Apparent deviation from altitude. In-flight variations noted on static instruments.

Attributes
ACN	123539
Time
Date	198909
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ave
State Reference	CA
Altitude	msl bound lower : 250 msl bound upper : 9400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zoa
Operator	general aviation : corporate
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : atp
Experience	flight time last 90 days : 120 flight time total : 17500 flight time type : 30
ASRS Report	123539
Person 2
Affiliation	government : faa
Function	controller : local
Qualification	controller : non radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	atc equipment other atc equipment : unspecified
Resolutory Action	none taken : detected after the fact
Consequence	faa : reviewed incident with flight crew Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

En route from yuma to monterey, the aircraft was occasionally difficult to fly precise altitude with the autoplt engaged--altitude would vary +/-200'. Xchk of altitude, heading, vertical velocity and altitude were indicative of wave action. Since a large frontal system had just passed through the flight area and the area was mountainous, I assumed it was wave action and reported it as such. Upon landing at mry, the tower said that maybe I should have my transponder checked as they were reading my altitude as 800' above the runway on T/D. I confirmed this as I watched my altimeter descend from 1000' to 250' during taxi. The altimeter and vertical speed were both sluggish. After analyzing the problem, I concluded that my normal static system was obstructed. I switched to alternate static system, filed a flight plan to vny to effect repair and departed. On climb out I leveled at 9000' assigned, but momentarily watched my altitude continue to increase to 9300', even though I was holding 80 degree nose down attitude and airspeed increased 30 KTS trying to maintain 9000'. Center asked me to check altitude, and I reported 9300' and he said he had 9400'. I slowly returned to 9000' because of the sluggish static instruments. On the VMC descent into vny, the altimeter and vertical speed continued to respond at a rate much less than actual performance. Investigation revealed a pinched static line downstream of both the airspeed indicator and the alternate static source valve. This resulted in sluggish operation of all static instruments, except the airspeed, on both normal and alternate static sources. Repair was quickly made and system static checked ok. The assumption that proper operation of the static system would be restored by selection of alternate static source was in error. I should have been more suspicious of the wave action I thought I was experiencing when center had no other reports and I was not experiencing more turbulence.

Original NASA ASRS Text

Title: APPARENT DEVIATION FROM ALT. INFLT VARIATIONS NOTED ON STATIC INSTRUMENTS.

Narrative: ENRTE FROM YUMA TO MONTEREY, THE ACFT WAS OCCASIONALLY DIFFICULT TO FLY PRECISE ALT WITH THE AUTOPLT ENGAGED--ALT WOULD VARY +/-200'. XCHK OF ALT, HDG, VERT VELOCITY AND ALT WERE INDICATIVE OF WAVE ACTION. SINCE A LARGE FRONTAL SYS HAD JUST PASSED THROUGH THE FLT AREA AND THE AREA WAS MOUNTAINOUS, I ASSUMED IT WAS WAVE ACTION AND RPTED IT AS SUCH. UPON LNDG AT MRY, THE TWR SAID THAT MAYBE I SHOULD HAVE MY XPONDER CHKED AS THEY WERE READING MY ALT AS 800' ABOVE THE RWY ON T/D. I CONFIRMED THIS AS I WATCHED MY ALTIMETER DSND FROM 1000' TO 250' DURING TAXI. THE ALTIMETER AND VERT SPD WERE BOTH SLUGGISH. AFTER ANALYZING THE PROB, I CONCLUDED THAT MY NORMAL STATIC SYS WAS OBSTRUCTED. I SWITCHED TO ALTERNATE STATIC SYS, FILED A FLT PLAN TO VNY TO EFFECT REPAIR AND DEPARTED. ON CLBOUT I LEVELED AT 9000' ASSIGNED, BUT MOMENTARILY WATCHED MY ALT CONTINUE TO INCREASE TO 9300', EVEN THOUGH I WAS HOLDING 80 DEG NOSE DOWN ATTITUDE AND AIRSPD INCREASED 30 KTS TRYING TO MAINTAIN 9000'. CENTER ASKED ME TO CHK ALT, AND I RPTED 9300' AND HE SAID HE HAD 9400'. I SLOWLY RETURNED TO 9000' BECAUSE OF THE SLUGGISH STATIC INSTRUMENTS. ON THE VMC DSCNT INTO VNY, THE ALTIMETER AND VERT SPD CONTINUED TO RESPOND AT A RATE MUCH LESS THAN ACTUAL PERFORMANCE. INVESTIGATION REVEALED A PINCHED STATIC LINE DOWNSTREAM OF BOTH THE AIRSPD INDICATOR AND THE ALTERNATE STATIC SOURCE VALVE. THIS RESULTED IN SLUGGISH OPERATION OF ALL STATIC INSTRUMENTS, EXCEPT THE AIRSPD, ON BOTH NORMAL AND ALTERNATE STATIC SOURCES. REPAIR WAS QUICKLY MADE AND SYS STATIC CHKED OK. THE ASSUMPTION THAT PROPER OPERATION OF THE STATIC SYS WOULD BE RESTORED BY SELECTION OF ALTERNATE STATIC SOURCE WAS IN ERROR. I SHOULD HAVE BEEN MORE SUSPICIOUS OF THE WAVE ACTION I THOUGHT I WAS EXPERIENCING WHEN CENTER HAD NO OTHER RPTS AND I WAS NOT EXPERIENCING MORE TURB.

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.