IBC SW4 near Arecibo on Dec 2nd 2013, impact with terrain after inflight break up

Last Update: December 15, 2020 / 21:44:24 GMT/Zulu time

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Incident Facts

Date of incident
Dec 2, 2013

Classification
Crash

Aircraft Registration
N831BC

ICAO Type Designator
SW4

An IBC Airways Swearingen SA-226 Metro III, registration N831BC performing freight flight II-405 from Santo Domingo (Dominican Republic) to San Juan (Puerto Rico) with 2 crew, was observed in low level flight near Arecibo (Puerto Rico) around 20:10L (00:10Z Dec 3rd). Soon after residents heard the sounds of an explosion. A search was initiated for the aircraft, the aircraft was located in steep forested terrain almost on the boundary between Florida and Arecibo districts near a cemetary at position N18.3832 W66.5878 the following morning, the aircraft had burst into flames upon impact with terrain, both crew perished in the crash.

Puerto Rico police reported they received 911 calls reporting an aircraft accident at around 20:15L and started a search for the aircraft, rescue teams reached the crash site the following morning at about 09:30L. Both occupants of the aircraft were found killed.

Radar data suggest the aircraft entered a descent of about 900 fpm until impact with terrain.

Arecibo Airport said the aircraft was not destined for their airport and was not in contact with the airport.

The FAA reported the aircraft crashed under unknown circumstances about 10nm from Arecibo, the occupant(s) were fatally injured.

Some time in the past the NTSB released their final report concluding the probable cause of the crash was:

The flight crew's excessive elevator input during a rapid descent under night lighting conditions, which resulted in the overstress and breakup of the airplane. Contributing to the accident was an initial loss of airplane control for reasons that could not be determined because postaccident examination revealed no mechanical anomalies that would have precluded normal operation.

The NTSB reported, that the aircraft did not carry a flight data or cockpit voice recorder nor was it required to do so and summarized the sequence of events:

A review of radio transmission transcripts indicated that the crew first contacted the San Juan Combined En route Approach Control (CERAP) facility at 1948, 13 nautical miles west of "MELLA" intersection at 11,000 feet. At 2001, the crew was told to descend to 7,000 feet at "pilot's discretion," and at 2007, the crew was advised to change frequency to the next CERAP sector controller. The crew subsequently contacted the next controller, "leaving one one thousand, descending to seven thousand." The controller then advised the crew to maintain 3,000 feet, expect the ILS (instrument landing system) approach, proceed direct to the "TNNER" fix, and that information "Tango" was in effect. After a crewmember read back the information at 2007:46, there were no further transmissions from the airplane. At 2011:52, the controller advised that radar contact was lost.

According to a Federal Aviation Administration (FAA) inspector interview (the inspector conducted the interview in Spanish and translated it into English), a witness stated that he heard some engine noise, and when he looked outside, he saw the airplane with the right wing down, "turning in a spiral form." He also noticed a red light "spinning." After that, he heard an "impact noise" and 5 seconds later, "another solid impact noise."

Radar data revealed that after crossing MELLA, the airplane proceeded toward TJSJ along a heading of about 085 degrees true, crossing the west coast of Puerto Rico just south of the town of Stella. The airplane maintained 11,000 feet until 2007, and had descended to 8,300 feet by 2010:08. The radar track then indicated a 20-degree turn to the left, and a descent to 7,300 feet by 2010:13. The radar track subsequently indicated about a 45-degree turn to the right, and a descent to 5,500 feet by 2010:18. There were no additional verifiable altitude positions.

Descent calculations between 2010:08 and 2010:13 indicated a rate of descent of about 12,000 feet per minute (fpm), and between 2010:13 and 2010:18, over 21,000 fpm. Groundspeed calculations indicated a fairly constant average of about 260 knots (provided in 10-knot increments) until the airplane initiated a descent. The last two calculations, 1 minute apart and just prior to the rapid descent, were 280 and 290
knots.

The NTSB analysed:

There was no evidence of any in-flight mechanical failures that would have resulted in the loss of control, and the airplane was loaded within limits. Evidence of all flight control surfaces was confirmed, and, to the extent possible, flight control continuity was also confirmed. Evidence also indicated that both engines were operating at the time of the accident, and, although one of the four propeller blades from the right propeller was not located after separating from the fractured hub, there was no evidence of any preexisting propeller anomalies. The electrically controlled pitch trim actuator did not exhibit any evidence of runaway pitch, and measurements of the actuator rods indicated that the airplane was trimmed slightly nose low, consistent for the phase of flight. Due to the separation of the wings and tail, the in-flight positions of the manually operated aileron and rudder trim wheels could not be determined.

Other similarly documented accidents and incidents generally involved unequal fuel burns, which resulted in wing drops or airplane rolls. In one case, the flight crew intentionally induced an excessive slide slip to balance fuel between the wings, which resulted in an uncontrolled roll. However, in the current investigation, the fuel cross feed valve was found in the closed position, indicating that a fuel imbalance was likely not a concern of the flight crew.

In at least two other events, unequal fuel loads also involved autopilots that reached their maximum hold limits, snapped off, and rolled the airplane. Although the airplane in this accident did not have an autopilot, historical examples indicate that a sudden yawing or rolling motion, regardless of the source, could result in a roll, nose tuck, and loss of control. The roll may have been recoverable, and in one documented case, a pilot was able to recover the airplane, but after it lost almost 11,000 ft of altitude.

During this accident flight, it was likely that, during the descent, the flight crew did regain control of the airplane to the extent that the flight control surfaces were effective. With darkness and the rapid descent at a relatively low altitude, one or both crewmembers likely pulled hard on the yoke to arrest the downward trajectory, and, in doing so, placed the wings broadside against the force of the relative wind, which resulted in both wings failing upward. As the wings failed, the propellers simultaneously chopped through the fuselage behind the cockpit. At the same time, the horizontal stabilizers were also positioned broadside against the relative wind, and they also failed upward. Evidence also revealed that, at some point, the flight crew lowered the landing gear. Although it could not be determined when they lowered the gear, it could have been in an attempt to slow or regain control of the airplane during the descent.

Although reasons for the loss of control could not be definitively determined, the lack of any preexisting mechanical anomalies indicates a likelihood of flight crew involvement. Then, during the recovery attempt, the flight crew's actions, while operating under the difficult circumstances of darkness and rapidly decreasing altitude, resulted in the overstress of the airplane.

No Metars are available for Arecibo, Aguadilla Airport 25nm west of Arecibo reported:
TJBQ 030150Z 00000KT 10SM SKC 24/ A2996 RMK RWY08 ATIS L
TJBQ 030050Z 16003KT 10SM FEW020 24/ A2994 RMK RWY08 ATIS K
TJBQ 022350Z 17003KT 10SM SCT020 SCT040 27/ A2993 RMK RWY08 ATIS J
TJBQ 022250Z 11005KT 10SM SCT010 SCT030 28/ A2991 RMK RWY08 ATIS I
TJBQ 022150Z 10005KT 10SM SCT020 29/ A2988 RMK RWY08 ATIS H
Incident Facts

Date of incident
Dec 2, 2013

Classification
Crash

Aircraft Registration
N831BC

ICAO Type Designator
SW4

This article is published under license from Avherald.com. © of text by Avherald.com.
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