B737-800
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Training / Qualification; Communication Breakdown
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Procedure
The Approach Controller issued clearance for a visual approach [to Runway 28R] with a [restriction] to cross 6 DME [of the I-IAP ILS DME] at 2;000 FT. I refused the clearance and stated that I could cross an intersection [TOLOC; on the ILS 28R ILS] at 7.7 ILS DME.  I was advised that [TOLOC] was 6 DME from the runway. Discussion: The intersection I was referring to is 7.7 DME from the [far] END of the runway. DME is measured from [a ground based transmitter to the aircraft DME receiver]; not from a physical point [TOLOC in this case] to the [approach end of the] runway. In this case the raw data DME being displayed to the pilot is [derived from the ILS Localizer transmitter at the far end of Runway 28R]. When the aircraft is over the [approach] end of this runway the DME measures 1.7 DME. By accepting this Controller's clearance as issued I would have placed the aircraft 4.3 miles from the end of the runway at 2;000 FT which would have caused an unstable approach.Actually; because I rejected his clearance as stated and corrected it there was NO event but if this Controller continues issuing this type of clearance there most certainly will be in the future.
When approach cleared a B737NG to cross 6 DME from the runway at 2;000 FT; cleared to land Runway 28R at PDX the Captain refused the clearance reminding the Controller that the ILS DME is transmitted from the far end of the runway by the localizer transmitter and; as a result; 6 DME was just 4.3 NM from the approach end of the runway and an approach from that altitude at that distance would; by definition; exceed stabilized approach criteria.