A middle-aged priest. A 26-year-old woman. A registered sex offender.

Three seemingly very different people with one thing in common: All were accused of sexually assaulting fellow passengers on airplanes.

Even before Jessica Leeds alleged that Donald Trump had touched her inappropriately during a flight in 1979, many frequent fliers had concluded that increasingly cramped planes with fewer flight attendants in the aisles seemed to embolden gropers.

“Sexual harassment and assault is happening on aircraft, and we believe it’s happening more often because of the conditions on board,” said Sara Nelson, the international president of the Association of Flight Attendants-CWA union. She cited cramped, confined spaces; alcohol and drugs; fewer flight attendants; and dark cabins on night flights as factors.

Prosecutors said that the Rev. Marcelo De Jesumaria testified that he considered touching his sleeping victim on a US Airways flight in 2014 “consensual because she did not reject his touches and he interpreted her silence, because she was asleep, as ‘coyness.’ ”

The woman said she awoke on the flight to feel De Jesumaria’s hand on the top of her leg and then on her breast, according to the U.S. attorney for the Central District of California.

When De Jesumaria relaxed his grip, the victim went to the restroom and summoned a flight attendant. Law enforcement was waiting when the plane landed.

De Jesumaria, 47, was sentenced to six months in prison and six months of home confinement after being convicted of abusive sexual contact.

De Jesumaria had not been seated next to his victim initially but switched seats by asking a flight attendant if he could “sit next to his wife.”

Heidi Anne McKinney, 26, was charged with touching another woman on the thigh and groin during an Alaska Airlines flight this year.

In another case, a woman was allegedly groped by Yoel Oberlander on an overnight El Al flight from Tel Aviv to Newark on May 29 while seated between him and her mother. She kept repositioning herself to shake his hand off her hand, thigh and breast. It wasn’t until her mother awoke that she asked her to switch seats, and she eventually reported to the crew what had taken place. Oberlander, 35, was charged with one count of abusive sexual contact on an airplane.

Hard to quantify

Just how frequent sexual assault is during air travel is difficult to determine, but FBI investigations into in-flight sexual assaults have risen 45 percent so far this year — 58 investigations from January through September 2016, compared with 40 for all of 2015. That increase doesn’t include incidents reported to local and airport police. It also doesn’t account for the 75 percent of sexual assaults that generally go unreported, according to the Bureau of Justice Statistics.

Alcohol or drugs were identified as a factor in 23 percent of the 10,854 disruptive incidents last year, the International Air Transport Association said. Those who commit sexual violence use alcohol to exploit their victims’ vulnerability, lower their own inhibitions and excuse their behavior, said Laura Palumbo, communications director for the National Sexual Violence Resource Center.

Crew members already receive training on serving alcohol responsibly. The trade association is now calling on airport bars and duty-free shops to voluntarily follow suit so that passengers aren’t drunk when they board planes.

Unless police are called to meet the flight, it is up to the crew to decide whether to report disruptive behavior to the Federal Aviation Administration.

“This is a unique crime,” said Nelson, a United Airlines flight attendant with 20 years of experience. “It’s really not the same as asking, ‘How much did that person hurt you when they hit you on the head?’ ”

An American Airlines spokesman, Ross Feinstein, said it was not up to the crew to assess whether a crime, or what type of crime, had occurred.

“We’re reporting misconduct that occurred on the aircraft. It’s up to law enforcement to determine if any criminal misconduct occurred,” he said. Regardless of the situation, all conflicts on aircraft are handled the same way: by separating those involved, deciding if a diversion of the plane is necessary, and calling ahead for law enforcement to meet it.

But the lack of data on airplane sexual assault makes it difficult to study.

“It’s hard to assess what’s going on if we don’t know the extent of what’s happening,” said Elizabeth L. Jeglic, an associate professor specializing in sex offender policy and treatment at John Jay College of Criminal Justice.

Still, with about 712 million passengers on U.S. flights in the past year, the number of passengers who are sexually assaulted is a tiny percentage of overall air travelers.

Legislation sought

The flight attendants’ union has been working with members of Congress and victim advocacy groups on legislation that would expand crew training to include dealing with victims of sexual assault on flights, as well as to create new industry reporting standards.

An earlier effort by Democratic Rep. Eleanor Holmes Norton, the District of Columbia’s nonvoting member, failed in 2014. Her bill would have required the FAA to collect and publish data on sexual assault.

Palumbo said that more could be done to address airline sexual assault.

“There is a strong body of research that lets us know when people are given the tools to understand what sexual violence is, how best to intervene in instances of sexual violence, and have training and policies as well as those steps, it can lower rates of sexual violence and can be in the best interest of passenger safety,” she said.