Bell 525 fatal accident July 2016
Well, the Bell 204/205/212/214 were known for their High Inertia Rotor Systems!
525 resumes flights
https://www.verticalmag.com/press-re...-test-program/
https://www.verticalmag.com/press-re...-test-program/
Cause here:
In case the link doesn't work for non FB friends: Edit: the FB link doesn't appear fro some reason.
NTSB has determined the probable cause of the fatal Bell Helicopter 525 crash that killed both test pilots in July 2016. As R&WI was told in March 2017 by three people briefed on the investigation by the NTSB, investigators found vibration to be the probable cause.
This probable cause was approved Tuesday:
“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.”
Bell said investigation activities were completed in 2017. The manufacturer and the NTSB studies the cause of the vibration, "which had never been encountered before," Bell said.
"The vibration was the result of an unanticipated combination of very high airspeed with a sustained low rotor rpm condition. The in-depth analysis of the flight data resulted in a thorough understanding of the corrective actions necessary, and appropriate changes to the aircraft have been implemented," Bell said. "Changes include a further enhancement of the filtering system on the pilot’s side-stick controller so that vibrations of the pilot stick are not passed to the rotor system. Additional control system filtering was also applied to the system that is used to stabilize the helicopter in gusts and maneuvers."
In December 2017, the FAA proposed special conditions for the 525. It believed the model should feature mode annunciation because of the fly-by-wire technology.
“This helicopter will have a novel or unusual design feature associated with fly-by-wire flight control system functions that affect the pilot awareness of the flight control modes while operating the helicopter,” the Federal Register stated. “The applicable airworthiness regulations do not contain adequate or appropriate safety standards for this design feature. These proposed special conditions contain the additional safety standards that the administrator considers necessary to establish a level of safety equivalent to that established by the existing airworthiness standards.”
Aural cues were available to the crew regarding low rotor rotation, including the master warning annunciation and the sound of decreasing rotation. The NTSB said the master warning aural tone would have annunciated at 12.5 seconds and 16.8 seconds, continuing until acknowledged by the crew. But the same tone was used with at least 21 other warning messages, the NTSB said, because audio files had not yet been developed — the flight test team had decided this type of aural annunciation would be sufficient and a distinct tone for low rotor rotation speed was not immediately needed to conduct flight tests.
The 525 impacted terrain near Italy, Texas, July 16, 2016, after the tail boom separated from the fuselage in flight. The aircraft was involved in a series of one-engine-inoperative tests, according to the NTSB. On the final test, rotor rotation slowed from 100% to some 91%. In response, the crew began lowering the collective recover. The NTSB said that after the crew stopped lowering the collective, rotation only recovered to about 92%. Then the helicopter began vibrating at a frequency of 6 Hz.
The NTSB said vibration was evident in both rotor systems, the airframe, the pilot seats and the control inputs. After rotor rotation remained somewhat constant for a few seconds, it began to fluctuate, consistent with collective control inputs. The NTSB said subsequent collective control input increases led to further decay in rotation. As the collective was raised, it decreased to some 80% and the main rotor blades began to flap. NTSB said that the flapping then occurred low enough to impact the tail boom, severing it.
According to the investigation body, the main rotor, tail rotor, flight controls, powerplants and rotor drive systems showed no evidence of preexisting malfunction before the vibrations began. Structural wreckage did not show evidence that the oscillations alone resulted in a structural failure leading to the in-flight breakup. NTSB also said examination of the wreckage revealed no indications that the helicopter had been improperly maintained.
Bell designed software filters in the cyclic control laws in consideration to biomechanical feedback. (NTSB defines biomechanical feedback as “unintentional control inputs resulting from involuntary pilot limb motions caused by vehicle accelerations.”) However, no filter was designed for the collective. This means that the 6-Hz oscillatory collective inputs by the pilot were not filtered, NTSB said. The result was a control feedback loop when the pilot-held collective stick commanded an oscillatory collective pitch input into the main rotor, increasing the 6-Hz vibration, which in turn increased the magnitude of the oscillatory collective pitch input.
In addition, the NTSB continued, the gain between the pilot movement and the collective control stick movement in the vertical axis had not been tested on a shake table before the accident. For the cyclic control, though, lateral vibration was introduced on a shake table.
According to the NTSB, since the accident, Bell has:
• designed a software filter for the collective control law to dampen biomechanical feedback due to oscillatory control inputs as the frequency of control input increases
• adjusted the aero-servo-elastic model with a correlation factor to incorporate the aerodynamic effects observed during flight test and the accident test to preclude such occurrences seen in the accident flight's telemetry data
• performed shake tests with pilots using a side-stick collective to determine and incorporate the transfer function for human biomechanical feedback
• modified the AHRS software filters to further reduce the AHRS response to a 6-Hz airframe vibration
• indicated that, for the accident helicopter model, cockpit audio is now being recorded by an onboard CVFDR, and communications to and from the ground monitoring station are recorded by the CVFDR and the telemetry system during all flights (cockpit video is also being recorded by the instrumentation system and archived at the ground station)
• issued a company-wide business directive to ensure that cockpit audio is recorded during all telemetered flight test activities across all flight test sites
• plans to conduct flight testing in the 95% to 100% range of Nr in an OEI condition
• plans to implement, for the accident helicopter model, the unique low Nr aural tone in their test aircraft, and a software update that includes a larger font size for the Nr numeric display on the PSI
• plans to implement a separate PBA specifically for low Nr and is incorporating more salient cues into the tactile cueing system
• plans to incorporate the automatic termination of OEI training mode should Nr fall below a certain limit; and
• incorporated a safety officer for the accident helicopter model test program who will have dedicated safety-related responsibilities.
Bell resumed flight testing July 2017 after receiving an experimental certificate renewal from the FAA. It said at that time it still hopes to certify the 525 this year.
In case the link doesn't work for non FB friends: Edit: the FB link doesn't appear fro some reason.
NTSB has determined the probable cause of the fatal Bell Helicopter 525 crash that killed both test pilots in July 2016. As R&WI was told in March 2017 by three people briefed on the investigation by the NTSB, investigators found vibration to be the probable cause.
This probable cause was approved Tuesday:
“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.”
Bell said investigation activities were completed in 2017. The manufacturer and the NTSB studies the cause of the vibration, "which had never been encountered before," Bell said.
"The vibration was the result of an unanticipated combination of very high airspeed with a sustained low rotor rpm condition. The in-depth analysis of the flight data resulted in a thorough understanding of the corrective actions necessary, and appropriate changes to the aircraft have been implemented," Bell said. "Changes include a further enhancement of the filtering system on the pilot’s side-stick controller so that vibrations of the pilot stick are not passed to the rotor system. Additional control system filtering was also applied to the system that is used to stabilize the helicopter in gusts and maneuvers."
In December 2017, the FAA proposed special conditions for the 525. It believed the model should feature mode annunciation because of the fly-by-wire technology.
“This helicopter will have a novel or unusual design feature associated with fly-by-wire flight control system functions that affect the pilot awareness of the flight control modes while operating the helicopter,” the Federal Register stated. “The applicable airworthiness regulations do not contain adequate or appropriate safety standards for this design feature. These proposed special conditions contain the additional safety standards that the administrator considers necessary to establish a level of safety equivalent to that established by the existing airworthiness standards.”
Aural cues were available to the crew regarding low rotor rotation, including the master warning annunciation and the sound of decreasing rotation. The NTSB said the master warning aural tone would have annunciated at 12.5 seconds and 16.8 seconds, continuing until acknowledged by the crew. But the same tone was used with at least 21 other warning messages, the NTSB said, because audio files had not yet been developed — the flight test team had decided this type of aural annunciation would be sufficient and a distinct tone for low rotor rotation speed was not immediately needed to conduct flight tests.
The 525 impacted terrain near Italy, Texas, July 16, 2016, after the tail boom separated from the fuselage in flight. The aircraft was involved in a series of one-engine-inoperative tests, according to the NTSB. On the final test, rotor rotation slowed from 100% to some 91%. In response, the crew began lowering the collective recover. The NTSB said that after the crew stopped lowering the collective, rotation only recovered to about 92%. Then the helicopter began vibrating at a frequency of 6 Hz.
The NTSB said vibration was evident in both rotor systems, the airframe, the pilot seats and the control inputs. After rotor rotation remained somewhat constant for a few seconds, it began to fluctuate, consistent with collective control inputs. The NTSB said subsequent collective control input increases led to further decay in rotation. As the collective was raised, it decreased to some 80% and the main rotor blades began to flap. NTSB said that the flapping then occurred low enough to impact the tail boom, severing it.
According to the investigation body, the main rotor, tail rotor, flight controls, powerplants and rotor drive systems showed no evidence of preexisting malfunction before the vibrations began. Structural wreckage did not show evidence that the oscillations alone resulted in a structural failure leading to the in-flight breakup. NTSB also said examination of the wreckage revealed no indications that the helicopter had been improperly maintained.
Bell designed software filters in the cyclic control laws in consideration to biomechanical feedback. (NTSB defines biomechanical feedback as “unintentional control inputs resulting from involuntary pilot limb motions caused by vehicle accelerations.”) However, no filter was designed for the collective. This means that the 6-Hz oscillatory collective inputs by the pilot were not filtered, NTSB said. The result was a control feedback loop when the pilot-held collective stick commanded an oscillatory collective pitch input into the main rotor, increasing the 6-Hz vibration, which in turn increased the magnitude of the oscillatory collective pitch input.
In addition, the NTSB continued, the gain between the pilot movement and the collective control stick movement in the vertical axis had not been tested on a shake table before the accident. For the cyclic control, though, lateral vibration was introduced on a shake table.
According to the NTSB, since the accident, Bell has:
• designed a software filter for the collective control law to dampen biomechanical feedback due to oscillatory control inputs as the frequency of control input increases
• adjusted the aero-servo-elastic model with a correlation factor to incorporate the aerodynamic effects observed during flight test and the accident test to preclude such occurrences seen in the accident flight's telemetry data
• performed shake tests with pilots using a side-stick collective to determine and incorporate the transfer function for human biomechanical feedback
• modified the AHRS software filters to further reduce the AHRS response to a 6-Hz airframe vibration
• indicated that, for the accident helicopter model, cockpit audio is now being recorded by an onboard CVFDR, and communications to and from the ground monitoring station are recorded by the CVFDR and the telemetry system during all flights (cockpit video is also being recorded by the instrumentation system and archived at the ground station)
• issued a company-wide business directive to ensure that cockpit audio is recorded during all telemetered flight test activities across all flight test sites
• plans to conduct flight testing in the 95% to 100% range of Nr in an OEI condition
• plans to implement, for the accident helicopter model, the unique low Nr aural tone in their test aircraft, and a software update that includes a larger font size for the Nr numeric display on the PSI
• plans to implement a separate PBA specifically for low Nr and is incorporating more salient cues into the tactile cueing system
• plans to incorporate the automatic termination of OEI training mode should Nr fall below a certain limit; and
• incorporated a safety officer for the accident helicopter model test program who will have dedicated safety-related responsibilities.
Bell resumed flight testing July 2017 after receiving an experimental certificate renewal from the FAA. It said at that time it still hopes to certify the 525 this year.
So to this dumb old pilot, this is what it sounds like happened:
They were doing high speed single-engine cuts. During the last one, the MRRPM decreased to 91%. The pilot lowered the collective but the NR only came back up to 92%. The blade spacing got out of phase (which we know happens anyway with multi-blade rotors in forward flight) and a bad vibration began, apparently a vertical because they got into "biomechanical feedback" (which is I guess a fancy way of saying PIO) in the collective channel.
Things happen fast when they go wrong in helicopters.
The pilot of the chase helicopter radioed that the 525 was getting some serious blade flapping, but apparently it was too late. The thing came apart.
Part of the problem was that the 525 pilots couldn't quickly and easily get the second engine back online to get the MRRPM up. Computers, gotta love 'em! In a normal helicopter they could've just rolled the throttle up or pushed the lever forward. Not in these newfangled crash-by-wire birds! Not only that, apparently they couldn't see how low the NR had gotten because the cues on the display are rather small. Not that it would've helped if the vibration was preventing them from pushing whatever (presumably small) button to take them out of "simulated OEI" operation.
But it makes you wonder, doesn't it? I mean, how does an R-22 pilot get his MR back if it droops too low? Right, he pulls back on the cyclic and loads the rotor! Sometimes just lowering the collective doesn't do the trick. And so, because the 525 ON A TEST FLIGHT *inexplicably* did not have a working CVR, we will never know what the pilots were talking about and why they didn't "simply" raise the nose and get the speed down and the rpm up.
Bell says that they did not anticipate how bad the vibration would get at high-speed and low NR. Really? Hmm. I wonder why not? Maybe they should've have consulted with Sikorsky. Those guys up in Connecticut have much more experience designing and building multi-blade, high-speed rotors, something that Bell seems kind of new and inexperienced at.
Heh. "Biomechanical feedback."
They were doing high speed single-engine cuts. During the last one, the MRRPM decreased to 91%. The pilot lowered the collective but the NR only came back up to 92%. The blade spacing got out of phase (which we know happens anyway with multi-blade rotors in forward flight) and a bad vibration began, apparently a vertical because they got into "biomechanical feedback" (which is I guess a fancy way of saying PIO) in the collective channel.
Things happen fast when they go wrong in helicopters.
The pilot of the chase helicopter radioed that the 525 was getting some serious blade flapping, but apparently it was too late. The thing came apart.
Part of the problem was that the 525 pilots couldn't quickly and easily get the second engine back online to get the MRRPM up. Computers, gotta love 'em! In a normal helicopter they could've just rolled the throttle up or pushed the lever forward. Not in these newfangled crash-by-wire birds! Not only that, apparently they couldn't see how low the NR had gotten because the cues on the display are rather small. Not that it would've helped if the vibration was preventing them from pushing whatever (presumably small) button to take them out of "simulated OEI" operation.
But it makes you wonder, doesn't it? I mean, how does an R-22 pilot get his MR back if it droops too low? Right, he pulls back on the cyclic and loads the rotor! Sometimes just lowering the collective doesn't do the trick. And so, because the 525 ON A TEST FLIGHT *inexplicably* did not have a working CVR, we will never know what the pilots were talking about and why they didn't "simply" raise the nose and get the speed down and the rpm up.
Bell says that they did not anticipate how bad the vibration would get at high-speed and low NR. Really? Hmm. I wonder why not? Maybe they should've have consulted with Sikorsky. Those guys up in Connecticut have much more experience designing and building multi-blade, high-speed rotors, something that Bell seems kind of new and inexperienced at.
Heh. "Biomechanical feedback."
Part of the problem was that the 525 pilots couldn't quickly and easily get the second engine back online to get the MRRPM up.
212man:
Yeahhhhh, I get that. In fact the NTSB report describes what they did, but not *how* they did it. And...not being familiar with all this fly-by-wire stuff, I just wasn't sure how they could simulate a single-engine failure and "fool" the FADECs into not trying to maintain 100% NR, which is their primary task. But I guess they did.
Then again, we understand that in a twin, if you pull the power of one engine completely off at a very high-speed cruise, the good engine is going to try very hard to keep the NR up, to the point of overtemping itself - or- if you have overtemp protection provided by FADEC, letting the rotor RPM droop.
And Bell obviously wanted to avoid unnecessary damage to those expensive CT-7's. Thus, GE must've come up with some program that removed/disabled the governing feature and limiting power to the equivalent of OEI. Simple!
When they did that at 185 knots, things did not go according to plan. The rotor RPM drooped to 91%. The pilots only lowered the collective to a 58% index, not even as low as they'd pushed it down in prior tests at lower speeds. But the NR did not come back up. Then what they're calling an unanticipated "scissor-mode" started in the main rotor and the pilots got a bad vibration. As the pilot's inputs got out-of-phase with the vibration, the NR decayed to about 80%. Then the AHRS, which was supposed to help with stability, started working against them. (Shades of the 609 crash!)
Yowsa!
From the NTSB report:
I suggest that there was a third way: RAISING THE NOSE. Every helicopter pilot knows that simply raising the nose and loading the rotor results in an increase in NR. But the crew probably didn't know how low the NR was. There was only one master caution tone, and it was the same for *21* separate items. The little PSI (power situation indicator) screen just had a green arc with no numbers between 90 and 100, AND the engineers at Bell admitted (presumably with some throat-clearing and shoe-shuffling) that the information for that green arc were taken from another model helicopter! Whaaat? See, they hadn't actually tested the 525 at really low NR's yet, powered or otherwise.
Which might be an indictment of Bell or might not. In a test program for a new-design aircraft, you can only test for so many things on any given flight. Maybe Bell just hadn't gotten around to testing the rotor at really low NR's yet. Althoooooough...didn't they spend a lot of time flying that rotor around on a 214ST?
The NTSB goes on and on about why the pilots did not exit the "OEI training mode" when they realized that not only was their NR really low but that it was causing this horrible vertical vibration. NTSB give the impression that the crew had all the time in the world to troubleshoot and respond to the problem. In reality, when the bad vibration started, the flight would be over in another nine seconds. Those nine seconds must've been a wild ride.
Finally, it is simply inconceivable that in 2016 a company like Bell would send a sophisticated, experimental ship like the 525 up on an envelope-expanding test flight with no CVFDR, no camera in the cockpit and no way to record the pilots' intercom channel. The mind boggles at Bell's arrogance and negligence there. I mean, couldn't someone have mounted a GoPro in the cabin of that thing? It seems that every damn R-44 comes equipped with such a device. That flight test department does not seem to be very well-managed.
When I'm critical of things like the goofy 505, people often say to me, "Don't you think Bell knows how to design and build a helicopter?" Programs like the 525 development really make me wonder. This ain't the 1940's; test pilots shouldn't be expendable anymore.
Not quite - both engines were driving the rotor but with a limited power to simulate OEI.
Then again, we understand that in a twin, if you pull the power of one engine completely off at a very high-speed cruise, the good engine is going to try very hard to keep the NR up, to the point of overtemping itself - or- if you have overtemp protection provided by FADEC, letting the rotor RPM droop.
And Bell obviously wanted to avoid unnecessary damage to those expensive CT-7's. Thus, GE must've come up with some program that removed/disabled the governing feature and limiting power to the equivalent of OEI. Simple!
When they did that at 185 knots, things did not go according to plan. The rotor RPM drooped to 91%. The pilots only lowered the collective to a 58% index, not even as low as they'd pushed it down in prior tests at lower speeds. But the NR did not come back up. Then what they're calling an unanticipated "scissor-mode" started in the main rotor and the pilots got a bad vibration. As the pilot's inputs got out-of-phase with the vibration, the NR decayed to about 80%. Then the AHRS, which was supposed to help with stability, started working against them. (Shades of the 609 crash!)
Yowsa!
From the NTSB report:
Interviews with the helicopter manufacturer test pilots and engineers suggest that there were two ways for the pilots to exit the low Nr and, correspondingly, the vibration condition: (1) lower/reduce the collective to increase Nr or (2) exit OEI training mode, which would increase power available from the engines.
Which might be an indictment of Bell or might not. In a test program for a new-design aircraft, you can only test for so many things on any given flight. Maybe Bell just hadn't gotten around to testing the rotor at really low NR's yet. Althoooooough...didn't they spend a lot of time flying that rotor around on a 214ST?
The NTSB goes on and on about why the pilots did not exit the "OEI training mode" when they realized that not only was their NR really low but that it was causing this horrible vertical vibration. NTSB give the impression that the crew had all the time in the world to troubleshoot and respond to the problem. In reality, when the bad vibration started, the flight would be over in another nine seconds. Those nine seconds must've been a wild ride.
Finally, it is simply inconceivable that in 2016 a company like Bell would send a sophisticated, experimental ship like the 525 up on an envelope-expanding test flight with no CVFDR, no camera in the cockpit and no way to record the pilots' intercom channel. The mind boggles at Bell's arrogance and negligence there. I mean, couldn't someone have mounted a GoPro in the cabin of that thing? It seems that every damn R-44 comes equipped with such a device. That flight test department does not seem to be very well-managed.
When I'm critical of things like the goofy 505, people often say to me, "Don't you think Bell knows how to design and build a helicopter?" Programs like the 525 development really make me wonder. This ain't the 1940's; test pilots shouldn't be expendable anymore.
Yeahhhhh, I get that. In fact the NTSB report describes what they did, but not *how* they did it. And...not being familiar with all this fly-by-wire stuff, I just wasn't sure how they could simulate a single-engine failure and "fool" the FADECs into not trying to maintain 100% NR, which is their primary task. But I guess they did.
