VFR general aviation aircraft receiving flight following from ATC and transitioning under class B wi...

Narrative:

We were flying under the chicago class bravo at 2;500 ft; (class B floor 3;000') using flight following. ATC advised us twice of traffic crossing from right to left about 1000' above; airliners departing from and arriving at midway. ATC then advised of traffic above 3000'; three aircraft in trail one of which we would pass nearly directly under. We reported traffic in sight and ATC then asked our altitude. We were showing 2500 on the altimeter and reported that to ATC. They reminded us of the current altimeter setting; (which we had been using; and read-back to them at this point) and ATC said they were showing us at 2;200'. They then reported us to the airliner as traffic at or below 2200. About the same time we passed under the airliner's flight path; we started a gradual climb; thinking that our lower altitude was causing spotty radar coverage. Passing through 2;600'; we heard a conversation between the airliner and ATC. As we leveled off ATC asked us why we had initiated a climb to 2;700'; and advised us that we had caused a TCAS RA for the airliner. I replied that since they had us reported as lower than our actual altitude; I thought we may have fallen off of their radar; due to our location. We must have actually been at 2;500' before our climb and probably popped up on ATC's radar and the airliner's TCAS screen shortly after we started to climb. Also at this time we were about 5 miles from where the floor of the class bravo goes down to 1;900' ATC probably expected us to stay lower so we could descend under that floor and about this same time; we did start to descend; realizing we had to be under 1'900 feet within a couple of miles; regardless of radar coverage. ATC also reminded us of this and we informed them we were descending to 1;800'. This was the last we heard from them until we requested to change frequencies for 06C. This could have been avoided by staying at 2500' since ATC was reporting us as lower and probably expected us to be at a lower altitude anyway due to us approaching a lower floor of the class bravo. Also; when flying near other traffic; it would probably be best to maintain a constant altitude until well clear; instead of just past.

Original NASA ASRS Text

Title: VFR general aviation aircraft receiving flight following from ATC and transitioning under Class B with several air carrier departures climbing above; elects to climb after being reported 300 FT low by C90; resulting in a TCAS RA event with one of the departure aircraft.

Narrative: We were flying under the Chicago class Bravo at 2;500 FT; (Class B floor 3;000') using flight following. ATC advised us twice of traffic crossing from right to left about 1000' above; airliners departing from and arriving at Midway. ATC then advised of traffic above 3000'; three aircraft in trail one of which we would pass nearly directly under. We reported traffic in sight and ATC then asked our altitude. We were showing 2500 on the altimeter and reported that to ATC. They reminded us of the current altimeter setting; (which we had been using; and read-back to them at this point) and ATC said they were showing us at 2;200'. They then reported us to the airliner as Traffic at or below 2200. About the same time we passed under the airliner's flight path; we started a gradual climb; thinking that our lower altitude was causing spotty radar coverage. Passing through 2;600'; we heard a conversation between the airliner and ATC. As we leveled off ATC asked us why we had initiated a climb to 2;700'; and advised us that we had caused a TCAS RA for the airliner. I replied that since they had us reported as lower than our actual altitude; I thought we may have fallen off of their radar; due to our location. We must have actually been at 2;500' before our climb and probably popped up on ATC's radar and the airliner's TCAS screen shortly after we started to climb. Also at this time we were about 5 miles from where the floor of the Class Bravo goes down to 1;900' ATC probably expected us to stay lower so we could descend under that floor and about this same time; we did start to descend; realizing we had to be under 1'900 feet within a couple of miles; regardless of radar coverage. ATC also reminded us of this and we informed them we were descending to 1;800'. This was the last we heard from them until we requested to change frequencies for 06C. This could have been avoided by staying at 2500' since ATC was reporting us as lower and probably expected us to be at a lower altitude anyway due to us approaching a lower floor of the Class Bravo. Also; when flying near other traffic; it would probably be best to maintain a constant altitude until well clear; instead of just past.

Data retrieved from NASA's ASRS site as of April 2012 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	843362
Time
Date	200907
Local Time Of Day	1801-2400
Place
Locale Reference	06C.Airport
State Reference	IL
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	None
Aircraft 2
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Person 1
Function	Single Pilot
Qualification	Flight Crew Private
Experience	Flight Crew Last 90 Days 15 Flight Crew Total 60 Flight Crew Type 12
Events
Anomaly	Conflict Airborne Conflict
Miss Distance	Horizontal 3000 Vertical 500

VFR general aviation aircraft receiving flight following from ATC and transitioning under class B with several air carrier departures climbing above; elects to climb after being reported 300 ft low by C90; resulting in a TCAS RA event with one of the departure aircraft.