UK AAIB Report - January 2013

[toptobottom]

January's UK AAIB report has been published; the only rotor craft incident is a 40+ year old jetbox doing aerial work in Scotland (http://www.pprune.org/is here), which suffered an engine failure and successfully auto-rotated from 600' agl. The axial compressor failed.

Why does the report say that the UK is considered to be a corrosive environment :confused:. According to the manufacturers Maintenance and Operations manual, all C20s operated in a corrosive environment should have a compressor wash every day...?!

[SilsoeSid]

http://www.aaib.gov.uk/cms_resources.cfm?file=/AAIB%20Bulletin%201-2013.pdf

...and the large interim report on the North Sea ditching of EC225 LP Super Puma G-CHCN 22-Oct-12

[Non-PC Plod]

The UK is generally considered to be a salt-laden environment, because of the proximity of the sea wherever you are in the country. I remember Eurocopter/Turbomeca stipulated daily compresor washes a few years back on EC135 after bits of compressor started detaching and ingesting through the rest of the engine.

[fisbangwollop]

He certainally woke me up with his Mayday call :cool:

[toptobottom]

FBW - a mayday at 600'?! :D

[fisbangwollop]

Yep...the next call was to say he was safely on the deck!!! The pipe and powerline boys and girls always talk to Scottish Info whilst on task over Scotland......it is times like this that it is nice to know someone is there should help be required. :cool:

[ericferret]

The issue of compressor washes and the Allison is an old one.
Without a manual to consult I believe Allison said a martime climate was anywhere within 60 miles of the sea. We reckoned that unless you flew around the Litchfield area permanently then there was a daily compressor wash requirement.
I assume the AAIB are just quoting the engine manufacturers manual.

A few years ago I sent a C20 compressor to Heliwork for overhaul.
I received a very guarded call from Eric Tombs asking me for a bit more background on the module.

I said that it was from a new aircraft to us and we had carried out no maintenance on the compressor.

He said "You know when we inspect the wheels we use magnifying equipment to look for corrosion pitting, well on this one you don't need a microscope, the smart money here says this unit was within 10 hours of failure"

He sent the bits back and the expression Swiss cheese springs to mind.

The problems we have in the UK are a mixture of a salt laden atmosphere and industrial pollution/ acid rain. I am sure everybody has flown through yellow smelly clouds.

So what if you dont water wash?

The minimum is higher costs on overhauls.

The impracticalities of daily washes for private owners are obvious. Two people required, probably a ground cart, what about away from base?
You pays your money and takes your choice.

[Savoia]

Another corrosion related accident:

GATINEAU, QC, Jan. 10, 2013 - The Transportation Safety Board of Canada (TSB) today released its investigation report (A11C0152) into the September 2011 accident during a training flight in a Bell 206B helicopter operated by Wisk-Air Limited, in Thunder Bay, Ontario. The instructor and student were practising an emergency landing exercise using a technique called autorotation.

On the afternoon of 13 September 2011, the helicopter was on a local training flight at the Thunder Bay International Airport with an instructor and a student on board. They were using the threshold of Runway 30 as a designated landing area. The student pilot entered a practice autorotation, which involved reducing the engine power setting, with intention to subsequently re-apply power when the autorotation exercise was completed. When the student pilot tried to re-apply power, the helicopter's transmission clutch did not engage, and the rotor speed decreased. The instructor took control and landed the helicopter. After the helicopter landed, the helicopter's clutch re-engaged and the main rotor mast severed just below the rotor head. The helicopter was then shut down and the crew exited without injuries. There was no fire.

The investigation found that moisture had contaminated the transmission oil causing corrosion of the internal components of the transmission-oil cooler and the clutch. Corrosion products from the oil cooler caused a blockage resulting in reduced oil flow to the clutch. The subsequent damage and overheating prevented the clutch from functioning properly.

SOURCE: Transportation Safety Board of Canada

[AnFI]

That's THE thing about the 'free wheel unit check'

with intention to subsequently re-apply power when the autorotation exercise was completed. When the student pilot tried to re-apply power, the helicopter's transmission clutch did not engage, and the rotor speed decreased. The instructor took control and landed the helicopter. After the helicopter landed, the helicopter's clutch re-engaged and the main rotor mast severed just below the rotor head.

It's not that 'needles don't split' - that they 'do rejoin' that we're checking, not that most see it that way.
The consquence of sudden re-engagement can be catastrophic as demoed here. (Another un-factored risk in Twin engined helicopters, (Ian Southridge AS355))

[Thomas coupling]

Firstly: good landing and job well done, especially from 600': pat on the back.
Second:

Comp washes: I had to pin this down during my time with the National Police EC135 User Group. Talking with the Turbomeca and PandW reps, the following is a technical requirement. Failing to comply will render engine turbine/nozzles and associated hardware warranties, null and void:

Salt laden atmosphere as recognised by RR, TM and P&W: up to 150 miles inland. IE: The whole of the UK.
Comp was MUST be done after last flight of the day. It is acceptable for this period to roll over to 'prior to first flight of the day'.

Comp wash must be done i.a.w. manufacturers recommendations. (Usually a timed run with a minimum amount of fluid thru the cent/axial/Power turbines.

Fluid used: MUST be distilled water! Tap water has too many contaminants and the contaminant level must be less than 20ppm.

Eventually we devised a trolley mechanism which when attached to the engine bleed valves, could be operated by ONE person from the pilots seat as they crank the motor over for the recommended time period. The kit even converted tap water to distilled water within minutes.

To be fair to the OEM's - the amount of accelerated wear and tear by NOT doing it is blatantly obvious after paying a visit to their workshops before and after or with and without comp washed engines.

[toptobottom]

AnFI - do you mean Ian Shoobridge, AS355 (my friend and instructor on B206)?

[AnFI]

ttb - thank you for the correction

[MightyGem]

He certainally woke me up with his Mayday call
I remember doing my "mayday" call for my RT licence. Being a single engine mil pilot at the time and used to being at less than a 1000', I gave a simple, "Army 123, engine failure, North of XYZ", and by that time, I'm probably on the ground. Not good enough. The examiner wanted the full spiel, problem, position, intentions etc. :hmm:

TC, P&W users can use tapwater for engine rinses.

[expatflyer]

FAA Airworthiness Circular AC43-4A pretty much sums it up in figure 4-20 when they put the whole of the UK at moderate to severe corrosion severity zone...
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2043-4A/$FILE/AC%2043-4a%20.pdf

[fisbangwollop]

MightyGem Being a single engine mil pilot at the time and used to being at less than a 1000', I gave a simple, "Army 123, engine failure, North of XYZ",

And in this case that was about it.......enough info though for ATC to get the cavalry moving towards the last known position!! In this instance though he was close to one of our radio aerials so was able to communicate with him that all was well after landing and no sign of fire.......so you see food for thought to those that poo hoo the idea of talking to ATC whilst at low level. :cool::cool:

[HeliStudent]

In post #8 it says that -

The investigation found that moisture had contaminated the transmission oil causing corrosion of the internal components of the transmission-oil cooler and the clutch. Corrosion products from the oil cooler caused a blockage resulting in reduced oil flow to the clutch. The subsequent damage and overheating prevented the clutch from functioning properly.

I have three questions please -

1. Can someone describe in simple terms what type of clutch is fitted to the Bell 206.

2. How can corrosion in the transmission oil result in the helicopter's main rotor mast being severed just under the rotorhead?

3. Would this severing of the main rotor mast have occurred in-flight had the clutch re-engaged before they landed?

[ericferret]

206 has a sprag clutch.

Not sure what you mean as the explanation given is fairly good.
However, as the pilot increaced power the N2 speed would increace rapidly.
Suddenly applying this power when the clutch re-engaged caused a massive load on the drivetrain. The drive train in this case failed just below the head. That is a huge suprise to me (and I suspect to the pilot).
I would expect a failure either within the engine or main gearbox but more likely the freewheel or the engine to MRGB driveshaft.

As to your third question the answer would appear to be yes which is a touch worrying!!!!!!

I wonder if there is some other factor involved.
Could be that as they attempted to recover with no power available the RRPM had decayed so that when the engine re-engaged the differential between the N2 speed and RRPM was well beyond normal and the overstresss ocurred.

Still surprised at the failure point.

[Aucky]

Could be that as they attempted to recover with no power available the RRPM had decayed so that when the engine re-engaged the differential between the N2 speed and RRPM was well beyond normal and the overstresss ocurred

If N2 is rising from around 65% as throttle is re-opened, and RRPM has decayed the differential would be smaller wouldn't it? Slightly off topic - is the reason for opening the throttle over 5+ secs when recovering to prevent a shock on the sprag clutch when N2 & RRPM meet, or prevent compressor stall or excessive N1...? I don't think there is a large change in torque (as there would be on the ground) as the engine is not having to spin up the blades in auto at 100%.

[ericferret]

No sure about that

If the rotor rpm has decayed the speed at the MRGB input will be lower than the normal input speed of round about 6000 rpm.
As the power is increaced with no load due to the freewheel not engaging the N2 would run up to max very rapidly so you would have a reverse situation.

Normally the MRGB input would be at 6000 and the N2 would come up to 6000 engine rpm smoothly to match it.

In this case the output of the N2 would already be 6000 and attempting to engage in a milisecond with an MRGB drive doing less than 6000.

This is the differential I meant. The N2 would normally be negative relative to the MRGB whereas in this case it would be positive.

Just thoughts, I am not an expert as some of this falls into the category referred to by a certain tech instructor as "pilot sh*t" !!!!!