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Wake Turbulence Separation and helicopters

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Wake Turbulence Separation and helicopters

Old 16th Apr 2021, 08:25
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Wake Turbulence Separation and helicopters

Sorry for intruding onto an ATC forum but I have a question regarding the application of wake turbulence separation between FW and RW.

The reasons for separating FW from FW are obvious since a few knots of differential airflow between one wing and another due to turbulence can be very dangerous leading to stall and loss of control.

On a helicopter, large differences of airspeed exist between one side of the rotor disc and the other all the time, with the rare exception of a still air hover in flat calm conditions, and that difference can be very large, even in the hover on a windy day.

The differences in airflow don't lead to stall or loss of control in a helicopter - even something as light as an R 22 can operate in gusty, bumpy conditions and winds varying far more than in the wake of a 747.

So my question is - why apply the same separation criteria, whether it be distance based or time based, between a FW and a RW?

A second question is, what alleviations are allowed to disregard wake turbulence separation when wind conditions - ie strong crosswind away from other traffic - mean the wake cannot possibly persist and be affecting other traffic?

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Old 16th Apr 2021, 09:56
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To my knowledge, the primary problem is flipping in case you enter the rotary wake generated around the wing tips of the aircraft ahead.

Other types of turbulence, the one generated from stuff hanging from the aircraft, from preceeding is relatively easy to deal with, as it would in most cases require a slight increase in speed to overcome, it's just as you say, gusty and bumpy.

But entering the rotary wake, which can be very forcefull, is also a problem when flying a helicopter.
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Old 16th Apr 2021, 12:36
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Even in severe turbulence ie much much worse than encountered near a large jet in take off or approach configuration, a helicopter remains perfectly controllable and has no desire to flip upside down. It might wobble a bit but it doesn't stall or spin.

After nearly 40 years of helicopter flying both mil and civil, mixing it with the big stuff when required, I have never been affected by wake turbulence from a FW.

A 3 min separation from a heavy jet is just massive overkill.
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Old 16th Apr 2021, 13:02
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I began flying military helicopters for a living in the first week of 1979 and am still doing so in civilian street. I also hold a CPLA and used to teach close formation in helicopters (no closer than 1 rotor span) and on fixed wing.

In London ATC are now reportedly getting concerned about having to physically separate helicopters from helicopters at the heliport, based on wake turbulence. It's total overkill imho, but as a mere pilot I probably know almost nothing about flying.

"Recommended spacing is..." is still the best way to deal with it. Let the pilot take responsibility and make the decisions.

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Old 16th Apr 2021, 13:46
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Originally Posted by [email protected] View Post
A 3 min separation from a heavy jet is just massive overkill.
Like I said, I really don't know.... I fly Piper 28 and control traffic, have no experience in helicopters. But my understanding of the wing tip vortices is that it creates a circular motion of the air, and that circular motion can be strong enough to flip stuff... though I don't see why it shouldn't affect helicopters?

Some countries allow for the pilots to take the responsibility for wake turbulence separation, if you are able to do so, you'll get "essential traffic information" about the wake turbulence, and a take-off clearance, and off you can go.
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Old 16th Apr 2021, 14:05
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jmmoric,

The wings on helicopters aren't fixed, for one thing. If the rotors fly up or down, the fuselage doesn't necessarily follow.

The only time in 42 years of helicopter flying that I had a problem, albeit very short lived, was about thirty years ago when we were on very short finals to pick up an underslung load alongside a runway and a Chinook flew directly over us, probably less than 100 feet above, when he misunderstood his landing clearance and approached the wrong landing spot.
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Old 16th Apr 2021, 14:12
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Originally Posted by ShyTorque View Post
jmmoric,

The wings on helicopters aren't fixed, for one thing. If the rotors fly up or down, the fuselage doesn't necessarily follow.
Makes sense.

Unfortunately I can't change rules.
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Old 16th Apr 2021, 15:07
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One of our low hour students got too close to a helicopter on finals a few months ago. The turbulence from the helicopter flipped the aircraft, about 30 degrees I'm told. Not the sort of thing you'd want if a student, or even an experienced plank winger. It seems that the student was allowed to get a bit too close to the heli, with the above result.
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Old 16th Apr 2021, 16:23
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BW, Yes, that situation is well documented and not at all surprising; the downwash from a helicopter can be surprisingly intense. There was a tragic, fatal accident at St. Mawgan/Newquay some years ago where the pilot of a light aircraft went around from an approach and its pilot subsequently lost control after allegedly being affected by downwash from a RAF Seaking operating alongside the runway. The RAF pilot was an acquaintance of mine and the burden of blame placed on him was, in the circumstances, unfair. I don't think he ever got over it and he passed away in sad circumstances at a rather young age.

Crab was referring to the other situation, i.e. how turbulence from a fixed wing tends to affect a following helicopter to a far lesser extent.
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Old 16th Apr 2021, 16:57
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Yeah, Iím with Crab and ST on this 1 as an ATCO it seems pointless - not least when you see helicopters on SAR tasking in Gale Force conditions and hocering happily (or operating the North Sea Rigs etc)

Rules are rules......... and they have added tiltrotors into that as well now. (Obvs we get a lot of them!)

Bizarelly, when we operated the Helipad at the Farnborough Airshow we had heavy depatures off runway 24 with the 25 FATO converging on the runway with no worries (and aircraft parking/manouevring inbetween)




Last edited by AlanM; 16th Apr 2021 at 17:42. Reason: Wrong photo
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Old 16th Apr 2021, 18:09
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Originally Posted by ShyTorque View Post
"Recommended spacing is..." is still the best way to deal with it. Let the pilot take responsibility and make the decisions.
At Aberdeen, still one of the busiest airports for helicopter movements even with the downturn, that is what happens, and in my experience, it is acknowledged and that is the end of it!
Is it ever mandatory?
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Old 17th Apr 2021, 06:35
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1) Because we’re told to (agree it’s overkill for helicopters)

2) Not in the civvy world, although there are a few concepts out there in the R and D phase to reduce or completely negate wake turbulence separation when there’s a crosswind greater than 7kts+, but these concepts are based on fixed wing pairs departing from a runway on SIDs.
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Old 17th Apr 2021, 06:49
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I get the impression that there is something of a grey area regarding this requirement to apply FW-FW separation rules to FW-RW - where are the rules that mandate this application of separation?

Shy - didn't realise you knew PB - that crew at St Mawgan were all my colleagues at Chivenor at the time and I still work with the Rad Op. The downwash allegation was nonsense and, when showed to be so, the family (seeking compensation) changed their argument to propose distraction caused by the proximity of the Sea King.
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Old 17th Apr 2021, 07:20
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I recall an awful accident at Oxford in the early 90's when a light aircraft (PA28, I think) was flipped on short finals by the downwash from a large helicopter (S61, I think, which was operating shuttles from there as it was British Grand Prix weekend).

I remember hearing about the accident on the radio news, and a few days later that the instructor killed had been an instructor of mine at Brize Norton Flying Club.
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Old 17th Apr 2021, 10:42
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Originally Posted by Wycombe View Post
I recall an awful accident at Oxford in the early 90's when a light aircraft (PA28, I think) was flipped on short finals by the downwash from a large helicopter (S61, I think, which was operating shuttles from there as it was British Grand Prix weekend).
Here is a link to an AAIB report for a PA-28 crash on short final believed caused by wake turbulence from an S-76 in 2009, for interested parties.

I have only a university aerospace engineering degree and yes, rotor blades flap - as I recall, to mitigate dissymmetry of lift as the aircraft moves - but is there a limit to the hinge angle, which, if exceeded, could cause the helicopter to roll anyway?

Could a wake vortex act on the rotor and fuselage independently? I suppose there must be a distance at which the vortex diameter is about the same as the length from the rotor head to the fuselage, so the top of the vortex could grab the rotor and the bottom the fuselage? Although I suspect this distance would be so close to the landing aeroplane that the helicopter pilot would have bigger problems anyway.

Edit: after a quick research, no, don't think so. I just read a paper where the vortex core was measured and it's only about a foot across for a 757 / a bit more for an MD-11. Presumably the velocity outside of the core rapidly decays to a point that wouldn't bother a helicopter


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Old 17th Apr 2021, 19:53
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Look here MATS Part 1 and see from page 103 onwards.
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Old 17th Apr 2021, 20:09
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I'm definitely not a helicopter pilot, I do find them interesting. I'm quite surprised that wake turbulence, that would easily down smaller jets and airplanes, isn't a worry in the RW world!
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Old 18th Apr 2021, 08:34
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I was going to highlight the MATS Part 1 text so thanks for that.

​​​​​​The problem is that whilst we know that helicopters can cause wake vortices, little or no research has been done on how helicopters are affected by these vortices and the turbulence they create and how well they can resist those effects. As such, the wake turbulence separations are applied equally to both fixed and rotary wing.
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Old 18th Apr 2021, 09:52
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The problem is that whilst we know that helicopters can cause wake vortices, little or no research has been done on how helicopters are affected by these vortices and the turbulence they create and how well they can resist those effects. As such, the wake turbulence separations are applied equally to both fixed and rotary wing.
And that seems to be the problem - no-one wants to listen to helicopter pilots who know the wake turbulence from a FW just isn't a problem - downwash from another helicopter is a different matter and we all know about how to avoid that.

Even the MATS pages (thanks for the links) specify in the opening paragraphs that wake turbulence vortices are most hazardous to aircraft with a small wingspan during the take-off, initial climb, final approach and landing phases of flight - nothing there about affecting helicopters at all.

It just seems a very lazy option to apply FW limitations to RW operations.

If you are trying to complete an instructional sortie within an allotted time (because the customer is paying) and you are constantly held and delayed for wake turbulence, it is a frustrating and inefficient restriction of airspace, but it seems the controllers hands are tied because the authorities can't be bothered to acknowledge that helicopters are different.

I am led to believe that the FAA don't apply wake turbulence separation to helicopters on approach behind a FW.
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Old 18th Apr 2021, 11:55
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It’s by no means unusual for ATC to hold our heli on the runway prior to departure, in CAVOK conditions, for a preceding fixed wing departure in a strong “away” crosswind; even though it’s perfectly obvious from the adjacent windsock that wake turbulence from that aircraft was never really over the runway and well within three minutes it wouldn’t even be over the airfield.

Thing is, when hover taxiing (or arguably, partly so, even when ground taxiing), a helicopter has already taken off to get to the runway departure point. To subsequently hold it for wake turbulence reasons is absurd.
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