Narrative:

Air carrier X at 11000 just south of detroit metropolitan, being vectored for an approach on runway 21. The controller was working a lot of traffic, and was very busy. We had a transmission blocked, was advised of that, and then came back in a hurried manner. His phraseology was poor, but we both thought he cleared us to 10000. We read back '10000', and our call sign. The controller went on to the next item at hand and we started down. At 10600 we saw air carrier Y at our 3 O'clock and about 1 1/2 mi, so we started to level off. The controller called at the same time, and told us to maintain 11000. We read back the clearance, and returned to 11000. He then told us that we had crossing traffic at 10000. We told him that we had understood we were cleared to 10000. He did not say anything else. We called on the phone, once we were down, and they said that the traffic was at 10000, and had gotten a TCASII alert. To resolve the alert, air carrier Y had to leave 10000 for 9600. The problem arose because of poor communication. This was brought about because there was too much radio traffic for the controller to listen to readbacks. The only solution is to reduce the radio workload for our ATC system. Too many planes, and not enough controllers. Supplemental information from acn 191032. Air carrier X experienced an altitude excursion which resulted in air carrier Y getting a RA from their TCASII. Detroit approach control was very busy handling numerous other aircraft in the area and there were few pauses between radio xmissions. We were performing our descent checklist at this point and the controller had a transmission that was blocked. Someone responded 'covered'. The controller then very rapidly read back what we both understood to be a clearance to descend to 10000 ft. Our response was '10000 air carrier X.' the controller then made several xmissions to other aircraft. As we were descending we saw traffic at our 2:00-2:30 position below our altitude that was crossing our position from right to left and immediately stopped our descent at approximately 10600 ft. The controller told us to maintain 11000 ft and that we had crossing traffic at 10000 ft. We rapidly climbed to 11000 ft and told the controller that we had understood we were previously cleared to 10000 ft. The controller did not respond. At the time this event occurred I feel the controllers workload was very high. After the controllers first transmission was blocked I believe the controller responded very rapidly with a clearance using non standard phraseology which my captain and I understood to be a descent clearance. This event could have been prevented had the controller spoken slower using standard phraseology.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR X UNAUTH DSCNT FROM ASSIGNED ALT TO OCCUPIED ALT HAD LTSS FROM ACR Y. PLTDEV. ACR Y TCASII RA EVASIVE ACTION TAKEN.

Narrative: ACR X AT 11000 JUST S OF DETROIT METRO, BEING VECTORED FOR AN APCH ON RWY 21. THE CTLR WAS WORKING A LOT OF TFC, AND WAS VERY BUSY. WE HAD A XMISSION BLOCKED, WAS ADVISED OF THAT, AND THEN CAME BACK IN A HURRIED MANNER. HIS PHRASEOLOGY WAS POOR, BUT WE BOTH THOUGHT HE CLRED US TO 10000. WE READ BACK '10000', AND OUR CALL SIGN. THE CTLR WENT ON TO THE NEXT ITEM AT HAND AND WE STARTED DOWN. AT 10600 WE SAW ACR Y AT OUR 3 O'CLOCK AND ABOUT 1 1/2 MI, SO WE STARTED TO LEVEL OFF. THE CTLR CALLED AT THE SAME TIME, AND TOLD US TO MAINTAIN 11000. WE READ BACK THE CLRNC, AND RETURNED TO 11000. HE THEN TOLD US THAT WE HAD XING TFC AT 10000. WE TOLD HIM THAT WE HAD UNDERSTOOD WE WERE CLRED TO 10000. HE DID NOT SAY ANYTHING ELSE. WE CALLED ON THE PHONE, ONCE WE WERE DOWN, AND THEY SAID THAT THE TFC WAS AT 10000, AND HAD GOTTEN A TCASII ALERT. TO RESOLVE THE ALERT, ACR Y HAD TO LEAVE 10000 FOR 9600. THE PROBLEM AROSE BECAUSE OF POOR COM. THIS WAS BROUGHT ABOUT BECAUSE THERE WAS TOO MUCH RADIO TFC FOR THE CTLR TO LISTEN TO READBACKS. THE ONLY SOLUTION IS TO REDUCE THE RADIO WORKLOAD FOR OUR ATC SYS. TOO MANY PLANES, AND NOT ENOUGH CTLRS. SUPPLEMENTAL INFO FROM ACN 191032. ACR X EXPERIENCED AN ALT EXCURSION WHICH RESULTED IN ACR Y GETTING A RA FROM THEIR TCASII. DETROIT APCH CTL WAS VERY BUSY HANDLING NUMEROUS OTHER ACFT IN THE AREA AND THERE WERE FEW PAUSES BTWN RADIO XMISSIONS. WE WERE PERFORMING OUR DSCNT CHKLIST AT THIS POINT AND THE CTLR HAD A XMISSION THAT WAS BLOCKED. SOMEONE RESPONDED 'COVERED'. THE CTLR THEN VERY RAPIDLY READ BACK WHAT WE BOTH UNDERSTOOD TO BE A CLRNC TO DSND TO 10000 FT. OUR RESPONSE WAS '10000 ACR X.' THE CTLR THEN MADE SEVERAL XMISSIONS TO OTHER ACFT. AS WE WERE DSNDING WE SAW TFC AT OUR 2:00-2:30 POS BELOW OUR ALT THAT WAS XING OUR POS FROM R TO L AND IMMEDIATELY STOPPED OUR DSCNT AT APPROX 10600 FT. THE CTLR TOLD US TO MAINTAIN 11000 FT AND THAT WE HAD XING TFC AT 10000 FT. WE RAPIDLY CLBED TO 11000 FT AND TOLD THE CTLR THAT WE HAD UNDERSTOOD WE WERE PREVIOUSLY CLRED TO 10000 FT. THE CTLR DID NOT RESPOND. AT THE TIME THIS EVENT OCCURRED I FEEL THE CTLRS WORKLOAD WAS VERY HIGH. AFTER THE CTLRS FIRST XMISSION WAS BLOCKED I BELIEVE THE CTLR RESPONDED VERY RAPIDLY WITH A CLRNC USING NON STANDARD PHRASEOLOGY WHICH MY CAPT AND I UNDERSTOOD TO BE A DSCNT CLRNC. THIS EVENT COULD HAVE BEEN PREVENTED HAD THE CTLR SPOKEN SLOWER USING STANDARD PHRASEOLOGY.

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.