A DHC8-400 Captain did not abort a takeoff while the takeoff warning horn was sounding because he had verified the items; which cause the horn were in position and he assumed the horn was a faulty overspeed warning horn which uses the exact same horn.

Narrative

Possible take-off with takeoff warning tone beeping that was recognized as an airspeed overspeed warning tone. Completed taxi and before take-off checklists with no anomalies including a pass of the takeoff warning test. I also conducted my visual inspection of the 5 items that trigger the takeoff warning horn as a technique I have adopted prior to Takeoff and all items were within parameters. Aircraft was set up for a practice low visibility takeoff. Once cleared for takeoff aircraft was aligned with runway and the take-off began. Once power was added for take-off the airspeed overspeed horn began its beeping. My first response to the First Officer was 'Why is the airspeed warning horn going off; do you see anything that I am missing here?' I verified the engine instruments and they were normal; I verified the Caution/Warning panel; and also was looking for warning tone generator failure messages on the ED. I saw nothing wrong with the aircraft and considering our speed and position on the runway announced 'Continue'. We continued the take-off as I was under the impression that it was a faulty airspeed warning tone and not a takeoff warning horn. After reaching cruise flight I began to review the situation and for the first time considered that the tone that was going off could have been the take-off warning tone. However I was not sure because the warning tone I heard was the airspeed horn. Upon landing I wanted to run the takeoff warn test to verify the tone associated with it and it in fact was the same tone we heard on takeoff which is also the same tone as an aircraft overspeed. I contacted the Duty Officer immediately as I now questioned whether I continued a takeoff with a takeoff warning horn going off which would require a take-off abort. After discussing the situation with the duty officer for guidance due to the complexity of the event we determined that a report was in order and to contact maintenance to further discuss the event and determine if a problem exists. After a phone discussion with maintenance and an explanation of the situation it was determined that it couldn't be determined whether it was a takeoff horn or an airspeed horn considering that the two are the same tone. It was believed to be an inadvertent tone and no write up was required. Other minor items to consider on this was: all circuit breakers were checked and they were all in; also the airplane had just been delivered from the maintenance hanger prior to this flight. There are multiple reasons why I believe this event occurred: Experience; Aircraft warning tone design; training. To start I am a somewhat new captain to the aircraft at 6 months in the airplane. Therefore I am still in the process of 'learning' this aircraft as is typical for a pilot. With that inexperience there is a mix of training and aircraft warning tone design. The biggest reason I think this event occurred was the recognition of a warning tone that covers two events. My first thought on takeoff when the tone was heard was 'Why is the airspeed overspeed horn beeping.' From my standpoint I had properly recognized a warning tone that is associated with an overspeed. For six months the only time I have heard this tone was when I run the Air Data Computer test on daily checks or if a pilot overspeeds the aircraft. That repetitive action had trained my brain to correlate that tone with an airspeed overspeed or an ADC test. (i.e. Pavlov's Dog) I truly believe that having one warning tone for two completely separate events is a design problem that should be examined. My other thought as I continued viewing this situation was looking back to my training. It occurred to me that there is a chance that I might not have heard the Takeoff Warning horn throughout my aircraft training. If I did hear the tone it might have only been for a brief moment; but I don't believe I ever heard this tone associated with a takeoff warning. On top of that; it occurredto me that a pilot could complete an entire training cycle without hearing a takeoff warning horn if they properly do their flows and checklists each simulator or aircraft flight. Also I have never practiced in the simulator an abort due to a takeoff warning horn in any aircraft that I have flown at this air carrier. This is a bit odd as we do practice aborts for most other failures; windshear; engine fire/failure; loss of HGS etc. but never a takeoff warning horn. While a pilot who has more experience in the aircraft might have recognized this tone as a takeoff warning horn it is also possible for a new pilot to the airplane to properly recognize the tone as an overspeed warning which would not require an abort. However; I have learned that these two tones are now the same and have the knowledge now to differentiate these tones depending on my phase of flight. If I hear this tone on the ground I will assume it is a takeoff warning horn and execute a takeoff abort; and if this tone happens in flight I will assume it is an overspeed and slow the aircraft down. Of course that assumes the systems are working properly and that the airspeed horn is not malfunctioning on the takeoff roll or vice versa.