Help understanding a R22 crash (hypothetical)

[needhelp]

Hey,
I’m a fixed wing PPL. I’m studying a university paper on accident investigation.
This assignment has me really stumped, and was wondering if anyone could point me in the right direction.



A Robinson R22 helicopter was operating some 10 nm east of the aerodrome with a flying instructor and student on board. During the flight the aircraft was seen to break apart and fall to the ground. No one saw the aircraft in flight immediately before the accident sequence began but several persons saw the aircraft descend in a flat attitude and pieces fall from it after the pilots were in trouble. Witnesses also observed that the main rotor was stationary and the blades were coned. At the accident site the first persons to arrive encountered a strong smell of fuel. Both occupants were killed in the accident but no fire occurred.

RELEVANT FACTS.
This was the instructor’s first flight with a student after qualifying as an instructor.
The instructor had flown 3000 hours on helicopters, held a CPL(H) and an R22 rating.
More than half of the instructor’s helicopter flying was in the Robinson R22 and R44 helicopters.
Both pilots passed a medical within the last 30 days and the autopsies after the accident revealed no medical factors which might have contributed to the accident.

The weather conditions observed at the aerodrome were:
Wind 270◦/12 knots gusting to 18 knots
Visibility: 25 nm
Cloud: Scattered at 2500 feet broken at 5000 and 20 000 feet
Ambient temperature at sea level: 14◦C dew point 5◦C
The helicopter had sufficient fuel for 2 hours flying [13gallons] when it took off 20 minutes before the accident.
There were indications that the carburettor heat was “OFF” when the aircraft collided with the ground with little forward motion.
The main rotor blades were bent into a “tulip” shape but still attached on impact.
The main rotor had flapped down in flight and chopped off the tail boom in the vicinity of the anti-collision light, at low rpm.
The aircraft was close to it maximum permitted all up weight when it took off.

REQUIREMENT.
Explain with the aid of diagrams two possible explanations for this accident which are in keeping with the facts listed above.

I can come up with one explanation. That is carburetor icing has caused the engine to fail, has gone unrecognized, leading to a blade stall, and chopping off the tail boom.

Can anyone else think of any other ideas that does not involve a blade stall?? Maybe the failure of a certain component, that would cause the rotor to strike the tail boom at slow RRPM?

Thank You!

[JimEli]

US FAA SFAR-73 is your friend.

[needhelp]

It couldnt have been caused by mast bumping as the rotor struck the boom at low RPM. When must bumping happens rotor RPM doesnt slow down?

[EN48]

This scenario is much like that described in this report:

http://www.ntsb.gov/publictn/1996/SIR9603.pdf

Loss of main rotor control due to large, abrupt control inputs by a new, very low time student have been found to cause a substantial number of R22 and some R44 accidents, virtually all fatal. In such a scenario, the main rotor flaps down so quickly that the instructor does not have time to intervene before it strikes and severs the tail boom.

[darrenphughes]

A few possibilities spring to mind, although most of them may be ruled out for different reasons.

1. Carb icing in the R22 can happen during flight below 18" of Manifold Pressure even when the Carb Temp indicator is out of the of the the yellow range. Due to the governor gradually opening the throttle up very slowly, the 2 guys may not have been aware and as the butterfly valve area of the carb ices up to the point where the fuel/air mixture no longer flows the engine may quit instantly. This would cause rapid decay in RRPM. If an autorotation isn't successfully entered, the low RRPM could lead to excessive flapping that could chop off the tail boom. As the rotor slows down towards the stall the blades start to cone up more than normal due to the loss of centrifugal force, they will then tulip upwards as the helo falls. This is unlikely, due to the amount of time the instructor had in Robbies. I'd like to think that anyone with that much experience in the R22 wouldn't let that carb ice scenario happen. But **** happens sometimes, so it can't be ruled out.

2. Surprise engine failure(throttle chop) by the instructor was met by the wrong reaction by the student, raising the collective. This would cause low RRPM, which could lead to excessive flapping that could chop off the tail boom just like above. This one may be most likely but could be argued against due to the carb heat not being on. I'm not a fan of throttle chops myself, but I know other instructors that will give them. I don't know any instructor who won't find some sort of sneaky way to apply full carb heat before a surprise auto(throttle chop or not), especially in those meteorological conditions. Although, maybe he forgot(nerves due to first instructional flight).

3. Retreating Blade Stall, can cause a rapid pitch up and roll to the left. If the person on the controls applies too much aft cyclic to try to slow the chopper down it could cause the already low rear of the rotor disk to come in contact with the tail, thus chopping it off. Proper recovery is to lower the collective pitch of the blades and then slow the helicopter down with small rearwards inputs to the cyclic while correcting the roll. From what I hear, RBS is quite a violent experience, so the wrong large inputs might be very plausible. This scenario is probably ruled out with the fact that the blades had tuliped upwards due to rotor stall. As with all helicopters, RBS is a greater risk at higher density altitudes of which there is no mention in your case study.

I'm no expert, but that's my wee attempt!

[blakmax]

Hi needhelp. While the usual approach especially for a low hours instructor and student is to blame the crew for rapid stick inputs or low rotor RPM. Any thorough investigation must also consider structural failures and exclude that evidence before assigning blame. There have been investigations which may be of interest to your studies. Google SIR9603 for one, and then check A08_25_29 recommendations.

Regards

blakmax

[needhelp]

Thanks to every one who has taken the time to reply.

just to clearify two points.

1) If the student incorrectly raised the collective during a simulated engine failure, this would reduce RRPM, and could lead to excessive flapping and the tail boom coud be chopped off. Whould the rotors stay attached and tulip?

2) Low RRPM (but the blades not stall) can lead to excessive flapping?

[Daysleeper]

Whould the rotors stay attached and tulip?

Blades are surprisingly strong and the structure is surprisingly weak.

You would expect to see blade damage from any impact with the tail boom and things like paint transfer would pin it down conclusively.... not much help for your essay though :hmm:

No matter how experienced before becoming an instructor the first few trips with live students are always an eye opener, just how many different ways there are for people to try to kill you. After a few weeks you soon settle down to assuming that EVERYONE is trying to kill you :}

As an example a mate of mine ended up in a wall at the end of a small field when fixed wing instructing when the stude pulled the fuel cut off instead of the carb heat. fortunately they were ok.


For carb icing info try the CAA safety sense leaflet 14 at This PDF doc (http://www.caa.co.uk/docs/33/ga_srg_09webSSL14.pdf)

14 and 5 puts you in moderate for cruise , serious for descent and on the edge of serious any power.

[VeeAny]

If Low RPM has to be a factor and it is in the requirements, how do we get to it.

Robinson tell us that the magic figure to avoid Rotor stall in the R22 is 80% [+1% per 1000ft] so to get the Low RPM at impact has the rotor system stalled ? Low RPM coud be anything out of the Green Arc, so the question seems a bit ambiguous at this point.

Like has been said before

Carb Ice causing the engine to stop is the obvious favourite.

A couple of new ideas.

1] Unlikely as you would hope this would be mentioned in the relevant fact but a drive Belt Failure, at the moment of failure this would lead the engine to overspeed (some will say the governor will contain it, but I am lead to believe it wont) but the rotors will react as though the engine has failed as they are no longer being driven.

2] Governor switched off (or failed) and and an abrupt (probably) forward cyclic input made, which in some (not all) phases of flight will cause an abrupt drop in RRPM.

3] A hovering demonstration at altitude (which would be consistent with the low forward speed at impact), with a high power setting overpitching (or a genuine engine problem) at this point could stall the rotor with the given outcome.

4] An Autorotation demonstration with a faulty Sprag Clutch, would prevent the rotor system from freewheeling.

Any of these would be componded by an inexperienced instructor (inexperienced as an instructor) who will have his attention divided between the act of flying, allowing his student to take the controls and talking about it all at the same time, this takes a while to get used to (longer that the course normally takes). Initial or Demo Flights can be fatal (http://www.robinsonheli.com/srvclib/rchsn20.pdf)

Have a look at

Robinson Helicopter Company Service Bulletins and Letters Page (http://www.robinsonheli.com/servelib.htm)


Helicopter Safety | Publications (http://www.helicoptersafety.org/publications.asp) in particular the link to the NTSB reports on loss of R22 Main Rotor control.

and

Helicopter Safety | Carb Icing (http://www.helicoptersafety.org/genericaccident.asp?Keyword=Carb%20Icing) the CASA carb ice link is broken but will be fixed by about 1000Z today.

[blakmax]

Hi needhelp

As you are a probationer, you may not have noticed the private message capability that PPRuNe kindly provides. It is at the top right corner above the log-out button.

[BoeingMEL]

memory slightly hazy now but this (hypothetical) accident description reminds me of Harry's. (Low time student but Harry had lots of Robinson instructing time of course). Just a thought. bm

[needhelp]

Thank you again to all those who have taken the time to reply.

One other question.

If a student was to panic and make a too abrupt movement resulting in the rotor slicing the boom at a lowish RRPM.
would raising the colective and pulling back on the cyclic have his effect????

[TiPwEiGhT]

Are you trying to find effective methods to bin your instructor? Haha.

TiP:}

[Genghis the Engineer]

That's a good question, out of interest -which university set it?

G