Inverters eyed in OSU King Air crash

 - May 7, 2008, 6:12 AM

Failed inverters appear to be the prime suspect in the NTSB’s investigation of the January 27 crash of a King Air 200 in Strasburg, Colo. The two crewmembers and eight Oklahoma State University (OSU) basketball team members and personnel aboard the twin turboprop were killed in the accident.

The Board released factual information in mid-August, and investigators revealed findings of broken fuses in each inverter, which they do not believe are attributed to post-crash damage. A 2,800-ft-long debris field and the length of time from last communication to the point of impact indicate the turboprop did not careen wildly out of the sky. Powerplant, system and structure analysis, beyond the electrical system, do not suggest mechanical failures contributed to the accident.

The Part 91 flight departed Jefferson County Airport, just west of Denver, at approximately 1718 MST. Weather conditions at the time of departure were an indefinite ceiling of 200 ft overcast and one mile visibility. Meteorological investigators estimate the cloud tops were around 27,000 ft and, although a sigmet for icing existed, no pireps were made regarding icing.

Further weather analysis provided by researchers at the National Center for Atmospheric Research (NCAR) showed little or no probability for icing or supercooled large droplets. In fact, one of the researchers, Ben Bernstein, was inbound to Denver at the time of the accident and noted the cloud structure was not conducive to icing. Bernstein told the NTSB that there were no layers and they broke out very low to the ground.

At 1719:55 the pilot of the King Air, registered as N81PF, checked in with Denver Departure and reported leaving 6,500 ft for 8,000 ft. The controller cleared them direct to epkee–located about 42 mi east of the Denver VOR on the Plains Two Departure–and up to FL 230. N81PF acknowledged the clearance. At 1724:07 the controller assigned a heading of 110 deg and N81PF acknowledged.

A minute later ATC transferred N81PF to a different sector controller. The pilot checked in at 1726, reporting an altitude of 16,300 ft climbing to FL 230. The controller questioned the latest clearance and asked if he was direct epkee. The pilot repeated the assigned heading of 110 deg, to which the controller reissued the direct epkee clearance.

Telling the controller he would be turning about three degrees to the left, the pilot replied he understood direct epkee. There was no further communication between the pilot and ATC, and the aircraft crashed less than 11 min later.

If the pilots lost both electrical inverters as is suspected–maintenance records show this happened in this aircraft once before in October 1990–the pilot would have been left with an airspeed indicator and a rate-of-turn indicator. The copilot would have had an altimeter, airspeed indicator, attitude indicator and rate-of-turn indicator.

Investigators found the inverters’ internal fuses broken but not melted or burned, leading them to believe they were broken in flight. Communication equipment was not discussed in the factual report. However, the right circuit-breaker panel, which suffered crash and fire damage, was found with all the breakers popped.

Investigators tested a similar King Air for consequences of AC power failure and found flags appear in most of the instruments, including the pilot’s altimeter, attitude indicator, HSI, radio altimeter and RMI. In addition, the copilot’s RMI, HSI and the altitude preselect show flags as well. The majority of this equipment found in the wreckage, when readable, was flagged.

Oxygen and pressurization equipment was found in various states of destruction. The pressure transmitter, found broken open and with internal components missing, sustained fire damage. According to investigators, the oxygen controller case was “compromised by impact and the faceplate was contaminated.” A section of the flow control valve was missing, while the metal pneumatic line remained attached but was bent and distorted. Investigators found only one emergency oxygen mask and hose; it had cuts, rips and holes “consistent with the bag being folded at impact. It was not inflated and the hose was still attached.”

The NTSB told AIN the cockpit sustained tremendous damage, but it appeared the mask was “folded” as it would be if stowed. The oxygen bottle, normally located on the backside of the aft pressure bulkhead, was found at the end of a 100-ft trajectory. It was empty, with the end fitting missing.

Denver Mills, the 55-year-old PIC, had more than 5,000 hr TT, including 762 hr in the King Air 200. His copilot, Bjorn Fahlstrom, 30, had 1,828 hr TT and some 1,200 hr multi. Neither pilot tested positive for drugs or alcohol and fatigue did not appear to be a factor.

While much of the investigative work is complete, probable cause is still pending.