Airlines are struggling to keep flights on time heading into the Memorial Day holiday weekend, but at least cancellations have been relatively modest so far. The big test comes Friday, which is expected to be the busiest day of the weekend for air travel. The Transportation Security Administration predicts nearly 3 million people will pass through airport checkpoints Friday, which could threaten the record of 2.9 million, set on the Sunday after Thanksgiving last year. Even more people will be on the roads. AAA says nearly 44 million people will travel at least 50 miles from home this weekend, and about 38 million will go by vehicle.