B737-700
Training / Qualification
Aircraft Equipment Problem Critical
Flight Crew Inflight Shutdown; Flight Crew Returned To Departure Airport; Flight Crew Landed in Emergency Condition; General Maintenance Action; General Declared Emergency
Aircraft
In cruise at FL 390; approximately 100 NM east of our departure airport; we experienced an engine seizure of the number one engine. We heard a pop and felt the aircraft shudder and yaw. We could feel the airframe shake and heard a grinding as the engine came to a stop. The autopilot was on and both the Captain and I put our hands on the controls as he stated we had lost the number one engine. He then gave me aircraft control as he began to coordinate with ATC for a descent. He pulled up single-engine drift down speed and we began running the Engine Seizure Checklist.  We declared our emergency and told ATC we wanted to return to our departure airport. We continued to run the QRH and prepared for a single-engine landing. We had instinctively agreed the departure airport was our preferred return point and; based on our distance at this point of around 95 miles and our altitude of around FL 330; it seemed like an easy straight line 3-1 descent back to the airport. I did briefly consider diverting to ZZZ1 but decided against suggesting it because ZZZ1 often requires a steep descent after clearing the mountains and I wasn't sure we could make it at Flaps 15. Also; we had just landed and departed with it fresh in our mind and with a near perfect descent profile I figured it would give us plenty of time to complete all the necessary items without feeling rushed. The Captain later told me he considered all the airports in the area and came to the same conclusion himself and so we continued without any hesitation. The checklist; approach; and landing went well. We made our way through the checklist methodically and the Captain briefed the flight attendants (who were calm and collected the whole time) and made a PA to the passengers explaining the situation. We called Operations to advise them of our return and situation.  Dispatch had been on the line and gave us a quick check-in; it seemed almost just to let us know they were in the loop which saved us some extra radio time and allowed us to focus on arrival. Center gave us great handling and I think we talked to one Approach before Tower but not sure. With all the checklists complete and the aircraft lined up on final; the Captain assumed the controls and brought us in for landing. The emergency vehicles were waiting and; after landing; we taxied clear of the runway. The Captain asked the fire patrol to check the engine/aircraft for any visible threats and when they said all clear we taxied to Gate XX with the fire truck following behind. We had some concern of trying to make a hard right turn into the number two engine on the approach to the gate and the Captain requested a tow-in for safety which Operations happily provided. They were ready and waiting on us when we pulled abeam the gate.  Maintenance also met the aircraft to perform their initial inspection as well. We had a bit of activity in the front galley cockpit area with Maintenance asking questions and passengers deplaning and the Captain and I trying to get ourselves situated. The Captain and I felt like there was too much going on so he asked for a little space for a minute as we both really wanted to be in front of the passengers so they could see everything was OK. The passengers were very appreciative to the crew as a whole; and I heard many compliments to the flight attendants as they exited the plane. It was easy to see many of the passengers needed some reassurance and I felt like they really wanted a personal engagement after going through; for many of them; a very stressful experience.Overall; I felt pretty good about the whole experience. I thought as a crew we worked well together. Procedures and checklists were done methodically and accurately. I felt like the Captain kept a calm; steady command of the situation while skillfully negotiating the steps for a successful conclusion. He showed genuine concern for the passengers during deplaning and we both took time afterward to talk with the flight attendants to make sure they were OK. I felt the Flight Attendants did a fine job. I usually have my service button engaged during flight and; at one point; they got on to talk to each other after completing their task.  I'll say I was proud to hear them talking in calm; reassuring tones leaving no doubt in mind everything was fine in the back and the Captain and I could concentrate on our tasks without any concerns.  The 'crew members' outside the airplane did a good job too and I feel this was really a 'normal-abnormal' event.
I felt a significant vibration and saw that Number 1 Engine Vibration indicator reading 4.9 momentarily and then quickly subsiding. All engine performance and indications were normal prior to this event. A few seconds later; more significant vibration and engine instruments on the Upper Display Unit (UDU) showed Engine Exceedence Indication white box and more significant vibrations with yaw. Immediately following; the Number 1 engine started to roll back and amber Engine Fail alert appeared. There was no indication of fire or smoke but very significant vibrations continued until the engine seized. I coordinated with ARTCC to declare an emergency and descend to a FMC directed engine out altitude of FL 285 and airspeed of 226 knots. As the aircraft slowed; the engine vibration lessened then stopped but the engine indicators were showing a seized engine soon after (No N1 or N2).Better engine monitor and Maintenance could forward engine trends to pilots; although we saw no abnormal indications. I would like to see our QRH boxed letters and number narratives to be bullet statements rather than full sentences to expedite and to highlight in time critical situations.
A B737-700 number one engine seized at FL390 following significant engine vibrations; so an emergency was declared; the QRH completed; and the flight returned to the departure airport.