From Punta Arenas, the two patients will be taken to a medical facility that can provide a level of care that is not available at Amundsen-Scott, the spokesman, Peter West, said.
The Twin Otter flew completed this leg of the trip from the British Antarctic Survey's Rothera Research Station on the Antarctic Peninsula, where it had arrived 1:15 p.m. ET after a 1,500 mile flight from the South Pole Wednesday, West said.
The flight from the South Pole Station was risky because of weather and distance. To depart, the Twin Otter aircraft operators needed to be sure the weather was clear at both stations after resting for 10 hours, West said.
"Following crew rest, the team checked the weather at both the pole and Rothera and decided conditions warranted flying immediately north," West said in an email.
The dangers of flying to Amundsen-Scott during its winter season were illustrated by the two salient features of the mission.
One, while the crew was at the South Pole base, it needed to determine whether the second staff member was sick enough to risk being put on the Twin Otter airplane and increasing the weight of the load, West said.
"They are balancing the health and safety of the flight crew and the health and safety of the patient," West said.
Two, a second Twin Otter aircraft is at the British base 1,500 miles away on the Antarctic Peninsula. Its mission: to rescue the rescuers if necessary. That plane, still at Rothera, will fly north at a later time, West said.
The National Science Foundation, which manages Amundsen-Scott, would not have risked a winter flight without the first staff member's life being in jeopardy, West said.
"After comprehensive consultation with outside medical professionals, agency officials previously decided that a medical situation at Amundsen-Scott warrants returning one member of the station's winter crew to a hospital that can provide a level of medical care that is unavailable at the station," a foundation statement said.
Flights are discouraged between February and October due to the extremely cold and dark conditions, the only lighting for the landing being provided by the moon and the aurora australis.
The Otters, driven by two propellers, are designed to fly in extreme cold and land on skis on the compacted snow.
A pilot from Calgary-based Kenn Borek Air landed about 5:30 p.m. ET Tuesday at Amundsen-Scott.
Temperatures were approaching minus 74 degrees Fahrenheit, but the wind chill factor made it more like minus 106 degrees F, according to the webcam page
for the South Pole station of the U.S. Antarctica Program website.
The foundation is not identifying either patient or releasing any medical information because of patient privacy except to say they are both seasonally employed through the Lockheed Martin Antarctic Support Contract.
On June 14, the Otters flew from Canada to South America and then to the Antarctic Peninsula, where they both landed at Rothera, the British Antarctic Survey said Monday.
Conditions were considered flyable Tuesday, and the pilot made the 1,500-mile journey to the South Pole station.
The second Otter remained at Rothera to provide search and rescue capabilities if necessary.
From Rothera, the plane will fly the patients on to Chile and then to wherever treatment is to be provided, West said.
There are 48 people -- 39 men and nine women -- at Amundsen-Scott, one of three year-round stations operated by the foundation in Antarctica.
Researchers there are studying the atmosphere and dark matter using two radio telescopes as well as an observatory that monitors subatomic particles produced by black holes and other cosmic incidents.
Other medical evacuations have taken place in recent years at Amundsen-Scott
and 850 nautical miles away at McMurdo Station
. Both flew to New Zealand for medical care.
Americans have occupied the South Pole for research purposes since 1956. Amundsen-Scott was built in 1957 but has been updated and redeveloped over the years.