A B-fund is a defined CONTRIBUTION plan (an amount given to you each payroll period as a percentage of your income), versus an A-fund or a defined BENEFIT plan (traditional pension that guarantees a benefit, regardless of amount invested during your employment).
The difference is in the word in CAPS above. Defined CONTRIBUTION means they are guaranteed to contribute a certain percentage of your income into your plan account each payroll period. This keeps them from robbing the kitty, so to speak, as the money is out of their control every pay period (twice per month for us), and into OUR account and no longer theirs to touch.
How is the B-fund contributed and how does it affect your tax liability year by year? That is a critical thing to understand, as your tax liability might very well be lower in the future than it is now... or it might be the exact other way around.
Each B-fund at each airline is a little different. The financial accounting rules it's administered under all have their pros and cons, as previously mentioned. You have to figure out what airline you're talking about specifically.
Here at AirTran, it's 10.5% and it's put into your Fidelity 401k account which you can manage any way you see fit. You aren't fully vested until year 5, but you can control the investment mix just like you would your own individual contribution which makes it an EXCELLENT plan - I like a little control over my investment mix as I can plan a little more aggressively than my older coworkers closer to retirement.
The downside is that there are caps on 401k contributions each year (as a function of a percentage of your income). So if you're a "max out my 401k" kind of guy, the company's 7, 8, 10, or whatever percent counts against your own desire to contribute the maximum of your paycheck into your 401k, IF their B-fund is a 401k rule fund.
The up-side to this whole equation is that even if your company goes bankrupt, they can't rob your 401k (ever) or B-fund monies (in most cases), even if the company liquidates, like they did with the A-fund (traditional defined BENEFIT pensions).
When ALPA goes back after the Legacy carriers that bit WAY too much off during bankruptcy, the smartest thing they could do would be to concentrate any retirement plans as B-fund plans, and not try to get back traditional defined benefit pensions... but I digress.
