B737-900
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Workload; Time Pressure; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
On descent into ZZZ we smelled fumes in the cockpit of possible electrical nature. Followed QRC checklist. alerted flight attendants and shortly after we smelled it they did too in mid cabin. We had a similar smell that lasted about a minute soon after level-off from ZZZ1; but lasted only about a minute then dissipated. So when it occurred again on descent we requested priority handling and got expedited handling to Runway XXL in ZZZ. The aircraft itself went through MX before we got it in ZZZ1.  Anti-ice valve issue; changing wheel well light; and enroute some galley lights went out. Landed without incident after being cleared by airport [support personnel] we continued to taxi to the gate. Passengers and crew did not complain about any air quality issues affecting health.  Flight control parameters exceeded. Speed brakes were extended with flaps greater than 15 degrees in my haste to make an expeditious approach and landing in case smoke fumes returned.
After top of climb about 100 NM from ZZZ2; we started to smell a slight burning odor. The Captain and I started to discuss it. He thought it smelled like a match; I thought more like a forest fire or maybe something from the galley. We called back and the flight attendants said they were smelling something too; very light; about mid cabin. We asked them about the galley. They said no smell was in the galley or near the bathroom. The flight attendants suggested and we all decided that they go row to row to investigate and ask all passengers to unplug their devices. During that time the smell got stronger and the Captain and I donned our oxygen masks. The flight attendants said the smell was strongest mid-cabin when they walked back and started to dissipate when they came forward. The Captain took off his mask and said that the smell was nearly gone. After a few minutes I took my mask off too and also found that there was no odor. The entire crew discussed possible causes of the odor; possible diversions if it comes back; and formed a consensus that we continue the flight; allow passengers to reconnect their devices and monitor the situation. The Captain and I both kept our oxygen masks unstowed.During the decent; on the ZZZZZ arrival into ZZZ; we noticed the odor started to come back and slowly get stronger. We both donned our masks and established communication. The Captain instructed me to tell ATC about our situation and request priority handling and we needed to land as soon as possible. While I was coordinating with ATC; the Captain called and told the cabin crew that we were requesting priority handling and were going to land as soon as possible. I think that he told them to call us if they see smoke; and also overheard him do portions of the safety briefing. ATC vectored us off the arrival and gave us a different runway. I ran the 'smoke; fire; or fumes in the passenger cabin or flight deck' QRC. The Captain elected not to go into the QRH as we were on downwind for the runway and the priority was to land. While I was programming the FMC; he pulled the oxygen mask away from his face and discovered that the odor had dissipated as it had done before. The Captain decided to keep his mask off as there was no evidence of fumes and I was having a certain amount of difficulty with communication. I elected to keep my oxygen mask on; just in case. We were cleared for a visual approach to XXL. The Captain kept the speed up and then slowed to configure the aircraft. I was monitoring the aircraft's energy state to ensure we would not have to go around and was looking for the [support] vehicles. We landed; the support vehicles followed us off the runway. They did an external inspection and found no signs of smoke or fire. The Captain thought it was safe to taxi to the gate; so we did. The [support] personnel met us at the gate and we briefed with them.  The smell was transient in nature both times.Communication is very difficult with the oxygen mask on and the old communication panel where the hot mike switch does not stay on.
B737-900 flight crew reported a fumes event during descent to destination.