A beech C90 in climb at FL200 noted 25 KT and 250 ft difference between #1 and #2 system. Returned to departure field. Found static drain valve partially open.

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Attributes
ACN	522497
Time
Date	200108
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl bound lower : 20000 msl bound upper : 25000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zse.artcc
Operator	general aviation : corporate
Make Model Name	King Air C90 E90
Operating Under FAR Part	Part 91
Flight Phase	climbout : vacating altitude cruise : enroute altitude change
Route In Use	arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 200 flight time total : 17000 flight time type : 750
ASRS Report	522497
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Qualification	pilot : instrument pilot : multi engine pilot : commercial
Events
Anomaly	aircraft equipment problem : critical maintenance problem : improper documentation maintenance problem : improper maintenance non adherence : published procedure non adherence : far other anomaly
Independent Detector	aircraft equipment other aircraft equipment : fo airspeed and altimeter other flight crewa
Resolutory Action	aircraft : equipment problem dissipated flight crew : landed as precaution flight crew : diverted to another airport
Consequence	other
Factors
Maintenance	contributing factor : schedule pressure performance deficiency : testing performance deficiency : repair performance deficiency : fault isolation
Supplementary
Problem Areas	Aircraft Maintenance Human Performance
Primary Problem	Maintenance Human Performance

Narrative:

Aircraft was out of maintenance check/avionics modification. It was test flown by pilots from the FBO and returned to service. On takeoff, a slight air noise was noted to the right of the pilot, but I thought it might be wind noise from new antennas installed during modification. During climb to FL250, aircraft was not performing up to PAR. Rate of climb was low and I was unable to maintain normal climb indicated airspeed. Engine parameters were normal. As a scanned the instruments on the r-hand panel, a split of 25 KIAS and an altitude difference of 500 ft was noted. I leveled off and received clearance to return to the departure airport. As I descended, the difference decreased 5 KTS and 100 ft/5000 ft of altitude and became 0 difference at sea level. Maintenance found a static drain valve had opened slightly. The aircraft passed a pitot static test the day before.

Original NASA ASRS Text

Title: A BEECH C90 IN CLB AT FL200 NOTED 25 KT AND 250 FT DIFFERENCE BTWN #1 AND #2 SYS. RETURNED TO DEP FIELD. FOUND STATIC DRAIN VALVE PARTIALLY OPEN.

Narrative: ACFT WAS OUT OF MAINT CHK/AVIONICS MODIFICATION. IT WAS TEST FLOWN BY PLTS FROM THE FBO AND RETURNED TO SVC. ON TKOF, A SLIGHT AIR NOISE WAS NOTED TO THE R OF THE PLT, BUT I THOUGHT IT MIGHT BE WIND NOISE FROM NEW ANTENNAS INSTALLED DURING MODIFICATION. DURING CLB TO FL250, ACFT WAS NOT PERFORMING UP TO PAR. RATE OF CLB WAS LOW AND I WAS UNABLE TO MAINTAIN NORMAL CLB INDICATED AIRSPD. ENG PARAMETERS WERE NORMAL. AS A SCANNED THE INSTS ON THE R-HAND PANEL, A SPLIT OF 25 KIAS AND AN ALT DIFFERENCE OF 500 FT WAS NOTED. I LEVELED OFF AND RECEIVED CLRNC TO RETURN TO THE DEP ARPT. AS I DSNDED, THE DIFFERENCE DECREASED 5 KTS AND 100 FT/5000 FT OF ALT AND BECAME 0 DIFFERENCE AT SEA LEVEL. MAINT FOUND A STATIC DRAIN VALVE HAD OPENED SLIGHTLY. THE ACFT PASSED A PITOT STATIC TEST THE DAY BEFORE.

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.