Date: 201911
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: For the second time for the day for flight ZZZ-ZZZ1; on climbout after flap retraction with the autopilot off accelerating through 225 knots the aircraft started to have mild oscillations for 10-15 seconds. Flight controls felt normal according to the Captain who was the Pilot Flying. The only reason we discussed this occurrence was because this was our second flight of the day in this aircraft. Our previous flight was ZZZ-ZZZ1. While departing ZZZ on the ZZZZZ Departure flying an ZZZZ1 departure profile; we were vectored off the departure by ATC early then given direct clearance to ZZZZZ1 Intersection. This caused us to fly over terrain the SIDs normal track avoids but because of our altitude being that of which cleared the terrain; as usual ATC vectors us for a short cut. Around this time we were retracting flaps; accelerating through approximately 225 knots while I was hand flying the aircraft and it began mild oscillations that ended as we approached 250 knots and I thought were due to turbulence from the terrain we were safely above. The flight to ZZZ3 continued without any further anomalies. As we leveled off at our cruise altitude; we began to discuss the oscillations. We both assumed the first flights were terrain induced and never gave it a single thought. Once discussing the second occurrence; we pieced together the similar air speeds and configurations of the two occurrences. There is no Flight Manual procedure for this type of thing and came to the conclusion that this is something Maintenance could address possibly; and the captain made the Electronic Logbook entry appropriately. Once reaching the gate and letting the passengers deplane; I was expecting to debrief Maintenance about the occurrence to give them as much information as possible so they could properly diagnose the problem. Which is what happened. After discussion with the Mechanic he informed us he would being filing the Maintenance equivalent to a report; so I am filing this report as well.


Second narrative: Departing ZZZ we encountered what seemed like localized light chop inducing subtle rhythmic pitch oscillations. This soon dampened and did not seem alarming. However; during the subsequent leg the same thing occurred at the same point in the flight profile/envelope. There was no airframe vibration or uncommanded yoke movement in either case. In both cases the aircraft was being hand flown and positive control was never in question. Again; this soon dampened and flight continued uneventfully. A Maintenance write up was submitted.


Synopsis: B737 flight crew reported airframe oscillations at similar airspeeds and configurations on multiple occasions.