Abby Sunderland, 16, is feared lost at sea today in her attempt to become the youngest sailor ever to circumnavigate the globe.
A support crew lost contact with Abby, who was in heavy seas in the Southern Indian Ocean, early this morning.
Sunderland's brother, Zac Sunderland, said his sister's boat was clearly in trouble.
"The boat is most likely not completely submerged because there's another alarm that sends off a signal if it goes 15 feet underwater," Zac Sunderland said in an interview with Ron Kilgore of KNX radio in Los Angeles. "So yeah, she's pretty banged up out there right now, and [we're] just trying to get the rescue teams out."
Abby's mother, MaryAnne Sunderland, told ABC News that Abby manually activated two emergency beacons sometime before 6 a.m. Pacific Time today.
A plane from Qantas air, based in Australia, planned to fly over Sunderland's location at first light Friday -- or late Thursday evening Eastern Time.
Watch the full story Friday on "20/20" at 10 p.m. ET
Abby was in 20-25 foot waves at the time of last contact, with 35-knot winds, said MaryAnne Sunderland, who is due to give birth at the end of the month. She was shaken but focused on trying to get a rescue effort together.
The closest land to Abby's boat was Reunion Island, which is east of Madagascar. The nearest ship was 400 miles away. Rescuers were trying to contact the ship.
Zac Sunderland told KNX that the support crew was "still trying to figure out the rescue situation."
"There's two boats headed out to her position. One of them's an estimated 40 hours, the other is 48. So right now we're just trying to figure out if there's any way faster. She's in the middle of nowhere pretty much, in the Southern Indian Ocean, there's nothing closer.
"We're just hoping everything's all right out there. There's nothing we can really know for sure out there right now."
A note posted to a blog tracking Abby's progress by her support team Thursday detailed her survival gear.
"Abby has all of the equipment on board to survive a crisis situation like this," the posting said. "She has a dry suit, survival suit, life raft, and ditch bag with emergency supplies. If she can keep warm and hang on, help will be there as soon as possible."
Click HERE to watch Abby give a tour of her boat.
Jeff Casher, an engineer on Sunderland's support team, told ABC News that he last spoke with the 16-year-old sailor before 6 a.m. PDT, after she had been knocked down twice during the night because of strong winds -- meaning that her sail had touched the water.
One of those knock-downs, Casher said, ripped the radar off the boat. She had been speaking with Casher on a satellite telephone earlier because of engine problems and was in the process of fixing those problems when she told Casher she'd call right back.
She has not been heard from since, except for the distress signals.
Abby is approximately 500 miles north of the Antarctic Islands on her bid to become the youngest to circumnavigate the globe in a sailboat, solo.
Abby herself last posted a blog entry from her boat, "Wild Eyes," on Wednesday night.
"It was a nice day today with some lighter winds which gave me a chance to patch everything up," she wrote. "Wild Eyes was great through everything but after a day with over 50 knots at times, I had quite a bit of work to do."
The weather was getting rough again, she noted.