Ahh...actually it was a VERY good question. Here is why...and it has been happening to some folks out there. More on this a minute...
Scenario - you are hired at XYZ Airline with very low hours. i.e. you are not even CLOSE to FAA ATP minimums. With me so far? Good. So, you stay with XYZ Airline until you have the required total time and go to upgrade. Great!
But hold on there kiddo...you waltz into your checkride ready to go, but the DE upon reviewing your 8710 says, "you are not qualified to be a Captain with XYZ Airlines". Uh-oh!! Big embarrassment, six weeks of ground school and sim wasted right? Wrong! Because good ol' bike21 is gonna save your day right now.
Here is what you do.
The requirement people seem to be getting tripped up on is the night PIC and cross country requirement as PIC. (25 hours and 100 hours) What you need to do as SIC before the flight, is agree with your Captain that you will be ACTING as PIC for this leg and will be essentially making all the decisions. You then keep a separate log of when these legs occurred so you can later prove to the FAA that you are indeed ready for the ATP checkride.
Here is the section out of the FARs...
250 hr. of flight time as PIC of an airplane, or as SIC performing the duties and functions of a PIC under the supervision of a PIC, or by any combination of the two. This requirement must include
1. 100 hr. of cross-country time
2. 25 hr. of night flight time
NOTE: Not more than 100 hr. of the total aeronautical experience requirements may be obtained in a flight simulator or a flight training device that represents an airplane, provided the experience was obtained in a course conducted under FAR Part 142.
Basically just make sure you meet the mins! If you hardly instruct (CFIs quit complaining that you HAD to instruct for six months, oh so agonizing...ok that is another for another thread) and get hired with low hours i.e. 300-500 hours, use caution!
****disclaimers****
#1 No this wasn't me, cause I already had ATP mins before I got hired on
#2 I cannot tell you exactly where I got this information. But it is happening.