Small aircraft has altitude deviation during IFR flight.

Attributes
ACN	292580
Time
Date	199501
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : stl
State Reference	MO
Altitude	msl bound lower : 11000 msl bound upper : 11300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zkc tower : bos
Operator	general aviation : personal
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : cfi pilot : instrument
Experience	flight time last 90 days : 21 flight time total : 1663 flight time type : 417
ASRS Report	292580
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera
Resolutory Action	flight crew : became reoriented
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Departed bartlesville, ok (bvo) after being cleared to lewis university (lot) as filed (direct bvo VOR RNAV direct jot direct). WX was clear with unlimited visibility at takeoff and during entire trip, except for last 15 mins of 2.5 hour trip when there was about a 200 ft thick broken deck at about 3000 ft MSL. The nebound flight paralleled a cold front which lay about 100 NM east along the entire route and the altimeter setting changed continuously from about 30.38 at bvo to 30.08 area setting at lot. The autoplt altitude hold and RNAV tracking were used at cruise (11000 ft) the entire route. To change altitude and continue route tracking, the altitude hold must be disengaged and the autoplt altitude trim adjusted for an altitude change. Then the altitude hold must be manually re-engaged when the desired altitude is reached. An ATC handoff about 1 hour 45 mins after takeoff (near quincey, il) resulted in an altimeter setting from 30.26 to 30.22. The 30.22 was set in the kohlsman window (resulting in 10900 ft indicated instead of the 11000 ft assigned) and a slight nose up trim was adjusted to climb to the assigned 11000 ft. Cockpit distrs (fuel management, navigation, map reading, etc) resulted in a climb through 11000 ft. About 10 mins later ATC curtly reiterated the 30.22 altimeter setting and said I was 300 ft high. An immediate check of the altimeter showed me 100 ft high and I so informed ATC and said I was returning to 11000 ft (which I did within 15- 30 seconds). Having the ability to monitor the transponder altitude sent to ATC, I confirmed the transponder altitude is about 100 ft higher than the altimeter. Both were checked within the last 60 days as part of the IFR biennial calibration check and are apparently ok. In the future I will request calibration so that the blind encoder and the altimeter agree to less than 100 ft and I will make altitude changes in such a manner that cockpit distrs are less likely to cause forgetting to re-engage altitude hold.

Original NASA ASRS Text

Title: SMA HAS ALTDEV DURING IFR FLT.

Narrative: DEPARTED BARTLESVILLE, OK (BVO) AFTER BEING CLRED TO LEWIS UNIVERSITY (LOT) AS FILED (DIRECT BVO VOR RNAV DIRECT JOT DIRECT). WX WAS CLR WITH UNLIMITED VISIBILITY AT TKOF AND DURING ENTIRE TRIP, EXCEPT FOR LAST 15 MINS OF 2.5 HR TRIP WHEN THERE WAS ABOUT A 200 FT THICK BROKEN DECK AT ABOUT 3000 FT MSL. THE NEBOUND FLT PARALLELED A COLD FRONT WHICH LAY ABOUT 100 NM E ALONG THE ENTIRE RTE AND THE ALTIMETER SETTING CHANGED CONTINUOUSLY FROM ABOUT 30.38 AT BVO TO 30.08 AREA SETTING AT LOT. THE AUTOPLT ALT HOLD AND RNAV TRACKING WERE USED AT CRUISE (11000 FT) THE ENTIRE RTE. TO CHANGE ALT AND CONTINUE RTE TRACKING, THE ALT HOLD MUST BE DISENGAGED AND THE AUTOPLT ALT TRIM ADJUSTED FOR AN ALT CHANGE. THEN THE ALT HOLD MUST BE MANUALLY RE-ENGAGED WHEN THE DESIRED ALT IS REACHED. AN ATC HDOF ABOUT 1 HR 45 MINS AFTER TKOF (NEAR QUINCEY, IL) RESULTED IN AN ALTIMETER SETTING FROM 30.26 TO 30.22. THE 30.22 WAS SET IN THE KOHLSMAN WINDOW (RESULTING IN 10900 FT INDICATED INSTEAD OF THE 11000 FT ASSIGNED) AND A SLIGHT NOSE UP TRIM WAS ADJUSTED TO CLB TO THE ASSIGNED 11000 FT. COCKPIT DISTRS (FUEL MGMNT, NAV, MAP READING, ETC) RESULTED IN A CLB THROUGH 11000 FT. ABOUT 10 MINS LATER ATC CURTLY REITERATED THE 30.22 ALTIMETER SETTING AND SAID I WAS 300 FT HIGH. AN IMMEDIATE CHK OF THE ALTIMETER SHOWED ME 100 FT HIGH AND I SO INFORMED ATC AND SAID I WAS RETURNING TO 11000 FT (WHICH I DID WITHIN 15- 30 SECONDS). HAVING THE ABILITY TO MONITOR THE XPONDER ALT SENT TO ATC, I CONFIRMED THE XPONDER ALT IS ABOUT 100 FT HIGHER THAN THE ALTIMETER. BOTH WERE CHKED WITHIN THE LAST 60 DAYS AS PART OF THE IFR BIENNIAL CALIBRATION CHK AND ARE APPARENTLY OK. IN THE FUTURE I WILL REQUEST CALIBRATION SO THAT THE BLIND ENCODER AND THE ALTIMETER AGREE TO LESS THAN 100 FT AND I WILL MAKE ALT CHANGES IN SUCH A MANNER THAT COCKPIT DISTRS ARE LESS LIKELY TO CAUSE FORGETTING TO RE-ENGAGE ALT HOLD.

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.