Difference in captain altimeter and first officer altimeter corrected several times during flight. W...

Attributes
ACN	111527
Time
Date	198905
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ztl
State Reference	GA
Altitude	msl bound lower : 25000 msl bound upper : 25800
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl
Operator	other
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : military
Function	flight crew : first officer
Qualification	pilot : military
Experience	flight time last 90 days : 20 flight time total : 3100
ASRS Report	111527
Person 2
Affiliation	government : military
Function	flight crew : captain oversight : pic
Qualification	pilot : military
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera other flight crewa
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

On second leg of day mission, after 1 hour at assigned FL250 with autoplt coupled on altitude hold, pilot corrected gradual 150' loss of indicated altitude. After 15 mins pilot again corrected 150' loss of ind altitude--comparison with copilot instruments showed pilot altitude at FL250, copilot altitude at FL253. Copilot queried center which verified mode C as FL250. 10 mins later another correction resulted in pilot altitude showing FL250 while copilot altitude showed FL255. Center verified mode C FL250. Pilot then noted gradual 25 KT decrease in IAS to 150 KIAS; copilot airspeed indicator remained steady at 175 KIAS. Switch to alternate static source caused pilot's instruments to jump to 180 KIAS and FL258. Center immediately called and said mode C showed us at FL258 and directed us to end mode C transmission. We did, reported probable static system malfunction, descended to FL250 using alternate static source and continue west/O further incident. Crew then discussed first leg of flight when 2 similar autoplt altitude corrections resulted in pilot's altitude showing FL260 while copilot's altitude showed FL263--and crew had, on 2 separate occasions, sighted FL270 traffic and remarked how they each seemed much closer in altitude than 1000' apart.

Original NASA ASRS Text

Title: DIFFERENCE IN CAPT ALTIMETER AND FO ALTIMETER CORRECTED SEVERAL TIMES DURING FLT. WHEN ALTERNATE SOURCE OF STATIC WAS SELECTED, 800' ERROR IN ALT WAS REPORTED BY ATC BY CENTER. USING ALTERNATE SOURCE OF STATIC, FLT CONTINUED.

Narrative: ON SECOND LEG OF DAY MISSION, AFTER 1 HR AT ASSIGNED FL250 WITH AUTOPLT COUPLED ON ALT HOLD, PLT CORRECTED GRADUAL 150' LOSS OF INDICATED ALT. AFTER 15 MINS PLT AGAIN CORRECTED 150' LOSS OF IND ALT--COMPARISON WITH COPLT INSTRUMENTS SHOWED PLT ALT AT FL250, COPLT ALT AT FL253. COPLT QUERIED CENTER WHICH VERIFIED MODE C AS FL250. 10 MINS LATER ANOTHER CORRECTION RESULTED IN PLT ALT SHOWING FL250 WHILE COPLT ALT SHOWED FL255. CENTER VERIFIED MODE C FL250. PLT THEN NOTED GRADUAL 25 KT DECREASE IN IAS TO 150 KIAS; COPLT AIRSPD INDICATOR REMAINED STEADY AT 175 KIAS. SWITCH TO ALTERNATE STATIC SOURCE CAUSED PLT'S INSTRUMENTS TO JUMP TO 180 KIAS AND FL258. CENTER IMMEDIATELY CALLED AND SAID MODE C SHOWED US AT FL258 AND DIRECTED US TO END MODE C XMISSION. WE DID, RPTED PROBABLE STATIC SYSTEM MALFUNCTION, DSNDED TO FL250 USING ALTERNATE STATIC SOURCE AND CONTINUE W/O FURTHER INCIDENT. CREW THEN DISCUSSED FIRST LEG OF FLT WHEN 2 SIMILAR AUTOPLT ALT CORRECTIONS RESULTED IN PLT'S ALT SHOWING FL260 WHILE COPLT'S ALT SHOWED FL263--AND CREW HAD, ON 2 SEPARATE OCCASIONS, SIGHTED FL270 TFC AND REMARKED HOW THEY EACH SEEMED MUCH CLOSER IN ALT THAN 1000' APART.

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.

Difference in captain altimeter and first officer altimeter corrected several times during flight. When alternate source of static was selected, 800' error in altitude was reported by ATC by center. Using alternate source of static, flight continued.