Hydraulic "Transparancy"?

[Winnie]

For #### and other who may know:

What is it really?

From what I understood in the POH it is a system to "warn" the pilot of possibly overstressing the machine, by stiffening up the controls in large angles of bank and or with sharp pullups? I remember an instance in Norway a couple of years back, when the pilot banked really steep, and almost lost control, and the AAIB-N atributed that to "transparency" and inadequate knowledge of the phenomenon, can anybody explain?

 

Thank you very much!

[407 Driver]

Winnie, I can't explain it as good as ####, but it's more to do with the flight loads from the M/R system overpowering the Hydraulic system....giving the pilot the feel of locked controls. It generally happens in higher G situations, ie turns.

 

I only had it happen once in my Astar days, not a pleasant experience. The key to recovering is to level the aircraft and / or reduce collective pitch. ( with very stiff controls).

 

 

[Jammed left]

It also happens during extreme turbulence. The way it was explained to me is exactly as described above. The load on the M/R exceeds the capabilities of the servo. The obvious solution to get out of the transparency is to reduce the load on the servos - the few times I''ve had it, it was fairly easy to get out of- pushing down on the stiff collective wasn''t too hard, and control is restored.

 

The key to preventing it from becoming a serious problem is to understand that it occurs in high disk loading situations. I think if you aren''t into high speed, low altitude, high g maneuvers, there aren''t too many situations (but maybe a select few) where it would be too much of a problem to deal with.

 

If there are some situations / experiences where it did become an issue, I''d be interested in hearing them

 

cheers

[407 Driver]

It may have become an issue in Canmore AB about 15 years ago. A fully loaded 350B crashed on Christmas day, all 6 on board were killed. The aircraft''s rear stinger made a 400'' mark in the snow prior to impact. A video camera on board showed that the aircraft conducted a fast and low ridge crossing. The aircraft may have experienced lee-side turbulence, lost HYD control, and was in a position that the pilot could not safely reduce collective. This pilot WAS NOT known as a cowboy, he may have simply been caught by this phenomonon.

 

On my one and only time in "transparency" I was in a

B model, AUW of around 4200#, gently circling a spot fire at 4,000'' +30c when the controls locked....no more than 15 degs of bank at 70 Kts.

[4961]

It's worse in the B or D compared to the rest of the type. Not much fun if your close to the ground.

[407 Driver]

Why would that be 4961? Those little green blades are so much smaller than the 355 grey blades? Is it a different HYD pump on the BA and larger?

[I FLY]

I had it happen to me in a gradule descent from 4000'' with a full load of siesmic boys in a B1. I had about 80% TQ, with AS at about 120KT and it was a turbulant and about a 500 to 1000 foot ROD with the turbulance. dropped the collective as it happened and it pushes to the right and quick.

[4961]

The smaller blades on the B are the problem (blue for the B and D grey for all the rest). The hydraulic systems are all the same from the pump to the servos. It seems that the smaller blades have a harder time with the forces and transfer it to the servos. Get up to altitude in a loaded B and you''ll feel some feedback at 80kts. The grey blades are a good start getting rid of the problem.

[Phil Croucher]

I believe they tried 2 minutes and found that, while there is a difference between 2 minutes and 30 seconds, it wasn''t worth the wait, and in some cases could be detrimental to the engine. The Allison, of course, is squished back on itself, so would have more hotspots which need stabilising.

 

The D model Astar needs 3 minutes

 

Phil

[Winnie]

Thank you, that explains that, and also If I ever experience it,  I Won''t mention it to the CAA/TC/FAA or whoever, Just mark it in the book as being not smooth enough. As an aside, an ASTAR plunged today in Norway, but apparently the pilot walked away, even if the airframe apparently was not too pretty!

 

Second Question, How come the 206 and most other machines have a 2 minute cooldown, when the french machines seems to have "no more" than 30 seconds??

 

Have me beat for sure! Well again, thanks for all replies