B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
ATC Issue All Types
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Procedure
An MD-11 was cleared for takeoff. I punched the clock for the 90 second separation for wake turbulence. We were cleared into position with Line up and Wait. I taxied into position. As soon as I was in position; we were cleared for takeoff. I looked at my clock and only 45 seconds had gone by since the MD-11 took off. We told tower that we needed at least 30 more seconds. The controller canceled our takeoff clearance and stated that by radar we had enough separation. We stated no; we need more time. We were then cleared for takeoff. I was astonished they were going by radar separation verses time. That is really an unsafe condition for aircraft behind a heavy taking off.
Air Carrier Captain reported ATC cleared them for takeoff prior to sufficient wake turbulence separation from a preceding departure.