NTSB Factual Report

HISTORY OF FLIGHT

On 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 INFORMATION

The 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 INFORMATION

The 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 engine deck. The helicopter was equipped with a triple-redundant fly-by-wire flight control system with a triplex hydraulic system. Additionally, the helicopter was equipped with retractable tricycle landing gear.

The cockpit was configured for two pilots in a side-by-side seated position and a center console between them. Each pilot had a cyclic side-stick controller forward of the seat's right armrest, a collective side-stick controller immediately forward of the seat's left arm rest, and a set of pedals forward of their feet. The instrument panel consisted of four identical primary flight display (PFD)/multifunction display (MFD) panels. The center console had two Garmin Touch Control (GTC) panels, the landing gear handle, the Nav/Com panel, and the flight test switch panel, which included some controls for the OEI special training mode software. Directly above the GTCs were the engine control COSIF (crank, off, start, idle, fly) knobs. Each pilot had an additional pilot display unit that provided real-time flight test instrumentation parameters such as DA, boom airspeed, mast airspeed, engine torque, load factor, pitch/yaw/roll rates, slip angle, and main rotor and tail rotor flapping angles.

OEI Training Mode

OEI training mode is a specific GE software-driven capability that permits simulation of a single-engine failure without actually rolling back or shutting down an engine in flight. When the flight crew engages the OEI training mode, both engines reduce power to represent the power available from a single engine. Consistent with normal operations and depending on the flight conditions, if the power demanded by the rotor exceeds the power available, Nr will droop. If single-engine power is insufficient to sustain the forward speed, the pilot must reduce the power demand by lowering the collective control, applying aft cyclic (to reduce speed), or using a combination of both. Nr increases to 103% when the power required matches the single-engine power available.

To engage OEI training, the pilot or copilot navigates to the OEI training page on the GTC and selects the engine to fail on the touch screen. Once selected, a green bar appears on the failed engine button to signal that OEI training mode was engaged (see figure 2). When OEI training mode is engaged, the pilot's side (right-seat) PFD displays simulated OEI engine values, and the copilot's side (left-seat) PFD displays the actual all-engines-operative (AEO) data.

Figure 2. OEI training page on the GTC.

Source: Bell Helicopter

The OEI special training mode that Bell used for the accident flight test did not incorporate an automatic disengagement of OEI training mode for low Nr. Bell modified the production version of OEI training mode software, originally created by GE, to eliminate a safeguard that automatically exited the OEI training mode when Nr fell below 90%. According to Bell, automatic disengagement at 90% Nr was not low enough to allow development and demonstration of OEI recovery across the flight envelope during testing, and a lower Nr value for automatic disengagement was deemed unnecessary due to the highly controlled test environment. To manually exit OEI training mode, the pilot could (1) press the engine fail button on the GTC (the same button used to engage OEI training mode), (2) exit the OEI training page on the GTC (using the BACK button), or (3) move the COSIF switch to a position other than "Fly" and then return the switch to "Fly." The Bell 525 lead test pilot indicated in a postaccident interview that the options to exit OEI training mode were not discussed formally with all the test pilots but were specifically discussed with the accident test pilot. Bell 525 test pilots interviewed said that they almost always press the engine fail button on the GTC to exit OEI training mode; some Bell pilots were aware of the other methods to exit OEI training mode while other test pilots were not. Disengaging OEI training mode would make both engines available to provide full power to restore the reference Nr to 100% if the rotor was in a drooped state.

The production OEI training mode, which will be used in Bell 525 production helicopters, includes an automatic disengagement of OEI training if Nr decays below 90% (pending validation via testing). In the production OEI training mode, automatic exit would occur in the following circumstances:

-Loss of an engine.

-Torques of the two engines are not within ~30 ft-lb of each other.

-There are any significant engine failures (any fault that would cause local channel degraded on any of the 4 channels). If the enable bit for training is set (bit 20) AND both engine request bits are set (bit 21 and 22). To engage training only one-engine request bit can be set.

-Power turbine speed (Np) is 5% below the reference value (having previously been within 1% of the reference while in training) or to a value below 90%.

-Np is above 106%.

-Real engine gas producer turbine speed is above 106%.

-Real engine measured gas temperature is above 1934.3° F/ 1056.8° C.

-Real single-engine torque is above 521 ft-lb (67.7%).

-Real engine oil temperature is above 148.89° C.

-Low oil pressure switch is tripped.

OEI training mode flight test risk analysis worksheets documented planned operational risk mitigation for OEI training. A worksheet approved on June 29, 2015, included a discussion of the risk of low Nr, and a worksheet approved on April 1, 2016, included a discussion about engine overtorquing.

Power Situation Indicator (PSI)

The PSI was located in the bottom left corner of the PFD for each pilot. The bars in the bottom right corner of the PSI represented Np for the number 1 engine, Nr, and Np for the number 2 engine, respectively. The arc in the center of the display depicted the percentage of engine value compared to its limit (see figure 3).

Figure 3. Example of PSI on the Bell 525.

Source: Bell Helicopter

Indications of Low Rotor Rpm in the Bell 525

Power Situation Indicator

The PSI displayed Nr as a vertical scale (center bar in lower right indicator) when Nr was above 90%, as shown in figure 3. If Nr dropped below 90%, the display changed to an analog needle that displayed a green arc for Nr between 100 and 90%, a yellow arc for Nr between 86 and 89%

NTSB Probable Cause

A severe vibration of the helicopter that led to the crew's inability to maintain sufficient rotor rotation speed (Nr), leading to excessive main rotor blade flapping, subsequent main rotor blade contact with the tail boom, and the resultant in-flight breakup. Contributing to the severity and sustainment of the vibration, which was not predicted during development, were (1) the collective biomechanical feedback and (2) the attitude and heading reference system response, both of which occurred due to the lack of protections in the flight control laws against the sustainment and growth of adverse feedback loops when the 6-hertz airframe vibration initiated. Contributing to the crew's inability to maintain sufficient Nr in the severe vibration environment were (1) the lack of an automated safeguard in the modified one-engine-inoperative software used during flight testing to exit at a critical Nr threshold and (2) the lack of distinct and unambiguous cues for low Nr.