A United Airlines flight was diverted to Albuquerque, New Mexico after the co-pilot passed out in the cockpit.
It comes just a day after the death of a American Airlines pilot mid-flight raised the scrutiny of the heath of flight crew.
Flight 1614 from Houston to San Francisco carrying 177 passengers and 10 crew members was rerouted when the first officer became ill and passed out.
The plane landed at Albuquerque without incident.
The co-pilot regained consciousness and was able to walk off the plane before being taken to hospital.
Traveler Lori Ferguson said passengers were initially told the co-pilot had suffered a massive seizure.
"We saw multiple fire engines, police cars, ambulances. It was quite something," Ferguson said, describing scene on the tarmac.
A day earlier, an American Airlines flight en route to Boston diverted to Syracuse, NY after pilot Michael Johnston became unresponsive and later died.
