Inexplicable Descent Of A Learjet 25, Part 2

When safety investigators interviewed the two sons of the Learjet 25 captain, they found that both were Embraer EMB-190 captains. One son had a late dinner at the Monterrey hotel with his father just before the accident flight on Dec. 9, 2012.

The captain had flown the accident jet before. He told the son that it was an old aircraft and he would not fly that night if there was fog at the destination. “I kind of don’t want to fly,” the son quoted him as saying. The cockpit instruments were old, and “the airplane flew a little crooked.” The horizontal situation indicator needed to be “cranked around.” He liked to fly the Learjet at Mach 0.80, but this jet could only achieve Mach 0.77.

The son said his father charged more than other pilots but was valued because of the way he flew and the way he treated the passengers. The owner of the jet liked to carry entertainers and encouraged them to buy their aircraft.

Rivera was not charged for the flight but might have been willing to pay $450,000 for the jet, according to the captain’s son. His opinion was that it was worth less than $100,000. He also reported that his father had exchanged his Mexican Airline Transport Pilot (ATP) certificate for a commercial certificate because in Mexico, the ATP expired at age 65.

The second son was president of the Mexican Pilots Union. He had flown Learjets and many other aircraft with his father. He said his father was one of the first pilots in Mexico to fly the Learjet, and he had extensive experience in the type.

The family owned a company that sold aircraft, parts and insurance. His father told him that the accident jet was not “flying straight” and that it “flew to the side.” He said another pilot claimed that the jet had a pitch trim problem, but it had been fixed.

Mexican aviation regulator DGAC did not seek background information on the Learjet 25 co-pilot, saying he was inexperienced and lacked the U.S. certification to fly the business jet.

Examination Of The Aircraft
To determine what kind of mechanical failure may have occurred, investigators had to establish some facts. The cockpit voice recorder was destroyed. The jet’s logbooks and maintenance records had disappeared.

The FAA was able to provide some help. The jet was manufactured in 1969. It had been listed on the Part 135 certificate of a company in Houston since 1986 and had been sold to Starwood Management LLC in the summer of 2012.

The Learjet had been involved in one prior accident, a landing mishap, in 2005. The FAA found the only documented inspection of the jet, and it happened just before it was sold. A mechanic in Houston who was familiar with the aircraft signed off “600, 300 hr., 24, 12-month inspections as per Lear 25 Chapter 8 Inspection Guide,” along with additional items. There was no discussion of the jet’s possible rigging or flight control anomalies. That was on July 10, 2012.

Investigators spoke to people who had flown and/or repaired the jet. The FAA found and interviewed the pilot who flew the aircraft from Houston to Toluca, Mexico, on Aug. 31, 2012. He denied the jet had any handling problems. However, he may not have been the most reliable witness. He had served time for drug smuggling and had a long association with the company that owned the Learjet.

The FAA’s security and special investigations teams were highly interested in Starwood LLC, since its owner, a man named Ed Nunez, was suspected of illegal activities and had previously been convicted of drug smuggling activities.

When investigators tracked down the address Nunez used to register aircraft, it was a post office box in Las Vegas. At the time of the accident, the FAA registry showed 30 aircraft registered to Starwood. Most were older business jets.

During an initial sweep of the accident site, an NTSB investigator found the horizontal stabilizer actuator jack screw. The jack screw has been an important clue in many investigations because damage to it may point to the reason for loss of control, and an intact jack screw can allow you to infer the angle of the stabilizer and the corresponding trim speed.

The jack screw was hand-carried to the NTSB’s materials lab in September 2013. It was shipped to Varian Medical Technologies in Lincolnshire, Illinois, for computed tomography scans, then returned to Washington, where the investigators examined it. They concluded there was no pre-existing damage, corrosion or fatigue of the jack screw components. There was a fracture, but it was caused by “single cycle bending overstress.” In other words, the part broke when it impacted the mountainside.

The NTSB report, issued in June 2014, eliminated one important theory in favor of mechanical failure causing the accident.

There was one other NTSB report that held some promise of an explanation as to why the Learjet 25 crashed into a mountainside in Mexico.

An aircraft performance specialist obtained radar data from U.S. sources. The sites used were far away from the Learjet’s flightpath, at 120 nm (138 mi.) and 170 nm, respectively, and the radars only recorded data at 12-sec. intervals. This meant data points could be as much as half a mile off. Nonetheless, the specialist was able to construct a good representation of the flightpath, including altitude, speed and climb angle.

The graphs showed the jet climbed steadily but experienced significant fluctuations in airspeed. The specialist attributed the fluctuations to data errors, but they might indicate the climb was not as smooth as it seemed. She concluded: “The aircraft was gaining speed to a final, pre-loss of control speed of about 400 kt. and Mach 0.65. The speed and altitude of the aircraft put it well within the performance envelope for the Lear 25.”

But was it? Mexico’s air traffic control radar, which was much closer to the jet, showed the aircraft hit a peak ground speed of 440 kt. at 28,700 ft. If that was also its true airspeed, the Mach number was 0.74 and the calibrated airspeed was 288 kt.

A FAA special certification review of early Learjets in 1981 provides some insight, in Part 3 of this article.

To Read Inexplicable Descent Of A Learjet 25, Part 1, click here.