Employer Contributions
United will make a contribution equal to 16% of your eligible
compensation each pay period. This company contribution will
be split into two contributions: 9% will go to the B-Plan source;
7% will go to the C-Plan source.
Another limit determines the combined maximum employee
and employer contributions that can be made to your PRAP
account each year; in 2017, that limit is $54,000 (or 100% of
your compensation if less) [415 limit].
The 415 limit applies to your combined pre-tax contributions,
Roth 401(k) contributions, post-tax contributions and any
company contributions made on your behalf to the PRAP. Any
“catch-up” contributions you make do not count towards the
415 limit.
The IRS also limits the amount of annual compensation that
may be taken into account for the PRAP. The limit for 2017
is $270,000. Any compensation over this amount cannot be
deferred into a qualified plan such as the PRAP.
Rollover contributions made into the PRAP are not subject to
any of these limits.
Important.

Until the Employee Makes $270K per year the 16% doesn't let them max out to the IRS limit. If I was in this position I would negotiate for the company to max out the B/C plan contribution every year from my first year to build my retirement savings early. Even if it required a few less $ per hour in pay (the IRS is going to get a large chunk of it anyway.)