A B737 flight crew receives an unclr clearance which almost leads to an altitude deviation.

Attributes
ACN	599855
Time
Date	200311
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : oak.vor
State Reference	CA
Altitude	msl single value : 7000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sct.tracon
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : approach descent : intermediate altitude
Route In Use	approach : visual
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : sct.tracon
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Flight Phase	descent : intermediate altitude descent : approach
Route In Use	approach : visual
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 220 flight time total : 23500 flight time type : 1500
ASRS Report	599855
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	other anomaly other
Independent Detector	other flight crewa
Resolutory Action	controller : issued new clearance
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

Norcal approach asked if we had oakland in sight. The first officer flying pointed to the airport. I told approach the airport was in sight. The controller then asked if we had visual with another carrier traffic at our 1 O'clock position, and 6 mi. Once again, the first officer pointed to the traffic and said, 'in sight.' I saw the traffic and reported, 'traffic in sight' to approach. The controller then said, 'company flight number, follow the traffic for the visual to runway 29.' a few seconds passed as we started out of our assigned altitude. Approach issued a clearance to 5000 ft. My response was, 'I thought you cleared us for the visual to runway 29.' the controller pointed out that he wanted us to follow the other carrier. Since our altitude had not really changed before the descent clearance was issued, we did not deviate from our clearance. We, however, would have. Although technically correct, the controller's use of unusual, if not 'non-standard' phraseology could have caused a serious altitude deviation, in fact a complete approach. The controller should have said, 'navigation visually, follow other carrier, maintain 7000 ft.'

Original NASA ASRS Text

Title: A B737 FLT CREW RECEIVES AN UNCLR CLRNC WHICH ALMOST LEADS TO AN ALTDEV.

Narrative: NORCAL APCH ASKED IF WE HAD OAKLAND IN SIGHT. THE FO FLYING POINTED TO THE ARPT. I TOLD APCH THE ARPT WAS IN SIGHT. THE CTLR THEN ASKED IF WE HAD VISUAL WITH ANOTHER CARRIER TFC AT OUR 1 O'CLOCK POS, AND 6 MI. ONCE AGAIN, THE FO POINTED TO THE TFC AND SAID, 'IN SIGHT.' I SAW THE TFC AND RPTED, 'TFC IN SIGHT' TO APCH. THE CTLR THEN SAID, 'COMPANY FLT NUMBER, FOLLOW THE TFC FOR THE VISUAL TO RWY 29.' A FEW SECONDS PASSED AS WE STARTED OUT OF OUR ASSIGNED ALT. APCH ISSUED A CLRNC TO 5000 FT. MY RESPONSE WAS, 'I THOUGHT YOU CLRED US FOR THE VISUAL TO RWY 29.' THE CTLR POINTED OUT THAT HE WANTED US TO FOLLOW THE OTHER CARRIER. SINCE OUR ALT HAD NOT REALLY CHANGED BEFORE THE DSCNT CLRNC WAS ISSUED, WE DID NOT DEVIATE FROM OUR CLRNC. WE, HOWEVER, WOULD HAVE. ALTHOUGH TECHNICALLY CORRECT, THE CTLR'S USE OF UNUSUAL, IF NOT 'NON-STANDARD' PHRASEOLOGY COULD HAVE CAUSED A SERIOUS ALTDEV, IN FACT A COMPLETE APCH. THE CTLR SHOULD HAVE SAID, 'NAV VISUALLY, FOLLOW OTHER CARRIER, MAINTAIN 7000 FT.'

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.