Flight crew of commuter aircraft experiences intermittent warning horn in level flight. During attempts to determine the cause of the warning, the aircraft is allowed to deviate from assigned altitude. A faulty flap selector is found to be the cause of the warning horn.

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Attributes
ACN	120666
Time
Date	198908
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : mkc
State Reference	MO
Altitude	msl bound lower : 8000 msl bound upper : 8500
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : mci
Operator	common carrier : air taxi
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : first officer
Qualification	pilot : cfi pilot : commercial
Experience	flight time last 90 days : 240 flight time total : 2520 flight time type : 310
ASRS Report	120666
Person 2
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

ATC issued clearance to climb and maintain 8000'. Shortly after leveling at 8000' an intermittent warning horn went off. Both the captain and I were unsure what was causing this alarm. After attempting to disable the stall warning system I noticed our altitude climbing through 8300'. The captain queried as to possible trim runway (similar warning horn). At the same time the captain noticed the flap selector in the full down position. I was unable to maintain the assigned altitude because of this. Upon consultation with the captain, factors contributing to the altitude excursion were: 1) worn flap position gate allowed selector to slip to full down position; 2) similar sounding warning horns for both sas, trim and flap/landing gear confign make it difficult to distinguish between them. Recommendations: 1) redesign flap selector to require a separate position for 1/4, 1/2, 3/4 and full flaps. 2) make warning horns more unique. 3) a warning light along with the warning horn for landing gear confign.

Original NASA ASRS Text

Title: FLT CREW OF COMMUTER ACFT EXPERIENCES INTERMITTENT WARNING HORN IN LEVEL FLT. DURING ATTEMPTS TO DETERMINE THE CAUSE OF THE WARNING, THE ACFT IS ALLOWED TO DEVIATE FROM ASSIGNED ALT. A FAULTY FLAP SELECTOR IS FOUND TO BE THE CAUSE OF THE WARNING HORN.

Narrative: ATC ISSUED CLRNC TO CLB AND MAINTAIN 8000'. SHORTLY AFTER LEVELING AT 8000' AN INTERMITTENT WARNING HORN WENT OFF. BOTH THE CAPT AND I WERE UNSURE WHAT WAS CAUSING THIS ALARM. AFTER ATTEMPTING TO DISABLE THE STALL WARNING SYS I NOTICED OUR ALT CLBING THROUGH 8300'. THE CAPT QUERIED AS TO POSSIBLE TRIM RWY (SIMILAR WARNING HORN). AT THE SAME TIME THE CAPT NOTICED THE FLAP SELECTOR IN THE FULL DOWN POS. I WAS UNABLE TO MAINTAIN THE ASSIGNED ALT BECAUSE OF THIS. UPON CONSULTATION WITH THE CAPT, FACTORS CONTRIBUTING TO THE ALT EXCURSION WERE: 1) WORN FLAP POS GATE ALLOWED SELECTOR TO SLIP TO FULL DOWN POS; 2) SIMILAR SOUNDING WARNING HORNS FOR BOTH SAS, TRIM AND FLAP/LNDG GEAR CONFIGN MAKE IT DIFFICULT TO DISTINGUISH BTWN THEM. RECOMMENDATIONS: 1) REDESIGN FLAP SELECTOR TO REQUIRE A SEPARATE POS FOR 1/4, 1/2, 3/4 AND FULL FLAPS. 2) MAKE WARNING HORNS MORE UNIQUE. 3) A WARNING LIGHT ALONG WITH THE WARNING HORN FOR LNDG GEAR CONFIGN.

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.