CNN  — 

Glimmers of a gradual return to air travel mean airlines are rolling out new policies aimed at protecting passengers and crew from coronavirus transmission and increasing consumer confidence in air travel safety.

Many airlines are requiring that passengers and crew wear face coverings, and some carriers are limiting seat selection options to allow more distance between passengers.

But social distancing on planes isn’t always possible – or guaranteed – on more crowded flights.

In a photo posted by a doctor aboard a recent United Airlines flight, many middle seats were occupied. A number of the passengers were medical professionals flying for free, according to the airline.

After a flurry of social media posts about the nearly full flight, United announced a policy change allowing passengers on flights that are near capacity to rebook on a different flight or get a credit.

The airline says currently 85% of its flights are less than half full.

Other airlines have adopted various protective policies – from mask requirements to social distancing measures to reductions in high-touch interactions.

In the United States, internal memos distributed by major airlines indicate that mask requirements may not be strictly enforced.

The US Transportation Security Administration has also announced that its officers will be required to wear facial coverings at airport security screening checkpoints.

Airline passengers should check airline policies before heading to the airport. Here’s where some major carriers stand so far:

United Airlines

All United Airlines flight attendants are required to wear face masks. United was the first major US carrier to announce the requirement.

The mask requirement was extended to United passengers, effective May 4.

The carrier has also modified in-flight service to minimize touchpoints and has moved primarily to prepackaged foods and sealed beverages.

United is also implementing changes to boarding and seat assignments to promote social distancing through at least May 31, although being seated next to an unoccupied seat is not guaranteed.

On May 11, United announced a policy change allowing passengers on flights that are near capacity to rebook on a different flight or get a credit.

The airline says currently 85% of its flights are less than half full.

A passenger prepares to board a flight departing Seattle-Tacoma International Airport on March 15.

Delta Air Lines

Delta Air Lines requires all passengers and employees to wear face masks or other coverings.

Delta is blocking middle seats and reducing the number of passengers on each flight, boarding customers 10 at a time and changing food and beverage service to reduce touchpoints between passengers and crew, according to measures outlined on the carrier’s website.

Delta Air Lines said on May 5 that it would cap capacity at 50% in first class and 60% in other classes.

American Airlines

American Airlines requires passengers to wear face coverings, effective May 11. This policy is in addition to an earlier announcement requiring flight attendants to wear face masks.

The airline has also turned off some computers and kiosks at ticket counters to encourage social distancing.

Signage at gates reminds passengers not to crowd together during the boarding process. American has also reduced food and beverage service and has limited the number of passengers on each aircraft

On flights through May 31, “American will not assign 50% of main cabin middle seats or seats near flight attendant jump seats on every flight, and will only use those middle seats when necessary,” the carrier’s temporary policy reads.

JetBlue Airways

JetBlue was the first major US carrier to require that passengers wear masks, effective May 4.

“Wearing a face covering isn’t about protecting yourself, it’s about protecting those around you,” said Joanna Geraghty, president and chief operating officer of JetBlue, in a statement. “This is the new flying etiquette.”

Crew members are also required to wear masks.

JetBlue has also limited the number of available seats on most flights and regularly reviews seat assignments to maximize space between passengers.

The carrier has adjusted food and beverage service to minimize touchpoints, with more limited offerings in many fare classes.

Southwest Airlines

Face coverings or masks are required for Southwest passengers starting May 11, and employees are required to wear masks while serving customers.

Southwest, the only major US carrier without assigned seats or different cabin classes, told CNN on May 5 that it is underselling each flight by “roughly a third.”

That allows each middle seat to potentially remain empty. But CEO Gary Kelly said the airline does not plan to mandate that all middle seats remain empty.

Southwest has suspended all onboard beverage and snack service until further notice and has modified its boarding procedure to groups of 10.

Frontier Airlines

In early May, Frontier Airlines said it would allow customers to buy out the middle seat in their row, guaranteeing some level of distancing. Those prices were to start at $39 per flight.

But a swift backlash from lawmakers led the carrier to reverse that policy.

Frontier is requiring passengers and its airport employees and flight crews to wear face coverings.

The airline plans to implement passenger and crew temperature screenings on June 1.

Air Canada

All Air Canada passengers are required to comply with a Canadian government mandate that all passengers flying through, to or from Canada wear a nonmedical face mask or face covering over their nose and mouth.

Air Canada has adjusted its boarding procedures and is reseating customers to enable social distancing. Where adequate spacing isn’t possible, passengers can choose to take a later flight at no additional cost.

Services and amenities have also been adjusted to mitigate the risk of viral transmission. Some food service has been suspended or shifted to prepackaged fare and bar service and pillows and blankets are no longer available on some flights.

Masks and other protective equipment common in Asia

All passengers on Singapore Airlines and SilkAir flights are required to wear face masks as of May 10 in accordance with a Civil Aviation Authority of Singapore directive.

Passengers on flights to Singapore are also required to undergo verbal health assessments and temperature checks.

Since April 23, Malaysia Airlines has required all passengers (excluding infants) to wear face masks.

Korean Air of South Korea is providing cabin crew with protective clothing, including goggles, masks and gloves.

“Protective gowns are being provided on medium- and long-haul inbound flights as overseas Covid-19 cases continue to increase significantly,” Korean Air says in outlining its protective policies.

All Korean Air passengers are required to wear masks, and the airline is providing surgical masks and hand sanitizer at airports and on aircraft.

Some European carriers are requiring masks

Passengers must wear face masks when flying with Lufthansa Group. The group includes Lufthansa, Swiss and Austrian Airlines. Flight attendants will also be required to wear masks, according to a Lufthansa Group news release.

Lufthansa added that leaving middle seats vacant will no longer be necessary.

“As a result of this new regulation, the middle seat that has thus far been left vacant in Economy and Premium Economy Class, will no longer remain a necessity, as the facial covering provides the necessary protection,” the news release said.

“Yet, currently, due to a decrease in flight demand, seats will be allocated as far apart as possible throughout the cabin.”

Air France is requiring all passengers to wear masks, effective May 11.

“Checking of passengers’ body temperature is being gradually implemented on all our flights,” an announcement on Air France’s website said.

Dutch carrier KLM is also requiring passengers to wear masks as of May 11. Cabin crews will also be required to wear face coverings.

CNN’s Greg Wallace, Pete Muntean, Harmeet Kaur, Carma Hassan and Alta Spells contributed to this report.