BE400 flight crew descended on GS and received TCASII RA.

Attributes
ACN	436850
Time
Date	199905
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : vny.airport
State Reference	CA
Altitude	msl single value : 6000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sct.tracon
Operator	general aviation : personal
Make Model Name	Beechjet 400
Operating Under FAR Part	Part 91
Navigation In Use	ils localizer & glide slope : 16r other
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : sct.tracon
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise : level
Person 1
Affiliation	other
Function	flight crew : captain
Qualification	pilot : atp
Experience	flight time last 90 days : 75 flight time total : 9000 flight time type : 350
ASRS Report	436850
Person 2
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : published procedure non adherence : far non adherence : required legal separation other anomaly other
Independent Detector	aircraft equipment : tcas other controllera other flight crewa
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Miss Distance	vertical : 500
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

The biggest mistake on this flight was to allow a factory pilot to fly without clearly establishing who was PIC. I was right seat and PNF. On approach to vny, we received a nonstandard instruction, 'intercept the GS at 6000 ft.' the PF intercepted the GS at 6000 ft and began to follow it. Passing 5500 ft the controller said 'beech jet, cleared ILS runway 16R vny.' simultaneously an RA sounded for traffic in our position but 500 ft lower. I had told the PF that I did not understand us to have approach clearance before he began descent, but did not take control of the airplane as we intercepted the GS. Basically, this is a retroactive attempt to read a controller's mind. Did the instruction authorize descent? I don't think so. Another, and very well qualified, BE400 captain thought it did. This type of ambiguous instruction, easily subject to pilot misunderstanding, should be avoided.

Original NASA ASRS Text

Title: BE400 FLC DSNDED ON GS AND RECEIVED TCASII RA.

Narrative: THE BIGGEST MISTAKE ON THIS FLT WAS TO ALLOW A FACTORY PLT TO FLY WITHOUT CLRLY ESTABLISHING WHO WAS PIC. I WAS R SEAT AND PNF. ON APCH TO VNY, WE RECEIVED A NONSTANDARD INSTRUCTION, 'INTERCEPT THE GS AT 6000 FT.' THE PF INTERCEPTED THE GS AT 6000 FT AND BEGAN TO FOLLOW IT. PASSING 5500 FT THE CTLR SAID 'BEECH JET, CLRED ILS RWY 16R VNY.' SIMULTANEOUSLY AN RA SOUNDED FOR TFC IN OUR POS BUT 500 FT LOWER. I HAD TOLD THE PF THAT I DID NOT UNDERSTAND US TO HAVE APCH CLRNC BEFORE HE BEGAN DSCNT, BUT DID NOT TAKE CTL OF THE AIRPLANE AS WE INTERCEPTED THE GS. BASICALLY, THIS IS A RETROACTIVE ATTEMPT TO READ A CTLR'S MIND. DID THE INSTRUCTION AUTHORIZE DSCNT? I DON'T THINK SO. ANOTHER, AND VERY WELL QUALIFIED, BE400 CAPT THOUGHT IT DID. THIS TYPE OF AMBIGUOUS INSTRUCTION, EASILY SUBJECT TO PLT MISUNDERSTANDING, SHOULD BE AVOIDED.

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.