Pilot of an small aircraft sel deviated from assigned altitude due to a frozen outside static source...

Attributes
ACN	270905
Time
Date	199405
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : har
State Reference	PA
Altitude	msl bound lower : 20000 msl bound upper : 21000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zny
Operator	general aviation : corporate
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 75 flight time total : 4500 flight time type : 450
ASRS Report	270905
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 other non adherence : clearance other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : overcame equipment problem other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

En route from richmond, va (ric) to nashua, nh (ash) at FL200 received an amended routing from center and was head down for a few mins reprogramming the navigation radios and looking over the route. I perceived a sound quality change in the aircraft and looked up to discover an indicated airspeed around 60 KTS and a 5 degree pitch up on the ADI. Power and engine gauges were normal. I disengaged the autoplt, levelled the aircraft off. Airspeed slowly increased (but not much) and indicated altitude was still 20000 ft. Suspecting static blockage, I selected alternate static and immediately the airspeed jumped to 140 KTS and the altimeter spun up to 21000 ft. Initiated an immediate descent and before I could call center, they called me. I explained the problem and heard nothing more about it. I believe moisture condensed in the static lines as I climbed, and froze at altitude. The outside air temperature at the time was minus 25 degrees C. The preflight static drain had resulted in no water. Had the blockage not occurred during my 'head down' period I think I would have noticed the problem prior to an altitude deviation. Interestingly, this flight was the second leg of a 6 hour flight. The first leg that day was 3.5 hours at FL200 with no problems of any kind. The static blockage occurred about 1.5 hours into the second leg, in clear air, with the only WX encountered being a layer from 8000 ft to 13000 ft during climb out from richmond.

Original NASA ASRS Text

Title: PLT OF AN SMA SEL DEVIATED FROM ASSIGNED ALT DUE TO A FROZEN OUTSIDE STATIC SOURCE CAUSING ERRONEOUS ALT AND AIRSPD INDICATIONS.

Narrative: ENRTE FROM RICHMOND, VA (RIC) TO NASHUA, NH (ASH) AT FL200 RECEIVED AN AMENDED ROUTING FROM CTR AND WAS HEAD DOWN FOR A FEW MINS REPROGRAMMING THE NAV RADIOS AND LOOKING OVER THE RTE. I PERCEIVED A SOUND QUALITY CHANGE IN THE ACFT AND LOOKED UP TO DISCOVER AN INDICATED AIRSPD AROUND 60 KTS AND A 5 DEG PITCH UP ON THE ADI. PWR AND ENG GAUGES WERE NORMAL. I DISENGAGED THE AUTOPLT, LEVELLED THE ACFT OFF. AIRSPD SLOWLY INCREASED (BUT NOT MUCH) AND INDICATED ALT WAS STILL 20000 FT. SUSPECTING STATIC BLOCKAGE, I SELECTED ALTERNATE STATIC AND IMMEDIATELY THE AIRSPD JUMPED TO 140 KTS AND THE ALTIMETER SPUN UP TO 21000 FT. INITIATED AN IMMEDIATE DSCNT AND BEFORE I COULD CALL CTR, THEY CALLED ME. I EXPLAINED THE PROB AND HEARD NOTHING MORE ABOUT IT. I BELIEVE MOISTURE CONDENSED IN THE STATIC LINES AS I CLBED, AND FROZE AT ALT. THE OUTSIDE AIR TEMP AT THE TIME WAS MINUS 25 DEGS C. THE PREFLT STATIC DRAIN HAD RESULTED IN NO WATER. HAD THE BLOCKAGE NOT OCCURRED DURING MY 'HEAD DOWN' PERIOD I THINK I WOULD HAVE NOTICED THE PROB PRIOR TO AN ALTDEV. INTERESTINGLY, THIS FLT WAS THE SECOND LEG OF A 6 HR FLT. THE FIRST LEG THAT DAY WAS 3.5 HRS AT FL200 WITH NO PROBS OF ANY KIND. THE STATIC BLOCKAGE OCCURRED ABOUT 1.5 HOURS INTO THE SECOND LEG, IN CLR AIR, WITH THE ONLY WX ENCOUNTERED BEING A LAYER FROM 8000 FT TO 13000 FT DURING CLBOUT FROM RICHMOND.

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.

Pilot of an small aircraft sel deviated from assigned altitude due to a frozen outside static source causing erroneous altitude and airspeed indications.