(CNN) -- We weren't supposed to see headlines like this anymore: "Passengers held on hot plane for hours" -- not after a tarmac delay rule went into effect in April imposing stiff fines on airlines that kept fliers stuck in grounded aircraft for a long time.
Yet the nightmare scenario unfolded again last week, prompting questions about the scope of the new rule.
The latest incident involved a Virgin Atlantic flight headed to Newark, New Jersey, from London, England, but diverted to Connecticut on the evening of June 22 because of bad weather, the airline said.
Some of the 300 passengers on board complained they were held in sweltering conditions for hours with no food or water until 1 a.m. as the jet sat on the tarmac at Bradley International Airport.
"It was like four hours on the ground without any air conditioning. It was crazy, just crazy," passenger Beth Willan said.
"There were babies on the plane. And we are in the dark and hot. You try to be patient, but people were yelling and screaming."
iReporter Nicholas Van Pittman, who was on the flight, said the experience was "terrifying." He took video of what it was like on the plane, and another showing a passenger saying that he was "on the verge of having a panic attack."
The U.S. Department of Transportation's aviation enforcement office is looking into the incident to see whether the airline violated any laws or regulations, said DOT spokesman Bill Mosley.
The episode also caught Secretary of Transportation Ray LaHood's attention.
"The events reported ... in Connecticut reinforce my belief that passengers have rights and are entitled to fair treatment when they fly," LaHood said in a statement.
Should protections be extended?
But the Virgin Atlantic flight wasn't covered by the tarmac delay rule -- which exposes airlines to fines of up to $27,500 per passenger if they keep air travelers on the tarmac for more than three hours -- because the protections only apply on domestic routes.
Critics say that needs to change.
The three-hour limit should be extended to international flights operating in the United States, said both Kate Hanni, founder of FlyersRights.org, and Brandon Macsata, executive director of the Association for Airline Passenger Rights.
"It makes sense that if these carriers are going to be doing business in the United States, they would have to comply with the regulations that are placed on domestic carriers as well," Macsata said.
"An excessive tarmac delay is an excessive tarmac delay and we just don't think it's humane to have passengers exposed to that sort of treatment."
But the airline industry counters that there isn't enough data on how often tarmac delays impact international flights to justify a three-hour limit.
Extending the rule could also have unintended consequences for passengers, said Steve Lott, a spokesman for the International Air Transport Association, which represents 230 airlines.
When the tarmac delay rule forces a domestic flight to be canceled, there may be several other flights that day on which passengers can be rebooked, Lott said. But there are fewer international flights, so travelers could be stranded for a day or two if there's a cancellation.
"The bottom line is, let's really define if some of these rules are in the customers' best interest. Obviously, nobody wants to be stuck on a plane for four or five hours at a time, but the concern is passengers are stranded at an airport for 24 hours or longer," Lott said.
There is also the matter of having proper immigration and customs personnel on hand at the airport to process passengers getting off international flights -- a non-issue on domestic routes.
In the case of the June 22 incident, Virgin Atlantic said there was a delay because U.S. immigration officers had to be put in place at an airport not accustomed to dealing with so many people.
Still, the government is taking a closer look at the issue.
Last month, the Department of Transportation proposed asking international airlines that operate in the U.S. to comply with the tarmac delay rule requirements.
While overseas carriers still wouldn't be subject to the three-hour limit, the DOT would require them to have contingency plans for lengthy tarmac delays, which is already a requirement for U.S. carriers operating overseas flights.
Those plans would call for international airlines to decide how long they would allow planes to sit on the tarmac at U.S. airports, make sure there is enough food and water for passengers in such cases, as well as provide "assurance of operable lavatories [and] medical attention if needed," Mosley said.
You can comment on the proposal until August 9 at regulations.gov.
FlyersRights.org's Hanni said any steps forward would be helpful, but she criticized the portion of the proposal that allows international carriers to choose their own tarmac time limit.
"They can pick any amount of time they want. They can pick 10 hours," Hanni said.
"It's like the fox guarding the henhouse. You can't let them set their own amount of time because I guarantee you it's going to be longer than is safe."
Some airlines skirting rule?
Hanni is also concerned about what she called "some manipulation of the rule" by domestic airlines.
Since the tarmac delay regulation went into effect, there have been reports of carriers keeping planes full of passengers at the gate for hours with the door open, thus skirting the tarmac passenger protections, Hanni said.
"Basically, they're loading the planes and they're keeping them at the gate, because the clock doesn't start ticking on the tarmac time until they push back from the gate," Hanni said.
The latest such incident happened last month in Chicago, Illinois, where a plane spent two-and-a-half hours at the gate, then two-and-a-half hours on the tarmac, Hanni said.
"We need to fix the loopholes in the rule. The rule should count as soon as the last butt hits the seat," she said.
The DOT's aviation enforcement office is looking into a number of complaints related to the tarmac delay rule, the agency said.
So far, the office has not found an incident where a flight waited at the gate with the door open for three or more hours and where there is clear evidence that passengers were not allowed to leave the aircraft.
The agency is asking any air travelers willing to confirm that they were stopped by an airline from getting off in such circumstances to contact the DOT.