Aircraft equipment problem, altitude deviation.

Attributes
ACN	96835
Time
Date	198810
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : dxr
State Reference	CT
Altitude	msl bound lower : 5000 msl bound upper : 5300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	general aviation : personal
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	climbout : intermediate altitude cruise other
Route In Use	enroute airway : n90
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : private pilot : instrument
Experience	flight time last 90 days : 65 flight time total : 4000 flight time type : 1200
ASRS Report	96835
Person 2
Affiliation	government : faa
Function	controller : departure
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

On climb out from 3000 to 5000' after departure from danbury, ct, on an instrument flight plan to charleston, wv, when the autoplt pitch trim wheel switch was rotated to level off at 5000' the aircraft continued to trim up and continued up to approximately 5300' before the uncontrolled trim up condition was corrected by disabling the autoplt with its main switch and by pulling the trim circuit breaker. The aircraft was hand flown back to the correct altitude while maintaining proper heading. The controller inquired about the altitude and was informed of the trim difficulties. The aircraft was in VFR conditions on top and not in conflict with any other traffic during the maneuver. When the autoplt was re-engaged it would hold course but not altitude. At destination the autoplt was checked and found to have a qty of water in a static line connected to the altitude diaphragm trim unit in the tail. Outside air temperature was approximately -10 degrees C, so it would appear that the water condensed in the static line to the altitude trim, froze and locked the trim full up. When it melted the trim worked ok. Condensation in that line has been a problem before and draining the line is a 50 hour maintenance item which I require of my service facility. It just built up quickly and froze this time. Callback conversation with reporter revealed the following: reporter states problem occurs when aircraft sits on ground in nice, warm moist environment and then climbs into cooler air and it 'rains' inside the tube. This tube is a loop (U tube) arrangement. In liquid form it would probably not cause much problem, but when it freezes and altitude hold is engaged, it can cause 200-300' erroneous reading. Also stated must be very careful when blowing line at servicing to protect the diaphragm. Reporter is considering installing a drain valve so can clear west/O removing whole back end of baggage compartment. Has discussed problem with mfr.

Original NASA ASRS Text

Title: ACFT EQUIPMENT PROBLEM, ALT DEVIATION.

Narrative: ON CLBOUT FROM 3000 TO 5000' AFTER DEP FROM DANBURY, CT, ON AN INSTRUMENT FLT PLAN TO CHARLESTON, WV, WHEN THE AUTOPLT PITCH TRIM WHEEL SWITCH WAS ROTATED TO LEVEL OFF AT 5000' THE ACFT CONTINUED TO TRIM UP AND CONTINUED UP TO APPROX 5300' BEFORE THE UNCONTROLLED TRIM UP CONDITION WAS CORRECTED BY DISABLING THE AUTOPLT WITH ITS MAIN SWITCH AND BY PULLING THE TRIM CB. THE ACFT WAS HAND FLOWN BACK TO THE CORRECT ALT WHILE MAINTAINING PROPER HDG. THE CTLR INQUIRED ABOUT THE ALT AND WAS INFORMED OF THE TRIM DIFFICULTIES. THE ACFT WAS IN VFR CONDITIONS ON TOP AND NOT IN CONFLICT WITH ANY OTHER TFC DURING THE MANEUVER. WHEN THE AUTOPLT WAS RE-ENGAGED IT WOULD HOLD COURSE BUT NOT ALT. AT DEST THE AUTOPLT WAS CHKED AND FOUND TO HAVE A QTY OF WATER IN A STATIC LINE CONNECTED TO THE ALT DIAPHRAGM TRIM UNIT IN THE TAIL. OUTSIDE AIR TEMP WAS APPROX -10 DEGS C, SO IT WOULD APPEAR THAT THE WATER CONDENSED IN THE STATIC LINE TO THE ALT TRIM, FROZE AND LOCKED THE TRIM FULL UP. WHEN IT MELTED THE TRIM WORKED OK. CONDENSATION IN THAT LINE HAS BEEN A PROB BEFORE AND DRAINING THE LINE IS A 50 HR MAINT ITEM WHICH I REQUIRE OF MY SVC FAC. IT JUST BUILT UP QUICKLY AND FROZE THIS TIME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATES PROB OCCURS WHEN ACFT SITS ON GND IN NICE, WARM MOIST ENVIRONMENT AND THEN CLBS INTO COOLER AIR AND IT 'RAINS' INSIDE THE TUBE. THIS TUBE IS A LOOP (U TUBE) ARRANGEMENT. IN LIQUID FORM IT WOULD PROBABLY NOT CAUSE MUCH PROB, BUT WHEN IT FREEZES AND ALT HOLD IS ENGAGED, IT CAN CAUSE 200-300' ERRONEOUS READING. ALSO STATED MUST BE VERY CAREFUL WHEN BLOWING LINE AT SERVICING TO PROTECT THE DIAPHRAGM. RPTR IS CONSIDERING INSTALLING A DRAIN VALVE SO CAN CLEAR W/O REMOVING WHOLE BACK END OF BAGGAGE COMPARTMENT. HAS DISCUSSED PROB WITH MFR.

Data retrieved from NASA's ASRS site as of August 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.