Frozen static source causes light twin to climb above assigned altitude and then descend below it.

Attributes
ACN	259750
Time
Date	199312
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : hyk
State Reference	KY
Altitude	msl bound lower : 25000 msl bound upper : 25700
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl
Operator	general aviation : personal
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : atp pilot : instrument pilot : commercial
Experience	flight time last 90 days : 60 flight time total : 2800 flight time type : 850
ASRS Report	259750
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude inflight encounter : weather
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : became reoriented flight crew : overcame equipment problem
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Filed FL220 from mlb to lex. Given FL210 as final. Started picking up light rime ice. Requested and got FL250 which temporarily put me above clouds (long enough for ice to sublimate). Soon back into clouds and again picking up ice. Advised center of development and that I would need lower if worsened. Continuous mild to moderate turbulence which at times was severe. (All airlines were complaining to ZTL about turbulence from FL200 to FL370 and perhaps higher). Cleared direct lex and shortly thereafter turbulence increased requiring both hands on yoke to control aircraft (hand-fly only, autoplt worse). Pitot heat on from beginning. Outside air temperature minus 25 degrees C. Picked up considerable intermittent induction ice, (even with alternate air on both engines) with surging of engines. Suddenly, on scan, IAS at 240 KTS increasing (was at 155- 160 KTS in cruise). Almost simultaneously altimeter was seen to be rapidly descending and was at 24300 ft. I pulled up elevator (back), as my initial thought was I was in a dive. Attitude indicator indicated same instruction (flight director). IAS needle went to and beyond red line (244 KTS). At this time, I went to alternate static source. Altimeter then indicated I was at 25700 ft. Asi remained the same. I immediately descended to FL250. ZTL asked what my altitude was and I told them 25000 ft, and also about the problems I have described and that I thought everything was under control. They said no problems. Pressurization gauges indicated correctly. Nevertheless, I went on oxygen as a precautionary measure and asked for and was given a descent in stages to 11000 ft. IAS did not return until passing through 17000 ft, where outside temperature was minus 20 degrees C (must be considerable moisture in both static system, which I am going to have investigated). Remainder of flight was essentially normal. I heard later that atl closed because of blowing snow.

Original NASA ASRS Text

Title: FROZEN STATIC SOURCE CAUSES LIGHT TWIN TO CLB ABOVE ASSIGNED ALT AND THEN DSND BELOW IT.

Narrative: FILED FL220 FROM MLB TO LEX. GIVEN FL210 AS FINAL. STARTED PICKING UP LIGHT RIME ICE. REQUESTED AND GOT FL250 WHICH TEMPORARILY PUT ME ABOVE CLOUDS (LONG ENOUGH FOR ICE TO SUBLIMATE). SOON BACK INTO CLOUDS AND AGAIN PICKING UP ICE. ADVISED CTR OF DEVELOPMENT AND THAT I WOULD NEED LOWER IF WORSENED. CONTINUOUS MILD TO MODERATE TURB WHICH AT TIMES WAS SEVERE. (ALL AIRLINES WERE COMPLAINING TO ZTL ABOUT TURB FROM FL200 TO FL370 AND PERHAPS HIGHER). CLRED DIRECT LEX AND SHORTLY THEREAFTER TURB INCREASED REQUIRING BOTH HANDS ON YOKE TO CTL ACFT (HAND-FLY ONLY, AUTOPLT WORSE). PITOT HEAT ON FROM BEGINNING. OUTSIDE AIR TEMP MINUS 25 DEGS C. PICKED UP CONSIDERABLE INTERMITTENT INDUCTION ICE, (EVEN WITH ALTERNATE AIR ON BOTH ENGS) WITH SURGING OF ENGS. SUDDENLY, ON SCAN, IAS AT 240 KTS INCREASING (WAS AT 155- 160 KTS IN CRUISE). ALMOST SIMULTANEOUSLY ALTIMETER WAS SEEN TO BE RAPIDLY DSNDING AND WAS AT 24300 FT. I PULLED UP ELEVATOR (BACK), AS MY INITIAL THOUGHT WAS I WAS IN A DIVE. ATTITUDE INDICATOR INDICATED SAME INSTRUCTION (FLT DIRECTOR). IAS NEEDLE WENT TO AND BEYOND RED LINE (244 KTS). AT THIS TIME, I WENT TO ALTERNATE STATIC SOURCE. ALTIMETER THEN INDICATED I WAS AT 25700 FT. ASI REMAINED THE SAME. I IMMEDIATELY DSNDED TO FL250. ZTL ASKED WHAT MY ALT WAS AND I TOLD THEM 25000 FT, AND ALSO ABOUT THE PROBS I HAVE DESCRIBED AND THAT I THOUGHT EVERYTHING WAS UNDER CTL. THEY SAID NO PROBS. PRESSURIZATION GAUGES INDICATED CORRECTLY. NEVERTHELESS, I WENT ON OXYGEN AS A PRECAUTIONARY MEASURE AND ASKED FOR AND WAS GIVEN A DSCNT IN STAGES TO 11000 FT. IAS DID NOT RETURN UNTIL PASSING THROUGH 17000 FT, WHERE OUTSIDE TEMP WAS MINUS 20 DEGS C (MUST BE CONSIDERABLE MOISTURE IN BOTH STATIC SYS, WHICH I AM GOING TO HAVE INVESTIGATED). REMAINDER OF FLT WAS ESSENTIALLY NORMAL. I HEARD LATER THAT ATL CLOSED BECAUSE OF BLOWING SNOW.

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.