Pilot training crew of a C172, lost correct indications from the airspeed, vertical speed and altime...

Attributes
ACN	383119
Time
Date	199710
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : ipt
State Reference	PA
Altitude	msl bound lower : 4100 msl bound upper : 5000
Environment
Flight Conditions	IMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zny
Operator	general aviation : personal
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	instruction : instructor
Qualification	other other : other pilot : cfi pilot : instrument pilot : commercial
Experience	flight time last 90 days : 50 flight time total : 1700 flight time type : 1000
ASRS Report	383119
Person 2
Affiliation	Other
Function	instruction : trainee
Qualification	pilot : private
Events
Anomaly	aircraft equipment problem : critical inflight encounter : weather other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	controller : issued new clearance flight crew : declared emergency flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft

Narrative:

Regarding: altitude excursion below assigned altitude. Conditions of flight: 1 hour of constant flight in very wet clouds and intermittent rain. Outside air temperature approximately 10 degrees C. Student IFR instructional flight. Type: C172 1997 model. Descending from 6000-5000 ft vectors to join the localizer for an ILS approach. Reported WX was about 300 ft above decision altitude and 3 mi. Actual visibility turned out to be about 1-1/2 mi. During the descent we experienced complete failure of the pitot static system. The airspeed indicator was indicating above red line. The altimeter was frozen at 5600 ft. The vertical speed indicator was returning to a level flight attitude. We immediately pulled the alternate air control and put on pitot heat even though the outside air temperature was at 10 degrees C. It had no immediate effect on the pitot static failure. As we started to troubleshoot the condition and we leveled the aircraft using the attitude indicator and notified ATC that we had a major problem. We were now below 5000 ft. The controller then cleared us to 4100 ft. We were able to approximate our altitude using the visual reading of pressure altitude shown on the transponder (bendix king). After about 2 mins all instruments returned to normal. ATC confirmed our altitude at 4100 ft. What probably happened was that the static line had filled with water. After we opened the alternate static port the water probably started to drain with the help of the changing pitch of the aircraft. After the flight we noticed that the carpet on the floor was soaking wet. After this incident I recalled, while flying older models of C172's, that there had been times when I had been flying in wet clouds that the pitot static instruments would occasionally jump. I attributed that to the static port being covered with water. I probably was wrong. It probably was an ingestion of water into the static line. I am not a mechanic and I'm only guessing based on my observations. We flew back with the alternate static port open and the pitot heat on. The instruments operated correctly.

Original NASA ASRS Text

Title: PLT TRAINING CREW OF A C172, LOST CORRECT INDICATIONS FROM THE AIRSPD, VERT SPD AND ALTIMETER CAUSING DSCNT BELOW ASSIGNED ALT DUE TO PITOT STATIC TUBE ICING. ALL INDICATIONS CAME BACK CORRECTLY WITHIN A COUPLE OF MINS AFTER THE PITOT TUBE HEAT WAS APPLIED.

Narrative: REGARDING: ALT EXCURSION BELOW ASSIGNED ALT. CONDITIONS OF FLT: 1 HR OF CONSTANT FLT IN VERY WET CLOUDS AND INTERMITTENT RAIN. OUTSIDE AIR TEMP APPROX 10 DEGS C. STUDENT IFR INSTRUCTIONAL FLT. TYPE: C172 1997 MODEL. DSNDING FROM 6000-5000 FT VECTORS TO JOIN THE LOC FOR AN ILS APCH. RPTED WX WAS ABOUT 300 FT ABOVE DECISION ALT AND 3 MI. ACTUAL VISIBILITY TURNED OUT TO BE ABOUT 1-1/2 MI. DURING THE DSCNT WE EXPERIENCED COMPLETE FAILURE OF THE PITOT STATIC SYS. THE AIRSPD INDICATOR WAS INDICATING ABOVE RED LINE. THE ALTIMETER WAS FROZEN AT 5600 FT. THE VERT SPD INDICATOR WAS RETURNING TO A LEVEL FLT ATTITUDE. WE IMMEDIATELY PULLED THE ALTERNATE AIR CTL AND PUT ON PITOT HEAT EVEN THOUGH THE OUTSIDE AIR TEMP WAS AT 10 DEGS C. IT HAD NO IMMEDIATE EFFECT ON THE PITOT STATIC FAILURE. AS WE STARTED TO TROUBLESHOOT THE CONDITION AND WE LEVELED THE ACFT USING THE ATTITUDE INDICATOR AND NOTIFIED ATC THAT WE HAD A MAJOR PROB. WE WERE NOW BELOW 5000 FT. THE CTLR THEN CLRED US TO 4100 FT. WE WERE ABLE TO APPROXIMATE OUR ALT USING THE VISUAL READING OF PRESSURE ALT SHOWN ON THE XPONDER (BENDIX KING). AFTER ABOUT 2 MINS ALL INSTS RETURNED TO NORMAL. ATC CONFIRMED OUR ALT AT 4100 FT. WHAT PROBABLY HAPPENED WAS THAT THE STATIC LINE HAD FILLED WITH WATER. AFTER WE OPENED THE ALTERNATE STATIC PORT THE WATER PROBABLY STARTED TO DRAIN WITH THE HELP OF THE CHANGING PITCH OF THE ACFT. AFTER THE FLT WE NOTICED THAT THE CARPET ON THE FLOOR WAS SOAKING WET. AFTER THIS INCIDENT I RECALLED, WHILE FLYING OLDER MODELS OF C172'S, THAT THERE HAD BEEN TIMES WHEN I HAD BEEN FLYING IN WET CLOUDS THAT THE PITOT STATIC INSTS WOULD OCCASIONALLY JUMP. I ATTRIBUTED THAT TO THE STATIC PORT BEING COVERED WITH WATER. I PROBABLY WAS WRONG. IT PROBABLY WAS AN INGESTION OF WATER INTO THE STATIC LINE. I AM NOT A MECH AND I'M ONLY GUESSING BASED ON MY OBSERVATIONS. WE FLEW BACK WITH THE ALTERNATE STATIC PORT OPEN AND THE PITOT HEAT ON. THE INSTS OPERATED CORRECTLY.

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 training crew of a C172, lost correct indications from the airspeed, vertical speed and altimeter causing descent below assigned altitude due to pitot static tube icing. All indications came back correctly within a couple of mins after the pitot tube heat was applied.