Date: 201510
Local time of day: 0601-1200
Location: SEA.Airport, WA
Flight conditions: IMC
Weather elements / visibility: Rain; 1
Light: Daylight
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower SEA
Second aircraft: A321
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Anomaly: Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Runway
Detector: Person Flight Crew
When detected: Taxi
Result: General None Reported / Taken
Contributing factors: Human Factors; Procedure; Weather
Primary problem: Human Factors

Narrative: After landing on runway 16R in SEA; we were given instructions to taxi via T; Q; Cross Runway 16C and Hold Short of runway 16L at Q. While crossing runway 16C and preparing to hold short of 16L; I observed that I was behind an A321 in front of me on Taxiway Quebec. To my left; on the parallel taxiway P holding short of 16L was a B737. As I was slowing to stop behind the A321; The B737 was cleared to cross runway 16L and immediately after this; the A321 was given clearance to cross runway 16L on Q and to give way to another aircraft on Taxiway B (I believe it was to give way to the B737 crossing simultaneously on Taxiway P). I was expecting a clearance to cross; but it didn't happen so I stopped; set the parking brake; and switched off my taxi light short of the hold-short line by about 15 feet. As the A321 neared the other side of the runway (the white runway edge line) with its nose; tower immediately cleared a Q400 flight for takeoff on 16L. The red lights across the runway on taxiway Q illuminated. I heard this transmission and began to be concerned. The A321 was still on 16L and instead of expediting across; they began to slow. They stopped in my estimation with the nose wheel on or near the hold short line on Q and taxiway B. The tail of the aircraft was close to the edge of the runway (white runway edge line) and likely ON the runway. As I reached for my PTT button; I barely saw the Q-400 landing lights and nosewheel already rotated approximately 3000 feet to my left. The Q-400 passed overhead of the tail of the A321 at a height of approximately 200 feet before they resumed their taxi onto taxiway B. I suspect they would have questioned the takeoff clearance had they not presumably already switched to ground frequency for the other side of the runway. I immediately began to wonder how this had happened and what the consequence would have been if the Q-400 were a heavier aircraft requiring more runway. Visibility on the ground in SEA was announced by the tower controller consistently as 'more than 6000 feet' though in my estimation it was around 4500 feet. I could barely see the silhouette of the tower against the low overcast. It was light rain and mist at my position and I doubt that visibility between the tower and my position was sufficient for them to see any of this event. 1. The A321 should have not switched to ground frequency before exiting the runway or switched back to announce themselves as stopped on the runway. 2. Tower should NOT HAVE CLEARED the Q400 for takeoff in such a quick manner. 3. Low visibility taxi procedures could have prevented this situation.




Synopsis: EMB-175 Captain noted that the A321 cleared to cross the active runway ahead of him was not completely clear of the runway when Tower cleared a Q400 for takeoff. The reporter was about to inform the Tower when the Q400 rotated about 3000 feet prior to the intersection in question.