B747-400
Air Traffic Control Fully Certified
Communication Breakdown
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance
Procedure; Human Factors
A B747-400 was flying IMB; BONVL7; PDX at FL380. In the vicinity of IMB; I issued the clearance 'B747 descend and maintain FL240.' I gave this clearance because I had crossing traffic at FL380 as well as addition arrivals ahead of the 747. The B747 asked me; 'Say again; Center?' I told him to descend and maintain FL240. He read back; 'Descend via the BONVL7 Arrival.' I corrected him saying; 'No; descend and maintain FL240.' He mumbled something about; 'Present Heading descending to FL240.' At IMB the BONVL7 makes a slight turn to the right. The B747 did not make that turn but he did start his descent. I called my Supervisor over and discussed what happened and we talked about what whether we should vector him; or give him a fix to fly direct to. The B747 was off course by about 5 miles; diverging by about 15 degrees for approximately 4 minutes when the pilot called up and asked; 'B747; verify direct PDX.' I tried to explain; 'No; you are supposed to fly the profile of the BONVL7 but descend and maintain FL240.' Then the B747 pilot read back; 'Roger; BONVL7'; I then cleared 'B747 direct DUFUR; descend and maintain FL240.' He read that back correctly and I gave him a frequency change to the next Controller. I'm at a loss for words on how to fix this situation. When clearances can be difficult to understand we are suppose to resort to using plain language; but I think plain language only drives the communication gap further apart. How can I possibly simplify my clearance any more than a simple control instruction as 'Descend and Maintain' an altitude? How can the pilot of a B747 mistake that for anything other than what it was? If he had continued descending direct PDX instead of flying the arrival; he would have descended into Mt. Hood.
ZSE Controller described a confused descent clearance interpretation by an air carrier arrival; noting the apparent confusion between 'descend via' vs. a conventional 'descend' instruction.