Date: 202012
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: EMB-505 / Phenom 300
Operator: Fractional
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Fractional
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Time Pressure; Situational Awareness; Distraction; Communication Breakdown; Confusion; Workload
Second reporter function: First Officer; Pilot Flying
Anomaly: Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Leaving ZZZ; we turned left on Taxiway XX out of FBO to taxi to [Runway] XXR. When we turned left; ground told us to expect 15 minute delay due to departures on the center runway. We therefore proceeded to wait to do the taxi checklist until we were holding short. Approaching [Runway] XXR; I switched to Tower; and within a matter of seconds we hear Company Traffic is on a 4 mile final; and from there I do not recall what I heard. I do not recall because we were then doing the taxi and before takeoff checklist. I remember going up and hitting all of the lights on; which in my mind I must have thought that we were cleared for takeoff; and not line up and wait. We both verified the runway; did all the checklists; took the runway and took off. We never got corrected by ATC to abort takeoff because we weren't cleared. Everything seemed normal; we then got handed off to Departure; and a few minutes later we got asked to copy down a phone number for a possible pilot deviation. At this point both my partner and I looked at each other confused; not knowing what we did. We arrive in ZZZ1; we call the number; they said if they needed us they would call us; they then called us on the Monday after the incident and told us that we took off without clearance. Apparently; I did read line up and wait correctly; though I do not remember what I said due to the last minute high work load. We were called the morning of Monday [Date] from the supervisor telling us what we did. I then told him our side of it. He told me he had to do a report; I told him we would do the same on our end.This occurred because of task saturation. We went from a very calm and slow environment; to a very fast; high workload and task saturated environment. Part of doing the checklists requires a lot of talking between the flight crew. Having to do two checklists; the end of a briefing; as well as listen to ATC all last minute; we clearly didn't hear/comprehend the full ATC instruction. Which; led to us taking off without a clearance. The fact that neither of us caught it; and still didn't know what happened until we got a call; just shows to me how fast we were working. That is a pure reminder to remember to take things slower to make sure this does not happen again.Both my partner and I immediately started doing a report from the incident; when we found out what we did. We made sure that we timely called the ATC facility on the day of the incident as instructed.This occurred because of task saturation. We were thrown off guard getting a clearance 10 minutes before we were expecting to; and were rushing to complete everything. Rushing is never a good thing. We should have stated that we need a minute to finish some checklists and not agreed to the clearance; and if there was ever a doubt in our mind that we didn't understand a clearance we should have double checked with the Tower.


Second narrative: We were given taxi instructions from FBO via [Taxiway] XX to Runway XXR and told to expect a 15 minute wait at the end of the runway. I was Pilot Flying and made the decision to hold the taxi checklist until we got to the runway due to the short taxi and long wait for takeoff. Once we arrived at [Runway] XXR we switched to Tower and immediately was given clearance onto the runway. Due to the rush to get through the taxi checklist and before takeoff checklist; remember hearing traffic on four mile final and all lights being turned on; I thought we were cleared for takeoff. Once all checklists were done I preceded to takeoff. I do not recall hearing the lineup and wait instructions as my mind was rushed to complete both checklists. Looking back I clearly should have denied instructions to take the runway and take my time to complete checklists.We were notified after takeoff of a possible deviation and even at that time we did not know what we did. We called ATC after landing in ZZZ1. They didn't tell us what the possible deviation was and that they would call us back if needed. They called morning of [Date].I allowed myself to be rushed and became task saturated at a critical phase of flight by trying to rush through two checklists before takeoff and knowing traffic was inside a four mile final behind us. This prevented me in clearly hearing our instructions from the tower which was line-up and wait; not cleared for takeoff.Completed report after speaking with ATC on the morning of [Date]I allowed myself to be rushed and task saturated; trying to run two check lists after ATC instructions at the runway hold short line. Even though we had a short taxi and an expected 15 minute wait; I should have still ran the taxi checklist on the taxiway enroute to [Runway] XXR. If we had to wait the entire 15 minutes; I could've reviewed the taxi checklist again if needed. At the hold short once we were given instructions by ATC; I should have denied instructions to allow sufficient time to run the taxi checklist.


Synopsis: EMB-505 / Phenom 300 flight crew reported taking off without a clearance.