Date: 202110
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Time Pressure; Troubleshooting
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Aircraft

Narrative: While on vectors off of the Arrival Runway XX slowing to assigned speed 170 knots; we felt a high vibration; 3.8 vibration indication; on the number 2 engine.  Turning on final to Runway XX and slowing to final landing configuration; flaps 30; number 2 engine high vibration increased to an unacceptable level; which lead us to retard the number 2 thrust lever to idle.  We decided to continue the approach using flaps 15 with the number 2 engine at idle.  Number 2 engine at idle had no abnormal vibrations and engine indications were normal.  We selected AB3; GND PROX Flap Inhibit switch to Flap Inhibit and completed a new landing checklist for flaps 15.  We discussed go-around would require the use of flaps 1.  PM (Pilot Monitoring) selected flaps 15 approach speed on the FMC.  PM flew an uneventful approach and landing using the number 1 engine for approach and thrust reverse after touchdown.  Exited taxiway 1X taxi via 2 hold short 2Y.  We reviewed the QRC Engine Limit or Surge or Stall.  After running clock for two minutes (perceived three plus minutes) I closed the number 2 engine start lever via normal post landing ground procedures.  I called the F/A's (Flight Attendants) and told them the condition of the number engine and asked them to look out the right side of the aircraft and let me know if they see anything abnormal.  FO (First Officer) notified ZZZ Maintenance and asked them to meet us at the aircraft.  Uneventful taxi to Gate XX.  Passenger noted while walking off the aircraft she saw two puffs of fire come out the back of the engine after the landing gear came down.Our previous training for engine failure on final approach led us to continue the approach and land using flaps 15 as the safest course of action. Postflight we debriefed the event.  We both agreed we should have declared an emergency.  This would have provided a warning to Tower had our situation escalated resulting in an engine fire on final or during taxi.  Crash/Fire/Rescue (CFR) shadowing our aircraft would have provided information such as fuel leak or a tailpipe fire.


Second narrative: While on vectors off of the Arrival Runway XX slowing to assigned speed 170 knots; we felt a high vibration; 3.8 vibration indication; on the number 2 engine.  Turning on final to Runway XX and slowing to final landing configuration; flaps 30; number 2 engine high vibration increased to an unacceptable level which led us to retard the number 2 thrust lever to idle.  We decided to continue the approach using flaps 15 with the number 2 engine at idle.  Number 2 engine at idle had no abnormal vibrations and engine indications were normal.  We selected AB3; Ground Proximity Flap Inhibit switch to Flap Inhibit and completed a new Landing Checklist for flaps 15.  We discussed go-around would require the use of flaps 1.  PM (Pilot Monitoring) selected flaps 15 approach speed on the FMC.  PF (Pilot Flying) flew an uneventful approach and landing using the number 1 engine for approach and thrust reverse after touchdown.  We reviewed the QRC Engine Limit or Surge or Stall.  After running clock for two minutes (perceived three plus minutes) I closed the number 2 engine start lever via normal post landing ground procedures.  I called the F/As (Flight Attendants) and told them the condition of the number 2 engine and asked them to look out the right side of the aircraft and report anything abnormal.  FO notified ZZZ Maintenance and asked them to meet us at the aircraft.  Uneventful taxi to gate. Female Passenger noted while walking off the aircraft she saw two puffs of fire come out the back of the engine after the landing gear came down.


Synopsis: Flight Crew reported excessive engine vibration and elected to continue the approach with the #2 engine thrust lever at idle.