If you’re booked on a Spirit Airlines flight this weekend, stay tuned. Your pilot might refuse to take off.  For three years, the low-cost carrier’s pilots have been in unsuccessful negotiations with Spirit for higher salaries, and with the deadline for a new union contract looming this Friday, the disgruntled employees might finally walk out of the cockpit. The reasoning: The estimated 500 pilots say they are paid 10 percent less than those at other budget carriers, while Spirit claims it has already upped compensation by double-digit percentages. All parties hope to settle before Friday, but if that doesn’t pan out, officials at Spirit’s Atlantic City hub have asked AirTran to pick up extra routes.