|37000 Feet||Browse and search NASA's
Aviation Safety Reporting System
|Local Time Of Day||1201 To 1800|
|Locale Reference||atc facility : ptk|
|Altitude||msl bound lower : 4000|
msl bound upper : 4500
|Controlling Facilities||tracon : ptk|
tower : chs
|Operator||general aviation : instructional|
|Make Model Name||Small Aircraft, Low Wing, 1 Eng, Retractable Gear|
|Flight Phase||cruise other|
|Function||instruction : instructor|
|Qualification||pilot : cfi|
pilot : atp
|Experience||flight time last 90 days : 120|
flight time total : 6000
flight time type : 500
|Function||instruction : trainee|
|Qualification||pilot : student|
|Anomaly||aircraft equipment problem : critical|
altitude deviation : excursion from assigned altitude
|Independent Detector||other controllera|
other flight crewa
|Resolutory Action||aircraft : equipment problem dissipated|
flight crew : returned to intended course or assigned course
|Air Traffic Incident||Pilot Deviation|
On an IFR cross country instructional flight from lan to ptk we were flying level at 4000', as assigned by detroit approach control. We were intermittently in IMC with broken clouds and snow showers. Very suddenly the altimeter and vertical speed indicator began to fluctuate erratically. The altimeter first dropped to 3500', then returned to 4000', where it stabilized for approximately 1 min. It then jumped very quickly to 4500'. At this time the approach controller inquired about our altitude. We advised that our altimeter was behaving erratically and that it read 4500'. He confirmed that our mode C read 4500'. We immediately descended to 4000' and the controller confirmed this altitude. We then completed the localizer backcourse approach to ptk west/O incident, mostly in VMC. The airplane was then inspected for anomalies in the static system. None were found. It is my opinion that moisture from the front of the heated pitot mast was able to flow around and block the static source, which is located on the rear side of the mast on many small aircraft airplanes.
Original NASA ASRS Text
Title: GA SMA ACFT EQUIPMENT PROBLEM SUSPECT BLOCKED STATIC SOURCE THAT RESULTED IN ALT DEVIATION EXCURSION FROM CLRNC ALT.
Narrative: ON AN IFR CROSS COUNTRY INSTRUCTIONAL FLT FROM LAN TO PTK WE WERE FLYING LEVEL AT 4000', AS ASSIGNED BY DETROIT APCH CTL. WE WERE INTERMITTENTLY IN IMC WITH BROKEN CLOUDS AND SNOW SHOWERS. VERY SUDDENLY THE ALTIMETER AND VERT SPD INDICATOR BEGAN TO FLUCTUATE ERRATICALLY. THE ALTIMETER FIRST DROPPED TO 3500', THEN RETURNED TO 4000', WHERE IT STABILIZED FOR APPROX 1 MIN. IT THEN JUMPED VERY QUICKLY TO 4500'. AT THIS TIME THE APCH CTLR INQUIRED ABOUT OUR ALT. WE ADVISED THAT OUR ALTIMETER WAS BEHAVING ERRATICALLY AND THAT IT READ 4500'. HE CONFIRMED THAT OUR MODE C READ 4500'. WE IMMEDIATELY DSNDED TO 4000' AND THE CTLR CONFIRMED THIS ALT. WE THEN COMPLETED THE LOC BACKCOURSE APCH TO PTK W/O INCIDENT, MOSTLY IN VMC. THE AIRPLANE WAS THEN INSPECTED FOR ANOMALIES IN THE STATIC SYSTEM. NONE WERE FOUND. IT IS MY OPINION THAT MOISTURE FROM THE FRONT OF THE HEATED PITOT MAST WAS ABLE TO FLOW AROUND AND BLOCK THE STATIC SOURCE, WHICH IS LOCATED ON THE REAR SIDE OF THE MAST ON MANY SMA AIRPLANES.
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.