Because the flight was unpressurized, the flight could have climbed at a rate no greater than 2000'/minute. Any higher climb rate would have been potentially uncomfortable or even dangerous for the occupants. This is why full-scale reading on the CLIMB rate meter at the FE console is 2000. The pressurization system would normally keep the change rate of the pressure in the cabin within these limits although the plane would typically rise and descend at a higher rate (the plane vertical speed indicator has full scale of 6000'/minute).
If the pilot was "pushing it," the climb rate might have been up around 1600ft/minute.
Between 7:40:06 and 7:40:37 the plane went from "through 6500 ft" to "leveling at 7000." This was an average climb rate of very closely 1000 ft/min, which is about average normal climb rate. This rate was probably reduced a bit by the flight already leveling off.
Assume the plane took off at exactly 7:36 (as the NWA incident report says, minus the "exactly"). This would mean the plane rose 6112' from 388' at SEATAC in 4.1 minutes, for an average climb rate of 1491 ft/min--about as much as the pilot would have gone for since the plane was unpressurized. If takeoff had been at 7:36:59, the average rate would have been 1972 ft/min--the maximum that would be safe, and almost "pegging" the cabin climb rate meter. Takeoff would probably not have been more than half past 7:36.
What about an "estimated" 7:33 takeoff (as on n467us.com)? This would be close to 860 ft/min getting to 6500'. Taking off almost a minute later would have brought the climb rate up to 1000'/min.
Getting up to altitude expeditiously, consistent with occupant safety/comfort, would minimize fuel usage and maximize range for the overall trip. So the 7:36 takeoff as stated in the NWA incident report seems most likely.
If the pilot was "pushing it," the climb rate might have been up around 1600ft/minute.
Between 7:40:06 and 7:40:37 the plane went from "through 6500 ft" to "leveling at 7000." This was an average climb rate of very closely 1000 ft/min, which is about average normal climb rate. This rate was probably reduced a bit by the flight already leveling off.
Assume the plane took off at exactly 7:36 (as the NWA incident report says, minus the "exactly"). This would mean the plane rose 6112' from 388' at SEATAC in 4.1 minutes, for an average climb rate of 1491 ft/min--about as much as the pilot would have gone for since the plane was unpressurized. If takeoff had been at 7:36:59, the average rate would have been 1972 ft/min--the maximum that would be safe, and almost "pegging" the cabin climb rate meter. Takeoff would probably not have been more than half past 7:36.
What about an "estimated" 7:33 takeoff (as on n467us.com)? This would be close to 860 ft/min getting to 6500'. Taking off almost a minute later would have brought the climb rate up to 1000'/min.
Getting up to altitude expeditiously, consistent with occupant safety/comfort, would minimize fuel usage and maximize range for the overall trip. So the 7:36 takeoff as stated in the NWA incident report seems most likely.
