Cessna Aircraft Company began its defense this week in a civil trial over the 2005 crash of one of its airplanes at Katama, calling witnesses who defended the plane’s seat-locking system, raised questions about the pilot’s actions, and disputed future medical costs and lost income the pilot and passengers could incur.

The trial in Dukes Superior Court will enter its fourth week Monday, as Cessna and three plaintiffs are asking the jury to determine the cause of the June 23, 2005 crash that injured pilot Alec Naiman and his two passengers, Jeffrey Willoughby, and Mr. Willoughby’s then-13-year-old daughter, Jessica.

All three are deaf and were participating in a Deaf Pilots Association “fly-in,” in which members of the group converge for day flights to nearby locations. That day, several planes flew from Plymouth to the Island.

The trial has been conducted in a crowded, makeshift meeting room at Edgartown’s town hall. Sign language interpreters have been on hand to translate the proceedings for the three plaintiffs, who seek damages for their injuries.

Lawyers for the pilot and passengers claim Mr. Naiman’s airplane seat system was defective, causing his seat to slide back during landing, the plane to stall and ultimately crash. Cessna has argued that the seat system was sound, and blamed the crash on pilot error.

Raymond J. Claxton, an engineer for Engineering Systems Inc. of Dallas, Tex., testified Thursday that he performed tests on the seat system for Cessna, and concluded the seat locking system was not defective.

He narrated an animation of the plane crash that purported to show the crash’s impact on the interior of the plane, including the seat’s locking system, which uses two steel pins to engage aluminum rails attached to the floor. Near the jurors’ feet, he lifted a seat from the same Cessna model and, with a loud bang, snapped it onto aluminum rails fastened to a wooden platform.

Mr. Claxton, who has a doctorate in mechanical metallurgy, examined the seat system on Mr. Naiman’s plane and also tested Cessna’s seat system for the same model plane. Over the objection of plaintiffs’ attorneys, who questioned the reliability and methodology of his tests, he testified that the seat system was not prone to slippage.

And even if it slipped, he testified, the seat would travel only a short distance before it locked again. “The pins would have relocked in the very next hole,” he said. Alternative seat locking systems proposed by the plaintiffs, he said, were unworkable.

Lawyers for the crash victims began their cross examination late Thursday, but Mr. Claxton responded to early questions from attorney Donald J. Nolan that he is paid by Cessna $440 per hour. That total is into the “six figures,” he said.

Other Cessna witnesses this week included Michael Creato, manager of the Katama airfield, who was pumping gas at the other end of the airfield when the plane crashed, and Paul Santopietro, pilot of a red antique biplane that was preparing to take off as Mr. Naiman’s Cessna 172-N descended for landing.

Mr. Naiman’s attempt to abort his landing because of Mr. Santopietro’s biplane on the ground started a chain of events that led to the crash, the plaintiffs have alleged. As Mr. Naiman attempted a “go-around,” his seat inexplicably slid back, causing him to lose control of the plane.

Both Mr. Santopietro and Mr. Creato suggested that while they knew the Deaf Pilots Association planned to use the Katama field, they did not know the exact day and were surprised when they started landing.

Mr. Santopietro, who divides his time between Florida and the Vineyard, says he repeatedly used the radio to announce his intentions to take off, and was confused when he saw two planes land without any radio response.

Recalling that June 23, 2005, was a “sunny day” with a typical summer “milky white” sky, Mr. Santopietro said that as he was turning around to take off, he saw Mr. Naiman’s plane approach for landing. “I, all of a sudden, had a windshield full of airplane,” he said.

He said he saw Mr. Naiman’s plane get a “little boost of power” and try to get out of the way — the “go-around” maneuver — and then he said he saw the airplane stall and crash.

Mr. Santopietro said “go-around” maneuvers are common at Katama due to traffic, and that he might do six to nine of them in an average day. “It’s just something you do all the time,” he said.

One issue during some of the past week’s testimony centered on what lost earnings and medical costs the plaintiffs should recover, should they prevail in the case.

Based on the testimony of a plaintiff’s witness, Mr. Willoughby, a computer-assisted designer who made about $36,000 annually before the crash, for example, lost the ability to work because of his crash injuries and will lose more than $1 million in future lost earnings. Defense witnesses disputed those assertions, insisting he would still be able to work in a “sedentary” job that would require a minimum of physical exertion.

Katie Moss, a registered nurse and life-care planner from Michigan, testified for the defense that she disagreed with the life care plan presented by the plaintiffs’ expert, disputing the inclusion of a cardiologist and a pulmonologist and how often and at what cost Mr. Naiman would need to have home assistance, hospitalizations, and replacements for his motorized wheelchair.

Out of the jury’s hearing, the lawyers sparred over the admissibility of questions about a federal lawsuit filed in St. Louis in 2008 in which the Willoughbys originally sued Mr. Naiman, as well as Cessna and others. Defense attorney Ralph G. Wellington was allowed by Judge C.J. Moriarty 2nd to ask limited questions about the lawsuit in front of the jury.

Mr. Wellington suggested that the Missouri lawsuit did not claim any seat slippage. He also asked Mr. Willoughby about the lawsuit’s claims that Mr. Naiman had been negligent. “I read it, but I didn’t say that and . . . the lawyer took care of that,” Mr. Willoughby responded, referring to an attorney who does not represent him in the current case.

The health problems of one of Cessna’s experts prompted indignant volleys between the two sides about the propriety of substituting another witness in the middle of the trial. The plaintiffs’ lawyers said they hadn’t had any opportunity to prepare for a new witness, especially the one suggested by the defense.

Defense expert James H. Raddin Jr., of Biodynamic Research Corporation (BRC) of San Antonio, Tex., was scheduled for surgery Thursday, which prompted Cessna’s lawyers to suggest James V. Benedict, the founder of BRC, as a possible alternative.

BRC is well known for testifying in automobile and aviation crash cases, primarily on behalf of manufacturers. According to cnnsi.com, which cited a Strategic Safety News article, BRC billed $65 million in 1991 to 1995 alone for work for automakers that had been sued by injured motorists.

Judge Moriarty agreed it would be unfair to substitute Dr. Benedict in the middle of the trial, but said that Dr. Raddin’s testimony could be recorded by videotape over the weekend, if he recovers sufficiently.

Testimony in the case is expected to conclude next week. A jury of nine women and three men is hearing the evidence.