Ron Twine who has worked for Air New Zealand as a flight attendant for 53 years is gearing up to take his last flight before retiring. Video / Air NZ
This article was one of Herald Travel’s best-read stories of 2023
Few things are better than boarding a plane only to spot an empty row.
However, most travellers know you can’t simply grab your carry-on bag and claim the free seats.
Firstly, it’s pretty embarrassing when the seats are actuallytaken and the passengers are simply the last to board. Secondly, a few minutes later, the crew will ask all passengers to be in their allocated seats for take-off.
It’s fair to assume this is so crew know where you are in case of an emergency but according to an Air New Zealand pilot, this isn’t the real reason.
The envelope is the range a plane’s centre of gravity can safely sit.
A simplified example of a plane's centre of gravity envelope. Photo / File
Think of it like a balance point; if the centre of gravity (COG) is too far forward or too far backward, the aircraft can become unstable, difficult to control or demand more fuel than necessary.
“We want the centre of gravity, if we can, towards the back of the aircraft because the aeroplanes produce less drag if it’s there,” Captain Morgan explained.
So, how do airlines ensure the COG stays within the envelope?
By distributing the weight of passengers, cargo and fuel accordingly.
To ensure it’s specific, the plane’s final COG is calculated right before departure when the exact number of passengers (who are assigned an average weight), cargo weight and placement of these items, is known.
“It’s important that the aircraft’s centre of gravity is actually where we expected it to be from the calculations,” Morgan said, as minutes before departure, the information is sent to pilots in what is called a “load sheet”.
This is used by pilots to help determine the required thrust and speed for takeoff.
Airlines know the weight of fuel decreases during flight, which pulls the COG forward, so they account for this and load the plane in a way that means it stays within the envelope.
“We predict where it’s going to be at the end of the flight because it has changed because the fuel has been consumed,” Morgan said.
What airlines cannot anticipate in calculations, he added, is a change in COG caused by passenger movement.
“If we have passengers moving from their allocated seats to different seats, at the end of the flight then the centre of gravity of the aircraft will have changed,” he explained. As a result, the aircraft’s “flying characteristics” will change too.
You may think you’re just one passenger who surely can’t influence the balance of a massive plane.
However, if 10 adult passengers (weighing an average of 80kg) with carry-on bags move from the back to the front of a plane, this can easily see up to a 900kg shift in balance, which could shift the COG outside a safe range.
“That’s why it’s really important to return to your original seat, it’s purely for that reason.”
It’s also why the airline may change your seat when you check in or go through the gate if other passengers have been added to the flight or changed flights.
