Date: 201811
Local time of day: 1201-1800
Location: OKC.Airport, OK
Flight conditions: VMC
Light: Daylight
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Landing
ATC / advisory: Tower OKC
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Instrument; Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Other / Unknown
Second reporter function: Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Ground Event / Encounter Ground Strike - Aircraft
Detector: Person Flight Crew
When detected: Other Post Flight
Result: Aircraft Aircraft Damaged
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: Normal flying of STAR to destination airport. Conducted the RNAV-Z in VMC conditions. Aircraft was stable at 1;000 feet AGL and on speed and glide path over the threshold. During landing; the tail of the aircraft contacted the ground as the aircraft settled and struts compressed. Touchdown was very soft and did not cause alert to either flight crew member. No abnormalities were detected by either flight crew member (tactile or audible). Neither Flight Attendant noted any abnormality during landing. Damage to a WiFi antenna; dump mast and an antenna occurred. The flight crew was unaware of the occurrence until two days later. Because the outbound crew was present upon arrival; we did not conduct a post-flight walk-around.We picked up the aircraft the same morning. During the first walk-around I noticed the right main strut seemed slightly lower than usual; though I didn't think it posed a threat. Aircraft have unique indications of wear and tear; and the strut varies greatly based on that and current weight on the aircraft. I did not indicate anything to the Captain as slight variances in aircraft are common.Takeoff at the departure airport was normal. No over-rotation occurred. During landing at the event airport; the touchdown was very soft and the ground spoilers did not immediately deploy. I was able to feel this; and I was also unable to initially deploy the thrust reversers. As the aircraft slowed; there was enough weight on wheels to deploy the ground spoilers and thrust reversers. As the aircraft settled onto the ground; I pulled back slightly to keep the nose wheel from slamming into the ground. Because of the slightly lower strut and the settling of the aircraft during the delayed spoiler deployment; I believe this to be when the tail of the aircraft contacted the ground. There were no audible indications and neither flight crew member felt anything abnormal.Possible guidance from Maintenance indicating how low of a strut is too low. This can be difficult as there is still a great bit of normal variance from aircraft to aircraft.Guidance for pilots regarding a technique to use during delayed spoiler deployment.


Second narrative: We had a stabilized landing at OKC Runway 35L in VMC conditions with light winds. The landing was soft enough to delay ground spoiler deployment. When the spoilers did deploy the aircraft settled on the runway and possibly scraped the tail. The aircraft's pitch at touchdown was normal with no 'nose-up' attitude.We do not know why an aircraft would scrap its tail on a perfectly flown landing.Contributing factors can only be guessed. They would include a delay in ground spoiler deployment due to WOW [Weight On Wheels] issues caused by an unusually soft landing and the possibilities of under-inflated struts. If a scraped tail was caused by a perfectly flown approach and landing; from a crew's perspective; I do not know how to avoid this from happening again.


Synopsis: Air carrier flight crew reported a tail strike during landing.