Narrative:

I was the captain of flight san to ord, on jul/sun/89. As we approached bdf VOR, holding procedures were initiated for the flight at FL330, with an efc time for approximately 30 mins later. Flts began to be cleared from the holding pattern, with flts at lower altitudes given vectors first. During these minutes as other aircraft were being vectored from the holding pattern, we were given clearance to descend from FL330 to FL290, then FL280, still flying the published holding pattern. The following clearance was given when our turn arose to leave the holding pattern, and to the best of my knowledge and recollection, this is the verbatim exchange: ord center: 'air carrier identification, turn right heading 270, maintain 270'. Air carrier identification, 'air carrier identification, roger, heading 270, FL270'. We then descended from FL280 to FL270. Shortly thereafter, the controller gave us another heading and said that we were not cleared to FL270 and to climb back to FL280. My response was that we were indeed, cleared to FL270 as per his previous instructions. Upon landing, I immediately called ord ATC and talked to an ATC controller who was not the controller involved but was familiar with the incident. Informed me that there had been no close proximity with another aircraft and that no action would be taken. I was not sure the, nor am I now, what was meant by the phrase 'maintain 270' if it is other than altitude. Did the controller mean 'heading 270' or 'airspeed 270'? In any event, both myself and the first officer unequivocally assumed that we were being cleared to fly a heading of 270 and to descend to FL270, which was read back to him. I believe this incident occurred because the controller's workload was extremely heavy during this period of time and precise terminology was not used for the ATC instructions for our flight.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION EXCURSION FROM CLRNC ALT DUE CLRNC INTERP.

Narrative: I WAS THE CAPT OF FLT SAN TO ORD, ON JUL/SUN/89. AS WE APCHED BDF VOR, HOLDING PROCS WERE INITIATED FOR THE FLT AT FL330, WITH AN EFC TIME FOR APPROX 30 MINS LATER. FLTS BEGAN TO BE CLRED FROM THE HOLDING PATTERN, WITH FLTS AT LOWER ALTS GIVEN VECTORS FIRST. DURING THESE MINUTES AS OTHER ACFT WERE BEING VECTORED FROM THE HOLDING PATTERN, WE WERE GIVEN CLRNC TO DSND FROM FL330 TO FL290, THEN FL280, STILL FLYING THE PUBLISHED HOLDING PATTERN. THE FOLLOWING CLRNC WAS GIVEN WHEN OUR TURN AROSE TO LEAVE THE HOLDING PATTERN, AND TO THE BEST OF MY KNOWLEDGE AND RECOLLECTION, THIS IS THE VERBATIM EXCHANGE: ORD CTR: 'ACR ID, TURN RIGHT HDG 270, MAINTAIN 270'. ACR ID, 'ACR ID, ROGER, HDG 270, FL270'. WE THEN DESCENDED FROM FL280 TO FL270. SHORTLY THEREAFTER, THE CTLR GAVE US ANOTHER HDG AND SAID THAT WE WERE NOT CLRED TO FL270 AND TO CLIMB BACK TO FL280. MY RESPONSE WAS THAT WE WERE INDEED, CLRED TO FL270 AS PER HIS PREVIOUS INSTRUCTIONS. UPON LNDG, I IMMEDIATELY CALLED ORD ATC AND TALKED TO AN ATC CTLR WHO WAS NOT THE CTLR INVOLVED BUT WAS FAMILIAR WITH THE INCIDENT. INFORMED ME THAT THERE HAD BEEN NO CLOSE PROX WITH ANOTHER ACFT AND THAT NO ACTION WOULD BE TAKEN. I WAS NOT SURE THE, NOR AM I NOW, WHAT WAS MEANT BY THE PHRASE 'MAINTAIN 270' IF IT IS OTHER THAN ALT. DID THE CTLR MEAN 'HDG 270' OR 'AIRSPEED 270'? IN ANY EVENT, BOTH MYSELF AND THE F/O UNEQUIVOCALLY ASSUMED THAT WE WERE BEING CLRED TO FLY A HDG OF 270 AND TO DSND TO FL270, WHICH WAS READ BACK TO HIM. I BELIEVE THIS INCIDENT OCCURRED BECAUSE THE CTLR'S WORKLOAD WAS EXTREMELY HEAVY DURING THIS PERIOD OF TIME AND PRECISE TERMINOLOGY WAS NOT USED FOR THE ATC INSTRUCTIONS FOR OUR FLT.

Data retrieved from NASA's ASRS site as of August 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.