Dauphin Service Letter request

[Geoffersincornwall]

Back in 2001 Whilst flying a Dauphin 'N' I had a problem with 'fenestron stall. This of course is not what EC call it but they did produce a Service Letter to deal with a series of similar incidents some of which ended badly. One of the points made in the letter was that any attempt to 'take the problem into the air' and resolve it in forward fight would be doomed to failure so don't even consider it.

I need to reinforce my instructional technique with some hard facts and would appreciate finding that Letter. Can anyone help?

Thanks

G.

[ec155mech]

you can use TIPI from eurocopters website for SIN's both on the engineering and flying side. its free and only requires you to create an account

https://www.eurocopter.com/techpub/F...myFO_login.php

[Geoffersincornwall]

EC155Mech

Thanks, my subscription expired but I'll try to get it reinstated.

Aye

G.

[cpt]

These "fenestron" stall situations really puzzles me .... (I'm not rated on the EC120)

I've flown several thousands hours on different variants of the 365 as well as a few hours on the "Gazelle" in various countries and operating conditions, including air work....and it is something that I only once "might" mildly have experienced on a 365C when taking off from an offshore rig.

It is a matter of fact that a "fenestron" becomes efficient at the end of the rudder pedal range,when countering an increasing torque, maybe is it what pilots experienced on conventional tail rotors can analyse as a tail rotor stall or an LTE...?
Normally this particularity is highlighted during the conversion course

[Geoffersincornwall]

EC Service Letter

The technician who gave me a copy of the SL pointed out that even back then in 2001 it was quite an old SL and had been around for quite a while. My memory may be off a bit but it could be that even then it was more than ten years old.

It is an LTE issue not a mechanical failure and is exacerbated by relative winds from Green 135, It was exactly that problem that snagged me on that hot day in Cameroon. I am trying to deal with some folk that seem to have a mindset that they have the skill to fly away and sort out a TR fail in the air. This SL may help to give them a more appropriate perspective.

G.

[Brilliant Stuff]

I think I know the letter you speak of it had did it also have a BO105 in it?

I was told by the old sweats that it's a question of not enough pedal in the Fenestron equiped model hence one should always make sure you have enough leg to put the boot in.....

[gnz]

Geoffers, check your PM.

[John Eacott]

Geoff,

Look for Service Letter 1518-67-01 dated 26th April 2001, and also SL 1673-67-04 dated 4th February 2004, which covers both fenestron and tail rotor equipped helicopters with clockwise rotating MRB and 1692-67-04 for anti-clockwise rotating MRB.

[John Eacott]

Also the AAIB report on G-BZXE, with references to the fenestron stall and from 1518-67-01:

EUROCOPTER remind you that in some configurations (hover flight, flight at low speed in light wind etc), starting a turn to the left can induce a high-rate turn if the pilot does not apply quickly the suitable position to the yaw pedal. If the pilot attempts to counter this high-rate turn by applying the amount of right yaw pedal that corresponds only to hover flight control, this is not sufficient to start actual deceleration, thus allowing the pilot to regain his bearings.
In this situation, right yaw pedal, and if necessary, full right yaw pedal, must be applied quickly, and held, to stop the turn to the left. Any delay in complying with this procedure will increase the time necessary to slow the helicopter. This effect is NORMAL and must not give rise to doubts as to the performance of the tail rotor. In all cases, the helicopter will stop turning.
A reminder is also given that all turns to the left, in hover flight conditions or at slow speed, must be made applying moderate left rudder.

[hihover]

Geoffers

You are quite right, as always, it is very much an LTE issue. There were several incidents involving Gazelles in the 70s and 80s. I was "fortunate" enough to be sitting in the back of one such incident as it unfolded before my eyes, which, by the third revolution had already popped out of my head.

In the hover at 5 ft into a 20kt wind, the clumsy (Sea King) pilot applied a bootful of Left pedal (the wrong pedal for the Gazelle). As we came through 180 degrees, the huge fin became a sail and the wind whipped us around even faster. He tried to stop the turn with full opposite pedal immediately....and off we went.

We did 5 or 6 revolutions in a very short period. He had managed to hold it level until about the second or third and then his instinct was to get away from the ground. As he applied power, (I believe) the increased downwash on the horizontal stabiliser pulled the nose up, which he corrected with forward cyclic and lowering the lever. However, this set up fore and aft oscillations, what a mess. He then opted for lowering the lever more and accepting a heavy landing (as specified in the FRCs for Tail Rotor Failure).

To reinforce the point you are trying to make, I learned the following points from that incident:

1. The whole thing was over inside 30 seconds (approx). There was no time for detailed analysis and the situation was getting worse. Putting down was the only option and we should have done it earlier.

2. We were not ready and were completely caught out. Simulator practice is absolutely invaluable. If no sim available, Tail Rotor issues should be a topic of discussion at regular intervals.

3. Every helicopter pilot has a better method of dealing with failures that happen to other helicopter pilots........ffs just follow the instructions in the handbook.

4. Not all Sea King pilots are clumsy, I just happened to have that one.

Tam

[Adroight]

Fenestron Stall
On early versions of the fenestron, as fitted to the Gazelle, there have been
instances of loss of tail rotor effectiveness during low speed manoeuvres. Extensive trials failed to pinpoint a precise flight regime that would produce this and with modern generations of fenestron, where the direction of rotation is reversed, the problem is not considered likely to occur. However if, during hovering, an undemanded yaw to the left occurs (not associated with loss of TR drive), the correct recovery action is to apply full right pedal, and maintain it until the yawing stops.

From a well known operators EC155 Part B.

[Anthony Supplebottom]

Putting down was the only option and we should have done it earlier.

Putting a spinning helicopter on the ground would not normally be my first choice, especially if the possibility of reducing/stopping the rotation exists.

[hihover]

Anthony

I agree. With hindsight, an additional 10,000 hours and a comfy chair, the picture now looks quite different.

However, on that particular day, as the situation was deteriorating rapidly, I was very grateful that he put us back on the ground and kept us upright.

[Geoffersincornwall]

SL has Arrived

Many thanks guys i have now received the SL concerned but don't let that stifle any debate you wish to have about LTE and TR Failures. I guess an LTE event holds some possibility of recovery and perseverance may be the order of the day but in the context of mechanical failure we teach lower the collective fully immediately and if the circumstances (altitude) give you time to reconsider then maybe the decision can be reviewed.

It is undoubtedly a quandary when close to the ground. Is it LTE? or mechanical failure? Me? I would rather bang it down early and minimise the dynamics. They can always buy me another machine but the co pilot and I and anyone else aboard would probably appreciate a few scrapes rather than a wooden box.

G.

[diginagain]

Putting a spinning helicopter on the ground would not normally be my first choice, ..........

My fellow student who destroyed Gazelle XZ319 didn't feel that he had much option.

[Anthony Supplebottom]

From the RAF AH1 HT2/3 Gazelle Aircrew Manual:

In light wind conditions, an extremely rapid build-up of yaw rate can follow a relatively small left pedal application during low speed flight or in the hover, particularly with the SAS disengaged. In this event, immediate and positive application of right pedal, up to the maximum, should be applied and maintained to arrest the rate of yaw.

Recovery action may be ineffective if the pedals are returned only to the hover position, and the yaw rate may initially continue to increase before deceleration and an eventual steady hover is established.

Furthermore, if the pedals are not returned as far as the original hover position, a steady hover will never be achieved and the aircraft will stabilise at a particular rate of yaw which may be very high. Pilots may misinterpret this as a loss of yaw control. Be warned that any delay in applying corrective action will require progressively larger right pedal inputs to achieve a steady state hover and may lead the pilot to believe that he has lost control.

Yaw rates up to 165° per second to the left can rapidly be arrested by applying full right pedal without any discernable loss of fenestron performance.

In the hover and at low forward speeds, ensure that pedal turns to the left are always made slowly and smoothly.

[212man]

From a well known operators EC155 Part B.

Looks familiar!

[Adroight]

Thought you may remember that 212man. These things are not S92's

[212man]

Thought you may remember that 212man

Hard to believe it's 12 years since I wrote it - time flies....