAL' SPLIT BTWN THE CAPT'S ALTIMETER AND THE STANDBY ALTIMETER HAS NEVER BEEN EXPLAINED AND IT CONTINUES TO THIS DAY. SINCE THEY DERIVE THEIR INFO FROM DIFFERENT SOURCES; THE SPLIT BTWN THE TWO SHOULD NOT HAVE STAYED CONSTANT OR NEARLY CONSTANT. THE CAPT'S ALTIMETER WAS SLOWLY DSNDING AND THE AUTOPLT; IN RESPONSE; WAS EVER SO SLOWLY CLBING IN RESPONSE TO MAINTAIN FL330 AND WE DIDN'T CATCH IT. LOOKING BACK; I SHOULD HAVE IMMEDIATELY QUERIED THE CTR CTLR AS TO WHAT HE WAS SHOWING ON HIS SCREEN AND NOW THAT DRVSM HAS BEGUN; REST ASSURE THAT TALKING TO THE CTLR WILL BE THE FIRST STEP SHOULD THIS OR SIMILAR PROB WITH ALT HOLDING OCCUR.

Data retrieved from NASA's ASRS site as of January 2009 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.