Stabilizer failure caused crash of Grob prototype

 - May 19, 2010, 12:17 PM

The 2006 fatal crash of the Grob G180A SPn midsize jet prototype was caused by failure of the twinjet’s horizontal stabilizer, according to the final report issued by the German Federal Bureau of Aircraft Accident Investigation (BFU). “The wreckage pattern allowed for only one conclusion: that the horizontal stabilizer had suffered an in-flight breakup due to aerodynamic flutter,” which rendered the aircraft uncontrollable, the report stated. While the cause of the flutter was not determined, the investigators considered a retrofitted elevator mass balance a possible culprit, along with prior damage to the elevator horn. The company’s chief pilot was killed in the crash.

The composite aircraft was not equipped with a flight data recorder or telemetry transfer system, which would have presented investigators with information on its flight profile, system performance and speeds. The only data for the accident was that collected through the local ATC radar traces of the final flight. Likewise, the aircraft was not furnished with a cockpit voice recorder.

At the time of the crash the SPn was being flown on a demonstration flight. According to company policy, a “reduced flight display” with a speed of less than 200 knots and a cloud-base minimum of 1,500 feet was called for under those circumstances, yet the aircraft was flying at up to 270 knots and it is likely that the company-mandated 500-foot vertical separation from clouds was not observed, since witnesses reported seeing it repeatedly enter and exit clouds. The report noted that under the stated policy the flight should not have been conducted due to the weather conditions, yet the investigators did not believe the failure to adhere to the limitations played a significant role in the accident.

As a result of the accident, the BFU issued several safety recommendations. In a request to both the Federal German Civil Aviation Authority and the EASA, the BFU recommended that all test aircraft with an mtow of more than 5,700 kilograms (12,566 pounds) be equipped with flight data recorders and cockpit voice recorders, or an uninterrupted telemetric datalink.

Design Changes Needed
The accident and resulting design changes caused delays that eventually derailed the aircraft’s certification and contributed heavily to the company’s insolvency. While development of the SPn has been shelved, plans still exist for the program’s resurrection, causing the BFU to recommend that the owners of the construction drawings, documentation and certificates of compliance thoroughly scrutinize the design, construction and technical arrangements of the horizontal stabilizer with respect to aerodynamic efficacy, strength and absence of flutter, redesigning it if necessary.

The agency also recommended that the EASA and other civil aviation authorities “check and monitor the integration of subcontractors in design organizations engaged in the design and construction of aircraft as described in EASA Part 21, 21A.239, and regulate as necessary.”

The accident board also found fault with the aircraft’s 406-MHz ELT, which failed due to impact damage. In response, the BFU called on ICAO, the FAA and EASA to mandate that all such systems have a supplemental internal antenna or an external antenna that is designed to continue to transmit after an accident. The transmitter was not a factor in locating the accident wreckage.