C172 VFR inbound to lvn described a confusing clearance and/or lack of same when first exiting class...

Narrative:

On a dual instruction VFR cross-country training flight to lvn; I was the cfii monitoring and providing dual instruction to a private pilot conducting cross-country training in preparation for an instrument rating. Upon establishing communication with minneapolis approach approximately 5 NM northwest of cfe; we were cleared into the minneapolis class B airspace on our direct routing with instructions to maintain VFR at or below 4;500 MSL. As we crossed cfe and continued southeast-bound; the controller issued us a vector for msp arrival traffic; instructing us to fly heading 170 and descend to maintain VFR at or below 3;500 msp. Our compliance with this ATC instruction placed us below the floor of the class B airspace in the area. Upon continuing on the VFR heading for 5-7 NM; we were instructed to 'resume own navigation direct lvn'. At this point; the aircraft was operating under the floor of the class B; which was at 4;000 MSL. Upon examining the new direct track to lvn from this point; the student and I both observed that the new direct routing would take us through the 3;000 MSL shelf of the class B. I thus requested approach to verify that we were cleared to transition the 3;000 MSL shelf; believing that descending below the class B automatically voided our class B clearance. The controller seemed surprised by my request for this clarification; and responded in a slightly dismissive tone that we were 'cleared into class B airspace'. We continued without incident; reported lvn in sight; and landed full stop. I believe there exists a certain ambiguity in the manner the controller handled our altitude assignments while we were under radar control within the msp class B airspace. We were not notified of leaving the class B even through we were instructed to descend below the associated shelf floor in our area. I suspect that the controller's surprise at my request for clarification stemmed from having the assumption that his initial class B clearance was still valid. If an aircraft is instructed by a controller to descend below a class B airspace floor; I believe more clarification should be placed upon informing the pilot(s) if re-entry into the class B is approved should further ATC instructions place the aircraft in a position to re-enter the airspace.

Original NASA ASRS Text

Title: C172 VFR inbound to LVN described a confusing clearance and/or lack of same when first exiting Class B and then routed back in without a Class B clearance; the reporter alleging ambiguity in the way ATC handled the flight.

Narrative: On a dual instruction VFR cross-country training flight to LVN; I was the CFII monitoring and providing dual instruction to a private pilot conducting cross-country training in preparation for an instrument rating. Upon establishing communication with Minneapolis Approach approximately 5 NM northwest of CFE; we were cleared into the Minneapolis Class B airspace on our direct routing with instructions to maintain VFR at or below 4;500 MSL. As we crossed CFE and continued southeast-bound; the Controller issued us a vector for MSP arrival traffic; instructing us to fly heading 170 and descend to maintain VFR at or below 3;500 MSP. Our compliance with this ATC instruction placed us below the floor of the Class B airspace in the area. Upon continuing on the VFR heading for 5-7 NM; we were instructed to 'resume own navigation direct LVN'. At this point; the aircraft was operating under the floor of the Class B; which was at 4;000 MSL. Upon examining the new direct track to LVN from this point; the student and I both observed that the new direct routing would take us through the 3;000 MSL shelf of the Class B. I thus requested Approach to verify that we were cleared to transition the 3;000 MSL shelf; believing that descending below the Class B automatically voided our Class B clearance. The Controller seemed surprised by my request for this clarification; and responded in a slightly dismissive tone that we were 'cleared into Class B airspace'. We continued without incident; reported LVN in sight; and landed full stop. I believe there exists a certain ambiguity in the manner the Controller handled our altitude assignments while we were under RADAR control within the MSP Class B airspace. We were not notified of leaving the Class B even through we were instructed to descend below the associated shelf floor in our area. I suspect that the Controller's surprise at my request for clarification stemmed from having the assumption that his initial Class B clearance was still valid. If an aircraft is instructed by a Controller to descend below a Class B airspace floor; I believe more clarification should be placed upon informing the pilot(s) if re-entry into the Class B is approved should further ATC instructions place the aircraft in a position to re-enter the airspace.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	996148
Time
Date	201202
Local Time Of Day	0601-1200
Place
Locale Reference	FCM.Airport
State Reference	MN
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	Descent
Route In Use	Direct
Flight Plan	VFR
Person 1
Function	Instructor Pilot Not Flying
Qualification	Flight Crew Flight Instructor Flight Crew Commercial Flight Crew Instrument
Experience	Flight Crew Last 90 Days 70 Flight Crew Total 615 Flight Crew Type 320
Events
Anomaly	ATC Issue All Types Airspace Violation All Types

C172 VFR inbound to lvn described a confusing clearance and/or lack of same when first exiting class B and then routed back in without a class B clearance; the reporter alleging ambiguity in the way ATC handled the flight.