CHI01MA163

During a test flight, the airplane encountered an elevator system oscillation while in a high-speed dive outside the normal operating envelope. The 17 second oscillation was recorded on the cockpit voice recorder and had an average frequency of 28 Hz. The aft elevator sector clevis (p/n 2331510-32) fractured due to reverse bending fatigue caused by vibration, resulting in a complete loss of elevator control. The flight crew reported that pitch control was established by using horizontal stabilizer pitch trim. The flightcrew stated that during final approach to runway 17 (13,337 feet by 200 feet, dry/asphalt) the aircraft's nose began to drop and that the flying pilot was unable to raise the nose using a combination of horizontal stabilizer trim and engine power. The aircraft landed short of the runway, striking an airport perimeter fence and a berm. The surface winds were from the south at 23 knots, gusting to 32 knots.

History of flight

On June 12, 2001, at 1300 central daylight time, a Learjet 25D, N333CG, operated by Avcon Industries, was destroyed on impact with terrain while landing at the Salina Municipal Airport (SLN), Salina, Kansas. The test flight was operating under the provisions of 14 CFR Part 91 and was on an instrument flight rules flight plan. The pilot and copilot sustained serious injuries. Visual meteorological conditions prevailed at the time of the accident. The flight departed Newton City County Airport (EWK), Newton, Kansas, at 1155.

According to the flight crew, the purpose of the test flight was to establish a performance baseline prior to a modification to the aircraft. The purpose of the modification was the development of a new supplemental type certificate (STC) for the Learjet model 25 airplane. The test flight profile was to execute a high-speed dive to verify and establish the vibration and buffeting characteristics of the unmodified aircraft prior to a delta-fin modification to the aft-fuselage. The flight crew stated that the airplane was to be put into a dive, at 43,000 feet pressure altitude (Palt) and that they were to follow a predetermined altitude versus speed schedule until reaching 20,400 feet, where they would execute a 1.5-G recovery. The pilot in the left seat was the pilot-in-command (PIC) and flying pilot (PF). The copilot handled the radio communications and other related pilot-not-flying (PNF) duties. According to Avcon's safety assessment procedures, the test flight was determined to be a high risk and was performed with a minimum crew. The test plan noted, "Caution, Learjets may experience aileron buzz and Mach tuck at these high speeds and Mach numbers. Discontinue evaluation if characteristics become objectionable."

The flight crew reported that during the high-speed descent, at approximately 24,000 feet, the control yoke began to vibrate rapidly for approximately 3-5 seconds after which elevator control was lost. The flight crew stated that they declared an emergency with the Federal Aviation Administration (FAA) Kansas City Air Route Traffic Control Center (ARTCC) and requested to divert to runway 17 (13,337 feet by 200 feet, dry/asphalt) at SLN. The flight crew stated that pitch control was established by using horizontal stabilizer pitch trim. The flight crew reported that during final approach to the runway the aircraft nose began to drop and that the flying pilot was unable to raise the nose using a combination of horizontal stabilizer trim and engine power. The aircraft landed short of the runway, striking an airport perimeter fence and a berm.

The accident airplane was equipped with a cockpit voice recorder (CVR), which was recovered from the wreckage and sent to the National Transportation Safety Board (NTSB) CVR Laboratory in Washington, D.C., for readout. The CVR recording began at 1228:22 (hhmm:ss) and continued up to the point-of-impact at 1300:00.

About 1229:35, Denver ARTCC cleared N333CG to descend from 43,000 feet to 14,000 feet.

The following table was compiled from the CVR Group Chairman Factual Report, the test flight card, FAA aircraft radar track data, and ARTCC transcripts of voice communications. The table depicts the high-speed dive portion of the flight:

Time (hhmm:ss) Altitude (feet Palt) Target Speed (Knots IAS) Actual Speed (Knots IAS)

1230:31 43,000 251 227

1231:05 40,000 268 266

1231:27 38,000 280 280

1231:44 36,000 294 293

1232:00 34,000 308 308

1232:18 32,000 323 322

1232:34 30,000 338 336

1232:41 29,000 346

1232:48 28,000 354 350

1233:02 26,000 369 367

1233:17 24,000 384 384

At 1233:31 a sound of an unidentified chattering noise starts and continues for a period of 17 seconds.

The following selections are from the CVR Group Chairman Factual Report, which is included with the docket material associated with this accident investigation. The labels HOT-1 and HOT-2 refer to the hot-microphone source for the pilot and copilot stations, respectively.

1233:55

HOT-2 I think....

1233:56

HOT-1 I think we lost our elevator.

1233:57

HOT-2 you okay?

1233:59

HOT-1 I think we lost our elevator.

1234:01

HOT-2 okay.

1234:02

HOT-1 I'm using the trim.

1234:03

HOT-2 all right t, keep using trim.

1234:13

HOT-2 okay, I'm gonna slow down.

1234:14

HOT-1 yeah.

1234:16

HOT-2 and we'll, go out here and experiment. I'm gonna go ahead and uh, declare and emergency. is the trim working?

1234:26

HOT-1 uh, trim's working but....

1234:27

HOT-2 okay, okay.

1234:27

HOT-1 ... no elevator.

1234:28

HOT-2 okay, all right. okay, we're gonna go into Salina. long runway, okay?

1234:34

HOT-1 uh, yeah.

1234:36

HOT-2 all right... let me get'em on the other side here. let me get myself hooked up here. we're gonna go out and do some slow flight and all kind of stuff before we try....

1235:03

HOT-1 yeah.

1235:04

HOT-2 ... and go in.

Between 1235:52 and 1236:26, the copilot declared an emergency to Kansas City ARTCC, requested to land at SLN, and N333CG was cleared direct to SLN.

At 1239:19 the copilot informs Kansas City ARTCC, "...we're flying on trim and uh, we'll proceed direct to Salina and uh try to make a, a landing using trim only." N333CG was cleared to descend to 9,000 feet msl.

Between 1239:40 and 1248:03, the pilot and copilot discuss aircraft configuration for the landing and how the approach will be flown.

1243:39

HOT-2 and what do you think? Maintain about a hundred and fifty knots or so on, on final....

1243:42

HOT-1 yeah.

1243:44

HOT-2 ... at, at least.

1243:44

HOT-1 yeah. and we got thrust reversers and we got the chute if we need it.

1243:48

HOT-2 yeah.

1243:48

HOT-1 I got the brakes.

1243:49

HOT-2 yep.

1243:50

HOT-1 we do have hydralics [unintelligible].

1243:51

HOT-2 we got thirteen thousand feet.

1243:52

HOT-1 yeah. yeah.

At 1243:59, Kansas City ARTCC cleared N333CG to descend to maintain 6,000 feet.

Between 1243:59 and 1253:29, the pilot and copilot discussed approach speeds, aircraft landing weight, landing flap settings, and the copilot began transferring fuel from the aft-fuselage tank.

At 1248:22, SLN tower tells N333CG, "... you can make a long final if you like straight in. give us a call five miles or right base your choice one seven." The copilot responded, "okay, we'll probably just circle out north and then try and set up a long straight in."

At 1249:48, SLN tower cleared N333CG for the visual approach to runway 17.

At 1253:29, N333CG was about 9.5 nm north of SLN and the copilot informed SLN tower that they are turning back toward the airport.

Between 1254:38 and 1255:53, the flightcrew located runway 17 and discussed the landing approach.

At 1256:45, the copilot reported being on a 3 nm final for runway 17 and SLN tower cleared N333CG for landing.

At 1257:15, SLN tower reported the wind as 200 degrees magnetic at 20 knots, gusting to 27 knots.

1257:18

HOT-2 okay, I'd main about a hundred and seventy if you can.

1257:21

HOT-1 okay.

1257:25

HOT-1 eeh, don't get much slower than that.

1257:37

HOT-2 everything's lookin' really good.

1257:43

HOT-2 got about, we're about plus five right now which is fine.

1257:45

HOT-1 oh, okay.

1257:46

HOT-2 about one seventy five. I'm gonna start bringing the power back just a little bit.

1257:50

HOT-1 (unintelligible word) okay.

1257:55

HOT-2 I was trying to set up a real gradual descent here.

1258:05

HOT-2 got a little bit of a rig