Narrative:

While flying a 1999 cessna 206, I noticed that the altimeter needle fluctuated +/- 800 ft in rapid fashion. This, however, occurred only while climbing. At straight and level and descents, the needle was steady. I had a theory that spiralling slipstream during climbs was affecting the air going into the static ports, so I decided to test my theory, and here is where I believe I violated far 91.205. I taped the static ports and used the alternate static supply to see if the altimeter needle still fluctuated. I climbed in VFR conditions and discovered that the needle was stable. I proved my theory. I have yet to contact cessna to see if they are getting reports from other 1999 C206 owners about this problem with fluctuating altimeter readings. If they (cessna) are getting additional complaints, I suspect that an ad may be required. Callback conversation with reporter revealed the following information: the reporter stated the altimeter problem of fluctuating in climb was causing some consternation with maintenance on the cause and fix. The reporter said the manufacturer was contacted and sent a service representative who advised balancing the propeller. The reporter said the propeller was balanced and the fluctuations disappeared. The reporter stated no propeller vibrations were felt, or written up in the flight log during the time the altimeter was fluctuating.

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Original NASA ASRS Text

Title: A CESSNA 206 H IN CLB ALTIMETER FLUCTUATED 800 FT UP AND DOWN, OK LEVEL, NOSE DOWN AND ALTERNATE STATIC SYS. CAUSED BY AN UNBALANCED PROP.

Narrative: WHILE FLYING A 1999 CESSNA 206, I NOTICED THAT THE ALTIMETER NEEDLE FLUCTUATED +/- 800 FT IN RAPID FASHION. THIS, HOWEVER, OCCURRED ONLY WHILE CLBING. AT STRAIGHT AND LEVEL AND DESCENTS, THE NEEDLE WAS STEADY. I HAD A THEORY THAT SPIRALLING SLIPSTREAM DURING CLBS WAS AFFECTING THE AIR GOING INTO THE STATIC PORTS, SO I DECIDED TO TEST MY THEORY, AND HERE IS WHERE I BELIEVE I VIOLATED FAR 91.205. I TAPED THE STATIC PORTS AND USED THE ALTERNATE STATIC SUPPLY TO SEE IF THE ALTIMETER NEEDLE STILL FLUCTUATED. I CLBED IN VFR CONDITIONS AND DISCOVERED THAT THE NEEDLE WAS STABLE. I PROVED MY THEORY. I HAVE YET TO CONTACT CESSNA TO SEE IF THEY ARE GETTING RPTS FROM OTHER 1999 C206 OWNERS ABOUT THIS PROB WITH FLUCTUATING ALTIMETER READINGS. IF THEY (CESSNA) ARE GETTING ADDITIONAL COMPLAINTS, I SUSPECT THAT AN AD MAY BE REQUIRED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ALTIMETER PROB OF FLUCTUATING IN CLB WAS CAUSING SOME CONSTERNATION WITH MAINT ON THE CAUSE AND FIX. THE RPTR SAID THE MANUFACTURER WAS CONTACTED AND SENT A SVC REPRESENTATIVE WHO ADVISED BALANCING THE PROP. THE RPTR SAID THE PROP WAS BALANCED AND THE FLUCTUATIONS DISAPPEARED. THE RPTR STATED NO PROP VIBRATIONS WERE FELT, OR WRITTEN UP IN THE FLT LOG DURING THE TIME THE ALTIMETER WAS FLUCTUATING.

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.