C172 PLT ENTERED DFW CLASS B WITHOUT CLRNC.

Narrative

I WAS ACTING AS A SAFETY PLT FOR MR X TO HELP HIM PREPARE FOR HIS INST CHK RIDE. WE DEPARTED GRAND PRAIRIE ARPT (GPM) AT ABOUT XA45; WITH A TARGET ALT OF 2500 FT MSL. OUR DEST WAS ALLIANCE FORTH WORTH (AFW). WE STAYED N OF ARLINGTON ARPT (GKY) BECAUSE OF HVY TFC. WE SWITCHED FREQS TO REGIONAL APCH TO REQUEST VECTORS FOR THE ILS APCH AT ALLIANCE. WE WERE GIVEN A SQUAWK CODE AND WERE ADVISED OF RADAR CONTACT. WE WERE THEN INSTRUCTED TO FLY A HDG OF 330 DEGS AND WE WERE NOT INSTRUCTED TO REMAIN CLR OF CLASS B AIRSPACE. A FEW MINS LATER; WE WERE GIVEN ANOTHER VECTOR OF 350 DEGS. APPROX XB00; THE CTLR ADVISED US THAT WE HAD PENETRATED CLASS B AIRSPACE WITHOUT A CLRNC; DIRECTLY FOLLOWED BY A CLRNC THROUGH CLASS B AIRSPACE. AFTER THE INCIDENT; WE WERE GIVEN VECTORS FOR 2 ILS APCHS INTO ALLIANCE. THE REST OF THE FLT WENT WITHOUT ANY PROBS. A CONTRIBUTING FACTOR TO THE INCIDENT WAS A MODE C XPONDER INDICATING 300 FT BELOW THE ALTIMETER; WHICH COULD HAVE CAUSED THE CTLR TO GIVE US A VECTOR THAT TOOK OUR RTE OF FLT UNDER THE SHELF OF CLASS B THAT STARTS AT 2000 FT MSL. ANOTHER WAS A HIGH PLT WORKLOAD BECAUSE OF HVY TFC AND A REQUEST FROM APCH THAT WE LISTEN TO THE ATIS. TO PREVENT A RECURRENCE; ATC SHOULD ISSUE A 'REMAIN CLR OF CLASS B AIRSPACE' OR A 'CLRED THROUGH CLASS B AIRSPACE' INSTRUCTION. THERE HAVE BEEN MANY FLTS BY MANY PLTS WHO HAVE BEEN CLRED THROUGH CLASS B AIRSPACE WITHOUT THE CTLR ACTUALLY SAYING 'CLRED THROUGH CLASS B.' SO EVEN UNDER VERY WORKLOADS; I BELIEVE THE CTLRS MUST NOT FORGET TO SAY THOSE MAGIC WORDS SO THAT PLTS DON'T GET COMPLACENT IN ASSUMING THAT THEY ARE CLRED THROUGH CLASS B. IF THE CTLR DOES NOT WANT THE PLT TO BE CLRED THROUGH CLASS B; THEN I BELIEVE THEY SHOULD ISSUE A 'MAINTAIN CLR OF CLASS B AIRSPACE' ALERT. WHEN NOTHING IS SAID; THE PLT WILL ASSUME HE IS CLRED AND THE CTLR MAY ASSUME OTHERWISE. BECAUSE OF THIS; I BELIEVE THAT EVEN IF OUR VECTORED HDG TOOK US THROUGH CLASS B AIRSPACE; IT WOULD BE OK; BECAUSE ATC TOLD US TO DO SO. FROM NOW ON WHEN I GO FLYING; I WILL DEMAND ATC TO GIVE ME A CLRNC SO OTHER PLTS ON THE FREQ COULD LEARN FROM ME TO NOT ASSUME THAT ATC HAS CLRED YOU THROUGH.