Hydraulic Systems

[Geoffersincornwall]

I am researching the failure modes of helicopter hydraulic systems and would appreciate any real-world feedback on this subject. I am particularly interested to know if anyone has experienced (or heard about) a failure that has generated additional problems beyond a simple loss of hydraulic pressure.

Thanks

G

[peterperfect]

ASW SeaKing has suffered cabin/cockpit fire caused by vapourised fluid from sonar winch high pressure pin-prick leak with presence of an ignition source.

[paco]

Ask all those AS 350 people who had cyclic hardovers

Phil

[Geoffersincornwall]

PACO

Please expand on this subject if you are able

G.

[212man]

A few examples that I can recall:

The Copterline S-76 accident is an obvious example where serious side effects led to loss of control. Worth googling the accident report.

I know of a BHL S-76A+ that suffered a pin-hole fracture in the pipe that supplied the VTA. The hole was in the inside of the knee bend that takes the pipe under the centre console and the 3000 psi atomised vapour filled the cockpit with fumes. Pretty unpleasant by all accounts but fortunately no ignition source!

I know the pilot of a B212 that had a hydraulic reservoir topped up with industrial strength detergent. After a short flight (marine pilot winching) they returned and shutdown. The first symptom all was not well was that the blades could not be turned to tie them down! Prolonged flight may well have resulted in MGB failure.

A certain GoM S-92 operator had an aircraft where a hydraulic pipe was chaffed by the engine heater mat wiring. When the heater was turned on during a drying run, the arcing ignited the now leaking fluid. The subsequent 4000 psi fed conflagration was quite a spectacle I believe! The roof melted and had it been in flight would have been catastrophic (one assumes.)

I once had an 'o' ring 'let go' during a ferry flight, in a factory fresh EC-155, in Cote D'Ivoire which dumped the system out the side. The only consequence was the mess it made of the airframe! Actually the other consequence was having to spend an unplanned night stop in a s**thole! Interestingly, the part was not shown in the IPC, so we just got a big bag of assorted 'o' rings sent. Once fixed we continued to Accra and got an MGB Chip enroute - great these new machines!

[Gray 14]

Hyd

Goeff,

Many years ago in a distant land....UH-1 loss of hydraulic fluid. Did a running landing to a psp runway without mishap but noticed a big difference in the control stiffness without fluid in the system. This as opposed to turning the system off during training flights.

Seems the fluid does act as lube. No fluid means harder controls.

Not a problem in the dual system aircraft I guess.

Stan

[Geoffersincornwall]

Keep it coming.

Thanks - that's the kind of stuff I need. Particularly would like to hear about any pump failures and/or servo jams.

Stan you old rascal are you still beating the sky to death or are you happily retired now?

G.

[Spanish Waltzer]

ASW SeaKing has suffered cabin/cockpit fire caused by vapourised fluid from sonar winch high pressure pin-prick leak with presence of an ignition source.

and that 'one in a million event' happened twice ISTR

[ericferret]

S76 rotorbrake fire caused after pressure blanks were fitted to the calipers after the failed hydraulic module had been removed/disconnected. In flight the fluid within the caliper got hot had nowhere to relieve to and put on the brake.

I think there has been more than one hydraulic brake related fire, Merlin, 139 ?????

Gazelle hydraulic system switching itself on and off in flight due to anti coll light wiring inducing a current in the hydraulic solenoid valve wiring.

[thecontroller]

http://www.pprune.org/rotorheads/233...k-stall-2.html

[paco]

Remote Helicopters in Slave Lake (in 2003) had an AS 350 cyclic hardover on the ground where during the preflight checks it motored over to the left and could not be pulled back. The same happened to a Canadian Helicopters one in Fort Nelson (or Fort St John). It may have been a contributory cause to the OMNR fatal accident in Ontario, and the USA news AStar that crashed so magnificently on top of a building.

There was a whole spate of these and even though there was a lot of testing I still don't know what the outcome was.

Phil

[Ascend Charlie]

Back in the 70s I had a hydraulic leak in the hell-hole in a Huey, detected it by smell and landed before any other symptoms showed themselves.

had a hydraulic lock-up in an AS350 during high-speed wing-over, but that is considered normal operations by those French.

[John Eacott]

Geoff,

The 206L has the hydraulic pump piggybacked off the Nr tacho, out of the MGB. Failure of the drive spline to the Nr tach will give an Nr failure warning (bloody horn!) plus hydraulic failure.

IIRC, the 206B has the Nr tach off the hyd pump so you'd have much the same symptoms if the drive to the hyd pump failed.

[JimL]

Hi Geoff,

Double hydraulic failure in the S55 series III, caused by a bridging of the 'primary off/secondary off' test switch (water ingestion).

Jim

[Shawn Coyle]

Hydraulic system failures are simulated in flight by turning off the hydraulics. This is not the only way systems fail, and one of the requirements in certification is to do a dry boost actuator flight. This often shows very different handling than just loss of pressure.

A Chinook in Canada had a hydraulic actuator blow out in one channel due to an unexpected failure in the AFCS circuitry. This put a much larger and faster hardover into one yaw channel actuator that was so strong it literally blew the side out of the actuator. The Chinook hydraulic system is duplicated, but the only thing between the two systems was a rubber O-ring. This was deemed sufficient because the failures predicted had said there would always be fluid on both sides of the O-ring, even if there was no pressure. Alas, no fluid, no safety and the pressurized fluid on the other side of the actuator blew through. Massive hardover in yaw (helicopter was reported to be rotating in yaw at 180 degrees per second).
Lots more stories about AS-350 hydraulic problems are out there.

[Geoffersincornwall]

Shawn et al

You rightly guessed that the simplistic approach to failure modes seen in the manufacturer's RFM is my agenda. At times they seem to ignore a comprehensive list of incidents and accidents that have occurred in the past.

Whilst I am not suggesting that every RFM should provide a solution to every possible failure I do believe that my students deserve a little more than 'Land As Soon As Practicable' following a system failure when there might be all hell let loose up there and maybe you won't find out until it's too late. My philosophy is to prefer embarassment at over reaction - land as soon as possible - rather than risk oblivion.

Keep the ammunition coming. I'm sure there are more situations where manufacturer's predict simplistic failure modes only to be confounded by poor maintenance, inappropriate overhaul processes and straightforward miscalculation.

G.

[peterperfect]

Also, wasn't there an early H3 incident where emergency air blow down of undercarriage caused the rotor brake (or at least it's warning light) to illuminate in flight ? Might have been due to a fault in a reservoir vent/or NRV; someone else online might confirm details.

[cpt]

I am with you Geoffer, I also think that the "land as soon as practicable" we may get in RFM when dealing with hydraulic malfunctions should rather be considered as "land as soon as possible" in some circumstances, especially whith dual body servo controls....a vital redundancy is lost and I find very unconfortable to intentionaly keep on flying (in most of the cases we cannot know the cause of the malfunction wich can affect the remaining operating system)

[Revolutionary]

A co-worker in Arizona once had the lock come off the retaining 'hook' on the collective in an AS350 whilst doing a hydraulics check on the hospital pad. With the hydraulics switched off, the collective immediately sprung almost full up, causing the helicopter to leap into the air in a wild spin.

He was able to turn the hydraulics back on and re-gain control only just in time to prevent the aircraft from crashing headfirst into the emergency room. Not that already being in the ER would have helped him any; he would have wiped out the entire staff.

[ShyTorque]

Was once flying as #2 in formation with a second Puma. We knew he had a major hydraulic failure because a red cloud appeared in front of us. I called to tell the other pilot, he said "No we haven't..... OH! Yes we have, PAN, PAN, PAN! A hose had popped.