Wichita Falls, Texas: Friday, 10 August 2001

By Jeff Keaton. Edited by Jay Carter, Jr.

Spinner Bracket Failure Causes Emergency Landing

On Sunday, August 5, 2001 while flight-testing near Olney, Texas, Rusty Nance, Chief Test Pilot of the CarterCopter Technology Demonstrator (CCTD), made an emergency landing after the bracket that holds the propeller spinner failed. Larry Neal was also on board at the time serving as Copilot and Flight Test Engineer. The CCTD was at an altitude of 4000 feet MSL and accelerating through 125 mph when the spinner support bracket failed, causing a loud bang and severe vibration. The spinner quickly disintegrated and its pieces impacted other parts of the aircraft including the tail booms, propeller, rotor, and cowl flap. The pilots immediately shut down the engine and executed a successful, very low speed dead stick landing on the runway. There were no injuries to the pilots or anyone on the ground. In three years of flight-testing, this is the first in-flight mechanical failure not associated with the powerplant and the second emergency landing.

This incident demonstrates some of the safety features designed into the CarterCopter. Because its ratio of lift to drag is better than any other rotary wing aircraft, it had plenty of range to easily glide back to the airport. The pilot was then able to transition to autogyro mode and make a very low speed, short landing.

This event ended the flight-test series that began on Friday, August 3 to evaluate a configuration change which moved the rotor aft in order to reduce the amount the aircraft would be pitched up at lower airspeeds. Another recent modification to prevent the landing gear from collapsing all the way if the air pressurization system failed, involved increasing the amount of oil in the landing gear system when extended. An engine-driven air compressor provides the principal air pressurization and pressurizes the system in less than 20 seconds. The backup electric compressor takes approximately 8 minutes to fully charge the landing gear and is also used as a backup to the control force trim system. With the engine shut off, there was not enough time to fully pressurize the landing gear. The extra oil in the landing gear kept the landing gear from fully retracting and prevented any additional damage from occurring during the forced landing.

Initial failure analysis indicates a fatigue failure in the spinner bracket caused by stress concentrations in a non-tempered welded joint. This steel part had been in use for approximately 200 hours. We normally stress relieve welded parts, but did not on this part. The new spinner support bracket will be heat-treated, tempered, and built from material with three times the tensile strength of the old bracket. The propeller will have to be rebuilt, but we may be able to reuse the spar which would save 3-4 days of work. Repairs and modifications will take four to six weeks before flight-testing can resume.

Spinner failure sequence 5 August 2001

In addition to making repairs, we plan to make two modifications. We will change the rotor teetering axis angle (delta 3) from 30º to 10º which will allow the rotor to track the spindle movement exactly and not momentarily move some 90º to the direction of the spindle travel. In order to go faster than 160 mph, we will also rework the aft section of the aircraft fuselage to reduce the amount of air separation. This separation is significant and could cause an equivalent several square feet of flat plate drag.