Bell 206B Hydraulic STC?

[Saber 09]

I fly both the TH-67A/A+ (206B3) and the OH-58C on a regular basis. The hydraulic systems are a bit different. For instance, the OH-58 has a T-valve to help with pressure differentials and there is filtration on both the pressure and return sides. The TH-67 does not have SCAS. Only the instrument version has force trim. :hmm:

I have noticed several NTSB reports over the past decade where the cause was undetermined, yet part of the accident sequence included a sharp, uncommanded roll to either the left or right. Some of these have resulted in fatalities. They appear to have occurred in all modes of flight from flat pitch, hover, slow flight, to cruise. Has anyone experienced one of these episodes, or have detailed knowledge of one occurring? I am curious, even if it did not result in an accident. I have not heard of any incidents like this with the OH-58A/C models. :confused:

On that note, does anyone have knowledge of an STC that would slightly modify the 206B3 hydraulics to make them more like the OH-58. This is a serious inquiry that may help save one of us one day. :8

Saber 09

[Shawn Coyle]

Many moons ago there was a Canadian CH-136 (OH-58A in a different identification number) that crashed at night. Long investigation determined that there was micro-FOD in the hydraulic system that basically rolled the helicopter inverted in very short order. Problem was due to the hydraulic test stand not having any filters or maintenance requirement to make sure the fluid was clean.
Other than that, I've not heard of any hydrualic related accidents on either series - but would be willing to look into them.

[Saber 09]

Shawn,

Thank you for your reply. That is actually helpful based on on avenues that I was heading down.

Saber 09

[NickLappos]

jam-proof pilot valves fix that problem in short order, but require new servos for an old design.

[gulliBell]

Out of curiosity, and following on from Nick Lappos' comment, is the pilot valve in the dual-servos on the S76 series of a jam proof design?

[unstable load]

The servo has a bypass valve in it that puts it to bypass if a pilot valve jams for any reason.

[Saber 09]

Shawn,

Do you have anymore details on the Canadien incident, or know where I can find them?

Saber 09

[Saber 09]

Nick,

Are these "jam-proof" pilot valves a factory installed item, or are they something that has done later under an STC? After the Canadien incident Shawn referenced, I am suprised there wasn't some kind of AD issued requiring them.

Do you know where I can find more information on them? I am trying to determine what is in our fleet of 206B3s produced between 1993 and now.

Saber 09

[Saber 09]

Unstable Load,

Is this bypass valve installed in all Bell 206B3s, just for customers that request it, or is a later installed item with an STC? There may still be an issue out there, and the non-existence of a "jam-proof" item may be the answer to a recent incident. I know I can find no recorded incident of a similar accident like Shawn describes with the OH-58A since that time frame. It leads me to believe that the military may have fixed its fleet, but not the civilian sector. Just an NTSB search from 1990 to present turned up over 8 similar incidents where the description was similar, yet the cause remains undetermined.

Saber 09

[NickLappos]

The jam proof servo valves have a dual spool pilot valve, where the inner spool is spring loaded so that if the outer spool is jammed by debris, the inner spool unseats and sends the whole servo to bypass.

The S76 has them, and so do most modern helos. The jam valve will signal a caution light when a jam displaces the valve.

Here is a diagram of one type of jam proof valve. Note how the inner spool, if displaced against the spring, will send the pressure to return:

http://webpages.charter.net/nlappos/jamproof.jpg

[Saber 09]

Nick,

The diagram did not show up on my end. However, diagrams of our servos that I have recently looked at showed what appeared like 3 spools. Are you able to send that diagram directly to me via email? If so, it is [email protected].

Saber 09

[NickLappos]

for those who can't view the diagram, the US Patent office has a search function that will find that patent number (and about 9 others). You can search under jam proof servo valve.

[Shawn Coyle]

Saber 09
You might try the Canadian Forces Flight Safety Office - don't have any contact details. Let me see who I can raise on that. May take a couple of days.
Shawn

[Saber 09]

Shawn,

Thanks, I will try to find their website.

Saber 09

[Saber 09]

Nick,

We tried searching by the patent # and by "jam proof servo valve". Both came back with no matches found. :ugh:

Saber 09

[NickLappos]

Yea, I did a search of the modern (post 1976) patents and then picked the right one from the references:

Patent Number = 3253613 granted in 1966

http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F3253613

[WhirlyBob]

Sabre 09-
In the first post on this thread you said:

"I have noticed several NTSB reports over the past decade where the cause was undetermined, yet part of the accident sequence included a sharp, uncommanded roll to either the left or right. "

Have you made any kind of a tabulation of the accidents in question? I'm not questioning your analysis, but I know someone who may be very interested in getting more details about the accidents and what they have in common. For that matter, I'm kinda interested myself, though I don't fly a B3 (at least not yet...) I gather you're suspecting an unrecognized pitch servo anomoly, but not being familiar with the B3 hydraulic system I couldn't guess any further. Care to expand on your thoughts/ suspicions?

Thx

[Saber 09]

Whirly Bob,

Sorry for the delayed response, I haven't been on for awhile. Well it just seems odd to me and some of my colleagues that there are several incidents of this happening with Bell 206 products both listed with the NTSB and with the military.

All have a few things in common:
Uncommanded pitch with roll that is sudden and violent at times.
Cyclic has no immediate, or a delayed effect to remedy the situation.
Cannot be duplicated during any post-crash analysis by the NTSB or military safety centers.

We have been unable to find any similar incidents with the OH-58 during the same time period. There are a few key differences in their systems, though.

With no duplication, there is only speculation and coincidence. I am really not able to discuss anything further at this time.

[NickLappos]

Servo jam and runaway can be eliminated as the cause, Saber, because the cyclic would also shove itself around the cockpit as the servo dragged it to new places, with forces that the pilot could not overcome. Bruises on the pilot's thighs are also likely. This is an unmistakable symptom, and would be described by the pilots as "The cyclic jumped around on me" and "the controls locked up and then went wild".

Your symptoms sound like main rotor stall, more than anything else. Any flight details?

[Saber 09]

Nick,

Actually, the cyclic did "jump" out of the pilot's hand, there was thigh bruising, and the cyclic was unresponsive to inputs for a couple of seconds. I say a couple of seconds, because it only took 4.5 seconds from onset to a totaled aircraft. There were 8 nearly identical incidents in the NTSB reports in the past 20 years. I know that is not many considering how many hours per year the Bell 206 flies, but most resulted in a totaled aircraft and permanent disability and/or fatalaties.

Saber

[NickLappos]

Saber,
Good info on the jumping around cyclic stick, but you left it out of the first post, and it does seem like an important detail! Do you have any references in the NTSC site?