An medium large transport crew responded to a TCASII descend command. The only problem -- the intruder traffic was below the reporting air carrier aircraft.

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Attributes
ACN	220779
Time
Date	199209
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : pit
State Reference	PA
Altitude	msl bound lower : 2800 msl bound upper : 3200
Environment
Flight Conditions	Mixed
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : pit
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent : approach
Route In Use	approach : straight in arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 240 flight time total : 7000 flight time type : 3000
ASRS Report	220779
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude conflict : airborne less severe
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : took evasive action flight crew : returned to intended course or assigned course other
Consequence	Other
Miss Distance	vertical : 700
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

On approach at pit on 28L ILS with no GS at 3000 ft about 10 DME. Approach called out traffic at 2500 ft. Shortly after, the TCASII warning sounded and then said 'descend, descend, crossing descend.' the captain started a descent and about that time we realized that the traffic was below us and the captain started a climb. The descent had only gone to 3800 and climb to 3200. The cause for this deviation was that the TCASII gave us faulty commands.

Original NASA ASRS Text

Title: AN MLG CREW RESPONDED TO A TCASII DSND COMMAND. THE ONLY PROBLEM -- THE INTRUDER TFC WAS BELOW THE RPTING ACR ACFT.

Narrative: ON APCH AT PIT ON 28L ILS WITH NO GS AT 3000 FT ABOUT 10 DME. APCH CALLED OUT TFC AT 2500 FT. SHORTLY AFTER, THE TCASII WARNING SOUNDED AND THEN SAID 'DSND, DSND, XING DSND.' THE CAPT STARTED A DSCNT AND ABOUT THAT TIME WE REALIZED THAT THE TFC WAS BELOW US AND THE CAPT STARTED A CLB. THE DSCNT HAD ONLY GONE TO 3800 AND CLB TO 3200. THE CAUSE FOR THIS DEV WAS THAT THE TCASII GAVE US FAULTY COMMANDS.

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.