Bell 525 fatal accident July 2016

[SansAnhedral]

Sounds like there was an incident today with a helicopter crash in the area where Bell performs 525 flight testing

[CertGuy]

Home | WFAA.com (http://Www.wfaa.com)

[CertGuy]

Fatalities. There is live stream on wfaa

[CertGuy]

Fatalities have been confirmed in a helicopter crash in Ellis County.

DPS and the Ellis County Sheriff's Office are on the scene of the crash near FM 876 and Bell Branch Road.

Bell Helicopter confirmed fatalities after its aircraft crashed. A source tells News 8 three helicopters were flying in formation when the incident happened.

Bell said in a statement the helicopter was "conducting developmental flight test operations on the Bell 525 at our Xworx facility in Arlington, Texas, that resulted in a helicopter accident."

Two farmers who witnessed the crash told News 8 they saw the helicopter flying from the northeast when it hit a power line and exploded.

One person was taken to a local hospital via EMS.

Stephanie Parker with the Ellis County Office of Emergency Management said on Twitter the FAA is closing the airspace above the crash location.

Check back for more on this developing story.

Copyright 2016 WFAA

[twinstar_ca]

damn.... RIP, brothers.... :(

[havoc]

Fatalities reported in Ellis Co. helicopter crash | WFAA.com (http://www.wfaa.com/news/local/helicopter-crash-reported-in-ellis-county/265212099)

[SplineDrive]

Bell Helicopter aircraft on test flight crashes in Ellis County | KXAN.com (http://kxan.com/2016/07/06/bell-helicopter-aircraft-on-test-flight-crashes-in-ellis-county/)

Has fairly clear video of the crash site and the tail section that appears to be separate from the rest of the crash site. My condolences to everyone involved.

[Vertical Freedom]

Rest in Peace Brothers (another sad, sad loss for our industry)

[Lonewolf_50]

Not sure if this figured into it, but a friend at work has a son living up in Denton area. Says that today therewas, due to a thermal inversion, a weather alert for "non-thunderstorm related wind shear." Unable at the moment to figure out how wide an area around Dallas/Fort Worth that weather pattern spread.

[Martin_Baker]

Vertical Mag is reporting the crash of a 525 during flight testing.

Condolences to the families of the 2 crew.

Very sad news.

[Mel Effluent]

Sad news here: https://www.flightglobal.com/news/articles/first-bell-525-test-aircraft-crashes-killing-crew-427063/.

[laurenson]

very sad indeed

Two killed as Bell 525 crashes during flight tests - Vertical (http://www.verticalmag.com/news/two-killed-bell-525-crashes-flight-tests/)

[Brother]

Horrible, RIP to those who test fly to bring us new technology aircraft.

Sounds insensitive, but I hope its not an aircraft problem. Our beleaguered industry does not need another airworthiness accident, especially with a new type which promises so much and will be a real jump in technology.

[sandiego89]

Very sad. Perhaps interesting in links above that local farmers reported a power line strike, but local law enforcement countered that.

[The Sultan]

The final flight. Ship was out at 190+ Knots.

https://www.flightradar24.com/data/aircraft/n525ta#a406614

The Sultan

[Johnny31297]

My condolences to friends and family of the lost crew members. :{

525 Relentless Prototype Crashes in Texas (http://www.heliweb.com/bell-525-relentless-prototype-crashes-texas/)

According to this, the tail boom was found 457.2 meters southeast of the main wreckage. Certainly don't wanna jump to conclusions, but mid-flight separation anyone?

[Reely340]

The final flight. Ship was out at 190+ Knots.

https://www.flightradar24.com/data/a...n525ta#a406614 (https://www.flightradar24.com/data/aircraft/n525ta#a406614)
How accurate is that speed info of flightradar24 ?

Is that some calculated "post processed" mess by flightradar24 or is it the actual a/c speed at a historic point in time verbatim as transmitted by ADSB-out?

[albatross]

Perhaps we should mourn the loss of the crew and refrain from speculation as to cause for the moment.
To all my friends at Bell.Sorry for your loss. Thoughts and prayers for the crew, their families and co-workers.

[alby3z]

RIP to our brothers. My thoughts goes to their families.

[picapart]

I would guess there was enough data transfer , black boxes and cvr as well as dash cam that it will be a big help on the investigation .
sad day though.

[megan]

Extract from one report, "It proceeded to the south, where it flew a pattern at altitudes between 2,000 and 3,000 feet. The last data point is at 1,975 feet at a speed of 199 knots at 11:47 a.m. A witness on the ground reported hearing a loud twin boom sound before seeing the helicopter make an immediate descent. Most of the helicopter debris is highly compact, though some is hundreds of feet away from the primary crash site. Dallas television station KXAS reports a section of the tail boom was located 1,500 feet southeast of the primary crash site."

[Three Lima Charlie]

No need to wait for the CVR and FDR data:
A Bell spokesperson said there was one "chase" helicopter present as well, which is standard in test flight scenarios.

[Lonewolf_50]

A witness on the ground reported hearing a loud twin boom sound before seeing the helicopter make an immediate descent Correlating that to what actually happened might be tricky given the broad range of audio interpretation on what a "boom sound" is. Megan, what is the source of that report?

[megan]

Lonewolf_50

Bell 525 Flight Test Vehicle Crashes; Two Crewmembers Dead | Business Aviation News: Aviation International News (http://www.ainonline.com/aviation-news/business-aviation/2016-07-06/bell-525-flight-test-vehicle-crashes-two-crewmembers-dead?eid=325861977&bid=1454679)

[PANews]

This morning the PR people at Bell announced they were suspending all promotion of the Bell 525 at Farnborough as a matter of 'respect'.

It was also announced that the CEO Mitch Snyder was no longer travelling to the UK for Farnborough for the same reason. Of course there were a number of other important Bell Helicopter programmes that he was there to promote besides the 525.

While perfectly understandable in the circumstances this is the second European media/PR event that Snyder has cancelled within a month. For the future health of the company there must surely be a limit to the number of times the CEO of any company can allow unfortunate circumstances to diminish his marketing role.

[Bell_ringer]

Considering the importance of the 525 to Bell and how much they have invested in it, his time is probably better spent overseeing their investigation into the accident and protecting the order book by reassuring customers.

[Sp 206]

Retreating blade stall. ??

[Lonewolf_50]

What leads you to that guess, Sp 206?

[henra]

What leads you to that guess, Sp 206?

It is much to early to conclude anything.
That said, 200kts, a new helicopter type being in initial flight envelope testing and alleged windy/turbulent conditions plus a chopped of tail could lead one to think in that direction.

[SASless]

Is that how a Rotor system would react to RBS.....Rotor system dipping to the rear or would the aircraft nose up and roll? How extreme would it be before the Pilot lowered the Collective and moved the Cyclic Aft upon realizing the aircraft had entered RBS?

Would there be any warning signs of the onset of RBS....what would they be?

[mnttech]

How accurate is that speed info of flightradar24 ?

Is that some calculated "post processed" mess by flightradar24 or is it the actual a/c speed at a historic point in time verbatim as transmitted by ADSB-out?



I wondered about the airspeed too. From the FAA website FAQ on ADS-B
ADS-B does not report vertical or horizontal airspeed. Instead, ADS-B reports horizontal and vertical velocity relative to the Earth. This velocity is useful for air traffic control functions and ADS-B applications. Airspeed can be provided by other aircraft sensors.
So that would ground speed, not an airspeed?

[Ascend Charlie]

Would a 5-blade head be susceptible to RBS anyway?? Always 2 blades on retreating side to share the load.

[IFMU]

When you look at RBS analytically it is generally with respect to the airspeed over the rotor disc. As you go faster eventually the whole retreating side has negative airspeed. RBS comes before that point as the blades need a higher AOA to preserve symmetry of lift. Extra blades are no guarantee against RBS.

Seems much too early to speculate. I am sure the data will tell its tale. My heart goes out to Bell, my former competitors yet we are brothers all.

[JohnDixson]

SAS, the modern multi-bladed rotors that I have experience with at Sikorsky do not exhibit the severe roll left tendency that I and perhaps others of the S-55/H-19 generation were schooled in.

I can only speak to the S-65,S-70, S-76 and S-92 series. If you drive these rotors into stall, the N/rev vibrations go up dramatically accompanied by non-linear ( with load factor ) increases in both the rotating ( push rods e.g. ) and stationary ( MR servo e.g. ) vibratory loads. A flight test team exploring stall limitations will of course be utilizing telemetry ( TM ) and the relevant loads will be a priority data stream for real time monitoring. Typically, the test team will work to a do-not-exceed limit ( DNE ) for these loads. The DNE is established based upon accelerated fatigue load testing of said components in the test lab, and are based on a very short component life, based in turn usually on a three sigma reduction from the component failure data. It is common for test teams to cycle count fatigue damage on these parts and they are replaced, discarded as required.

As you can see, this approach and taking into account that the published report is that the subject 525 was using TM, and against the personal experience just covered, makes it seem unlikely the cause was as simple as an entry into a rotor stall condition. But I hasten to add that I don't know anything about the 525 rotor, thus I'm out on a limb even going that far. It would be fair to add that during the 39 years at SA, there were a few rotor stability issues that cropped up, each with a different cause and a different solution, and I'd guess that other OEM's are no stranger to those occurrences.

Henra had it right: let the pro's at Bell who know their rotor and new machine do what they alone are equipped to do.

[SASless]

Brother Dixson as usual makes a very difficult explanation sound childishly simple!

The only helicopter I know for sure I had RBS happen was in a CH-47A due to a slightly over gross external load......and it occurred in the 46-48 knot range. Being Tandem Rotor....it was a very unusual result.

We had managed to trundle off with just over 16,000 pounds of 105 MM Howitzer Ammo due to a great amount of indiscretion. We realized our mistake when the FE counted the Projectiles.....and we tweaked to the fact we had grabbed a CH-54 Sky Crane Sling Load by mistake.

[JohnDixson]

SAS, just out of flight school, having wangled an assignment to the Test Board at Ft Rucker, I got to be instructed in the CH-47 by Boeing Test Pilot Jim Campbell*. Asking him re stall ( I had had two very realistic H-19 events post flight school ) in the Chinook, Jim said he had flown some of that work, and due to the lift split between the fwd and aft rotor, the aft operating with higher thrust, thus higher angle of attack, the aft rotor would stall first. He said it was therefore a sort of self-correcting situation: the aft rotor would stall first, the ship would go nose up, decelerating the aircraft and installing the rear rotor. That sound right?
* Heard later that unfortunately, Jim left Boeing for Air America and was KIA in SE Asia.

Your " CH-54 Pretend " flight might provoke some of the uninitiated to carp about failure to fill out a 365F Weight and Balance Form etc etc , and I thought you might comment re the very real issues of flying cargo in an active combat situation.

[Hot and Hi]

Would a 5-blade head be susceptible to RBS anyway?? Always 2 blades on retreating side to share the load.
No sure about that explanation. Typically you would say that for the same lift a rotor with more blades can be smaller in diameter, and can turn faster. As the retreating blade stalls when the airspeed of the a/c approaches the speed of the rotor blade relative to the airframe, that point it reached at higher airspeeds in a rotor that spins faster.

[Viper 7]

I had an RBS experience in a Sea King, high and fast and we got a very rapid onset 1:1 lateral oscillation.


It is 5 blades, fully articulated.


RIP to the lads, and condolences to the families.

[JohnDixson]

Viper, that sounds like pitch-lag instability, not stall.

Part 2: I looked in vain online for an old H-3 flight manual. There used to be a write-up on pitch-lag instability in the USN military manual.

Anyway, re H-3 pitch-lag: The H-3 rotor has some alpha-1 coupling in the geometry. In the H-3, as the blade lag angle increases ( which occurs with increased power ), the pitch angle decreases a bit. Hardly noticeable, but coming back into a hover on the 61, the pilot needs to nudge the collective up a bit more when he applies collective to come to the hover. Really a non-issue.

However, if a blade damper has a problem ( e.g. sticky relief valve ) then at higher speeds, and having nothing to do with stall*, the rotor can excite a pitch to lag angle instability. Pilots unfamiliar with the excitation may well call it a 1:1 or 1/rev, but in fact it is 2/3 per rev. Fix is to look at the dampers and check their timing.

* USN H-3 had a cruise guide indicator. The main rotor servo ( typically highest loads on the newer machines have highest loads on the aft longitudinal servo, but I honestly don't recall which servo on the H-3 had the highest loads ) had an LVDT ( linear,variable,differential transducer ) which measures the loads on that servo and feeds an indicator with range markings. Like the same system on the S-65 series, anything over 30% in indicative of increasing degrees of stall.

On the H-3, with a damper problem, one can see pitch-lag when the cruise guide will be barely indicating anything. It will be at higher power, thus faster speed, but its not a simple 1/rev. An easy way to evaluate the difference is that with a simple 1/rev, ( and assuming it is large enough to get one's attention )there is usually one blade out of track and quite visible from the cockpit. With pitch-lag, the main rotor feels and appears to wobble at a slightly slower frequency, and one can see that.

Rotor heads subsequent to the S-61 have a very flat alpha-1 geometry, that is, almost zero blade pitch change as the lag angle changes, and as a result have been absent this particular instability.

[NickLappos]

The flight test of a new aircraft involves a great deal of instrumentation and real-time monitoring by excellent pilots and engineers. Mysteries to field pilots, like Blade Stall, are easily measured and diagnosed as they start to occur in test aircraft, and watching these conditions develop is the craft of a modern test team. On all the test aircraft I flew, we had cockpit indications of the degree of stall, giving plenty of warning prior to any large stall event.
It is highly unlikely that the cause will be determined by a group of guys throwing darts at this forum, much more likely that a tragic set of individually obscure circumstances produce an unlikely chain of events that led to the mishap.
The craft of development helicopter flying is similar to clearing a mine field, where the object is to find the hazards before they trigger massive problems, using tools that find the problems before they become catastrophic. It takes about 1600 flight test hours to certify a helicopter type, and every one of those hours has a finite probability of catastrophe, even the mundane maintenance test flights and avionics check flights.
I know the Bell team well, they are pros, as good as anyone at this game. This has probably shaken them up because we all believe with confidence that our systems work well, and that data, careful test procedures and training will win every time. None the less, the nature of the unknown is to be unknown, and every now and then one of those mines goes off.
This accident is probably not an measure of the professionalism of the team, or of the innate airworthiness of the aircraft, it is a measure of the uncertainty of exploration, and the cost of being surprised.


A quote that helps explain it comes from racing driver Jackie Stewart, "Was I ever scared while driving? If I wasn't scared, I wasn't driving fast enough."

[oleary]

As always, thank you Nick, for your learned explanation.

I have stalled a 47, 205, 206, 212, 214ST, 500C, 500D, 55, 55T, 58, 58T and 61 (never stalled the 76).

Many have probably encountered it whether they recognize it as RBS or not.

It is easy to induce (any combination of high DA, high GW, high speed, high power, rapid change in attitude and/or turbulence).

It is also easy to recognize and recover from.

[gulliBell]

@NickLappos
199kts seems to me a pretty extreme space to occupy for a test flight. Did the S76 (or S70) ever go that fast during developmental test flying?

[JohnDixson]

Nick can best answer for the S-76.

The S-70 has a Vne of 193 KIAS and was tested there in accordance with US Army requirements,

Prior to production, the Army desired a test to achieve a free stream advancing tip Mach number of 1.0, and that was accomplished flying out of Burlington Vt, and the test instrumentation recorded a KIAS of 199.

[NickLappos]

John is right, many helicopters are tested in a dive to around 200 knots, the absolute requirement is to go to at least 1.2 Vh for a military machine (so if Vh is 161, Vdive is 193, in case you're wondering where John Dixson got the 193 from!)
For civil helos it is 1.11 times Vne (or actually, you operators get to fly to 90% of the maximum speed tested).
The S-76 flew a number of world records at 186 to 188 kts in level flight, and the most it has gone in a dive is about 210 knots (John, remember the Kollsman airspeed gauge with the stop at 155 knots, with Pasquarello and Mills flying? There is a great story there......)
The test point at 199 for the 525 tells us they have probably got about 165 max level flight speed, and that test point is fairly normal.

[John Eacott]

Nick,

Going a little OT: Happy Birthday :ok:

remember the Kollsman airspeed gauge with the stop at 155 knots, with Pasquarello and Mills flying? There is a great story there......

You can't possibly make that comment and not tell the story ;)

[SASless]

Nick and John probably remember stories of WPB Approch /Tower asking Airliners to increase their airspeed on the ILS due to overtaking aircraft behind them.....and the responses from them when told the traffic was a helicopter.

[HLCPTR]

Ah, yes....

"Helicopter 12345, reduce airspeed 20 kts for the Lear on final two miles ahead."

[blackdog7]

No further word yet from Bell?
High stakes when a multi billion dollar new platform has an inflight separation vs crew making contact with a power line, or both.
Textron has said the aircraft was exceeding expectations and had achieved over 200 knots, so speculation is speculation, be it RBS, separation, or wire strike.
What would your shareholders like to hear?
Condolences to all involved.

[Lonewolf_50]

High stakes when a multi billion dollar new platform has an inflight separation vs crew making contact with a power line, or both. Textron has said the aircraft was exceeding expectations and had achieved over 200 knots, so speculation is speculation, be it RBS, separation, or wire strike. If you take a look at the news reports, you'll find that emergency personnel who arrived on scene disagreed with an early report of the helicopter hitting the power lines, even though the aircraft's parts came down near some power lines.
http://www.verticalmag.com/news/two-killed-bell-525-crashes-flight-tests/
Photos of the crash site show a compact debris field, with only small fragments of the fuselage visible. WFAA reported eyewitness claims that the aircraft hit a power line and exploded, but the Texas Department of Public Safety said the aircraft did not strike the line and electricity transmission was unaffected.
http://www.heliweb.com/bell-525-relentless-prototype-crashes-texas/
The NTSB was headed to the crash scene at the time of the report. Early media coverage at the scene detailed an interview with two farmers that claim to have seen the helicopter come into contact with wires or a utility pole before the aircraft impacted the ground, however, followup reports stated that there had been no power outages in the area and all wires in the vicinity appeared to be intact. The crash resulted in a substantial fire that consumed a majority of the helicopter wreckage and singed the top of a utility pole close by. Helicopter debris was mainly localized at the crash site, however the tail boom of the 525 was reported to have been located 1,500 feet southeast of the main wreckage and appears from media footage shot from overhead to have no fire damage and a serrated angled tear at the point of separation.

[blackdog7]

So a chase aircraft, a platform stuffed full of sensors, and we have to base conclusions on conflicting news reports?

[JohnDixson]

SAS, there is a true and related anecdote re a 1972 hard IFR S-67 and Frankfurt Tower and an ILS clearance that I'll send via PM or email.

[The Sultan]

Blackdog7

The accident is being investigated by the NTSB. You obviously do not know what that means. Bell has turned over all data and only the NTSB can issue updates. When I say all I mean all, even copies. Failure to do this is a criminal offense. Look at the Gulfstream Roswell crash. Bell and vendors will assist, but the reports are NTSB.

The Sultan

[Viper 7]

Viper, that sounds like pitch-lag instability, not stall.

Part 2: I looked in vain online for an old H-3 flight manual. There used to be a write-up on pitch-lag instability in the USN military manual.

Anyway, re H-3 pitch-lag: The H-3 rotor has some alpha-1 coupling in the geometry. In the H-3, as the blade lag angle increases ( which occurs with increased power ), the pitch angle decreases a bit. Hardly noticeable, but coming back into a hover on the 61, the pilot needs to nudge the collective up a bit more when he applies collective to come to the hover. Really a non-issue.

However, if a blade damper has a problem ( e.g. sticky relief valve ) then at higher speeds, and having nothing to do with stall*, the rotor can excite a pitch to lag angle instability. Pilots unfamiliar with the excitation may well call it a 1:1 or 1/rev, but in fact it is 2/3 per rev. Fix is to look at the dampers and check their timing.

* USN H-3 had a cruise guide indicator. The main rotor servo ( typically highest loads on the newer machines have highest loads on the aft longitudinal servo, but I honestly don't recall which servo on the H-3 had the highest loads ) had an LVDT ( linear,variable,differential transducer ) which measures the loads on that servo and feeds an indicator with range markings. Like the same system on the S-65 series, anything over 30% in indicative of increasing degrees of stall.

On the H-3, with a damper problem, one can see pitch-lag when the cruise guide will be barely indicating anything. It will be at higher power, thus faster speed, but its not a simple 1/rev. An easy way to evaluate the difference is that with a simple 1/rev, ( and assuming it is large enough to get one's attention )there is usually one blade out of track and quite visible from the cockpit. With pitch-lag, the main rotor feels and appears to wobble at a slightly slower frequency, and one can see that.

Rotor heads subsequent to the S-61 have a very flat alpha-1 geometry, that is, almost zero blade pitch change as the lag angle changes, and as a result have been absent this particular instability.


Interesting stuff - I have no doubt that you are correct. Our machines were 1960s vintage and while very well maintained, tended to be cranky when pushed too hard. Best, V7 :ok:

[Lonewolf_50]

So a chase aircraft, a platform stuffed full of sensors, and we have to base conclusions on conflicting news reports?
Who is this we?
And why are you jumping to conclusions?

That asked, I concur with your point that good information should be available due to both telemetry and the chase plane. I am not sure that Bell is required to release that to the press, so all we have to work with is what is available through the press. (Sultan, I just realized, made this point somewhat differently).

I am confident that the NTSB is already working with Bell on assessing that information Bell should have on hand ... per Sultan's post.

[blackdog7]

"We" would be everyone who has speculated or commented on this thread.
Certainly not jumping to conclusions and am well aware of NTSB requirements and investigations.
Also aware of the warped reality of the value of human life and how multi-billion dollar programs can steamroll right over them to soothe investors.
Best of luck to Bell and TXT with the 525 program.

[NickLappos]

blackdog7,
I am afraid you have no idea what you are talking about. I well know the people at Bell and at the NTSB, I can assure others (not you) that they want nothing but the truth to come out of all this.
As for you, blackdog7 (+10 points for an apt name), I have often felt that people who have deep belief in unethical conspiracies are simply showing us what is inside their own heads, a squirming mess of conflict, lies and few scruples, so that these troubled people project that same unseemly world onto others they deal with.
I pity you.

[blackdog7]

Sorry Nick
A little too close to home I guess.
I pity you too!

[riff_raff]

The software used on almost every new FBW aircraft model has resulted in some problems. The most recent example is that of the A400 turboprop engine software which resulted in a fatal crash. The software code of the F-35 has also been plagued with problems. It would not seem unusual for Bell's first effort with a commercial FBW aircraft model to experience similar software problems.

[212man]

The software used on almost every new FBW aircraft model has resulted in some problems. The most recent example is that of the A400 turboprop engine software which resulted in a fatal crash

Ergo, nothing to do with FBW.....

[alby3z]

Ergo, nothing to do with FBW.....

Probably yes, but in FBW aircrafts the FCS interacts with the engine control system (e.g. fuel control laws scheduling as function of the altitude/airspeed), although I'm not aware of which kind of issue led to that particular accident.

[Gomer Pylot]

During a crash investigation the company can't release any information even to employees not directly involved in the investigation. Bell cannot even comment on the investigation other than to confirm that one is in progress. Soothing investors and customers is not a license to release anything. After the NTSB releases a report, Bell may be able to make comments, but not before.

[Lonewolf_50]

Given that this is a helicopter model in development, and not one that anybody is currently flying beyond the OEM and development team, the need or urgency to release an interim report or bulletin with alerts to users as the investigation progresses (like in the recent Bergen crash) isn't present. I'd suggest patience to all.

[griffothefog]

We only go forward because these men didn't go backwards...
RIP brave men 👏👏👏

[henra]

Given that this is a helicopter model in development, and not one that anybody is currently flying beyond the OEM and development team, the need or urgency to release an interim report or bulletin with alerts to users as the investigation progresses (like in the recent Bergen crash) isn't present. I'd suggest patience to all.

I don't see why speed of investigation should depend on the question if it is a Helicopter in development or in operational use.
It rather depends on the complexity of the task on hand. And this is obviously more complicated for a new model especially when it crashed during expansion of the envelope. The difficult task in that case is to find out whether it was due to a mechanical malfunction an unanticipated effect (aerodynamic, aero-elastic, etc.) or something in the interaction man -machine while in yet unproven terrain. On the plus side they might have more data due to instrumentation and chase aircraft.
For Bell finding the real cause will indeed be urgent because it will be important for the future of this type.

[Lonewolf_50]

I am not referring to speed of investigation, but the urgency (in time) of issuing any bulletin or report. Since nobody else is flying this helicopter the only people with an interest are already involved in the investigation.

[henra]

I am not referring to speed of investigation, but the urgency (in time) of issuing any bulletin or report. Since nobody else is flying this helicopter the only people with an interest are already involved in the investigation.


OK, fair enough!
Indeed for any one else besides Bell themselves there is no urgency in obtaining the information what went wrong.

[The Sultan]

From Rotor And Wing:

The No. 1 525 prototype’s main rotor blades appear to have struck its tail and nose during a high-speed, engine-out test that ended with the July 6 crash that killed its two pilots, Rotor & Wing International has learned.
“We saw signs consistent with a blade strike on the tailboom,” the National Transportation Safety Board’s investigator-in-charge for the probe, John Lovell, told R&WI, adding that investigators also believe “the nose was struck.”
The fly-by-wire Bell Helicopter super-medium, twin-engine helicopter broke up in midair during a flight to expand its operating envelope that included a test of the 525’s performance in one-engine-inoperative conditions as the aircraft approached its never-exceed speed (Vne), Lovell said. Bell early last March said it had flown the 525 above 200 kt in a shallow dive. A flight tracking service, Flightradar24, had reported July 6 that its last data set on the 525 flight put the aircraft at 199 kt at 1,975 ft.
Prior to the breakup, Lovell said, “data indicates that main rotor rpm dropped significantly.” He also said some of the aircraft’s main rotor blades “appeared to have dropped from their normal plane” of rotation.
Test pilots in a Bell 429 chase aircraft reported that some of the main rotor blades were moving out of plane before the aircraft broke up over Italy, Texas, about 30 nm south-southwest of the flight’s launch point, Bell’s Xworx research center at Arlington Municipal Airport.

Pretty well says it al. 40 knots beyond original Vne doing engine out tests, loss rpm, and the rotor stalled and/or experienced blow back.

The Sultan

[dangermouse]

FR24 shows groundspeed

A flight flown at Vne in IAS terms can give a completely different TAS or Groundspeed value depending on local ambient winds and density altitude.

There is no credible reason why an aircraft should be operating for certification purposes at Vne+40 kts, but in any case the report quoted states that they were approaching Vne, so the speeds quoted are meaningless or misleading.

Nobody here has the info required to work out what happened, so it would be better to stay quiet on any reasons why the aircraft and crew were lost, both on this programme and on the AW609 (but then again this is a rumour site so anything stated here should be taken with a huge dose of salt).

Thoughts from the southwest of the UK are of course with the crews families and the Bell employees involved.

DM

[The Sultan]

DM

Bell stated they were out at 200 kts and that would be from the card not FR24. As to reasons why the requirement is to demonstrate operations at 1.11 Vne. So if Bell was looking to expand to 180 knots, 200 would be the test point. Being out their testing engine failure is high risk, not something a line or armchair pilot would ever get close to. Some of us have.

The Sultan

[JohnDixson]

Sultan, are you suggesting there was a mixup re the specific tests to be conducted at the power on Vne ( actually 1.11 times that value ) and the OEI Vne as described in Advisory Circular 29-2C Para 29-1505?

As you implied, that testing is rather critical and typically the flight test cards and briefings are carefully ( very carefully ) constructed, so I kind of suspect I've misinterpreted your writing.

[SASless]

Sultan's ego must place a large load on his Rotor System....perhaps the Vibrations are getting to him.

I suppose he fails to grasp the experience level and background of a great many who attend this Forum.

Save your gratuitous remarks and stick to what ol' Joe Friday insisted upon will you...or as close to facts as you can get anyway.

Do you know exactly what the Task was that day, the details of the plan and brief, and the profile and Test Points being sought when the accident happened or are you just passing along some talk overheard at a Tavern somewhere?

[The Sultan]

JD

160 was the original Vne so I was just responding to why would anyone fly beyond VNE. Two reasons are expansion of the original envelope and the need to demo 1.1 Vne. As to AEO vs OEI Vne's those are defined in the end by testing the actual characteristics of the aircraft. That appears to be what Bell was doing. They may actually have a goal of a 200 Kt Vne. I was just throwing out the rationale of why testing at such speeds.

Finally. Not an expert on the nuances of Part 29, but you have to be able to transition from AEO Vne to OEI Vne so engine loss at Vne is required testing.

The Sultan

[JohnDixson]

Sultan: Not at 1.11Vne.

Look The Advisory Circular defines the testing requirements and is online. Have a read and you'll see what they had to do, and, important to your implication, not do.

Re the R/W note above. Doing the OEI Vne testing by itself would explain some Nr droop, but typically not result in the comment " significantly "*. The comments re the blades going out of track and the possibility of both nose and tail strikes opens up all sorts of possibilities, but one needs the telemetry data to make sense of what was going on.

*The FAA does not require simultaneous twin engine cuts on a twin engine Cat A machine. The military does, dependent on the rules set up for the particular aircraft. One day the Project Pilot on the CH-53D asked me if I was doing anything ( I wasn't ) and he needed me to help him do the twin cuts at max speed, max weight, with appropriate pilot delay time. Now that defined significant rotor speed droop ( high 70 % as I recall ). I still have a copy of the data traces. The aircraft rolled left very quickly and in fact my right leg got whacked pretty good when Frank put in full right cyclic. But the blades remained in track etc. Now, we had done a buildup at incremental speeds up to the max speed point and things had gone quietly, so the reaction at the final data point was without prior evidence that we were on the edge ( of rather severe stall ). This is not a suggestion at all re the Bell 525 situation, simply an observation that significant Nr droop ( depending on how the writer defines " significant " ) needs an explanation.

We can be certain that all of these aspects have already been examined in detail by the Bell investigative folks.

[212man]

Also, OEI is not AEI so it's not clear if this was being flown with one engine or not. I'd venture to suggest that at only 3000' or so, it would not be a dual cut. If it was indeed following a dual cut, it would be interesting to see if there are any similarities to the fatal AB139 accident that occurred during high speed autorotation testing (both bailed but FTE struck).

[JohnDixson]

212, for a Cat A aircraft, Part 29 does not require a simultaneous dual engine cut. It does require single engine cuts from dual engine flight and requires cutting the second engine from whatever speed the applicant selects for the OEI maximum speed ( usually OEI VH ). Part 29 also requires testing to whatever the applicant selects for the Vmax autorotation speed. That can have its own set of things to watch out for. After one reads all the testing required in that sub part G of the advisory circular and then comparing to the limited info in the referenced article, there is no certainty of exactly what the initial conditions were.

[The Sultan]

JD

Nothing I said or read say both engines were cut. As to Part 29 Sikorsky should read it for 30 minute loss of lube.

This was development/envelope expansion. Is not AEO all engines operating?

The Sultan

[212man]

Yes AEO is - I wrote AEI.

[HeliTester]

As to Part 29 Sikorsky should read it for 30 minute loss of lube.Wow! Startling comment!

[212man]

Not that startling given who wrote it!

[Lonewolf_50]

Yes AEO is - I wrote AEI. You can go back and edit your post to reflect the correct term so that what you wrote fits what you meant to write. (Heh, I just edited that last word since I wrote "wrote" instead of "write" and missed it the first time. Super example, eh?)


@TheSultan: John D has been sharing in good faith ... I'd encourage all of us to do likewise.
@SASless: winding up Sultan? Let's keep it classy.

[JohnDixson]

Nor did I imply such, Sultan.

The John Lovell quote re significant rotor speed droop, with no indication of what " significant " means in this instance, naturally provokes thought about what can cause that. My post 29 had a few thoughts about that. What I can add from experience is that cutting one engine from a two engine condition at the higher helicopter speeds, and under the assumption it is done in a controlled incremental build-up manner, shouldn't result in an Nr droop that would be abnormal. The speeds you mentioned, I.e., the 200 kt range, would be a dive point, hands on controls, with normal pilot reaction time. But as I wrote earlier, the information provided publicly to date does not allow the formation of any hypothesis that is defensible.

As to SA ( and I guess as a retiree that includes me ) reading Part 29 re the 30 min dry run Reg's , please check your PM's.

[The Sultan]

JD

I have heard of a number of 60's that ECU lockout checks on one engine resulted in damage to or loss of the aircraft. The highest visibility one was the HMX-1 ship. So the prescident has been established of unintended consequences when testing engine failure modes and those made it to the presidential fleet.

Report still remains doing engine out (one) testing at extreme of the airspeed range rpm was lost resulting in loss of the aircraft. As to what the crew felt or perceived prior to the event is still being analyzed.

The Sultan

[SASless]

Lone....gratuitous caustic comments elicit the same.

Just being a mirror.

[Lonewolf_50]

The Sultan:
Precedent.

The matter of the S-70 / H-60 ECU lockout, and the hazards thereof for the unwary (with which I am familiar) may be worthy of its own thread. It doesn't belong in this conversation. John D is not trying to start a Bell V Sikorsky thing, and quite frankly I think I speak for all of us when I say none of us participating in this thread wants to go there. This crash, this loss, where the state of the art of rotary wing aviation was in the process of being advanced (and it cost our industry two fine men) is of interest to a great many regardless of favorite model or favorite manufacturer.

[DOUBLE BOGEY]

Lonewolf, well said!

[212man]

Quote:
Originally Posted by 212man View Post
Yes AEO is - I wrote AEI.
You can go back and edit your post to reflect the correct term so that what you wrote fits what you meant to write. (Heh, I just edited that last word since I wrote "wrote" instead of "write" and missed it the first time. Super example, eh?)


@TheSultan: John D has been sharing in good faith ... I'd encourage all of us to do likewise.
@SASless: winding up Sultan? Let's keep it classy.
Last edited by Lonewolf_50; 27th Jul 2016 at 23:13. Reason: typing spazz
Lonewolf_50 is offline Report Post

Or you could just read more carefully next time! If you look at the bottom of your post it says 'edited' (as all edited posts do) - mine does not and is original!

[Lonewolf_50]

Or you could just read more carefully next time! If you look at the bottom of your post it says 'edited' (as all edited posts do) - mine does not and is original!
I saw that, and thus recommended the idea of editing the original. No worries. I suppose that the use of that forum feature is optional. :ok:

[Rotor George]

"Bell 525 Relentless broke up in mid-flight, NTSB says"

Bell 525 Relentless broke up in mid-flight, NTSB says | The Star-Telegram (http://www.star-telegram.com/news/business/aviation/sky-talk-blog/article92196487.html)

[OldblokeTH53]

https://www.flightglobal.com/news/articles/main-rotorblades-struck-nose-and-tail-of-crashed-bel-428027/

Main rotorblades struck nose and tail of crashed Bell 525, says NTSB

01 AUGUST, 2016 BY: DOMINIC PERRY LONDON
Investigators probing the fatal crash on 6 July of the first Bell 525 Relentless prototype believe the main rotorblades struck both its nose and tail boom as it performed a high-speed simulated engine-out test in Texas.

Both experimental test pilots died in the accident which took place as the super-medium twin approached its never-exceed (Vne) speed. The tracking website FlightRadar24 suggests the Relentless was travelling at about 200kt (370km/h) at the time of the crash.

Confirming quotes originally reported on theRotor & Wingnews website, officials from the US National Transportation Safety Board (NTSB) say they saw signs consistent with blade strikes to the nose and tail boom of the 525 during simulated one-engine-inoperative trials.

Data from the helicopter indicates that main rotor RPM had also fallen significantly, the NTSB says, with some of its five composite main rotorblades appearing to drop from their normal plane.

However, the agency declines to speculate on any potential cause. “We have a great deal more work to do until we get into the analysis phase of the investigation where we will put all the data together to hone in on the factors that contributed to the crash,” it says.

The GE Aviation CT7-powered helicopter subsequently broke up and crashed around 30nm (56km) south of Bell’s facility in Arlington, Texas.

Although the majority of the wreckage was consumed by a post-crash fire, the 525’s flight-data recorder was recovered and data successfully downloaded, the NTSB says.

Bell declines to comment on the NTSB statements.

Flight tests using the remaining two prototypes of the fly-by-wire rotorcraft remain suspended, although the company continues to perform ground-based certification work.

[Ian Corrigible]

Some additional insight into the aircraft's airspeed at the time of the accident, from NTSB: Bell 525 suffered main blade strikes (https://www.ainonline.com/aviation-news/general-aviation/2016-08-04/ntsb-bell-525-suffered-main-blade-strikes)
Flightradar24 shows the last data point for FTV1 was at 1,975 feet at a groundspeed of 199 knots. At the time, it was tracking south to north with a 20-knot tailwind, equating to an approximate airspeed of 179 knots.

I/C

[Lonewolf_50]

Ian, near the bottom of that article is the following:
The accident investigation will include copious data from real-time telemetry associated with the flight-test process, as well as the crew observations and video recordings from the chase helicopter, a Bell 429. GS and FR24 estimates were discussed up thread. NTSB has access to far better data from Bell, don't you think?

[Ian Corrigible]

Lone,

Granted, and I don't usually post on accident threads (preferring to wait for the official report), but the tailwind 'information' quoted in the AIN article hadn't been posted before, and I figured it might help resolve the debate over IAS vs. TAS vs. GS, etc.

Cheers,
I/C

[Tatischeff]

Hello to all,

I'm quite a neophyte on this thread but as I read it many times, It appears that some (not all) of the blades had left their track. I'm wondering how it can be possible without considering the integrity of the pitch rods ?
Maybe I'm wrong but It's quite unusual to see blades living their life independtly each other...

[skadi]

Hello to all,

I'm quite a neophyte on this thread but as I read it many times, It appears that some (not all) of the blades had left their track. I'm wondering how it can be possible without considering the integrity of the pitch rods ?
Maybe I'm wrong but It's quite unusual to see blades living their life independtly each other...

One possibilty to get one blade out of track is to change the aerodynamic, p. e. skin delamination. Even loosening of leadingedge protection tape would eventually change the track on one blade, I had that experience once and it wasnt nice...

skadi

[henra]

I'm quite a neophyte on this thread but as I read it many times, It appears that some (not all) of the blades had left their track.


Where did you read that it was not all blades that left their intended track?
So far I did not understand it that way.
The signs so far rather seem to point to a Retreating Blade Stall and resulting blowback of the entire Rotor disc. Maybe I overlooked something.

[Tatischeff]

[the NTSB says] some of its five composite main rotorblades appearing to drop from their normal plane.
Quote coming from the Flight Global article :https://www.flightglobal.com/news/articles/main-rotorblades-struck-nose-and-tail-of-crashed-bel-428027/

It could be a misunderstanding from the journalist but when NTSB says "some", I'm pretty sure It does not mean "all".

@henra : What you say is interesting: a retreating blade stall can result in the rollback of the entire rotor ? So the maximal loss of lift is on the back of the H/C (the phase shift is still applicable) ?

[henra]

@henra : What you say is interesting: a retreating blade stall can result in the rollback of the entire rotor ? So the maximal loss of lift is on the back of the H/C (the phase shift is still applicable) ?

Retreating Blade stall will cause maximum loss of Lift on the side of the retreating blade but with the phase shift the disc will mainly tilt to the rear.
A severe retreating blade stall usually is a reliable way to surgically chop off the tail of any helicopter. Teetering Rotor systems are significantly more prone to it but in a very violent case you will achieve this even in a fully articulated system. The only type of rotor system where it might be more or less impossible would be a rigid head (Bo105/EC135). I'm not aware of a case where someone managed to chop off the tail in one of those with a retreating blade stall.

[CertGuy]

henra,
Didn't the NTSB say it hit the nose and tail?

[Lonewolf_50]

henra,
Didn't the NTSB say it hit the nose and tail?
It's not beyond reason to believe that the same blade had enough momentum to go through the tail section and then hit the nose. Also, the helicopter was in powered flight so the head would keep turning ...

[Ascend Charlie]

In 1981 in Oz, a B-model Huey had a mast bump and blade separation. One blade entered the cockpit on the left side, killed the left-seater, sliced off the left cabin door and the long-range fuel tanks on their mount and still had enough energy to take off the tailboom too.

[SASless]

Well, the Bell 204/205/212/214 were known for their High Inertia Rotor Systems!

[gmrwiz]

Any news about the last July B525 accident?

[skadi]

Any news about the last July B525 accident?

3 days ago:

https://www.flightglobal.com/news/articles/bell-works-to-keep-525-on-track-despite-flight-test-431024/

skadi

[HeliHenri]

525 resumes flights
https://www.verticalmag.com/press-releases/bell-525-relentless-resumes-flight-test-program/

[MightyGem]

Cause here:
https://www.facebook.com/robert.lloyd.12935/posts/1568033639947533

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.

[FH1100 Pilot]

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."

[212man]

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.
Not quite - both engines were driving the rotor but with a limited power to simulate OEI.

[FH1100 Pilot]

212man:Not quite - both engines were driving the rotor but with a limited power to simulate OEI.

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:

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.

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.

[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.

Pretty standard way of simulating OEI these days, for both test and some operational types. Don't disagree with much of what you say though.