N525TA

HISTORY OF FLIGHTOn July 6, 2016, about 1148 central daylight time, an experimental research and development Bell 525 helicopter, N525TA, broke up in flight and impacted terrain near Italy, Texas. The two test pilots received fatal injuries, and the helicopter was destroyed. The helicopter, which was owned by Bell Helicopter Textron, Inc., was being operated under the provisions of 14 Code of Federal Regulations (CFR) Part 91 as a developmental flight test. Visual meteorological conditions prevailed at the time of the accident. The flight originated from Arlington Municipal Airport, Arlington, Texas.

About 0630 on the morning of the accident, the two test pilots, flight test engineers, and a chase helicopter flight crew briefed the planned flight. The brief detailed that the accident helicopter, accompanied by a chase helicopter, would proceed to the Arlington Initial Experimental Test Area (about 30 miles south of Arlington Municipal Airport) to perform the in-flight portion of the tests. The purpose of the flight was to evaluate engine loads at maximum continuous power, two-to-one-engine simulated engine failures, longitudinal roll oscillations, and run-on landings in the heavy, forward center-of-gravity configuration.

The test card for the two-to-one-engine simulated engine failure detailed that the pilots would simulate the loss of engine power from one engine while keeping both engines operating by using one-engine-inoperative (OEI) special training mode software, which reduced the power output of both engines to represent the maximum power that can be produced by one engine. When the OEI special training mode was engaged and a loss of power was simulated, the pilot would monitor rotor rotation speed (Nr) and intentionally delay his response by about 1 second before recovering from the maneuver by lowering the collective to reduce the power demanded by the rotor (and increase Nr). The lowest allowable Nr limit was identified as 86%; if Nr went below 86%, the test would be halted, and the crew would recover Nr to 103%, exit OEI special training mode, and return to steady level flight. A Bell structural engineer stated that flight below 86% Nr would result in the helicopter returning to base. During test flights, flight test engineers monitor real-time telemetry data from the helicopter under the oversight of the flight test director, who was in direct radio communications with both the test helicopter pilots and the chase helicopter pilots.

About 0959, weather conditions were determined to be acceptable for the flight, and about 1038, the helicopter departed for the test area, followed by the chase helicopter. About 1048, the pilots established the helicopter's maximum level flight airspeed (Vh) at 4,000 ft density altitude (DA) as 148 knots calibrated airspeed (KCAS). After performing steady-heading sideslips, the pilots performed a series of level turns and then began the two-to-one-engine simulated engine failures.

About 1108, the pilots set the OEI training mode shaft horsepower to a value predetermined by the flight engineers. The first three tests were performed in level flight at 102 KCAS, 131 KCAS, and 145 KCAS. The pilots then performed tests at 155 knots true airspeed (KTAS), 160 KTAS, 165 KTAS, and 175 KTAS, which required the helicopter to be in a shallow descent to achieve the required airspeed. These OEI tests had resulted in a rotor speed decay of 5 to 13% Nr. During these tests, to allow Nr to recover to 97% or greater, the crew lowered the collective input to near or below 50%. (100% is the full-up collective position, and 0% is the full-down collective position.) Data recorded on the helicopter's flight test recorder system, which was typically downloaded after each test flight and also transmitted via a telemetry stream to Bell's flight-test facility for real-time analysis and recording, indicate the build-up tests and recovery time required (see table 1). (Record 45 was a void record, and record 49 was aborted because of two engine torque spikes typical of wind gust encounters.)

Table 1. Build-up tests and recovery time required.

During the build up to the final test, the flight test engineers received warning and alert notifications, most of which related to main rotor and tail rotor pitch link loads, pylon loads, and tail boom loads. These alerts and warnings were expected as the airspeed increased and the dynamic loads on the rotor system and airframe also increased. During most of the OEI transitions, the pilot responded by lowering the collective between 1 and 2 seconds after the simulated loss of engine power. However, with each increase in airspeed, the time the crew took to recover Nr to the target value of 103% was longer. Bell test pilots indicated that they interpreted this trend as the tendency of the crew to be more judicious while applying collective at successively higher airspeeds in order to avoid recovering too fast and overspeeding the rotor or damaging the transmission.

About 1148, the final test was performed at 185 KTAS, which was the helicopter's never-to-exceed speed (Vne) at the time of the test flight; the set up and entry were the same as the previous tests. OEI was engaged, and Nr drooped to about 91% within 1.5 seconds. The Nr decay was stopped by the pilot's reduction of collective, and Nr began to recover and leveled out around 92%. The crew stopped lowering the collective at the 58% collective stick position. About 7 seconds after arresting the Nr decay (about 12 seconds into the test), the structural dynamics engineer noticed increased engine vibrations, at which point he called "knock-it-off." The test director radioed to the Bell 525 pilots to "knock-it-off," while other engineers in the telemetry room were receiving warnings and alerts and were reinforcing the "knock-it-off" call.

The crew of the chase helicopter, which was positioned about 100 ft above and on the right side of the Bell 525 about 3 to 4 rotor diameters away, heard the test director call "knock-it-off" about the same time they observed the 525's rotor blades flying high and the rotor looking wobbly and slow. The chase helicopter crew radioed, "Hey, you're flapping pretty good," but the 525 pilots did not respond. About 21 seconds into the test, the main rotor severed the tail boom, and the telemetry signal was lost. The chase helicopter crew observed the helicopter's tail and fuselage jack-knife and debris separate from the helicopter. The chase helicopter crew radioed to the test director, "We've had a major accident," and landed near the wreckage to attempt assistance. PERSONNEL INFORMATIONThe pilot held a letter of authorization (LOA) from the Federal Aviation Administration (FAA) dated December 2, 2015, authorizing him to act as pilot-in-command (PIC) of the Bell experimental helicopter designated model 525. He completed crew resource management (CRM) training on January 12, 2015. The pilot graduated from the United States Naval Test Pilot School (USNTPS) in 2010. He then worked on numerous flight test projects involving the Bell AH-1W (SuperCobra, a twin-engine attack helicopter) and UH-1Y (Venom/Super Huey, a twin-engine utility helicopter). On September 23, 2013, he was hired by the Bell Helicopter flight test department as a pilot for the Bell 525 program.

The copilot held an LOA from the FAA dated December 2, 2015, authorizing him to act as PIC of the Bell experimental helicopter designated model 525. He completed CRM training on January 12, 2015. The copilot completed US Navy flight training in 2000 and graduated from the USNTPS in 2006. He then worked on numerous AH-1W and UH-1Y test programs. On August 2, 2010, he was hired by the Bell Helicopter flight test department as a pilot for the Bell 525 program. AIRCRAFT INFORMATIONThe accident helicopter was a conventional main rotor and tail rotor design (see figure 1). On April 25, 2016, the helicopter received its latest experimental research and development airworthiness certificate from the FAA. The helicopter was a manufacturing prototype being developed for certification as a transport-category helicopter in compliance with 14 CFR Part 29. As part of the airworthiness certificate, the FAA issued an operating limitations document (also dated April 25, 2016) that specified the following: pilots operating the helicopter must hold a temporary LOA issued by an FAA flight standards operations inspector to act as PIC, the helicopter must be maintained by an FAA-approved inspection program, day visual flight rules flight operations are authorized, and all flights must be conducted within the Arlington Initial Experimental Test Area. The helicopter was estimated to weigh about 19,975 lbs at the time of the accident.

Figure 1. Accident helicopter (Bell 525, N525TA).

Source: Bell Helicopter

The Bell 525 helicopter had a five-bladed main rotor that provided helicopter lift and thrust and rotated in a counterclockwise direction when viewed from above. The main rotor was a fully articulated system that used elastomeric bearings to accommodate blade feathering, flapping, and lead-lag motions. Fluid-elastic dampers moderated lead-lag motion of the blades. The five main rotor blades were identified by colored stickers, presented in order of advancing rotation (when seated in the pilot seat and observing the blades pass from right to left): blue, orange, red, green, and white. The Bell 525 also had a four-bladed, fully articulated, canted tail rotor that provided thrust to counteract main rotor torque effect, control helicopter yaw, and provide lift. The four tail rotor blades were identified by colored stickers, presented in order of advancing rotation: blue, orange, red, and green. The helicopter was equipped with two General Electric (GE) CT7-2F1 turboshaft engines, mounted aft of the main transmission, and one Honeywell RE100BR auxiliary power unit (APU), mounted between the two engines at the aft end of the eng...