Stephen Bradley Mell, 53 (pictured), pleaded guilty to federal charges of engaging in interstate travel to engage in illicit sexual conduct and receiving child pornography in December 2018 and faces multiple years in jail at his sentencing on Tuesday

A New Jersey millionaire could get jail time after pleading guilty to engaging in sex acts with a 15-year-old girl, once even putting his private plane in autopilot for that express purpose.

Stephen Bradley Mell, 53, of Bedminster, New Jersey, pleaded guilty to federal charges of engaging in interstate travel to engage in illicit sexual conduct and receiving child pornography in December 2018 and is now due for sentencing on Tuesday.

Mell, a father-of-three who had a brokerage firm, was accused of committing sex acts with a 15-year-old girl whose mother had approached him so that he could give her flying lessons, according to legal documents obtained by the Bridgewater Courier News.

It has not been made clear exactly what type of aircraft Mell was operating when he took the victim for her lesson, but a member of staff at Somerset airport located in Bedminster said all aircraft used to teach lessons at the airport are Pipers.

Ahead of his sentencing on Tuesday, Mell's legal team has argued he is a 'humble man' who was on 'multiple anti-depressants' after falling into 'a spiral of depression' brought on by survivors' guilt over giving up his seat on a fatal helicopter trip with friends in 2012.

Mell also faces sentencing in July on a state charge related to sexual activity with the teen, which he pleaded guilty to earlier this month.

Authorities said that Mell (right) once put his private airplane in autopilot so that he could have sex with the teen girl while they were flying back from a trip to Cape Cod

A member of staff at Somerset airport said all aircraft used to teach lessons at the airport located in Bedminster are Pipers

It has not been made clear exactly what type of aircraft Mell was operating when he took the victim for her lesson. Mell owns multiple aircraft and had his own helipad at his Bedminster home. A file photo of Piper aircraft is shown

Mell, who has now pleaded guilty to engaging in sex acts with a minor, started his own charity, Air LifeLine, which flew children with medical needs anywhere in the US for treatment.

Air LifeLine later merged with Angel Flight NE. A representative for Angel Flight NE told DailyMail.com the organization had no comment on the federal and state charges Mell has pleaded guilty to.

Mell, who went by Brad, was also actively involved in other charities serving children, including sitting on the board of the Far Hills Day School where he was once a student, USA Today reported. He was also board member at the preparatory school where his son is a student, Westminster School in Connecticut, and volunteered with the New Jersey Conservation Foundation.

Mell had a helipad at his home which as recently been listed as sold on April 24 for an estimated $3.8million, according to Redfin.

In addition to the helipad, Mell's lavish former residence included a theater room, a marble bathroom, and a pool adjacent to a veranda.

The financier also owned several aircraft, at least one of which is said to have provided the backdrop for his sexual encounter with the teen.

Court documents state that Mell, who was said to have received his pilot's license when he was 16, started talking to the girl, who is not being named, in 2017, using text messages and Snapchat.

Their communication was said to have turned sexual when Mell, then 51, asked the teen 'if she knew how to perform oral sex.'

Court documents state that Mell (right), who was said to have received his pilot's license when he was 16, started talking to the girl, who is not being named, in 2017, using text messages and Snapchat

Mell allegedly 'performed oral sex acts' on the girl in June 2017 and engaged in sex acts with her again in July 2017, after which he supposedly bought her an emergency contraceptive pill.

He was said to have sent the teen a text message stating that, 'If you are nervous it will hurt more,' and added that 'When you are turned on is when it will feel OK.'

On July 20, 2017, Mell allegedly flew the girl on his private plane from the Somerset airport to Cape Cod 'for the purpose of engaging in illicit conduct, specifically, a sexual act with a person under the age of eighteen.'

It was during the return flight that he supposedly put the plane on autopilot so that he could have sex with the girl.

Mell, who started his own charity, Air Lifeline, which flew children with medical needs anywhere in the US for treatment, had a helipad at his home (shown) which as recently been listed as sold on April 24 for an estimated $3.8million

Mell's multi-million dollar Bedminster home which was listed as sold in April is shown

In addition to the helipad, Mell's lavish former residence included a theater room, a marble bathroom, and a pool adjacent to a veranda

The veranda leading out to the pool at Mell's former home, which was sold in April, in shown

The financier also owned several aircraft, at least one of which is said to have provided the backdrop for his sexual encounter with the teen. Mell's former bathroom is shown

Mell was said to have performed sex act with the teen on several other occasions over the course of the next month, entreating the girl to send him images of herself in sexually-explicit conduct via text messages, SnapChat photos or FaceTime video chats.

Over the summer of 2017, Mell sent the minor a birthday card for her 16th birthday. On August 23, 2017 he sent her a message referring to arranging a hotel room for the two of them in the same town where the teen was having dinner, writing, 'Now let me get a room and we are all set.'

Around that same time during a trip to New York City where they had sex, Mell supposedly took a photo of the girl with his cell phone, as she stood naked with 'genital area clearly visible.'

In September 2017, Mell was said to have asked the girl to go on birth control, texting her to 'Please get the IUD' and 'I know you are trying.'

In October 2017, Mell texted the girl, 'Trust me the sex is amazing but so is being around you.'

The final event detailed in the federal complaint against Mell occurred in November 2017, when he was said to have asked the girl to send him 'a nice naked pic cute thing.'

In a legal brief, Mell's defense noted the October 9, 2012 deaths of William W. Ellsworth, 52, of Tewksbury, New Jersey, and Tighe Sullivan, 51, of Darien, Connecticut, which allegedly caused Mell to suffer survivor's guilt.

A third man, Stephen Barral of Bernardsville who was 51 at the time, was seriously injured but survived, according to NJ.com.

In a legal brief, Mell's defense noted the October 9, 2012 deaths of William W. Ellsworth, 52 (right), of Tewksbury, New Jersey, and Tighe Sullivan, 51 (left), of Darien, Connecticut, which allegedly caused Mell to suffer survivors guilt

The legal brief said, 'It was in therapy that Mr. Mell was able to see how much he was doing for so many people and how the burden of his responsibility to so many people was taking its toll on him emotionally.' The document stated that during his therapy following the crash Mell was prescribed multiple anti-depressants, and noted that he is a 'humble man' who 'doesn't announce the accolades he has received in his lifetime.' The wreckage from the fatal crash is shown

Mell couldn't make the golfing trip to New York on his own helicopter at the last moment so he sent another friend instead, who was one of the men who died when the aircraft went down just outside Mount Pocono in Pennsylvania.

The legal brief said, 'It was in therapy that Mr. Mell was able to see how much he was doing for so many people and how the burden of his responsibility to so many people was taking its toll on him emotionally.'

The document stated that during his therapy following the crash Mell was prescribed multiple anti-depressants, and noted that he is a 'humble man' who 'doesn't announce the accolades he has received in his lifetime.'

It's not clear if Mell was allegedly on the anti-depressants while engaging in sexual acts with the teen.

Mell, whose wife Kimberly Ann (Ruggles) has filed for divorce, is currently out of jail on a $1million bond.

The two were married at the First Church of Christ Congregational in West Hartford, Connecticut in June of 1992.

At the time of their wedding, Mell was a 26-year-old graduate of Boston University with a master's degree in psychology and working as a municipal trader at W. H. Mell Associates in Summit, New Jersey, where his father was the president and chief executive officer.

Mell later took over as president of the 22-person company. DailyMail.com attempted to call the firm seeking a comment on Mell's sentencing but the telephone line has been disconnected.

Mell is currently staying with his mother, who her South Carolina home up as security for her son's release.

Mell's mother put her South Carolina home on Kiawah Island up as security for his release

After pleading guilty to federal charges Mell was made to turn over his pilot license and told to stay 1,000 feet away from the teen girl's home and workplace. In addition, he is restricted from using computers and must take periodic polygraph tests to ensure his compliance with the release conditions.

On Tuesday, Mell could be sentenced to five years in federal prison and five years of supervised release for the December 2018 charges he pleaded guilty to, according to the New York Daily News.

In addition to those charges, Mell pleaded guilty on May 13 in state court to third-degree endangering the welfare of a child by engaging in sexual relations with a female victim under the age of 16.

His sentencing for those charges will occur on July 12, when he could face a multi-year state prison sentence, too.