Air carrier medium large transport altitude deviation excursion from clearance altitude in response to GPWS terrain warning. Aircraft was being vectored back to the STAR after WX deviation.

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Attributes
ACN	159794
Time
Date	199010
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : pdx
State Reference	OR
Altitude	msl bound lower : 5500 msl bound upper : 5500
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : pdx
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : atp
Experience	flight time last 90 days : 220 flight time total : 10000 flight time type : 1150
ASRS Report	159794
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter other non adherence : clearance other spatial deviation
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued new clearance
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

On vector of 230 degrees at 5500' MSL descending with pdx approach control on pdx 095 degree right 23 DME in IMC conditions the mode 2 GPWS warning occurred for 2 cycles. We immediately initiated climb to 8000' and notified RAPCON. No further incidents to landing. Note: we had deviated for WX to the north of the bonyl 1 STAR and we had cleared the WX. We were being vectored now by ATC for arrival to the airport. Higher terrain is located just south of the bonyl 1. We were northeast of mt hood. After landing we phoned ATC (pdx RAPCON) and they became very defensive assuring us that their minimum vector altitudes had not been violated. They advised us they were filing a TCAS report on the incident. They seemed to not understand the problem which was a GPWS (ground proximity) warning. How did TCAS even enter the picture? Maybe it would help to provide ATC with more information on the GPWS--how it works, and what responses flight crews must make when in IMC conditions it goes off. It was not a case of obvious false warning so we could not just disregard it, yet ATC was offended! We're alive!

Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION EXCURSION FROM CLRNC ALT IN RESPONSE TO GPWS TERRAIN WARNING. ACFT WAS BEING VECTORED BACK TO THE STAR AFTER WX DEVIATION.

Narrative: ON VECTOR OF 230 DEGS AT 5500' MSL DSNDING WITH PDX APCH CTL ON PDX 095 DEG R 23 DME IN IMC CONDITIONS THE MODE 2 GPWS WARNING OCCURRED FOR 2 CYCLES. WE IMMEDIATELY INITIATED CLB TO 8000' AND NOTIFIED RAPCON. NO FURTHER INCIDENTS TO LNDG. NOTE: WE HAD DEVIATED FOR WX TO THE N OF THE BONYL 1 STAR AND WE HAD CLRED THE WX. WE WERE BEING VECTORED NOW BY ATC FOR ARR TO THE ARPT. HIGHER TERRAIN IS LOCATED JUST S OF THE BONYL 1. WE WERE NE OF MT HOOD. AFTER LNDG WE PHONED ATC (PDX RAPCON) AND THEY BECAME VERY DEFENSIVE ASSURING US THAT THEIR MINIMUM VECTOR ALTS HAD NOT BEEN VIOLATED. THEY ADVISED US THEY WERE FILING A TCAS RPT ON THE INCIDENT. THEY SEEMED TO NOT UNDERSTAND THE PROB WHICH WAS A GPWS (GND PROX) WARNING. HOW DID TCAS EVEN ENTER THE PICTURE? MAYBE IT WOULD HELP TO PROVIDE ATC WITH MORE INFO ON THE GPWS--HOW IT WORKS, AND WHAT RESPONSES FLT CREWS MUST MAKE WHEN IN IMC CONDITIONS IT GOES OFF. IT WAS NOT A CASE OF OBVIOUS FALSE WARNING SO WE COULD NOT JUST DISREGARD IT, YET ATC WAS OFFENDED! WE'RE ALIVE!

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.