During the National Day holiday, when traveling by air, many airlines recommend passengers arrive at the airport 2 hours early for check-in, with some international flights suggesting arrivals 3 hours in advance.

Once you miss the check-in deadline, even if you've reached the airport and feel confident you could make it to the aircraft door before takeoff, airports won't make exceptions. This isn't due to airports or airlines being inflexible, but because numerous preparations must be completed before takeoff, some of which can only be performed once passenger and baggage counts are finalized.

**The Critical Manifest**

When consulting pilots and flight attendants from two airlines about this issue, despite their different pre-flight responsibilities, their answers all pointed to the same crucial document: the manifest.

Pilots must input manifest data into the aircraft's computer system and can only request takeoff clearance from the control tower once the manifest is complete, then queue for departure according to instructions.

A key step in creating the manifest requires the airline's or airport's load planning department to calculate the aircraft's weight and balance based on aircraft type, fuel consumption for the route, passenger count (weight), and baggage weight, ensuring the aircraft's center of gravity remains within safe limits.

This calculation can only be performed when passenger and baggage weights are completely determined. If passengers continue boarding, both passenger and baggage weights change, requiring recalculation of the aircraft's weight and balance.

Therefore, flights stop accepting check-ins 30 minutes (or longer) before departure to avoid delaying other passengers' journeys.

**The Critical Importance of Weight Balance and Center of Gravity**

While passengers don't physically sense "weight balance" and "center of gravity" during flight, they're crucial for flight safety.

For example, the 2002 China Northern Airlines Flight 6136 crash was related to changes in the aircraft's center of gravity. The accident began with an onboard fire. Even with the cabin burning, the aircraft could still fly for some distance. In this emergency, the pilot decided to avoid densely populated urban areas and attempt an ocean landing to minimize ground casualties. However, after the cabin caught fire, passengers instinctively rushed toward the front of the aircraft where flames hadn't spread, causing the aircraft's center of gravity to shift forward suddenly, resulting in the plane diving nose-first into the ocean.

Additionally, in 2013, National Airlines Flight 102 crashed because cargo in the hold wasn't properly secured and shifted during flight, causing the aircraft's center of gravity to move aft, resulting in a fatal tragedy.

According to 2016 research data, 77% of flight accidents are related to aircraft center of gravity exceeding safe limits, with 57% of these accidents having fatal consequences.

Given the critical importance of weight balance and center of gravity for flight safety, providing adequate time for calculations and proper arrangement of baggage, fuel, and ballast loads is essential.

Incidentally, when purchasing tickets, even if flights aren't full, airlines don't allow passengers complete freedom in seat selection to prevent all passengers from choosing seats on one side of the aircraft or concentrating in the front or rear sections.

During flight, even if forward seats are empty, crew members won't allow passengers to change seats arbitrarily. In 2015, an incident occurred where a passenger refused to sit in their assigned seat and insisted on moving to a front seat, prompting other passengers to follow suit, ultimately affecting the aircraft's weight balance.

**How Airlines Determine Weight Without Weighing Passengers**

Since weight balance is so important, how do crew members know passenger and baggage weights? After all, during boarding, only checked baggage is weighed; passengers and carry-on luggage aren't.

The answer is simple: estimation.

While weight balance and center of gravity are important, they don't require absolute precision. According to China's Civil Aviation Administration's "Aircraft Weight and Balance Control Regulations," most commercial flights fall under the "large cabin aircraft" category. For these aircraft, "standard average weights" for passengers and baggage can be used for calculations.

The larger the aircraft and the more passengers, the more accurate standard average weight estimates become. For instance, the standard average weight for adults is 75 kilograms, including clothing and carry-on baggage. Children average 38 kilograms, and infants 10 kilograms.

Of course, China's "standard average passenger weight" may not apply in other countries and regions. According to the U.S. Centers for Disease Control and Prevention (CDC), American adult males average 90.63 kilograms (199.8 pounds) and females 77.47 kilograms (170.8 pounds). Adding clothing and baggage weight far exceeds 75 kilograms.

Some airlines also restrict carry-on baggage size and weight, partly to ensure luggage fits in overhead bins, but also to prevent excessively heavy carry-on bags that could affect weight estimates.

**Last-Minute Corrections**

Although aircraft stop check-in 30 minutes to 1 hour before departure, special situations may arise. Some passengers who've completed check-in might get absorbed shopping in duty-free stores and fail to reach the gate before boarding closes.

When gates are about to close, airports broadcast these passengers' names as reminders. If passengers still don't arrive after announcements, captains have the authority to order immediate departure, as an entire aircraft of passengers cannot wait indefinitely.

Train and high-speed rail travel is similar—though advance ticket checking stops just briefly before departure, holiday travelers must pay attention to timing.

Returning to aircraft check-in, when passengers don't arrive despite announcements, aircraft don't necessarily need to recalculate weight balance. There's a term called "last-minute corrections," referring to minor load adjustments that can be made after manifest completion but before takeoff without recreating the entire manifest.

This includes fuel weight adjustments, crew and passenger changes, and cargo modifications. These adjustments only require entries in the manifest's remarks section, with confirmation and signatures from the captain and relevant weight balance personnel.

Last-minute corrections can adjust values related to maximum zero fuel weight, maximum takeoff weight, and maximum landing weight. For A320-200 aircraft, adjustments can accommodate up to 6 passengers or 500 kilograms of load.

If changes involve too many people or excessive load modifications, weight balance must be recalculated and a new manifest created. This would undoubtedly delay other passengers, so remember to board promptly after check-in.

During pilot discussions, one noteworthy detail emerged: if all personnel are present, cargo is loaded, aircraft doors are closed, and no route restrictions exist, early departure requests are possible. While prerequisites are numerous, you might catch such an opportunity!

Finally, it's worth reiterating: arrive at airports early for air travel and pay attention to boarding times after check-in. This ensures smooth personal travel while showing respect for fellow passengers and crew members.