They arrive at Yaoundé's airport, walk across the tarmac and board a plane -- but they won't be going anywhere.
Instead, they scrub in for surgery and begin a day of operations aboard a fully equipped hospital that just happens to have wings. It's known as the Flying Eye Hospital.
The airplane is an MD-10 that once belonged to shipping company FedEx. The company donated the plane to Orbis, a nonprofit organization that aims to promote eye health around the world.
This plane is Orbis' third, taking over as the Flying Eye Hospital last year. It travels on missions anywhere it's needed and is almost completely self-contained. It carries all necessary supplies, runs off its own power generators and produces its own hospital-grade oxygen.
The only things the plane needs are water, aviation fuel and a runway large enough to land it.
First mission: surgery
For this particular trip, the Flying Eye Hospital spent four weeks in Cameroon, a country with a high need for ophthalmologists: doctors specializing in the eye.
According to Orbis, Cameroon has only 73 registered ophthalmologists for a population of roughly 24 million people. That means only one eye doctor for every 329,000 Cameroonians.
Serge Tchakounte, 40, was one of those patients in need of care.
He was injured in a motorbike accident in February. He is unable to remember much about the accident, as it knocked him out cold, but when he woke up, Tchakounte had a facial fracture in the orbital bones around his left eye.
"It impacted my sight in the sense that I cannot see well backward, up, down and to the sides," he said. "When I try to look up, I see the image double, even from backward and the sides."
Over time, without the bone to lift and stabilize it, Tchakounte's eye has sunk. His doctor knew that Orbis was planning to return to Cameroon in November for the organization's third trip to the country and referred Tchakounte to the program.
When Tchakounte arrived at Yaoundé Central Hospital for a consultation with the Orbis doctors, he met Dr. Thomas Johnson, a longtime volunteer and specialist in ocular plastic surgery at the University of Miami's Bascom Palmer Eye Institute in Florida. This was Johnson's 12th mission with Orbis.
"I think you need to be very flexible," Johnson said. "You're traveling to a different country, working with different people that you haven't worked with before, in different operating rooms and with different equipment, so you have to be very flexible (and) be able to improvise."
Second mission: teaching
Orbis' main mission is to teach, so the Flying Eye Hospital is first and foremost a teaching hospital. That means its priority isn't necessarily seeing as many patients as possible but seeing patients who represent a variety of cases. For a diverse specialty like ocular plastics, Johnson planned to do several types of operations over the course of the week.
"We're trying to show a variety of different techniques in a limited amount of time, so I think that's the most difficult part," Johnson said. "We can't really do surgery on every single patient."
Johnson and his colleagues planned to train local Cameroonian doctors, both on the plane and at the central hospital.
The plane itself is equipped with training simulators and cameras in the operating room. At the front, rows of airplane seats face a television monitor where trainees can watch live surgeries and ask questions.
In Cameroon, a medical translator spoke French, the country's national language, and provided real-time translation.
"The Flying Eye Hospital is sort of a United Nations," said Dr. Jonathan Lord, global medical director for Orbis. "We have, I think on this program, about 19 different nationalities."
The Flying Eye Hospital isn't the only mobile hospital undertaking missions around the world. Large ships like the US Navy's USNS Comfort and USNS Mercy are deployed both on humanitarian missions and to assist after natural disasters, such as the Haiti earthquake in 2010.
"With the opportunity to do long-term training and capacity building and not simply short-term surgery, these platforms can provide sustainable development in global health," said Dr. Mark Shrime, research director for Harvard Medical School's program in global surgery and social change. He is not involved in any Orbis projects.
"Because these platforms tend to be self-contained, they are less reliant on local infrastructure, which means they can sometimes do more complex procedures," he said.
However, Shrime added, "on a day-to-day basis, these platforms can be expensive. With something like Orbis, they also require an airport able to accommodate a plane."
According to the World Health Organization, an estimated 253 million people have vision impairment worldwide, of which 80% can be prevented or cured. The key is awareness and early detection, whether it is cataracts -- a clouding of the eye -- or glaucoma, a pressure on the nerve of the eye.
Both are very common in Cameroon and globally.
"There are over 500,000 people here in Cameroon who are either blind or have low vision," Lord said. "75 percent of them, 375,000 people, have treatable disease."
For Tchakounte, who had the facial fracture, surgery to repair it meant the chance to work again. He helps animals as a veterinary technician and hadn't been able to work since the accident because he couldn't see to drive a motorbike.
As the surgical team prepped Tchakounte, he was confident about the outcome.
"To have my eye as before the accident, if I can see without problem, I can drive again, it would be nice," he said.
In the airplane's operating room, Johnson placed an implant in the orbital floor to lift Tchakounte's eye, aligning it with his right eye. That was expected to correct the double vision.
After an hour and a half, the surgery was over. Tchakounte was wheeled out of the operating room and into the recovery room.
The only sign that the surgery took place on an airplane was the reflection of the plane windows on the glass lining the operating room.