Delta flights from South Africa have been forced to make stops in places like Boston and Puerto Rico because of 'payload' issues

• Two Delta Air Lines flights from South Africa have had to make fuel stops in the past week.
• A mix of worsening weather conditions for aircraft and increased passenger and/or cargo count might be to blame, experts say.

Two Delta Air Lines flights from South Africa to the US have had to make unscheduled stops for fuel in recent days on their way back to the US. The first diversion occurred on November 28 when Delta flight 201 from Johannesburg, South Africa to Atlanta made a technical stop in Boston to be refueled.

Another flight from Johannesburg to Atlanta that departed December 2 diverted to San Juan, Puerto Rico, where it also refueled before continuing on. Delta said both stops were "pre-planned" and new flight crews were required to fly the final legs to Atlanta.

"The redirection of the flight had to do with the technical specifications of our A350 aircraft and the payload of this particular flight," Delta told Insider of the Boston diversion, in a statement. "This can happen on ultra-long-haul flights when optimal operating conditions can't be met."

Numerous factors can play a role in a diversion including weather and payload, especially on an ultra-long-haul flight such as Johannesburg to Atlanta. Delta hasn't seen a diversion on the route since September 6, the earliest date available for viewing using FlightAware tracking data, and the route only relaunched on August 1.

One possibility could be a newfound demand for flights out of South Africa leading to fuller flights. The discovery of the Omicron coronavirus variant and the start of travel restrictions imposed by the US government has prompted some travelers to leave South Africa for the US.

"You have some unusual market conditions posed by the emergency of the Omicron variant and changes to travel policies in the US," Henry Harteveldt, travel industry analyst and president of Atmosphere Research Group, told Insider.

Delta declined to comment on how full its flights were on both occasions. But fuller flights is just one possibility out of many that could affect an ultra-long-haul route such as Atlanta-Johannesburg.

The route may be seeing increased cargo demand, Harteveldt said, and weather might also be playing a role as the weather gets warmer in South Africa and colder in the US.

Johannesburg is a "hot and heavy" city for airlines meaning that aircraft are operating in high temperatures at a high elevation airport, both of which affect an aircraft's performance capabilities. November and December are in the middle of the spring season in South Africa when temperatures rise and aircraft performance suffers.

And in Northern Hemisphere, November and December is the start of the cold season and when winds become stronger. An ultra-long-haul flight such as Johannesburg to Atlanta is at the top end of the Airbus A350-900 XWB's performance capabilities already, and any adverse condition could affect its range.

Delta's flight 201 on November 29 was able to make the 7,300-nautical-mile journey non-stop in 17 hours and 16 minutes, FlightAware data shows. Delta formerly flew the Boeing 777-200 on the Atlanta-Johannesburg route, the longest route in Delta's network, until the aircraft was retired in October 2020. As the only aircraft in Delta's fleet capable of flying the route, the A350 was used to relaunch the route in August and Delta is still adjusting to using the aircraft on ultra-long-haul routes.

"There might be a little bit of a learning curve here for Delta if they've not used this aircraft on this route before," Bruce McClellan, an aviation analyst at Teal Group, told Insider. "I just think, probably, they had some bad luck and they had pretty bad weather that came at them that caused the plane to divert and refuel."

Both experts agree that making fuel stops on scheduled non-stop flights can have reputational issues for Delta if they continue. United is currently Delta's main competition as the only other airline flying non-stop between South Africa and the US.

One solution might be to restrict how many passengers and how much freight is carried on the plane even more than it is now. Delta will then need to assess how much it can restrict before the flight becomes unprofitable.

If the A350 can't reliably perform the route without making stops and without significantly restricting its payload, Harteveldt said, then, "the airline has a fiduciary responsibility to reassess whether that route should be operated."