AC90 pilot descends from assigned altitude due to equipment malfunction while performing air ambulan...

Attributes
ACN	514148
Time
Date	200106
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : zse.artcc
State Reference	WA
Altitude	msl bound lower : 6000 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zse.artcc tower : sea.tower
Operator	common carrier : air taxi
Make Model Name	Turbo Commander 690C
Operating Under FAR Part	Part 135
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 60 flight time total : 9600 flight time type : 60
ASRS Report	514148
Person 2
Affiliation	other
Function	observation : passenger
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	controller : issued advisory controller : issued new clearance flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft Passenger Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While in cruise at 12000 ft on a direct course to hio, our pressurization was not maintaining cabin altitude of sea level as desired. We had a small child on board that was in need of sea level cabin altitude due to respiratory problems. The pressurization briefly 'bumped' in excess of 2000 FPM climb. The medical staff asked us to get a sea level cabin as quickly as possible. Copilot requested lower. However, due to opposite direction lower traffic, lower was not approved. Meanwhile, the autoplt inadvertently was turned off, causing us to descend to 11600 ft before we leveled off. While this happened, we were given a turn to the west and I requested a VFR on top descent and it was approved. We then descended VFR to 9500 ft just above the cloud layer. Then we received a turn back on course and an assigned altitude of 8000 ft. We then got lower to 6000 ft where the pressurization was satisfactory for the patient. I don't believe that any warnings for aircraft proximity were sounded with ZSE as the controller did not mention the altitude deviation.

Original NASA ASRS Text

Title: AC90 PLT DSNDS FROM ASSIGNED ALT DUE TO EQUIP MALFUNCTION WHILE PERFORMING AIR AMBULANCE SVC.

Narrative: WHILE IN CRUISE AT 12000 FT ON A DIRECT COURSE TO HIO, OUR PRESSURIZATION WAS NOT MAINTAINING CABIN ALT OF SEA LEVEL AS DESIRED. WE HAD A SMALL CHILD ON BOARD THAT WAS IN NEED OF SEA LEVEL CABIN ALT DUE TO RESPIRATORY PROBS. THE PRESSURIZATION BRIEFLY 'BUMPED' IN EXCESS OF 2000 FPM CLB. THE MEDICAL STAFF ASKED US TO GET A SEA LEVEL CABIN AS QUICKLY AS POSSIBLE. COPLT REQUESTED LOWER. HOWEVER, DUE TO OPPOSITE DIRECTION LOWER TFC, LOWER WAS NOT APPROVED. MEANWHILE, THE AUTOPLT INADVERTENTLY WAS TURNED OFF, CAUSING US TO DSND TO 11600 FT BEFORE WE LEVELED OFF. WHILE THIS HAPPENED, WE WERE GIVEN A TURN TO THE W AND I REQUESTED A VFR ON TOP DSCNT AND IT WAS APPROVED. WE THEN DSNDED VFR TO 9500 FT JUST ABOVE THE CLOUD LAYER. THEN WE RECEIVED A TURN BACK ON COURSE AND AN ASSIGNED ALT OF 8000 FT. WE THEN GOT LOWER TO 6000 FT WHERE THE PRESSURIZATION WAS SATISFACTORY FOR THE PATIENT. I DON'T BELIEVE THAT ANY WARNINGS FOR ACFT PROX WERE SOUNDED WITH ZSE AS THE CTLR DID NOT MENTION THE ALTDEV.

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.

AC90 pilot descends from assigned altitude due to equipment malfunction while performing air ambulance service.