As Americans grow larger and also bring more carry-on items onboard, the FAA has instructed airlines to re-calculate average weight, previously based on decades-old assumptions that are no longer reliable. The practical impact is a number of seat blocks on United Airlines this winter onboard the 757 aircraft, which you can use to your advantage to ensure an open seat next to you.
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https://liveandletsfly.com/united-airlines-blocks-seats-757/