SA227 flight crew experienced slow pressurization leak climbing out of bil.

Attributes
ACN	437027
Time
Date	199905
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : bil.airport
State Reference	MT
Altitude	msl bound lower : 9300 msl bound upper : 20700
Environment
Flight Conditions	Mixed
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zlc.artcc
Operator	common carrier : air carrier
Make Model Name	SA-227 AC Metro III
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : atp pilot : commercial pilot : multi engine
Experience	flight time last 90 days : 750 flight time total : 3100 flight time type : 180
ASRS Report	437027
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical altitude deviation : overshoot non adherence : clearance non adherence : far
Independent Detector	aircraft equipment other aircraft equipment : cabin alt warning other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance flight crew : landed as precaution flight crew : exited adverse environment flight crew : diverted to another airport
Consequence	other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

I was the PF and the first officer on an SA227 for a commuter airline. We were en route to wolf point, mt, cleared to FL210, still climbing. 60 mi northeast of the bil VOR, the cabin altitude annunciator light came on climbing through FL207. The captain requested a return to bil and descent to 10000 ft. ATC approved. I initiated a rapid descent although not of the emergency variety. We still had cabin pressure -- it just had started climbing through 10000 ft which sets off the annunciator light. We had a slow leak. We covered the action items for a cabin altitude annunciator light and each performed various tasks during the descent. The captain was busy reading the checklist and handling communications. My descent rate and momentary inattn caused me to fly through 10000 ft assigned, to 9300 ft. We immediately climbed back to 10000 ft, but not before ATC queried us on our altitude. By that time we were going through 9500 ft to 10000 ft and reported such. ZLC said 'roger and handed us off shortly after to bil approach. We initiated the ILS to runway 28R and landed without further incident.

Original NASA ASRS Text

Title: SA227 FLC EXPERIENCED SLOW PRESSURIZATION LEAK CLBING OUT OF BIL.

Narrative: I WAS THE PF AND THE FO ON AN SA227 FOR A COMMUTER AIRLINE. WE WERE ENRTE TO WOLF POINT, MT, CLRED TO FL210, STILL CLBING. 60 MI NE OF THE BIL VOR, THE CABIN ALT ANNUNCIATOR LIGHT CAME ON CLBING THROUGH FL207. THE CAPT REQUESTED A RETURN TO BIL AND DSCNT TO 10000 FT. ATC APPROVED. I INITIATED A RAPID DSCNT ALTHOUGH NOT OF THE EMER VARIETY. WE STILL HAD CABIN PRESSURE -- IT JUST HAD STARTED CLBING THROUGH 10000 FT WHICH SETS OFF THE ANNUNCIATOR LIGHT. WE HAD A SLOW LEAK. WE COVERED THE ACTION ITEMS FOR A CABIN ALT ANNUNCIATOR LIGHT AND EACH PERFORMED VARIOUS TASKS DURING THE DSCNT. THE CAPT WAS BUSY READING THE CHKLIST AND HANDLING COMS. MY DSCNT RATE AND MOMENTARY INATTN CAUSED ME TO FLY THROUGH 10000 FT ASSIGNED, TO 9300 FT. WE IMMEDIATELY CLBED BACK TO 10000 FT, BUT NOT BEFORE ATC QUERIED US ON OUR ALT. BY THAT TIME WE WERE GOING THROUGH 9500 FT TO 10000 FT AND RPTED SUCH. ZLC SAID 'ROGER AND HANDED US OFF SHORTLY AFTER TO BIL APCH. WE INITIATED THE ILS TO RWY 28R AND LANDED WITHOUT FURTHER INCIDENT.

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.